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                  THE INTERNATIONAL SCIENTIFIC SERIES
                               VOLUME LV




                  THE INTERNATIONAL SCIENTIFIC SERIES




                              EARTHQUAKES
                                  AND
                         OTHER EARTH MOVEMENTS


                                  BY
                              JOHN MILNE

PROFESSOR OF MINING AND GEOLOGY IN THE IMPERIAL COLLEGE OF ENGINEERING,
                             TOKIO, JAPAN

                      _WITH THIRTY-EIGHT FIGURES_

                               NEW YORK
                        D. APPLETON AND COMPANY
                        1, 3, AND 5 BOND STREET
                                 1886




                               PREFACE.

                   •       •       •       •       •


In the following pages it has been my object to give a systematic
account of various Earth Movements.

These comprise _Earthquakes_, or the sudden violent movements of the
ground; _Earth Tremors_, or minute movements which escape our attention
by the smallness of their amplitude; _Earth Pulsations_, or movements
which are overlooked on account of the length of their period; and
lastly, _Earth Oscillations_, or movements of long period and large
amplitude which attract so much attention from their geological
importance.

It is difficult to separate these Earth Movements from each other,
because they are phenomena which only differ in degree, and which are
intimately associated in their occurrence and in their origin.

                   •       •       •       •       •

Because Earthquakes are phenomena which have attracted a universal
attention since the earliest times, and about them so many observations
have been made, they are treated of at considerable length.

As very much of what might be said about the other Earth Movements is
common to what is said about Earthquakes, it has been possible to make
the description of these phenomena comparatively short.

The scheme which has been adopted will be understood from the following
table:—

                         I. EARTHQUAKES.

  1. Introduction.
  2. Seismometry.
                                     {(_a_) Theoretically.
  3. Earthquake Motion.              {(_b_) As deduced from experiments.
                                     {(_c_) As deduced from actual
                                             Earthquakes.
  4. Earthquake Effects.             {(_a_) On land.
                                     {(_b_) In the ocean.
  5. Determination of Earthquake origins.
                                     {(_a_) In space.
  6. Distribution of Earthquakes.    {(_b_) In time (geological time,
                                     {       historical  time, annual,
                                     {       seasonal, diurnal, &c.)
  7. Cause of Earthquakes.
  8. Earthquake prediction and warning.

                        II. EARTH TREMORS.

                       III. EARTH PULSATIONS.

                        IV. EARTH OSCILLATIONS.

In some instances the grouping of phenomena according to the above
scheme may be found inaccurate, as, for example, in the chapters
referring to the effects and causes of Earthquakes.

This arises from the fact that the relationship between Earthquakes
and other Earth phenomena are not well understood. Thus the sudden
elevation of a coast line and an accompanying earthquake may be
related, either as effect and cause, or _vice versâ_, or they may both
be the effect of a third phenomenon.

Much of what is said respecting Earthquake motion will show how little
accurate knowledge we have about these disturbances. Had I been writing
in England, and, therefore, been in a position to make references to
libraries and persons who are authorities on subjects connected with
Seismology, the following pages might have been made more complete,
and inaccuracies avoided. A large proportion of the material embodied
in the following pages is founded on experiments and observations
made during an eight years’ residence in Japan, where I have had the
opportunity of recording an earthquake every week.

The writer to whom I am chiefly indebted is Mr. Robert Mallet. Not
being in a position to refer to original memoirs, I have drawn many
illustrations from the works of Professor Karl Fuchs and M. S. di
Rossi. These, and other writers to whom reference has been made, are
given in an appendix.

For seeing these pages through the press, my thanks are due to
Mr. Thomas Gray, who, when residing in Japan, did so much for the
advancement of observational Seismology.

For advice and assistance in devising experiments, I tender my thanks
to my colleagues, Professor T. Alexander, Mr. T. Fujioka, and to my
late colleague. Professor John Perry.

For assistance in the actual observation of Earthquakes, I have to
thank my friends in various parts of Japan, especially Mr. J. Bissett
and Mr. T. Talbot, of Yokohama. For assistance in obtaining information
from Italian sources I have to thank Dr. F. Du Bois, from German
sources Professor C. Netto, and from Japanese sources Mr. B. H.
Chamberlain. For help in carrying out experiments, I am indebted to
the liberality of the British Association, the Geological Society of
London, the Meteorological and Telegraph departments of Japan, and to
the officers of my own institution, the Imperial College of Engineering.

And, lastly, I offer my sincere thanks to those gentlemen who have
taken part in the establishment and working of the Seismological
Society of Japan, and to my publishers, whose liberality has enabled me
to place the labours of residents in the Far East before the European
public.

                                                          JOHN MILNE.

  TOKIO, JAPAN; _June 30, 1883_




                               CONTENTS.

                   •       •       •       •       •

                              CHAPTER I.

                             INTRODUCTION.

                                                                   PAGE
  Relationship of man to nature—The aspect of a country
    is dependent on geological phenomena—Earthquakes an
    important geological phenomenon—Relationship of seismology
    to the sciences and arts—Earth movements other than
    earthquakes—Seismological literature—(Writings of Perrey,
    Mallet, Eastern writings, the Philosophical Transactions of
    the Royal Society, the ‘Gentleman’s Magazine,’ the Bible,
    Herodotus, Pliny, Hopkins, Von Hoff, Humboldt, Schmidt,
    Seebach, Lasaulx, Fuchs, Palmieri, Bertelli, Seismological
    Society of Japan)—Seismological terminology                        1


                              CHAPTER II.

                             SEISMOMETRY.

  Nature of earthquake vibrations—Many instruments called
    seismometers only seismoscopes—Eastern seismoscopes, columns,
    projection seismometers—Vessels filled with liquid—Palmieri’s
    mercury tubes—The ship seismoscope—The cacciatore—Pendulum
    instruments of Kreil, Wagner, Ewing, and Gray—Bracket
    seismographs—West’s parallel motion instrument—Gray’s conical
    pendulums, rolling spheres, and cylinders—Verbeck’s ball and
    plate seismograph—The principle of Perry and Ayrton—Vertical
    motion instruments—Record receiver—Time-recording
    apparatus—The Gray and Milne seismograph                          12


                             CHAPTER III.

              EARTHQUAKE MOTION DISCUSSED THEORETICALLY.

  Ideas of the ancients (the views of Travagini, Hooke, Woodward,
    Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
    and vibrations—Possible causes of disturbance in the earth’s
    crust—The time of vibration of an earth particle—Velocity
    and acceleration of a particle—Propagation of a disturbance
    as determined by experiments upon the elastic moduli of
    rocks—The intensity of an earthquake—Area of greatest
    overturning moment—Earthquake waves—Reflexion, refraction,
    and interference of waves—Radiation of a disturbance              41


                              CHAPTER IV.

             EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.

  Experiments with falling weights—Experiments with
    explosives—Results obtained from experiments—Relative
    motion of two adjacent points—The effect of hills and
    excavations upon the propagation of vibrations—The intensity
    of artificial disturbances—Velocity with which earth
    vibrations are propagated—Experiments of Mallet—Experiments
    of Abbot—Experiments in Japan—Mallet’s results—Abbot’s
    results—Results obtained in Japan                                 57


                              CHAPTER V.

             EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION
                            ON EARTHQUAKES.

  Result of feelings—The direction of motion—Instruments as
    indicators of direction—Duration of an earthquake—Period
    of vibration—The amplitude of earth movements—Side of
    greatest motion—Intensity of earthquakes—Velocity and
    acceleration of an earth particle—Absolute intensity of an
    earthquake—Radiation of an earthquake—Velocity of propagation     67


                              CHAPTER VI.

            EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.

  The destruction of buildings is not irregular—Cracks in
    buildings—Buildings in Tokio—Relation of destruction
    to earthquake motion—Measurement of relative motion of
    parts of a building shaken by an earthquake—Prevention of
    cracks—Direction of cracks—The pitch of roofs—Relative
    position of openings in a wall—The last house in a row—The
    swing of buildings—Principle of relative vibrational periods      96


                             CHAPTER VII.

            EFFECTS PRODUCED UPON BUILDINGS (_continued_).

  Types of buildings used in earthquake countries—In Japan,
    in Italy, in South America, in Caraccas—Typical houses
    for earthquake countries—Destruction due to the nature of
    underlying rocks—The swing of mountains—Want of support
    on the face of hills—Earthquake shadows—Destruction due
    to the interference of waves—Earthquake bridges—Examples
    of earthquake effects—Protection of buildings—General
    conclusions                                                      122


                             CHAPTER VIII.

                    EFFECTS OF EARTHQUAKES ON LAND.

  1. Cracks and fissures—Materials discharged from
    fissures—Explanation of fissure phenomena. 2. Disturbances
    in lakes, rivers, springs, wells, fumaroles, &c.—Explanation
    of these latter phenomena. 3. Permanent displacement of
    ground—On coast lines—Level tracts—Among
    mountains—Explanation of these movements                         146


                              CHAPTER IX.

                      DISTURBANCES IN THE OCEAN.

  Sea vibrations—Cause of vibratory blows—Sea waves:
    preceding earthquakes; succeeding earthquakes—Magnitude
    of waves—Waves as recorded in countries distant from the
    origin—Records on tide gauges—Waves without earthquakes—Cause
    of waves—Phenomena difficult of explanation—Velocity
    of propagation—Depth of the ocean—Examples of
    calculations—Comparison of velocities of earthquake waves
    with velocities which ought to exist from the known depth of
    the ocean                                                        163


                              CHAPTER X.

                 DETERMINATION OF EARTHQUAKE ORIGINS.

  Approximate determination of an origin—Earthquake-hunting
    in Japan—Determinations by direction of motion—Direction
    indicated by destruction of buildings—Direction
    determined by rotation—Cause of rotation—The use of time
    observations—Errors in such observations—Origin determined
    by the method of straight lines—The method of circles, the
    method of hyperbolas, the method of co-ordinates—Haughton’s
    method—Difference in time between sound, earth, and water
    waves—Method of Seebach                                          187


                              CHAPTER XI.

                  THE DEPTH OF AN EARTHQUAKE CENTRUM.

  The depth of an earthquake centrum—Greatest possible depth of
    an earthquake—Form of the focal cavity                           213


                             CHAPTER XII.

            DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.

  General distribution of earthquakes—Occurrence along
    lines—Examples of distribution—Italian earthquake of 1873—In
    Tokio—Extension of earthquake boundaries—Seismic energy in
    relation to geological time; to historical time—Relative
    frequency of earthquakes—Synchronism of earthquakes—Secondary
    earthquakes 226


                             CHAPTER XIII.

  DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_)                  234


                             CHAPTER XIV.

          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).

  The occurrence of earthquakes in relation to the position of
    the heavenly bodies—Earthquakes and the moon—Earthquakes
    and the sun; and the seasons; the months—Planets and
    meteors—Hours at which earthquakes are frequent—Earthquakes
    and sun spots—Earthquakes and the aurora 250


                              CHAPTER XV.

  BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN
    TEMPERATURE AND EARTHQUAKES                                      266


                             CHAPTER XVI.

              RELATION OF SEISMIC TO VOLCANIC PHENOMENA.

  Want of synchronism between earthquakes and volcanic
    eruptions—Synchronism between earthquakes and volcanic
    eruptions—Conclusion                                             270


                             CHAPTER XVII.

                       THE CAUSE OF EARTHQUAKES.

  Modern views respecting the cause of earthquakes—Earthquakes
    due to faulting—To explosions of steam—To volcanic
    evisceration—To chemical degradation—Attractive influence of
    the heavenly bodies—The effect of oceanic tides—Variation in
    atmospheric pressure—Fluctuation in temperature—Winds and
    earthquakes—Rain and earthquakes—Conclusion                      277


                            CHAPTER XVIII.

                      PREDICTION OF EARTHQUAKES.

  General nature of predictions—Prediction by the observation of
    unusual phenomena (Alteration in the appearance and taste of
    springs; underground noises; preliminary tremors; Earthquake
    prophets—warnings furnished by animals, &c.)—Earthquake
    warning                                                          297


                             CHAPTER XIX.

                            EARTH TREMORS.

  Artificially produced tremors—Observations of Kater, Denman,
    Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
    M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
    seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
    Count Malvasia, M. S. di Rossi—Instruments employed in
    Italy—Tromometers, microseismographs, microphones—Results
    obtained in Italy—In Japan—Cause of microseismic motion          306


                              CHAPTER XX.

                           EARTH PULSATIONS.

  Definition of an earth pulsation—Indications of
    pendulums—Indications of levels—Other phenomena indicating
    the existence of earth pulsations—Disturbances in lakes and
    oceans—Phenomena resultant on earth pulsations—Cause of earth
    pulsations                                                       326


                             CHAPTER XXI.

                          EARTH OSCILLATIONS.

  Evidences of oscillation—Examples of oscillation—Temple of
    Jupiter Serapis—Observations of Darwin—Causes of oscillation     344


  APPENDIX                                                           349


  INDEX                                                              359




                             EARTHQUAKES.

                   •       •       •       •       •




                              CHAPTER I.

                             INTRODUCTION.

  Relationship of man to nature—The aspect of a country is dependent
    on geological phenomena—Earthquakes an important geological
    phenomenon—Relationship of seismology to the sciences and
    arts—Earth movements other than earthquakes—Seismological
    literature—(Writings of Perrey, Mallet, Eastern writings, the
    Philosophical Transactions of the Royal Society, the ‘Gentleman’s
    Magazine,’ the Bible, Herodotus, Pliny, Hopkins, Von Hoff,
    Humboldt, Schmidt, Seebach, Lasaulx, Fuchs, Palmieri, Bertelli,
    Seismological Society of Japan)—Seismological terminology.


In bygone superstitious times lightning and thunder were regarded
as supernatural visitations. But as these phenomena became better
understood, and men learned how to avoid their destructive power, the
superstition was gradually dispelled. Thus it is with Earthquakes:
the more clearly they are understood, the more confident in the
universality of law will man become, and the more will his mental
condition be advanced.

In his ‘History of Civilisation in England,’ Buckle has laid
considerable stress upon the manner in which earthquakes, volcanoes,
and other of the more terrible forms in which the workings of
nature reveal themselves to us, have exerted an influence upon the
imagination and understanding; and just as a sudden fright may affect
the nerves of a child for the remainder of its life, we have in the
annals of seismology records which indicate that earthquakes have not
been without a serious influence upon the mental condition of whole
communities.

To a geologist there are perhaps no phenomena in nature more
interesting than earthquakes, the study of which is called Seismology.
Coming, as shocks often will, from a region of volcanoes, the study
of these disturbances may enable us to understand something about
the nature and working of a volcano. As an earthquake wave travels
along from strata to strata, if we study its reflections and changing
velocity in transit, we may often be led to the discovery of certain
rocky structures buried deep beneath our view, about which, without the
help of such waves, it would be hopeless ever to attain any knowledge.

By studying the propagation of earthquake waves the physicist is
enabled to confirm his speculations respecting the transmission of
disturbances in elastic media. For the physicist earthquakes are
gigantic experiments which tell him the elastic moduli of rocks as
they exist in nature, and when properly interpreted may lead him to
the proper comprehension of many ill-understood phenomena. It is not
impossible that seismological investigation may teach us something
about the earth’s magnetism, and the connection between earthquakes and
the ‘earth currents’ which appear in our telegraph wires. These and
numerous other kindred problems fall within the domain of the physicist.

It is of interest to the meteorologist to know the connections which
probably exist between earthquakes and the fluctuations of the
barometer, the changes of the thermometer, the quantity of rainfall,
and like phenomena to which he devotes his attention.

Next we may turn to the more practical aims of seismology and ask
ourselves what are the effects of earthquakes upon buildings, and
how, in earthquake-shaken countries, the buildings are to be made to
withstand them. Here we are face to face with problems which demand
the attention of engineers and builders. To attain what we desire,
observation, common sense, and subtle reasoning must be brought to bear
upon this subject.

In the investigation of the principle on which earthquake instruments
make their records, in the analysis of the results they give, in
problems connected with astronomy, with physics, and with construction,
seismology offers to the mathematician new fields for investigation.

A study of the effects which earthquakes produce on the lower animals
will not fail to interest the student of natural history.

A study like seismology, which leads us to a more complete knowledge
of earth-heat and its workings, is to be regarded as one of the
corner-stones of geology. The science of seismology invites the
co-operation of workers and thinkers in almost every department of
natural science.

We have already referred to the influence exerted by earthquakes over
the human mind. How to predict earthquakes, and how to escape from
their dangers, are problems which, if they can be solved, are of
extreme interest to the world at large.

In addition to the sudden and violent movements which we call
earthquakes, the seismologist has to investigate the smaller motions
which we call earth tremors. From observations which have been made
of late years, it would appear that the ground on which we dwell is
incessantly in a state of tremulous motion.

A further subject of investigation which is before the seismologist is
the experimental verification of the existence of what may be called
‘earth-pulsations.’ These are motions which mathematical physicists
affirmed the existence of, but which, in consequence of the slowness of
their period, have hitherto escaped observation.

The oscillations, or slow changes in the relative positions of land and
sea, might also be included; but this has already been taken up as a
separate branch of geology.

These four classes of movements are no doubt interdependent, and
seismology in the widest sense might conveniently be employed to
include them all. In succeeding chapters we will endeavour to indicate
how far the first three of these branches have been prosecuted, and
to point out that which remains to be accomplished. It is difficult,
however, to form a just estimate of the amount of seismological work
which has been done, in consequence of the scattered and uncertain
nature of many of the records. Seismology, as a science, originated
late, chiefly owing to the facts that centres of civilisation are
seldom in the most disturbed regions, and that earthquake-shaken
countries are widely separated from each other.

As every portion of the habitable globe appears to have been shaken
more or less by earthquakes, and as these phenomena are so terrible in
their nature, we can readily understand why seismological literature is
extensive. In the annals of almost every country which has a written
history, references are made to seismic disturbances.

An idea of the attention which earthquakes have received may be
gathered from the fact that Professor Alexis Perrey, of Dijon, who
has published some sixty memoirs on this subject, gave, in 1856, a
catalogue of 1,837 works devoted to seismology.[1] In 1858 Mr. Robert
Mallet published in the Reports of the British Association a list of
several hundred works relating to earthquakes. Sixty-five of these
works are to be found in the British Museum. So far as literature is
concerned, earthquakes have received as much attention in the East as
in the West. In China there are many works treating on earthquakes, and
the attention which these phenomena received may be judged of from the
fact that in A.D. 136 the Government appointed a commission
to inquire into the subject. Even the isolated empire of Japan can
boast of at least sixty-five works on earthquakes, seven of which
are earthquake calendars, and twenty-three earthquake monographs.[2]
Besides those treating especially of earthquakes, there are innumerable
references to such disturbances in various histories, in the
transactions of learned societies, and in periodicals. To attempt to
give a complete catalogue of even the books which have been written
would be to enter on a work of compilation which would occupy many
years, and could never be satisfactorily finished.

In the ‘Philosophical Transactions of the Royal Society,’ which were
issued in the eighteenth century, there are about one hundred and
eighty separate communications on earthquakes; and in the ‘Gentleman’s
Magazine’ for 1755 there are no less than fifty notes and articles on
the same subject. The great interest shown in earthquakes about the
years 1750–60 in England, was chiefly due to the terrible calamity
which overtook Lisbon in 1755, and to the fact that about this time
several shocks were experienced in various parts of the British
Islands. In 1750, which may be described as the earthquake year of
Britain, ‘a shock was felt in Surrey on March 14; on the 18th of the
same month the whole of the south-west of England was disturbed. On
April 2, Chester was shaken; on June 7, Norwich was disturbed; on
August 23, the inhabitants of Lancashire were alarmed; and on September
30 ludicrous and alarming scenes occurred in consequence of a shock
having been felt during the hours of Divine service, causing the
congregations to hurry into the open air.’[3] As might be expected,
these occurrences gave rise to many articles and notes directing
attention to the subject of earthquakes.

Seismic literature has not, however, at all times been a measure of
seismic activity: thus, in Japan, the earthquake records for the
twelfth and sixteenth centuries scarcely mention any shocks. At first
sight it might be imagined that this was owing to an absence of
earthquakes; but it is sufficiently accounted for by the fact that
at that time the country was torn with civil war, and matters more
urgent than the recording of natural phenomena engaged the attention of
the inhabitants. Professor Rockwood, who has given so much attention
to seismic disturbances in America, tells us that during the recent
contest between Chili and Peru a similar intermission is observable. We
see, therefore, that an absence of records does not necessarily imply
an absence of the phenomena to be recorded.

Perhaps the earliest existing records of earthquakes are those which
occur in the Bible. The first of these, which we are told occurred in
Palestine, was in the reign of Ahab (B.C. 918–897).[4] One of the most
terrible earthquakes mentioned in the Bible is that which took place
in the days of Uzziah, king of Judah (B.C. 811–759), which shook the
ground and rent the Temple. The awful character of this, and the deep
impression produced on men’s minds, may be learned from the fact that
the time of its occurrence was subsequently used as an epoch from which
to reckon dates.

The writings of Herodotus, Pliny, Livy, &c., &c., show the interest
which earthquakes attracted in early ages. These writers chiefly
devoted themselves to references and descriptions of disastrous shocks,
and to theories respecting the cause of earthquakes.

The greater portion of the Japanese notices of earthquakes is simply
a series of anecdotes of events which took place at the time of
these disasters. We also find references to superstitious beliefs,
curious occurrences, and the apparent connection between earthquake
disturbances and other natural phenomena. In these respects the
literature of the East closely resembles that of the West. The
earthquake calendars of the East, however, form a class of books which
can hardly be said to find their parallel in Europe;[5] while, on the
other hand, the latter possesses types of books and pamphlets which do
not appear to have a parallel elsewhere. These are the more or less
theological works—‘Moral Reflections on Earthquakes,’ ‘Sermons’ which
have been preached on earthquakes, ‘Prayers’ which have been appointed
to be read.[6]

Speaking generally, it may be said that the writings of the ancients,
and those of the Middle Ages, down to the commencement of the
nineteenth century, tended to the propagation of superstition and to
theories based on speculations with few and imperfect facts for their
foundation.

Among the efforts which have been made in modern times to raise
seismology to a higher level, is that of Professor Perrey, of Dijon,
who commenced in 1840 a series of extensive catalogues embracing
the earthquakes of the world. These catalogues enabled Perrey, and
subsequently Mallet in his reports to the British Association, to
discuss the periodicity of earthquakes, with reference to the seasons
and to other phenomena, in a more general manner than it had been
possible for previous workers to accomplish. The facts thus accumulated
also enabled Mallet to discuss earthquakes in general, and the various
phenomena which they present were sifted and classified for inspection.
Another great impetus which observational seismology received was Mr.
Mallet’s report upon the Neapolitan earthquake of 1857, in which new
methods of seismic investigation were put forth. These have formed
the working tools of many subsequent observers, and by them, as well
as by his experiments on artificially produced disturbances, Mallet
finally drew the study of earthquakes from the realms of speculation by
showing that they, like other natural phenomena, were capable of being
understood and investigated.

In addition to Perrey and Mallet, the nineteenth century has produced
many writers who have taken a considerable share in the advancement of
seismology. There are the catalogues of Von Hoff, the observations of
Humboldt, the theoretical investigations of Hopkins, the monographs of
Schmidt, Seebach, Lasaulx, and others; the books of Fuchs, Credner,
Vogt, Volger; the records and observations of Palmieri, Bertelli,
Rossi, and other Italian observers. To these, which are only a few
out of a long list of names, may be added the publications of the
Commission appointed for the observation of earthquakes by the Natural
History Society of Switzerland, and the volumes which have been
published by the Seismological Society of Japan.

Before concluding this chapter it will be well to define a few of the
more ordinary terms which are used in describing earthquake phenomena.
It may be observed that the English word _earthquake_, the German
_erdbeben_, the French _tremblement de terre_, the Spanish _terremoto_,
the Japanese _jishin_ &c., all mean, when literally translated,
_earth-shaking_, and are popularly understood to mean a sudden and more
or less violent disturbance.

Seismology (σειμός an earthquake, λόγος a discourse) in its simplest
sense means the study of earthquakes. To be consistent with a Greek
basis for seismological terminology, some writers have thrown aside
the familiar expression ‘earthquake,’ and substituted the awkward word
‘seism.’

The source from which an earthquake originates is called the ‘origin,’
‘focal cavity,’ or ‘centrum.’

The point or area on the surface of the ground above the origin is
called the ‘epicentrum.’ The line joining the centrum and epicentrum is
called the ‘seismic vertical.’

The radial lines along which an earthquake may be propagated from the
centrum are called ‘wave paths.’

The angle which a wave path, where it reaches the surface of the earth,
makes with that surface is called the ‘angle of emergence’ of the wave.
This angle is usually denoted by the letter _e_.

As the result of a simple explosion at a point in a homogeneous medium,
we ought, theoretically, to obtain at points on the surface of the
medium equidistant from the epicentrum, equal mechanical effects.
These points will lie on circles called ‘isoseismic’ or ‘coseismic’
circles. The area included between two such circles is an ‘isoseismic
area.’ In nature, however, isoseismic lines are seldom circles.
Elliptical or irregular curves are the common forms.

The isoseismic area in which the greatest disturbance has taken place
is called the ‘meizoseismic area.’ Seebach calls the lines enclosing
this area ‘pleistoseists.’

These last-mentioned lines are wholly due to Mallet and Seebach.

Many words are used to distinguish different kinds of earthquakes from
each other. All of these appear to be very indefinite and to depend
upon the observer’s feelings, which, in turn, depend upon his nervous
temperament and his situation.

In South America small earthquakes, consisting of a series of rapidly
recurring vibratory movements not sufficiently powerful to create
damage, are spoken of as _trembelores_.

The _terremotos_ of South America are earthquakes of a destructive
nature, in which distinct shocks are perceptible. It may be observed
that shocks which at one place would be described as _terremoto_
would at another and more distant place probably be described as
_trembelores_.

The _succussatore_ are the shocks where there is considerable vertical
motion. The terrible shock of Riobamba (February 4, 1797), which is
said to have thrown corpses from their graves to a height of 100 feet,
was an earthquake of this order.

The _vorticosi_ are shocks which have a twisting or rotatory motion.

Another method of describing earthquakes would be to refer to
instrumental records. When the vibrations of the ground have only
been along the line joining the observer and the epicentrum, the
disturbance might be called ‘euthutropic.’ A disturbance in which the
prominent movements are _transverse_ to the above direction might be
called ‘diagonic.’ If motions in both of these directions occur in the
records, the shock might be said to be ‘diastrophic.’ If there be much
vertical movement, the shock might be said to be ‘anaseismic.’ Some
disturbances could only be described by using two or three of these
terms.




                              CHAPTER II.

                             SEISMOMETRY.

  Nature of earthquake vibrations—Many instruments called
    seismometers only seismoscopes—Eastern seismoscopes, columns,
    projection seismometers—Vessels filled with liquid—Palmieri’s
    mercury tubes—The ship seismoscope—The cacciatore—Pendulum
    instruments of Kreil, Wagner, Ewing, and Gray—Bracket
    seismographs—West’s parallel motion instrument—Gray’s conical
    pendulums, rolling spheres, and cylinders—Verbeck’s ball and
    plate seismograph—The principle of Perry and Ayrton—Vertical
    motion instruments—Record receivers—Time-recording apparatus—The
    Gray and Milne seismograph.


Before we discuss the nature of earthquake motion, the determination
of which has been the aim of modern seismological investigation, the
reader will naturally look for an account of the various instruments
which have been employed for recording such disturbances. A description
of the earthquake machines which have been used even in Japan would
form a bulky volume. All that we can do, therefore, is to describe
briefly the more prominent features of a few of the more important of
these instruments. In order that the relative merits of these may be
better understood, we may state generally that modern research has
shown a typical earthquake to consist of a series of small tremors
succeeded by a shock, or series of shocks, separated by more or less
irregular vibrations of the ground. The vibrations are often both
irregular in period and in amplitude, and they have a duration of from
a few seconds to several minutes. We will illustrate the records of
actual earthquakes in a future chapter, but in the meantime the idea
that an earthquake consists of a single shock must be dismissed from
the imagination.

To construct an instrument which at the time of an earthquake shall
move and leave a record of its motion, there is but little difficulty.
Contrivances of this order are called _seismoscopes_. If, however,
we wish to know the period, extent, and direction of each of the
vibrations which constitutes an earthquake, we have considerable
difficulty. Instruments which will in this way measure or write down
the earth’s motions are called _seismometers_ or _seismographs_.

Many of the elaborate instruments supplemented with electro-magnetic
and clockwork arrangements are, when we examine them, nothing more than
elaborate seismoscopes which have been erroneously termed seismographs.

The only approximations to true seismographs which have yet been
invented are without doubt those which during the past few years have
been used in Japan. It would be a somewhat arbitrary proceeding,
however, to classify the different instruments as seismoscopes,
seismometers, and seismographs, as the character of the record given
by certain instruments is sometimes only seismoscopic, whilst at other
times it is seismometric, depending on the nature of the disturbance.
Many instruments, for instance, would record with considerable accuracy
a single sudden movement, but would give no reliable information
regarding a continued shaking.

_Eastern Seismoscopes._—The earliest seismoscope of which we find any
historical record is one which owes its origin to a Chinese called
Chôko. It was invented in the year A.D. 136. A description is given
in the Chinese history called ‘Gokanjo,’ and the translation of this
description runs as follows:—

‘In the first year of Yōka, A.D. 136, a Chinese called Chôko invented
the seismometer shown in the accompanying drawing. This instrument
consists of a spherically formed copper vessel, the diameter of which
is eight feet. It is covered at its top, and in form resembles a
wine-bottle. Its outer part is ornamented by the figures of different
kinds of birds and animals, and old peculiar-looking letters. In the
inner part of this instrument a column is so suspended that it can
move in eight directions. Also, in the inside of the bottle, there is
an arrangement by which some record of an earthquake is made according
to the movement of the pillar. On the outside of the bottle there
are eight dragon heads, each of which holds a ball in its mouth.
Underneath these heads there are eight frogs so placed that they appear
to watch the dragon’s face, so that they are ready to receive the ball
if it should be dropped. All the arrangements which cause the pillar to
knock the ball out of the dragon’s mouth are well hidden in the bottle.’

[Illustration: FIG. 1.]

‘When an earthquake occurs, and the bottle is shaken, the dragon
instantly drops the ball, and the frog which receives it vibrates
vigorously; any one watching this instrument can easily observe
earthquakes.’

With this arrangement, although one dragon may drop a ball, it is
not necessary for the other seven dragons to drop their balls unless
the movement has been in all directions; thus we can easily tell the
direction of an earthquake.

‘Once upon a time a dragon dropped its ball without any earthquake
being observed, and the people therefore thought the instrument of
no use, but after two or three days a notice came saying that an
earthquake had taken place at Rōsei. Hearing of this, those who doubted
the use of this instrument began to believe in it again. After this
ingenious instrument had been invented by Chōko, the Chinese Government
wisely appointed a secretary to make observations on earthquakes.’

Not only is this instrument of interest on account of its antiquity,
but it is also of interest on account of the close resemblance it bears
to many of the instruments of modern times.

Another earthquake instrument also of Eastern origin is the magnetic
seismoscope of Japan.

On the night of the destructive earthquake of 1855, which devastated
a great portion of Tokio, the owner of a spectacle shop in Asakusa
observed that a magnet dropped some old iron nails and keys which had
been attached to it. From this occurrence the owner thought that the
magnet had, in consequence of its age, lost its powers. About two hours
afterwards, however, the great earthquake took place, after which the
magnet was observed to have regained its powers. This occurrence led
to the construction of the seismoscope, which is illustrated in a book
called the ‘Ansei-Kembun-Roku,’ or a description of the earthquake of
1855, and examples of the instrument are still to be seen in Tokio.
These instruments consist of a piece of magnetic iron ore, which holds
up a piece of iron like a nail. This nail is connected, by means of a
string, with a train of clockwork communicating with an alarm. If the
nail falls a catch is released and the clockwork set in motion, and
warning given by the ringing of a bell. It does not appear that this
instrument has ever acted with success.

_Columns._—One of the commonest forms of seismoscope, and one which
has been very widely used, consists of a round column of wood, metal,
or other suitable material, placed, with its axis vertical, on a level
plane, and surrounded by some soft material such as loose sand to
prevent it rolling should it be overturned. The fall of such a column
indicates that a shaking or shock has taken place. Attempts have been
made by using a number of columns of different sizes to make these
indications seismometric, but they seldom give reliable information
either as to intensity or direction of shock. The indications as to
intensity are vitiated by the fact that a long-continued gentle shaking
may overturn a column which would stand a very considerable sudden
shock, while the directions in which a number of columns fall seldom
agree owing to the rotational motion imparted to them by the shaking.
Besides, the direction of motion of the earthquake seldom remains in
the same azimuth throughout the whole disturbance.

An extremely delicate, and at the same time simple form of seismoscope
may be made by propping up strips of glass, pins, or other easily
overturned bodies against suitably placed supports. In this way bodies
may be arranged, which, although they can only fall in one direction,
nevertheless fall with far less motion than is necessary to overturn
any column which will stand without lateral support.

_Projection Seismometers._—Closely related to the seismoscopes and
seismometers which depend on the overturning of bodies. Mallet has
described two sets of apparatus whose indications depend on the
distance to which a body is projected. In one of these, which consisted
of two similar parts arranged at right angles, two metal balls rest
one on each side of a stop at the lower part of two inclined \/ like
troughs. In this position each of the balls completes an electric
circuit. By a shock the balls are projected or rolled up the troughs,
and the height to which they rise is recorded by a corresponding
interval in the break of the circuits. The vertical component of the
motion is measured by the compression of a spring which carries the
table on which this arrangement rests. In the second apparatus two
balls are successively projected, one by the forward swing, and the
other by the backward swing of the shock. Attached to them are loose
wires forming terminals of the circuits. They are caught in a bed
of wet sand in a metal trough forming the other end of the circuit.
The throw of the balls as measured in the sand, and the difference
of time between their successive projections as indicated by special
contrivances connected with the closing of the circuits, enables
the observer to calculate the direction of the wave of shock, its
velocity, and other elements connected with the disturbance. It will be
observed that the design of this apparatus assumes the earthquake to
consist of a distinct isolated shock.

Oldham, at the end of his account of the Cachar earthquake of 1869,
recommends the use of an instrument based on similar principles. In his
instrument four balls like bullets are placed in notches cut in the
corners of the upper end of a square stake driven into the ground.

_Vessels filled with liquid._—Another form of simple seismoscope is
made by partially filling a vessel with liquid. The height to which the
liquid is washed up the side of the vessel is taken as an indication of
the intensity of the shock, and the line joining the points on which
maximum motion is indicated, is taken as the direction of the shock. If
earthquakes all lasted for the same length of time, and consisted of
vibrations of the same period, such instruments might be of service.
These instruments have, however, been in use from an early date. In
1742 we find that bowls of water were used to measure the earthquakes
which in that year alarmed the inhabitants of Leghorn. About the same
time the Rev. S. Chandler, writing about the shock at Lisbon, tells us
that earthquakes may be measured by means of a spherical bowl about
three or four feet in diameter, the inside of which, after being dusted
over with Barber’s puff, is filled very gently with water. Mallet,
Babbage, and De la Bêche have recommended the same sort of contrivance,
but, notwithstanding, it has justly been criticised as ‘ridiculous and
utterly impracticable.’[7]

An important portion of Palmieri’s well-known instrument consists
of horizontal tubes turned up at the ends and partially filled with
mercury. To magnify the motion of the mercury, small floats of iron
rest on its surface. These are attached by means of threads to a pulley
provided with indices which move in front of a scale of degrees. We
thus read off the intensity of an earthquake as so many degrees, which
means so many millimetres of washing up and down of mercury in a tube.
The direction of movement is determined by the azimuth of the tube
which gives the maximum indication, several tubes being placed in
different azimuths.

This form of instrument appears to have been suggested by Mallet, who
gives an account of the same in 1846. Inasmuch as the rise and fall
of the mercury in such tubes depend on its depth and on the period
of the earthquake together with its duration, we see that although
the results obtained from a given instrument may give us means of
making approximate comparisons as to the relative intensity of various
earthquakes, it is very far from yielding any absolute measurement.

Another method which has been employed to magnify and register the
motions of liquid in a vessel has been to float upon its surface a
raft or ship from which a tall mast projected. By a slight motion of
the raft, the top of the mast vibrated through a considerable range.
This motion of the mast as to direction and extent was then recorded by
suitable contrivances attached to the top of the mast.

A very simple form of liquid seismoscope consists of a circular trough
of wood with notches cut round its side. This is filled with mercury
to the level of the notches. At the time of an earthquake the maximum
quantity of mercury runs over the notches in the direction of greatest
motion. This instrument, which has long been used in Italy, is known as
a Cacciatore, being named after its inventor. It is a prominent feature
in the collection of apparatus forming the well-known seismograph of
Palmieri.

_Pendulum instruments._—Mallet speaks of pendulum seismoscopes and
seismographs as ‘the oldest probably of seismometers long set up in
Italy and southern Europe.’ In 1841 we find these being used to record
the earthquake disturbances at Comrie in Scotland.

These instruments may be divided into two classes: first, those which
at the time of the shock are intended to swing, and thus record the
direction of movement; and second, those which are supposed to remain
at rest and thus provide ‘steady points.’

To obtain an absolutely ‘steady point’ at the time of an earthquake,
has been one of the chief aims of all recent seismological
investigations.

With a style or pointer projecting down from the steady point to
a surface which is being moved backward and forward by the earth,
such a surface has written upon it by its own motions a record of
the ground to which it is attached. Conversely, a point projecting
upwards from the moving earth might be caused to write a record on the
body providing the steady point, which in the class of instruments
now to be referred to is supposed to be the bob of a pendulum. It is
not difficult to get a pendulum which will swing at the time of a
moderately strong earthquake, but it is somewhat difficult to obtain
one which will not swing at such a time. During the past few years,
pendulums varying between forty feet in length and carrying bobs of
eighty pounds in weight, and one-eighth of an inch in length, and
carrying a gun-shot, have been experimented with under a great variety
of circumstances. Sometimes the supports of these pendulums have been
as rigid as it is possible to make a structure from brick and mortar,
and at other times they have intentionally been made loose and
flexible. The indices which wrote the motions of these pendulums have
been as various as the pendulums themselves. A small needle sliding
vertically through two small holes, and resting its lower end on a
surface of smoked glass, has on account of its small amount of friction
been perhaps one of the favourite forms of recording pointers.

The free pendulums which have been employed, and which were intended
to swing, have been used for two purposes: first, to determine the
direction of motion from the direction of swing, and second, to see if
an approximation to the period of the earth’s motion could be obtained
by discovering the pendulum amongst a series of different lengths which
was set in most violent motion, this probably being the one which had
its natural period of swing the most nearly approximating to the period
of the earthquake oscillations.

Inasmuch as all pendulums when swinging have a tendency to change the
plane of their oscillation, and also as we now know that the direction
of motion during an earthquake is not always constant, the results
usually obtained with these instruments respecting the direction of
the earth’s motion have been unsatisfactory. The results which were
obtained by series of pendulums of different lengths were, for various
reasons, also unsatisfactory.

Of pendulums intended to provide a steady point, from which the
relative motion of a point on the earth’s surface could be recorded,
there has been a great variety. One of the oldest forms consisted of
a pendulum with a style projecting downwards from the bob so as to
touch a bed of sand. Sometimes a concave surface was placed beneath the
pendulum, on which the record was traced by means of a pencil. Probably
the best form was that in which a needle, capable of sliding freely up
and down, marked the relative horizontal motion of the earth and the
pendulum bob on a smoked glass plate.

It generally happens that at the time of a moderately severe earthquake
the whole of these forms of apparatus are set in motion, due partly to
the motion of the point of support of the pendulum, and partly to the
friction of the writing point on the plate.

Among these pendulums may be mentioned those of Cavallieri, Faura,
Palmieri, Rossi, and numerous others. It is possible that the
originators of some of these pendulums may have intended that
they should record by swinging. If this is so, then so far as the
determination of the actual nature of earthquake motion is concerned,
they belong to a lower grade of apparatus than that in which they are
here included.

A great improvement in pendulum apparatus is due to Mr. Thomas Gray
of Glasgow, who suggested applying so much frictional resistance to
the free swing of a pendulum that for small displacements it became
‘dead beat.’ By carrying out this suggestion, pendulum instruments
were raised to the position of seismographs. The manner of applying
the friction will be understood from the following description of a
pendulum instrument which is also provided with an index which gives a
magnification of the motion of the earth.

B B B B is a box 113 cm. high and 30 cm. by 18 cm. square. Inside this
box a lead ring R, 17 cm. in diameter and 3 cm. thick, is suspended as
a pendulum from the screw S. This screw passes through a small brass
plate P P, which can be moved horizontally over a hole in the top of
the box. These motions in the point of suspension allow the pendulum to
be adjusted.

[Illustration: FIG. 2.]

Projecting over the top of the pendulum there is a wooden arm W
carrying two sliding pointers H H, resting on a glass plate placed on
the top of the pendulum. These pointers are for the purpose of giving
the frictional resistance before referred to. If this friction plate
is smoked, the friction pointers will write upon it records of _large_
earthquakes independently of the records given by the proper index,
which only gives satisfactory records in the case of shocks of ordinary
intensity. Crossing the inside of the pendulum R there is a brass bar
perforated with a small conical hole at M. A stiff wire passes through
M and forms the upper portion of the index I, the lower portion of
which is a thin piece of bamboo. Fixed upon the wire there is a small
brass ball which rests on the upper side of a second brass plate also
perforated with a conical hole, which plate is fixed on the bar O O
crossing the box.

If at the time of an earthquake the upper part of the index I remains
steady at M, then by the motion at O, the lower end of the index which
carries a sliding needle at G, will magnify the motion of the earth in
the ratios M O: O G. In this instrument O G is about 17 cm.

The needle G works upon a piece of smoked glass. In order to bring
the glass into contact with the needle without disturbance, the glass
is carried on a strip of wood K, hinged at the back of the box, and
propped up in front by a loose block of wood Y. When Y is removed
the glass drops down with K out of contact with the needle. The box
is carried on bars of wood C C, which are fixed to the ground by the
stakes A A.

The great advantage of a pendulum seismograph working on a stationary
plate is, that the record shows at once whether the direction of motion
has been constant, or whether it has been variable. The maximum extent
of motion in various directions is also easily obtained.

The disadvantage of the instrument is, that at the time of a large
earthquake, owing perhaps to a slight swing in the pendulum, the
records may be unduly magnified.

On such occasions, however, fairly good records may be obtained from
the friction pointers, provided that the plates on which they work have
been previously smoked. It might perhaps be well to use two of these
instruments, one having a comparatively high frictional resistance, and
hence ‘dead beat’ for large displacements.

Many attempts have been made to use a pendulum seismograph in
conjunction with a record-receiving surface, which at the time of the
earthquake should be kept in motion by clockwork. In this way it was
hoped to separate the various vibrations of the earthquake, and thus
avoid the greater or less confusion which occurs when the index of the
pendulum writes its backward and forward motion on a stationary plate.
Hitherto all attempts in this direction, in which a single multiplying
index was used, have been unsuccessful because of the moving plate
dragging the index in the direction of its motion for a short
distance, and then allowing it to fall back towards its normal position.

In connection with this subject we may mention the pendulum
seismographs of Kreil, Wagener, Ewing, and Gray.

In the bob of Kreil’s pendulum there was clockwork, which caused a disc
on the axis of the pendulum to continuously rotate. On this continually
revolving surface a style fixed to the earth traced an unbroken circle.
At the time of an earthquake, by the motion of the style, the circle
was to be broken and lines drawn. The number and length of these lines
were to indicate the length and intensity of the disturbance.

Gray’s pendulum consisted of a flat heavy disc carrying on its upper
surface a smoked glass plate. This, which formed the bob of the
pendulum, was supported by a pianoforte steel wire. When set ready to
receive an earthquake, the wire was twisted and the bob held by a catch
so arranged that at the time of the earthquake the catch was released,
and the bob of the pendulum allowed to turn slowly by the untwisting of
the supporting wire. Resting on the surface of this rotating disc were
two multiplying indices arranged to write the earth’s motions as two
components.

In the instruments of Wagener and Ewing, the clockwork and moving
surface do not form part of the pendulum, but rest independently on
a support rigidly attached to the earth. In Wagener’s instrument one
index only is used, while in Ewing’s two are used for writing the
record of the motion.

A difficulty which is apparent in all pendulum machines is that when
the bob of such a pendulum is deflected it tends to fall back to its
normal position. To make a pendulum perfect it therefore requires
some compensating arrangement, so that the pendulum, for small
displacements, shall be in neutral equilibrium, and the errors due to
swinging shall be avoided.

Several methods have been suggested for making the bob of an ordinary
pendulum astatic for small displacements. One method proposed by Gray
consists in fixing in the bob of a pendulum a circular trough of
liquid, the curvature of this trough having a proper form. Another
method which was suggested, was to attach a vertical spiral spring
to a point in the axis of the pendulum a little below the point of
suspension, and to a fixed point above it, so that when the pendulum is
deflected it would introduce a couple.

Professor Ewing has suggested an arrangement so that the bob of the
pendulum shall be partly suspended by a stretched spiral spring, and at
the same time shall be partly held up from below by a vertically placed
strut, the weight carried by the strut being to the weight carried by
the spring in the ratio of their respective lengths. As to how these
arrangements will act when carried into practice yet remains to be seen.

Another important class of instruments are _inverted pendulums_. These
are vertical springs made of metal or wood loaded at their upper end
with a heavy mass of metal. An arrangement of this sort, provided at
its upper end with a pencil to write on a concave surface, was employed
in 1841 to register the earthquakes at Comrie in Scotland. In Japan
they were largely employed in series, each member of a series having
a different period of vibration. The object of these arrangements
was to determine which of the pendulums, with a given earthquake,
recorded the greatest motion, it being assumed that the one which was
thrown into the most violent oscillation would be the one most nearly
approximating with the period of the earthquake. The result of these
experiments showed that it was usually those with a slow period of
vibration which were the most disturbed.

_Bracket Seismographs._—A group of instruments of recent origin which
have done good work, are the bracket seismographs. These instruments
appear to have been independently invented by several investigators:
the germ from which they originated probably being the well-known
horizontal pendulum of Professor Zöllner. In Japan they were first
employed by Professor W. S. Chaplin. Subsequently they were used by
Professor Ewing and Mr. Gray. They consist essentially of a heavy
weight supported at the extremity of a horizontal bracket which is free
to turn on a vertical axis at its other end. When the frame carrying
this axis is moved in any direction excepting parallel to the length
of the gate-like bracket, the weight causes the bracket to turn round
a line known as the instantaneous axis of the bracket corresponding to
this motion of the fixed axis. Any point in this line may therefore be
taken as a steady point for motions at right angles to the length of
the supporting bracket. Two of these instruments placed at right angles
to each other have to be employed in conjunction, and the motion of
the ground is written down as two rectangular components. In Professor
Ewing’s form of the instrument, light prolongations of the brackets
form indices which give magnified representations of the motion, and
the weights are pivoted round a vertical axis through their centre.

In the accompanying sketch B is a heavy weight pivoted at the end of
a small bracket C A K, which bracket is free to turn on a knife-edge,
K, above, and a pivot A, below, in the stand S. At the time of an
earthquake B remains steady, and the index P, forming a continuation
of the bracket, magnifies the motion of the stand, in the ratio of
A C : C N.

[Illustration: FIG. 3.]

In an instrument called a double-bracket seismograph, invented by Mr.
Gray, we have two brackets hinged to each other, and one of them to a
fixed frame. The planes of the two brackets are placed at right angles,
so as to give to a heavy mass supported at the end of the outer bracket
two degrees of horizontal freedom.

In all bracket machines, especially those which carry a pivoted weight,
it is doubtful whether the weight provides a truly steady point
relatively to the plate on which the record is written for motion
parallel to the direction of the arm.

[Illustration: FIG. 4.]

_Parallel motion Instrument._—A machine which writes its record as
two components, and which promises great stability, is one suggested
by Professor C. D. West. Like the bracket machines it consists of
two similar parts placed at right angles to each other, and is as
follows: A bar of iron A is suspended from both sides on pivots at C C
by a system of light arms hinging with each other at the black dots,
between the upper and lower parts of the rigid frame B C. The arms are
of such a length that for small displacements parallel to the length
of the bar, C C practically move in a straight line, and the bar is
in neutral equilibrium. A light prolongation of the bar _d_ works the
upper end of the light index _e_, passing as a universal joint through
the rigid support F. A second index _e′ _ from the bar at right angles
also passes through F. The multiplying ends of these indices are
coupled together to write a resultant motion on a smoked glass plate S.

_Conical Pendulums._—Another group of instruments which have also
yielded valuable records are the conical pendulum seismographs. The
idea of using the bob of a conical pendulum to give a steady point in
an earthquake machine was first suggested and carried into practice by
Mr. Gray. The seismograph as employed consists of a pair of conical
pendulums hung in planes at right angles to each other. The bob of each
of these pendulums is fixed a short distance from the end of a light
lever, which forms the writing index, the short end resting as a strut
against the side of a post fixed in the earth. The weight is carried by
a thin wire or thread, the upper end of which is attached to a point
vertically above the fixed end of the lever.

_Rolling Spheres and Cylinders._—After the conical pendulum
seismographs, which claim several important advantages over the bracket
machines, we come to a group of instruments known as rolling sphere
seismographs. Here, again, we have a class of instruments for the
various forms of which we are indebted to the ingenuity of Mr. Gray.

The general arrangement and principle of one of these instruments will
be readily understood from the accompanying figure. S is a segment
of a large sphere with a centre near C. Slightly below this centre a
heavy weight B, which may be a lead ring, is pivoted. At the time of
an earthquake C is steady, and the earth’s motions are magnified by
the pointer C A N in the proportion of C A : A N. The working of this
pointer or index is similar to that of the pointer in the pendulum.

[Illustration: FIG. 5.]

Closely connected with the rolling sphere seismographs, are Gray’s
rolling cylinder seismographs.

These are two cylinders resting on a surface plate with their axes at
right angles to each other. Near to the highest point in each of these
cylinders, this point remaining nearly steady when the surface plate
is moved backwards and forwards, there is attached the end of a light
index. These indices are again pivoted a short distance from their ends
on axes connected with the surface plate. In order that the two indices
may be brought parallel, one is cranked at the second pivot.

_Ball and Plate Seismograph._—Another form of seismograph, which is
closely related to the two forms of apparatus just described, is
Verbeck’s ball and plate seismograph. This consists of a surface plate
resting on three hard spheres, which in turn rests upon a second
surface plate. When the lower plate is moved, the upper one tends to
remain at rest, and thus may be used as a steady mass to move an index.

_The Principle of Perry and Ayrton._—An instrument which is of interest
from the scientific principle it involves is a seismograph suggested
by Professors Perry and Ayrton, who propose to support a heavy ball on
three springs, which shall be sufficiently stiff to have an exceedingly
quick period of vibration. By means of pencils attached to the ball by
levers, the motions of the ball are to be recorded on a moving band of
paper. The result would be a record compounded of the small vibrations
of the springs superimposed on the larger, slower, wave-like motions
of the earthquake, and, knowing the former of these, the latter might
be separated by analysis. Although our present knowledge of earthquake
motion indicates that the analysis of such a record would often present
us with insuperable difficulties, this instrument is worthy of notice
on account of the novelty of the principle it involves, which, the
authors truly remark, has in seismometry been a ‘neglected’ one.

_Instruments to record Vertical Motion._—The instruments which have
been devised to record vertical motion are almost as numerous as those
which have been devised to record horizontal motion. The earliest
form of instrument employed for this purpose was a spiral spring
stretched by weight, which, on account of its inertia, was supposed
at the time of a shock to remain steady. No satisfactory results have
ever been obtained from such instruments, chiefly on account of the
inconvenience in making a spring sufficiently long to allow of enough
elongation to give a long period of vibration. Similar remarks may
be applied to the horizontally placed elastic rods, one end of which
is fixed to a wall, whilst the opposite end is loaded with a weight.
Such contrivances, furnished with pencil on the weight to write a
record upon a vertical surface, were used in 1842 at Comrie, and we
see the same principle applied in a portion of Palmieri’s apparatus.
Contrivances like these neither give us the true amplitude of the
vertical motion, insomuch as they are readily set in a state of
oscillation; nor do they indicate the duration of a disturbance, for,
being once set in motion, they continue that motion in virtue of their
inertia long after the actual earthquake has ceased. They can only be
regarded as seismoscopes.

[Illustration: FIG. 6.]

The most satisfactory instrument which has yet been devised for
recording vertical motion is Gray’s horizontal lever spring seismograph.

This instrument will be better understood from the accompanying sketch.
A vertical spring S is fixed at its upper end by means of a nut _n_,
which rests on the top of the frame F, and serves to raise or lower
the spring through a short distance as a last adjustment for the
position of the cross-arm A. The arm A rests at one end on two sharp
points, _p_, one resting in a conical hole and the other in a V-slot;
it is supported at B by the spring S, and is weighted at C with a lead
ring R. Over a pin at the point C a stirrup of thread is placed which
supports a small trough, _t_. The trough _t_ is pivoted at _a_, has
attached to it the index _i_ (which is hinged by means of a strip of
tough paper at _h_, and rests through a fine pin on the glass plate
_g_), and is partly filled with mercury.

Another method of obtaining a steady point for vertical motion is that
of Dr. Wagener, who employs a buoy partly immersed in a vessel of
water. This was considerably improved upon by Mr. Gray, who suggested
the use of a buoy, which, with the exception of a long thin style, was
completely sunk.

Among the other forms of apparatus used to record vertical motion may
be mentioned vessels provided with india-rubber or other flexible
bottoms, and partially filled with water or some other liquid. As the
vessel is moved up and down, the bottom tends to remain behind and
provides a more or less steady point. Pivoted to this is a light index,
which is again pivoted to a rigid frame in connection with the earth.
Instruments of this description have yielded good records.

_Record Receivers._—A large number of earthquake machines having been
referred to, it now remains to consider the apparatus on which they
write their motions. The earlier forms of seismographs, as has already
been indicated, recorded their movements in a bed of sand; others
wrote their records by means of pencils on sheets of paper. Where we
have seismographs which magnify the motion of the earth, it will be
observed that methods like the above would involve great frictional
resistances, tending to cause motion in the assumed steady points of
the seismographs. One of the most perfect instruments would be obtained
by registering photographically the motions of the recording index by
the reflection of a ray of light. Such an instrument would, however,
be difficult to construct and difficult to manipulate. One of the best
practical forms of registering apparatus is one in which the record is
written on a surface of smoked glass. This can afterwards be covered
with a coat of photographer’s varnish, and subsequently photographed by
the ‘blue process’ so well known to engineers.

To obtain a record of all the vibrations of an earthquake it is
necessary that the surface on which the seismograph writes should at
the time of an earthquake be in motion. Of record-receiving machines
there are three types. First, there are those which move continuously.
The common form of these is a circular glass plate like an old form of
chronograph, driven continuously by clockwork. On this the pointers of
the seismograph rest and trace over and over again the same circles.
At the time of an earthquake they move back and forth across the
circles, which are theoretically fine lines, and leave a record of
the earthquake. Instead of a circular plate, a drum covered with
smoked paper may be used, which, after the earthquake, possesses the
advantage, after unrolling, of presenting the record in a straight
line, instead of a record written round the periphery of a circle, as
is the case with the circular glass plates. Such records are easily
preserved, but they are more difficult to photograph.

The second form of apparatus is one which is set in motion at the
time of a shock. This may be a contrivance like one of those just
described, or a straight smoked glass plate on a carriage. By means
of an electrical or a mechanical contrivance called a ‘starter,’ of
which many forms have been contrived, the earthquake is caused to
release a detent and thus set in motion the mechanism which moves the
record receiver.

The great advantage of continuously-moving machines is that the
beginning and end of the shock can usually be got with certainty, while
all the uncertainty as to the action of the ‘starter’ is avoided.
Self-starting machines have, of course, the advantage of simplicity and
cheapness, while there is no danger of the record getting obliterated
by the subsequent motion of the plate under the index.

_Time-recording Apparatus._—Of equal importance with the instruments
which record the motion of the ground, are those instruments which
record the time at which such motion took place. The great value of
time records, when determining the origin from which an earthquake
originates, will be shown farther on. The most important result which
is required in connection with time observations, is to determine
the interval of time taken by a disturbance in travelling from one
point to another. On account of the great velocity with which these
disturbances sometimes travel, it is necessary that these observations
should be made with considerable accuracy. The old methods of adapting
an apparatus to a clock which, when shaken, shall cause the clock to
stop, are of little value unless the stations at which the observations
are made are at considerable distances apart. This will be appreciated
when we remember that the disturbance may possibly travel at the rate
of a mile per second, that its duration at any station may often extend
over a minute, and that one set of apparatus at one station may stop,
perhaps, at the commencement of the disturbance, and the other near
the end. A satisfactory time-taking apparatus will therefore require,
not only the means for stopping a clock, but also a contrivance which,
at the same instant that the clock is stopped, shall make a mark on a
record which is being drawn by a seismograph. In this way we find out
at which portion of the shock the time was taken.

[Illustration: FIG. 7.]

Palmieri stops a clock in his seismograph by closing an electric
circuit. Mallet proposes to stop a clock by the falling of a column
which is attached by a string to the pendulum of the clock. So long as
the column is standing the string is loose and the pendulum is free
to move; but when the column falls, the string is tightened and the
pendulum is arrested. The difficulty which arises is to obtain a column
that will fall with a slight disturbance. The best form of contrivance
for causing a column to fall, and one which may also be used in drawing
out a catch to relieve the machinery of a record receiver, is shown in
the accompanying sketch.

S is the segment of a sphere about 4·5 cm. radius, with a centre
slightly above C. L is a disc of lead about 7 cm. in diameter resting
upon the segment. Above this there is a light pointer, P, about 30
cm. long. On the top of the pointer a small cylinder of iron, W, is
balanced, and connected by a string with the catch to be relieved. When
the table on which W P S rests is shaken, rotation takes place near
to C, the motion of the base S is magnified at the upper end of the
pointer, and the weight overturned. This catch may be used to relieve
a toothed bar axled at one end, and held up above a pin projecting
from the face of the pendulum bob. When this falls it catches the
projecting pin and holds the pendulum.

Another way of relieving the toothed bar is to hold up the opposite
end to that at which it is axled by resting it on the extremity of a
horizontal wire fixed to the bob of a conical pendulum—for example, one
of the indices of a conical pendulum seismograph. The whole of this
apparatus, which may be constructed at the cost of a few pence, can be
made small enough to go inside an ordinary clock case.

The difficulty which arises with all these clock-stopping arrangements
is that it is difficult for observers situated at distant stations
to re-start their clocks so that their difference in time shall
be accurately known. Even if each observer is provided with a
well-regulated chronometer, with which he can make comparisons, the
rating of these instruments is for all ordinary persons an extremely
troublesome operation.

In order to avoid this difficulty the author has of late years used a
method of obtaining the time without stopping the clock. To do this a
clock with a central seconds hand is taken, and the hour and minute
hands are prolonged and bent out slightly at their extremities at right
angles to the face, the hour hand being slightly the longest. Each
hand is then tipped with a piece of soft material like cork, which is
smeared with a glycerine ink. A light flat ring, with divisions in it
corresponding to those on the face of the clock, is so arranged that
at the time of a shock it can be quickly advanced to touch the inked
pads on the hands of the clock and then withdrawn. This is accomplished
by suitable machinery, which is relieved either by an electro-magnet
or some other contrivance which will withdraw a catch. In this way an
impression in the form of three dots is received on the disc, and the
time known without either stopping or sensibly retarding the clock.

For ordinary observers, if a time-taker is not used in conjunction
with a record receiver, as good results as those obtained by ordinary
clock-stopping apparatus are obtainable by glancing at an ordinary
watch. Subsequently the watch by which the observation was made should
be compared with some good time-keeper, and the local time at which the
shock took place is then approximately known.

From what has now been said it will be seen that for a complete
seismograph we require three distinct sets of apparatus—an apparatus to
record horizontal motion, an apparatus to record vertical motion, and
an apparatus to record time. The horizontal and vertical motions must
be written on the same receiver, and if possible side by side, whilst
the instant at which the time record is made a mark must be made on
the edge of the diagram which is being drawn by the seismograph. Such
a seismograph has been constructed and is now erected in Japan. It is
illustrated in the accompanying diagram.

_The Gray and Milne Seismograph._—In this apparatus two mutually
rectangular components of the horizontal motion of the earth are
recorded on a sheet of smoked paper wound round a drum, D, kept
continuously in motion by clockwork, W, by means of two conical
pendulum seismographs, C. The vertical motion is recorded on the same
sheet of paper by means of a compensated-spring seismograph, S L M B.

The time of occurrence of an earthquake is determined by causing the
circuit of two electro-magnets to be closed by the shaking. One of
these magnets relieves a mechanism, forming part of a time-keeper,
which causes the dial of the timepiece to come suddenly forwards on
the hands and then move back to its original position. The hands are
provided with ink-pads, which mark their positions on the dial, thus
indicating the hour, minute, and second when the circuit was closed.
The second electro-magnet causes a pointer to make a mark on the paper
receiving the record of the motion. This mark indicates the part of the
earthquake at which the circuit was closed.

[Illustration: FIG. 8.]

The duration of the earthquake is estimated from the length of the
record on the smoked paper and the rate of motion of the drum. The
nature and period of the different movements are obtained from the
curves drawn on the paper.

Mr. Gray has since greatly modified this apparatus, notably by the
introduction of a band of paper sufficiently long to take a record for
twenty-four hours without repetition. The record is written in ink by
means of fine siphons. In this way the instrument, which is extremely
sensitive to change of level, can be made to show not only earthquakes,
but the pulsations of long period which have recently occupied so much
attention.




                             CHAPTER III.

              EARTHQUAKE MOTION DISCUSSED THEORETICALLY.

  Ideas of the ancients (the views of Travagini, Hooke, Woodward,
    Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves
    and vibrations—Possible causes of disturbance in the Earth’s
    crust—The time of vibration of an earth particle—Velocity and
    acceleration of a particle—Propagation of a disturbance as
    determined by experiments upon the elastic moduli of rocks—The
    intensity of an earthquake—Area of greatest overturning
    moment—Earthquake waves—Reflexion, refraction, and interference
    of waves—Radiation of a disturbance.


_Ideas of Early Writers._—One of the first accounts of the varieties of
motion which may be experienced at the time of an earthquake is to be
found in the classification of earthquakes given by Aristotle.[8] It is
as follows:—

1. Epiclintæ, or earthquakes which move the ground obliquely.

2. Brastæ, with an upward vertical motion like boiling water.

3. Chasmatiæ, which cause the ground to sink and form hollows.

4. Rhectæ, which raise the ground and make fissures.

5. Ostæ, which overthrow with one thrust.

6. Palmatiæ, which shake from side to side with a sort of tremor.

From the sixth group in this classification we see that this early
writer did not regard earthquakes as necessarily isolated events, but
that some of them consisted of a succession of backward and forward
vibratory motions. He also distinguishes between the total duration of
an earthquake and the length of, and intervals between, a series of
shocks. Aristotle had, in fact, some idea of what modern writers upon
ordinary earthquakes would term ‘modality.’

The earliest writer who had the idea that an earthquake was a
pulse-like motion propagated through solid ground appears to have been
Francisci Travagini, who, in 1679, wrote upon an earthquake which
in 1667 had overthrown Ragusa. The method in which the pulses were
propagated he illustrated by experiments.

Hooke, who, in 1690, delivered discourses on earthquakes before the
Royal Society, divides these phenomena according to the geological
effects they have produced; thus there is a _genus_ producing
elevations, a _genus_ producing sinkings, a _genus_ producing
conversions and transportations, and a _genus_ which produces what, in
modern language, we should term metamorphic action.

Woodward, in his ‘Natural History,’ written in 1695, speaks of
earthquakes as being agitations and concussions produced by water in
the interior of the earth coming in contact with internal fires.

Stukeley observed that an earthquake was ‘a tremor of the earth,’ to
be explained as a vibration in a solid. The Rev. John Mitchell, writing
in 1760, says that the motion of the earth in earthquakes is partly
tremulous and partly propagated by waves.

From these few examples, to which might be added many more, it will
be seen that an earthquake disturbance has usually been regarded as
a concussion, vibration, trembling, or undulatory movement. Further,
it can be seen in narratives of earthquakes that it had been often
observed that these tremblings and shakings continued over a certain
period of time. Although it had been noticed that large areas were
almost simultaneously affected by these disturbances, no definite idea
appears to have existed as to how earthquake motion was propagated.
Usually it was assumed that the disturbance spread through subterranean
channels.

The first true conception of earthquake motion and the manner of its
propagation is due to Dr. Thomas Young, who suggested that earthquake
motion was vibratory, and it might be ‘propagated through the earth
nearly in the same manner as a noise is conveyed through the air.’ The
same idea was moulded into a more definite form by Gay Lussac.

The first accurate definition of an earthquake is due to Mr. Robert
Mallet, who, after collecting and examining many facts connected with
earthquake phenomena, and reasoning on these, with the help of known
laws connected with the production and propagation of waves of various
descriptions, formulated his views as follows:—

An earthquake is ‘_the transit of a wave or waves of elastic
compression in any direction from vertically upwards to horizontally,
in any azimuth, through the crust and surface of the earth, from any
centre of impulse or from more than one, and which may be attended
with sound and tidal waves, dependent upon the impulse and upon
circumstances of position as to sea and land_.’

In brief, so far as motion in the earth is concerned. Mallet defined
an earthquake as being a motion due to the transit of waves of elastic
compression. In many cases it is possible that this is strictly true,
but in succeeding pages it will be shown that earthquake motion may
also be due to the transit of waves of elastic distortion.

To obtain a true idea of earthquake motion is a matter of cardinal
importance, as it forms the key-stone of many investigations.

If we know the nature of the motion produced by an earthquake, we are
aided in tracking it to its origin, and in reasoning as to how it was
produced. If our knowledge of the nature of the motion of an earthquake
is incorrect, it will be impossible for us intelligently to construct
buildings to withstand the effects of these disturbances. We have thus
to consider, in this portion of seismology, a point of great scientific
importance, and shall deal with it at some length.

_Nature of Elastic Waves and Vibrations._—When it is stated that an
earthquake consists of elastic waves of compression and distortion, the
student of physics has a clear idea of what is meant and a knowledge
of the mechanical laws which govern such disturbances. The ordinary
reader, however, and the majority of the inhabitants of earthquake
countries, who of all people have the greatest interest in this matter,
may not have so clear a conception, and it will, therefore, not be out
of place to give some general explanation on this point.

The ordinary idea of a wave is that it is a disturbance similar to
that which we often see in water. Waves like these must not, however,
be confounded with elastic waves. A disturbance produced in water,
say, for instance, by dropping a stone into a pond, is propagated
outwards by the action of gravity. First, a ridge of water is raised
up by the stone passing beneath the surface. As this ridge falls
towards its normal position in virtue of its weight, it raises a second
ridge. This second ridge raises a third ridge, and so on. The water
moves vertically up and down, whilst the wave itself is propagated
horizontally.

To understand what is meant by elastic waves, it is first necessary
to understand what is meant by the term elastic. In popular language
the term elastic is confined to substances like india-rubber, and but
seldom to rock-like materials, through which earthquake waves are
propagated. India-rubber is called elastic because after we remove a
compressive force it has a tendency to spring back to its original
shape. The elastic force of the india-rubber is in this case the force
which causes it to resist a change of form. Now, a piece of rock may,
up to a certain point, like the india-rubber, be compressed, and when
the compressing force is removed it will also tend to resume its
original form. However, as the rock offers more resistance to the
compressing force than the india-rubber offers, we say that it is the
more elastic. It may be here observed that a substance like granite
offers great resistance, not only to compression or a change of volume,
but also to a change of form or shape; whereas a substance like air,
which is also elastic, only offers resistance to compression, but not
to a change of shape.

With these ideas before us we will now proceed to consider how, after
a body has been suddenly compressed or distorted, this disturbance
is propagated through the mass. For the elastic body let us take a
long spiral spring hung from the ceiling of a room and kept slightly
stretched by a weight. If we give this weight an upward tap from below,
say with a hammer, we shall observe a pulse-like wave which runs up the
spring until it reaches the ceiling of the room. Here it will, so to
speak, rebound, like a billiard ball from the end of a table, and run
towards the weight from which it started. Whilst this is going on we
may also observe that the weight is moving up and down.

Here, then, we have two distinct things to observe—one being the
transmission of motion up to the ceiling, which we may liken to the
transmission of an earthquake wave between two distant localities on
the earth’s surface, and the other being the up and down motion of our
weight, which we may compare to the backward and forward swinging which
we experience at the time of an earthquake.

These two motions—namely, the pulse-like wave produced by the
transmission of motion, and the backward and forward oscillation of the
weight or of any point on the spring—must be carefully distinguished
from each other.

First, we will consider the backward and forward motion of the weight.
The distance through which the weight moves depends upon the force
of the blow. The number of up and down oscillations it makes, say
in a second, depends upon the stiffness of the spring. The weight,
supposing it to be always the same, will move more quickly at the end
of a stiff spring than at the end of a flaccid one; that is to say, its
velocity is quicker. As in any given spring the number of up and down
oscillations are always the same in a given interval of time, if these
oscillations are of great extent, the weight must move more quickly
with large than with small oscillations.

At the time of an earthquake the manner in which we are moved backward
and forward is very similar to the manner in which the weight is moved.
If we stand on a hard rock-like granite, we are to a great extent
placed as if we were attached to a stiff quickly-vibrating spring.
If, however, we are on a soft rock, it is more like being on a loose
flaccid spring.

All that has thus far been considered has been a backward and forward
kind of motion, where there is a rectilinear _compression_ and
_extension_ amongst the particles on which we stand.

We might, however, imagine our rock, which for the moment we will
consider to be a square column, to be twisted, and thus have its
_shape_ altered. When the twisting force is taken off it seems evident
that the column would endeavour to untwist itself or regain its
original form. Now the force which a body offers against a change of
_volume_ may be very different from that which it offers against a
change of _form_.

In disturbances which take place in the rocky crust of our earth,
it would seem possible that we may have vibrations set up which are
either compressions and extensions or twistings and distortions. These
may take place separately or simultaneously, or we may have resultant
motions due to their combination.

The following are examples of possible causes which might give rise to
these different orders of disturbance:—

1. Imagine a large area stretched by elevation until it reaches the
limit of its elasticity and cracks. After cracking, in consequence
of its elasticity, it will fly back over the whole area like a
broken spring, and each point in the area will oscillate round its
new position of equilibrium. In this case there will be no waves
of distortion excepting near the end of the crack, where waves are
transmitted in a direction parallel to the fissure.

2. The ground is broken and slips either up, down, or sideways, as
we see to have taken place in the production of faults. Here we get
distortion in the direction of the movement, and waves are produced
by the elastic force of the rock, causing it to spring back from its
distorted form. In a case like this the production of a fissure running
north and south might give rise to north and south vibrations, which
would be propagated end on towards the north and south, but broadside
on towards the east and west. With disturbances of this kind, on
account of the want of homogeneousness in the materials in which
they are produced, we should expect to find waves of compression and
extension.

3. A truly spherical cavity is suddenly formed by the explosion of
steam in the midst of an elastic medium. In this case all the waves
will be those of compression, each particle moving backward and forward
along a radius.

Should the cavity, instead of being truly spherical, be irregular, it
is evident that, in addition to the normal vibration of compression,
transverse waves of distortion will be more or less pronounced,
depending upon the nature of the cavity.

The combination of these two sets of vibrations may cause a point in
the earth to move in a circle, an ellipse, the form of a figure eight,
and in other curves similar to these, which are produced by apparatus
designed to show the combination of harmonic motion. From these
examples it will be seen that we have therefore to consider two kinds
of vibrations—one produced by compression or the alteration of volume,
and the other produced by an alteration in shape.

Now the resistance which a body offers, either to a change in its
volume or in its shape, is called its elasticity, and the law which
governs the backward and forward motion of a particle under the
influence of this elasticity may be expressed as follows:

If T be the time of vibration, or the time taken by a particle to make
one complete backward and forward swing, D the density of the material
of which this particle forms a part, and E the proper modulus of
elasticity of the material, then,

                                  _____
                          T = 2π √ D/E

                           _____
From this formula, T = 2π √ D/E, we see that the time of
vibration of the earth during an earthquake, or the rate at which we
are shaken backwards and forwards, varies directly as the square root
of the density of the material on which we stand, and inversely as the
square root of a number proportional to its elasticity.

_Velocity and Acceleration of an Earth Particle._—Another important
point, which the practical seismologist has often brought to his
notice, is the question of the velocity with which an earth particle
                                         _____
moves. According to the formula, T = 2π √ D/E, we should expect
that a particle would make each semi-vibration in an equal time, and
from a knowledge of the density and elastic moduli of a body this
time might be calculated. Although the time of a semi-oscillation may
be constant, we must bear in mind that, like the bob of a pendulum
during each of its swings, the particle starts from rest, increases in
velocity until it reaches the middle portion of its half swing, from
which it gradually decreases in speed until it reaches zero, when it
again commences a similar motion in the opposite direction.

These pendulum-like vibrations are sometimes spoken of as simple
harmonic motions. If we know the distance through which an earthquake
moves in making a single swing, and the time taken in making this
swing, on the assumption that the motion is simple harmonic we can
easily calculate the maximum velocity with which the particle moves.

Thus, if an earth particle takes one second to complete a
semi-oscillation, half of which, or the amplitude of the motion, equals
_a_, the maximum velocity equals π × _a_.

Again, assuming the earth vibrations to be simple harmonic, the maximum
acceleration or rate of change in velocity will come about at the ends
of each semi-oscillation; and if V be the maximum velocity of
the particle, and _a_ the amplitude or half semi-oscillation, then the
                            V^2
maximum acceleration equals ———.
                            _a_

Later on it will be shown, as the result of experiment, that certain of
the more important earth oscillations in an earthquake are not simple
harmonic motion. Nevertheless the above remarks will be of assistance
in showing how the velocity and other elements connected with the
motion of an earth particle, which are required by the practical
seismologist, may be calculated, irrespective of assumptions as to the
nature of the motion.

_Propagation of a Disturbance._—We may next consider the manner in
which a disturbance, in which there are both vibrations of compression
and of distortion, is propagated. The first or normal set of vibrations
are propagated in a manner similar to that in which sound vibrations
are propagated. From a centre of disturbance these movements approach
an observer at a distant station, so to speak, end on. The other
vibrations have a direction of motion similar to that which we believe
to exist in a ray of light. These would approach the observer broadside
on.

If the disturbance passed through a formation like a series of
perfectly laminated slates, each of these two sets of vibration might
be subdivided, and we should then obtain what Mallet has termed
ordinary and extraordinary normal and transverse vibrations.

In consequence of the difference in the elastic forces on which the
propagation of these two kinds of vibration depends, the normal
vibrations are transmitted faster than the transversal ones—that is
to say, if an earthquake originated from a blow, the first thing that
would be felt at a point distant from the origin of the shock would be
a backward and forward motion in the direction to and from the origin,
and then, a short interval afterwards, a motion transversal, or at
right angles to this, would be experienced.

From the mathematical theory of vibratory motions it is possible to
calculate the velocity with which a disturbance is propagated. As the
result of these investigations it has been shown that normal vibrations
travel more quickly than transverse vibrations.

Deductions from experiments on small specimens are, however,
invalidated by the fact that the specimens used for experiments are,
of course, nearly homogeneous, whilst the earthquake passes through
a mass which is heterogeneous and more or less fissured. Mallet, by
experiments ‘on the compressibility of solid cubes of these rocks,
obtained the mean modulus of elasticity,’ with the result that ‘nearly
seven-eighths of the full velocity of wave-transit due to the material,
if solid and continuous, is lost by reason of the heterogeneity and
discontinuity of the rocky masses as they are found piled together in
nature.’ The full velocities of wave-transit, as calculated by Mallet
from a theorem given by Poisson, were—

  For slate and quartz transverse to lamination, 9,691 feet per second.
        „         „    in line of lamination,    5,415  „         „

This more rapid transmission in a direction transverse to the
lamination, Mr. Mallet observes, may be more than counterbalanced by
the discontinuity of the mass transverse to the same direction.

_The Intensity of an Earthquake._—The intensity of an earthquake
is best estimated by the intensity of the forces which are brought
to bear on bodies placed on the earth’s surface. These forces are
evidently proportional to the rate of change of velocity in the body,
and, as the destructive effect will be proportional to the maximum
forces, we may consistently indicate the intensity of an earthquake by
giving the maximum acceleration to which bodies were subject during
the disturbance. On the assumption that the motion of a point on
the earth’s surface is simple harmonic, the maximum acceleration is
directly as the maximum velocity and inversely as the amplitude of
              _v_^2
motion, or as ————— where _v_ indicates velocity and _a_ amplitude.
               _a_

The next question of importance is to determine the manner in which
earthquake energy becomes dissipated—that is, to compare together
the intensity of an earthquake as recorded at two or more points at
different distances from the origin. First let us imagine the origin
of our earthquake to be surrounded by concentric shells, each of
which is the breadth of the vibration of a particle. Going outwards
from the centre, each successive shell will contain a greater number
of particles, this number increasing directly as the square of the
distance from the origin. Let the blow have its origin at the centre,
and give a vibratory movement to the particles in one of the shells
near the centre.

This shell may be supposed to possess a certain amount of energy,
which will be measured by its mass and the square of the velocity of
its particles. In transferring this energy to the neighbouring shell
which surrounds it, because it has to set in motion a greater number
of particles than it contains itself, the energy in any one particle
of the second layer will be less than the energy in any one particle
in the first layer; the total energy in the second shell, however,
will be equal to the total energy in the first shell. Neglecting the
energy lost during the transfer, if the energy in a particle of the
first shell at any particular phase of the motion be K_{1},
and the energy in a particle of the second shell K_{2}, these
quantities are to each other inversely as the masses of the shells—that
is, inversely as the squares of the mean radii of the shells.

                  K_{2}   _r__{1}^2
  In symbols,     ————— = —————————                   (1)
                  K_{1}   _r__{2}^2

Assuming that energy is dissipated,

              K_{2}   _r__{1}^2       _r__{1}^2
              ————— > ————————— = _f_ —————————       (2)
              K_{1}   _r__{2}^2       _r__{2}^2

where _f_ < 1 is the rate of dissipation of energy which is assumed to
be constant.

_Area of greatest Overturning Moment._—Although the rate of dissipation
of the impulsive effects of an earthquake may follow a law like that
just enumerated, it must be remembered that if the depth of the origin
is comparable with the radius of the area which is shaken, the maximum
impulsive effect as exhibited by the actual destruction on the surface
may not be immediately above the origin where buildings have simply
been lifted vertically up and down, but at some distance from this
point, where the impulsive effort has been more oblique.

At the _epicentrum_ we have the maximum of the true intensity as
measured by the acceleration of a particle, or the height to which
a body might be projected, but it will be at some distance from
this where we shall have the maximum intensity as exhibited by an
overturning effort.

This will be rendered clear by the following diagram.

In the accompanying diagram let O be the origin of a shock, and O C the
seismic vertical equal to _r_. Let the direct or normal shock emerge at
C, C_{1}, C_{2}, and at the angles θ_{1}, θ_{2}, &c.

Assuming that the displacement of an earth particle at C equals C
B, and at C_{1} equals _c__{1} _b__{1}, and at C_{2} equals _c__{2}
_b__{2}, &c., and let these displacements C B, _c__{1} _b__{1}, _c__{2}
_b__{2}, &c., for the sake of argument, vary inversely as _r_, _r__{1},
_r__{2}, &c.

[Illustration: FIG. 9.]

The question is to determine where the horizontal component C A of
these normal motions is a maximum.

First observe that the triangle O C _c_ is similar to _a_, _b_, _c_.

            _h_
Also _r_ = —————, and therefore the normal component _c__{1}
           sin θ

                               sin θ
_b__{1} at C_{1} is equal to C —————.
                                _h_

Also _c__{1} _a__{1} = _c__{1} _b__{1}, cos θ.

                       sin θ cos θ    _c_    sin 2θ
      ∴ C_{1} _a_ = C ———————————— = ———— ∙ ———————,
                          _h_         _h_      2

and sin 2θ is greatest when 2θ = 90° or θ = 45°.

That is to say, the horizontal component reaches a maximum where the
angle of emergence equals 45°.

This question has been discussed on the assumption that the amplitude
of an earth particle varies inversely as its distance from the origin
of the shock. Should we, however, assume that this amplitude varies
inversely as the square of the distance from the origin, we are led to
the result that the area of greatest disturbance is nearer to the point
where the angle of emergence is 55° 44′  9″. Both of these methods are
referred to by Mallet, but the first is considered as probably the more
correct.

_Earthquake Waves._—Hitherto we have chiefly considered earthquake
vibrations; now we will say a few words about earthquake waves. If
we strike a long iron rod at one end, we can imagine that, as in the
long spring, a pulse-like motion is transmitted. If the rod be struck
quickly, the pulses will rapidly succeed each other, and if struck
slowly the pulses will be at longer intervals. Each individual pulse,
however, will travel along the rod at the same rate, and hence the
distance between any two will remain constant; but that distance will
depend on the interval between the blows producing these pulses being
equal to the distance travelled by one pulse before the next blow is
struck.

From this we see that an irregular disturbance will produce an
irregular succession of motions; some will be like long undulations in
a wide deep ocean, whilst others will be like ripples in a shallow bay.
Again, consider the bar to be struck one blow only, and then left to
itself. The bar will propagate a series of pulses along its length, due
to the out and in vibration of its end. These will succeed each other
at regular intervals, and will be mixed up with the pulses we have
previously considered.

From this we see that in an earthquake, if it be produced by one blow,
the motion will be isochronous in its character; but if it be due to
a succession of blows at regular intervals, the motion will be the
resultant of a series of isochronous motions, and will be periodical.
If the impulses are irregular, you have a motion which is the resultant
of a number of isochronous motions due to each impulse, but these
compounded together in a different manner at each instant during the
earthquake, and giving as a result a motion which is in no sense
isochronous. This approaches more nearly to the actual motions we feel
as earthquakes.

If we can imagine the ground shaken by an earthquake, made of a
transparent material which transmitted less light when compressed,
and we could look down upon a long extent of this at the time of an
earthquake, we should see a series of dark bands indicating strips of
country which were compressed. The distances between these bands might
be irregular. Keeping our attention on one particular band, this would
be seen to travel forward in a direction from the source. If we kept
our eye on one particular point, it would appear to open and shut,
becoming light and dark alternately.

As to the existence of these elastic waves in actual earthquakes we
have no direct experimental evidence. The only kind of wave with which
we are familiar is a true surface undulation, which, although having
the appearance of a water-wave, may nevertheless represent a district
of compression.




                              CHAPTER IV.

             EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.

  Experiments with falling weights—Experiments with
    explosives—Results obtained from experiments—Relative motion
    of two adjacent points—The effect of hills and excavations
    upon the propagation of vibrations—The intensity of artificial
    disturbances—Velocity with which earth vibrations are
    propagated—Experiments of Mallet—Experiments of Abbot—Experiments
    in Japan—Mallet’s results—Abbot’s results—Results obtained in
    Japan.


_Experiments with Falling Weights._—A series of experiments, as the
nature of the disturbance produced in the surface of the earth when
a heavy weight is allowed to fall on it, was begun in November 1880
by Mr. T. Gray and the author. These experiments were carried out at
the Akabane Engineering Works in Tokio. The weight used was a ball
of iron weighing about a ton, which in the different experiments was
allowed to fall from heights varying between ten and thirty-five feet.
The position of the place where the ball was allowed to fall was such
that in one direction the vibrations were transmitted up the side of
a steep hill, in another direction across a pond with perpendicular
sides, and in another direction across a level plain the material of
which consisted for the most part of hardened mud extending to a very
considerable depth. The vibrations produced by the fall of the ball
were transmitted through this hard mud with considerable intensity to a
distance of between 300 and 400 feet.

The object of the experiment was to find the nature of the vibrations
produced in the crust of the earth by such a blow, the velocity of
transmission through this comparatively soft material, the effect of
hills and excavations in cutting off such disturbances, and the law
according to which the amplitude of the vibrations diminishes with the
distance from the source.

A considerable variety of apparatus was used during these experiments,
but the most reliable results were obtained from the records of a
rolling sphere seismograph, which wrote the vibrations on a stationary
plate, and from the records of two bracket seismographs, similar to
Professor Ewing’s horizontal lever seismographs, which gave a record
of the vibrations as two rectangular components on a moving plate of
smoked glass.

[Illustration: FIG. 10.]

The general result as to the nature of the disturbance was that two
distinct sets of vibrations were set up by the blow. In one set the
direction of motion was along a line joining the point of observation
with the point from which the disturbance emanated; in the other set
the direction of motion was at right angles to that line. The nature
of the resultant motion will be gathered from fig. 10, which is taken
from the records drawn by the rolling sphere seismograph at a distance
of 50 feet, 100 feet, and 200 feet respectively from the point where
the ball struck the ground. The direct or normal vibrations reached
the instrument first, and were followed at an interval depending on
the distance of the instrument from the origin by the transverse
vibrations. From the records of these two sets of vibrations as
separated by the bracket seismographs, combined with the known rate of
motion of the glass plate, the velocity of transmission was found to
be, for normal vibrations 446–438 feet per second, and for transverse
vibrations 357–353 feet per second.

The effect of the hill in cutting off the disturbance seemed to be
slight, but the direction of the vibrations which ascended the side
was mostly transverse. The pond, on the other hand, seemed completely
to cut off the disturbance, which, however, gradually crept round the
side, so that only a comparatively small triangular area was in shadow.

The amplitude of the vibrations diminished directly as the distance
increased for some distance from the origin, but at greater distance
the rate of diminution seemed to be slower. The transverse vibration
seemed to die out less quickly than the normal vibrations.[9]

These experiments were afterwards very considerably extended by the
author. In these later experiments charges of from one to two pounds of
dynamite were placed in bore-holes of various depths and exploded by
means of electricity. The results obtained confirmed the conclusions
already arrived at from the former experiments. The experiments on
velocity, however, seemed to indicate that the higher the initial
impulse the greater was the velocity. The velocity of propagation of
the transverse vibrations seemed to approach more and more to that of
the directed vibrations as the distance from the origin of disturbance
increased. Fig. 11 shows the nature of the record obtained from the
explosion of two pounds of dynamite at the bottom of a bore-hole eight
feet deep. These records show the interval of time which elapsed
between the arrival of the normal and the transverse vibrations at
points distant 100, 250, and 400 feet from the bore-hole. In the case
of the 100-feet station it will be observed that the motion towards the
origin is greater than that from the origin. It is also to be noticed
that the period of vibration becomes greater as the distance from the
origin increases.

[Illustration: FIG. 11.—Records obtained at three stations
of the motion of the ground produced by the explosion of 2 lbs. of
dynamite.]

_The Intensity of Artificial Disturbances._—The data which we have at
our disposal for determining the intensity of an earth particle which
has been caused to vibrate by the explosion of a charge of dynamite are
a series of records similar to that given on p. 60. These disturbances
are practically surface movements, and may be compared with the
movements of an earthquake which spreads over an area the radius of
which is great as compared with its depth.

To find the mean acceleration of an earth particle, which quantity
has been taken to represent intensity, during any simple backward or
forward motion of the earth, it will be first necessary to determine
the amplitude of this motion and its maximum velocity, the mean
                            V^2
acceleration being equal to ———.
                            2A

[Illustration: FIG. 12.]

The second and third movements in a shock invariably exhibited the
greatest intensity, and to a distance of 400 feet from the origin,
where about three pounds of dynamite had been exploded in a bore-hole
about six feet deep, these intensities decreased directly as the
distance from the origin. The less intense movements also decreased
directly as the distance from the origin to a certain point, but after
that they decreased more slowly. A mean result of the more prominent
vibrations in four sets of experiments is shown in the curve, fig. 12,
where the horizontal measurements represent distance from the origin
in feet, and the vertical measurements mean acceleration in thousands
of millimetres per second.

This curve approximates to an equi-angular hyperbola. The area between
the curve and its asymptotes is proportional to the whole energy of
the shock. The area of the diagram is proportional to the energy given
up to the ground by the explosion of three pounds of dynamite. If we
call the unit shock the effect produced by the explosion of one pound
of dynamite, the above artificial earthquake had an intensity equal to
three.

The only other investigations which have been made in this interesting
branch of observational seismology are those by Mr. Robert Mallet,[10]
and those by General Henry L. Abbot.[11]

_Mallet’s Results._—The velocity with which earth vibrations were
transmitted as deduced by Mr. Mallet were as follows:—

                                                  Feet per second
  In sand                                              824·915
  In contorted stratified rock, quartz, and slate
      at Holyhead                                    1,088·669
  In discontinuous and much shattered granite        1,306·425
  In more solid granite                              1,664·574

A striking result which was obtained by Mallet in his experiments at
Holyhead was that the transit velocity increases with an increase in
the intensity of the initial shock. Thus with a charge of 12,000 pounds
of powder the transit rate was 1,373 feet per second, whilst with 2,100
pounds the transit rate was 1,099 feet per second. In these experiments
tremors were observed as preceding and following the main shock.

_Abbot’s Results._—The important results obtained by General Abbot are
contained in the following table:—

  A. No. of Observation
  B. Distance to Station in miles
  C. Type of Seismometer
  D. Velocity in feet per second
  +--+----------------+--------------------------+--------+---+-------+
  | A|      Date      |      Cause of Shock      |   B    | C |   D   |
  +--+----------------+--------------------------+--------+---+-------+
  | 1| Aug.  18, 1876 | 200 lbs. of dynamite     |  5 ±   | B | 5,280 |
  | 2| Sept. 24, 1876 | Hallet’s Point Explosion |  5·134 | A | 3,873 |
  | 3|        „       |   „        „       „     |  8·330 | B | 8,300 |
  | 4|        „       |   „        „       „     |  9·333 | A | 4,521 |
  | 5|        „       |   „        „       „     | 12·769 | B | 5,309 |
  | 6| Oct.  10, 1876 |  70 lbs. dynamite        |  1·360 | A | 1,240 |
  | 7| Sept.  6, 1877 | 400  „      „            |  1·169 | A | 3,428 |
  | 8|        „       |  „   „      „            |  1·169 | B | 8,814 |
  | 9| Sept. 12, 1877 | 200  „      „            |  1·340 | A | 6,730 |
  |10|        „       |  „   „      „            |  1·340 | B | 8,730 |
  |11|        „       |  70  „      „            |  1·340 | A | 5,559 |
  |12|        „       |  „   „      „            |  1·340 | B | 8,415 |
  +--+----------------+--------------------------+--------+---+-------+

A seismometer of type A means that the telescope used in observing the
tremor produced on the surface of a vessel of mercury by the passage of
the shock had a magnification of 6, whilst a telescope of the type B
had a magnification of 12.

The mean velocity given by six observations with type A is 4,225 feet
per second, while that given by the same number with type B is 7,475
feet per second.

[Illustration: FIG. 13.]

If we assume that the first tremor observed in the mercury is to
determine the true rate of transmission, General Abbot tells us that we
must reject all observations made with type A, inasmuch as they do not
reveal the velocity of the leading tremor. However, he also tells us
that a still higher power above 12 might have detected still earlier
tremors.

When gunpowder was the explosive, the observers noted that the
disturbance observed in the mercury took a much longer time to reach a
maximum than it did when dynamite was employed.

It was also observed that explosions fired beneath deep water gave
a higher velocity than similar explosions which took place beneath
shallow water. In the latter case much of the energy was probably
expended in throwing a jet of water into the air.

Another point which was observed appears to have been that the rate
varied with the initial shock. Thus:—

                                         Feet per second
  400 lbs. of dynamite  gave                  8,814
  200  „         „                            8,730
   70  „       powder (deep) gave             8,415

Also it is probable that the rate of a wave diminished with its
advance. For,

                                           Feet per second
    200 lbs. of dynamite gave for 1  mile      8,730
     „   „         „      „       5  miles     5,250
  50,000 „         „      „       8    „       8,300
    „    „         „      „      13½   „       5,300

General Abbot’s general conclusions are:—

1. A high magnifying power of telescope is essential in seismometric
observations.

2. The more violent the initial shock the higher is the velocity of
transmission.

3. This velocity diminishes as the general wave advances.

4. The movements of the earth’s crust are complex, consisting of many
short waves first, increasing and then decreasing in amplitude; and
with a detonating explosive the interval between the first wave and
the maximum wave, at any station, is shorter than with a slow burning
explosive.

_Results obtained in Japan._—From some experiments made by the author
in the grounds of the Meteorological Department in Tokio, the following
results were obtained:—

  A. Velocity in feet per second for the first 200 ft. (A to B)
  B. Velocity in feet per second for the second 200 ft. (B to C)
  C. Velocity in feet per second for 400 ft. (A to C)
  D. Number of Cartridges of Dynamite (6 = 1 lb.)

  +--------------------+-----+-----+-----+------+
  |  No. of Explosion  |  A  |  B  |  C  |  D   |
  +--------------------+-----+-----+-----+------+
  |            {    I. | 464 | 186 | 265 |  8·3 |
  |  Vertical  {  III. |  -- | 211 |  -- | 10·1 |
  | vibrations {   IV. | 352 | 234 | 281 |  7·1 |
  |            {    V. | 343 | 232 | 277 |  5·0 |
  |  Normal    {   VI. |  -- |  -- | 407 | 10·0 |
  | vibrations {  VII. |  -- |  -- | 516 | 12·5 |
  | Transverse } VIII. |  -- |  -- | 344 | 12·5 |
  | vibrations }       |     |     |     |      |
  +--------------------+-----+-----+-----+------+

The general results to be deduced from the above appear to be:—

  1. For vertical motion.
    (_a_) For the first 200 feet. The velocity depends upon the initial
        force—the greater the charge of dynamite the greater the
        velocity.
    (_b_) For the second 200 feet. The above law only appears in
        experiments IV. and V., but it must be remembered that the
        origins of I. and III. were farther removed from A than IV.
        and V.
  The speed of the wave during the second 200 feet is always less
        than during the first 200 feet.

  2. For normal vibrations.
    Here the speed between A and C is all that was measured, but we
        again see that the greater the initial force, or the nearer
        we are to the origin of the disturbance, the greater is the
        velocity. This velocity is greater than the velocity of the
        vertical or transverse vibrations.

  3. For transverse vibrations.
    If we assume that the vertical vibrations are a component of the
        transverse motions we see the same law as before—namely, that
        the nearer we are to the origin of the disturbance the greater
        is the speed with which that disturbance is propagated.

It will be observed that the chief law here enunciated respecting the
decrease in speed of earth vibrations is the same as that pointed
out by General Abbot, from which it only differs by its being in all
cases proved without the introduction of personal errors, for the same
explosion, along the same line of ground and for different kinds of
vibrations.




                              CHAPTER V.

     EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON EARTHQUAKES.

  Result of feelings—The direction of motion—Instruments as indicators
    of direction—Duration of an earthquake—Period of vibration—The
    amplitude of earth movements—Side of greatest motion—Intensity
    of earthquakes—Velocity and acceleration of an earth
    particle—Absolute intensity of an earthquake—Radiation of an
    earthquake—Velocity of propagation.


_Result of Feelings._—As the result of our experiences, and by
observations upon the movements produced in various bodies, we can
say that an ordinary earthquake consists of a number of backward and
forward motions of the ground following each other in quick succession.
Sometimes these commence and die out so gently that those who have
endeavoured to time the duration of an earthquake have found it
difficult to say when the shock commenced and when it ended. This was
a difficulty which Mr. James Bissett in Yokohama, and the author in
Tokio, had to contend against when, in 1878, they commenced to time
shocks between these two places.

Sometimes these motions gradually increase to a maximum and then die
out as gradually as they commenced.

Sometimes the maximum comes suddenly, and at other times during an
earthquake our feelings distinctly tell us that there are several
maxima.

These have been the experiences of many observers, and have been
recorded by writers since the earliest times. Mallet devotes a
chapter to a consideration of the tremulous motion that precedes and
follows a shock, and he tells us that a single shock is an absolute
impossibility. In speaking of earthquakes, he says: ‘The almost
universal succession of phenomena recorded in earthquakes is, first a
trembling, then a severe shock, or several in quick succession, and
then a trembling gradually but rapidly becoming insensible.’

A quantitative and exact knowledge of the nature of earthquake
motion has only been attained of late years. The chief results which
investigators have aimed at have been the measurement of the amplitude,
the period, the direction, and the duration of the motions which
constitute an earthquake. Attention has also been given to the velocity
with which a disturbance is propagated.

_The Direction of Motion._—One of the most ordinary observations
which are made about an earthquake is its direction. If we were to
ask the inhabitants of a town which had been shaken by an earthquake
the direction of the motion they experienced, it is not unlikely that
their replies would include all the points of the compass. Many,
in consequence of their alarm, have not been able to make accurate
observations. Others have been deceived by the motion of the building
in which they were situated. Some tell us that the motion had been
north and south, whilst others say that it was east and west. A certain
number have recognised several motions, and amongst the rest there will
be a few who have felt a wriggling or twisting. Leaving out exceptional
cases, the general result obtained from personal observation as to
the direction of an earthquake of moderate intensity is extremely
indefinite, and the only satisfactory information to be obtained is
that derived from instruments or from the effects of the earthquake
exhibited in shattered buildings and bodies which had been overturned
or projected.

By the direction in which walls, columns, and other objects had been
overthrown or fractured, Mallet was enabled to determine the position
of the origin of the Neapolitan earthquake. Similar phenomena have many
times been taken advantage of by other investigators of earthquake
phenomena. Effects produced upon structures are, however, only to be
observed as the results of a destructive earthquake, at which time
cities may be regarded as collections of seismometers. (_See_ chapter
on Effects in Buildings.)

To determine the direction of movement during a small earthquake, the
most satisfactory method appears to be an appeal to instruments.

_Instruments as Indicators of Direction._—The relative values of
different kinds of instruments, such as columns, pendulums, and the
like, as indicators of direction have already been discussed.

By the use of pendulum seismographs it has been shown that during an
earthquake the ground may move in one, two, or several directions (see
p. 21); and it is, generally speaking, only in those cases where we
experience a decided shock in the disturbance that we can determine
with any confidence the direction in which the motion has been
propagated. Such directions are usually indicated by the major axis of
certain more or less elliptical figures which have been drawn, which
in themselves appear to indicate the combination of two rectilinear
movements.

Results similar to those indicated by the records of pendulum
seismographs have also been obtained upon moving plates with a double
bracket seismograph. Thus, in the earthquake which shook Tokio at 6
A.M. on July 5, 1881, there were indications of the following
motions:—

Near the commencement of the shock the motion was N. 112° E. One and a
half second after this, the direction of motion appears to have been
N. 50° E. In three-fourths of a second more it gradually changed to a
direction N. 145° E., and after a similar interval to N. 62° E. Half a
second after this it was N. 132° E., and four seconds later the motion
was again in the original direction—namely, N. 112° E.

These particular directions of motion have been selected because they
were so definitely indicated.

The commonest type of earthquake which is experienced in Japan, and
probably also in other earthquake-shaken districts, is the compound or
diastrophic form.

That earthquakes often have motions compounded of two sets of
vibrations, has also been proved by the analysis of the records
obtained from two component seismographs. From an analysis of a record
of this description, Professor Ewing has shown that in the earthquake
felt in Tokio on March 11, 1881, there were approximate circular
(somewhat spiral) movements.

This leads us to the consideration of the twisting and wriggling
motions which are said to be experienced by some observers. Motions
like these, which by the Italians and Mexicans are called _vorticosi_,
are usually supposed to be the cause of objects like chimneys and
gravestones being rotated. These phenomena, it will be seen from what
is said in the chapter upon the effects produced in buildings, can be
more easily explained upon the supposition of a simple rectilinear
movement.

That at the time of an earthquake there may be motion in more than one
direction has been recognised since the time of Aristotle; and it is
possible that two sets of rectilinear motion, as, for instance, the
normal and transverse movements, may have led observers to imagine that
there has been a twisting motion taking place, and this especially when
the two sets of movements have quickly succeeded each other.

Persons inside flexible buildings may possibly have experienced more
or less of a rotatory motion, although the shock was rectilinear; the
building assuming such a motion in consequence of its construction and
its position with regard to the direction of the shock.

In the case of destructive earthquakes, especially at points situated
practically above the origin, the universal testimony, Mallet tells
us, is that a twisting, wriggling motion in different planes, attended
by an up-and-down movement of greater range, is experienced. To such
disturbances the word _sussultatore_ is sometimes applied. Mallet has
given many elliptical and other closed curves to illustrate the nature
of such motions.

_Duration of an Earthquake._—When reading accounts of earthquakes
it is often difficult to determine the length of time a shaking was
continuous. In Japan, in A.D. 745, there was a shaking which is said
to have lasted sixty hours; and in A.D. 977 there were a series of
shakings lasting 300 days. Often we meet with records of disturbances
which have lasted from twenty to seventy days.

At San Salvador, in 1879, more than 600 shocks were felt within ten
days; in 1850, at Honduras, there were 108 shocks in a week; in 1746,
at Lima, 200 shocks were felt in twenty-four hours; at the island of
St. Thomas, in 1868, 283 shocks were felt during about ten hours.

Disturbances like these, which succeed each other with sufficient
rapidity to cause an almost continual trembling in the ground, may be
regarded as collectively forming one great seismic effort which may
last a minute, an hour, a day, a week, or even several years. Strictly
speaking, they are a series of separate earthquakes, the resultant
vibrations of which more or less overlap. Whenever a large earthquake
occurs it is generally succeeded by a large number of smaller shocks.

The seismic disturbance as regards time is, as Mallet remarks, very
often ‘like an occasional cannonade during a continuous but irregular
rattle of musketry.’ In the New Zealand earthquake of 1848, shocks
continued for nearly five weeks, and during a large portion of the time
there were at least 1,000 shocks per day.[12]

The earthquake of Lisbon, which in five minutes destroyed the whole
town, was followed by a series of disturbances lasting over several
months. After Basle had, on October 18, 1356, been laid in ruins,
it is stated shocks followed each other for a period of a year. The
Calabrian earthquake was continued with considerable strength for a
year, and it is said that the earth did not come completely to rest for
ten years. During this cannonade the heavy shocks announced, as they
do in most earthquake countries at the present day, a series of weaker
disturbances. In certain exceptional cases this order of events has
been inverted, and slight shocks have announced the coming of heavy
ones. Fuchs gives an example of this in the earthquake of Broussa, when
the first shock was on February 28, 1855. On March 9 and 23 there were
heavier shocks, but the heaviest did not arrive until March 28.

Under certain conditions it is possible to have a sensible vibration
produced in the ground which is practically of unlimited duration;
thus, for instance, it has been noticed that the falling of water
at certain large waterfalls, by its continuous rhythmical impact on
the rocks, produces in them tremors which are to be observed at
great distances. Of this the author convinced himself at the Falls
of Niagara, where he observed the reflected and ever-moving image of
the sun in a pool of water. Under favourable circumstances almost
continual condensation of steam might take place in volcanic foci, each
condensation giving rise to a blow sufficiently powerful to produce
vibrations in the surrounding ground. Those who have stood near a large
geyser, like the one in Iceland, when it makes an ineffectual effort
to erupt, will recognise how powerful such a cause might be. Humboldt
has remarked shocks on Vesuvius and Pichincha which were periodic,
occurring twenty to thirty seconds before each ejection of vapour and
ashes.

Earthquakes like these may be of vast extent, gradually spreading
further and further outwards. This spreading of earth vibrations may
be observed at a large factory containing heavy machinery or a steam
hammer. After the machinery comes to rest, it is probably some time
before the ground returns to rest. Examples of disturbances of this
nature are spoken of under the head of Earth Tremors.

The record of the duration of an ordinary earthquake as observed at a
given point is dependent upon the sensibility of our instruments.

Continuous motions perceptible to our senses without the aid of
instruments usually last from thirty seconds to about two or three
minutes. In Japan the shocks, as timed by watches, usually last from
twenty to forty seconds. Occasionally a continuous shaking is felt for
more than one and a half minutes, and cases have been recorded where
the motion has continued for as much as four minutes and thirty-three
seconds.

Seismometers having a multiplication of 6 to 12 usually indicate that
motion continues longer than is perceptible to the senses.

_Period of Vibration._—When an earthquake contains several prominent
vibrations which might be called the _shocks_ of the disturbance, our
feelings tell us that these have occurred at unequal intervals.

About the time which is taken for the complete backward and forward
oscillation of the ground which constitutes the shock a little has
already been said. This was deduced from the records of disturbances as
drawn by seismographs. From the same sources we can readily obtain the
period of all the prominent vibrations in a disturbance.

In any given earthquake there are irregularities in period, and
different earthquakes differ from each other. About the early attempts
to determine the period of earth vibrations something has been said in
the chapter on Earthquake Instruments.

In the earthquake of March 11 (referred to on p. 70) we find that both
components commenced with a series of small vibrations, about five or
six to the second; next came the shock, consisting of two complete
vibrations executed in two seconds. In this it is to be observed that
the motion eastwards was performed much more quickly than the motion
westwards. Next, by reference to the east and west component, it is
seen that there are a number of large vibrations, about one per second,
on which a number of smaller motions are superposed. As the motion
proceeds, these become less and less definitely pronounced and more
irregular in their intervals, until finally the motion dies away.

This earthquake, as recorded at the author’s house in Tokio, lasted
about one and a half minute.

The same earthquake, as recorded by Professor Ewing at a station
situated about one and a half mile distant, but on flat ground, appears
to have lasted four and a half minutes. The largest wave had a period
of 0·7 second.

In the earthquake of March 8, 1881, there were on an average 1·4
vibrations per second. These vibrations were executed in a direction
transverse to the line joining the observing station and the locality
from which the disturbance must have originated as determined by time
observations. It can, therefore, be assumed that these vibrations,
having so slow a period, were transverse motions, this slowness or
sluggishness being due to the fact that the modulus for distortion
is less than the modulus which governs the propagation of normal
vibrations.

_The Amplitude of Earth Movements._—In making estimates of the
distances through which we are moved backward and forward at the time
of an earthquake, if we judge by our feelings, we may often be misled.
If a person is out of doors and walking, an earthquake may take place
sufficiently strong to cause chimneys to fall and unroof houses, which,
so far as the actual shaking of the ground is concerned, will be passed
by unnoticed. On the other hand, to persons indoors, especially on an
upper story, it is impossible even for a tremor to pass by without
creating considerable alarm by the angular movement that has been taken
up by the building.

Many observers have endeavoured to make actual measurements of the
maximum extent through which the earth moves at the time of an
earthquake. Among the reports of the British Association for 1841 is
the report of a committee which had been appointed ‘for obtaining
instruments and registers to record shocks of earthquakes in Scotland
and Ireland’. We read that in one earthquake which had been measured
the displacement of the ground had been half an inch, and in another
it had been less than half an inch. The instruments used to make these
observations depended upon the inertia of pendulums which at the time
of the disturbance were supposed to remain at rest. Observations
similar to these have been made in Japan. One long series were made by
Mr. E. Knipping for Dr. Gr. Wagener. They extended from November 1878
to April 1880, and were as follows:—

   Number of     Maximum horizontal
  Earthquakes    motion of the ground
      10            ·0   to  0·15 mm.
       7            ·15   „  0·5  „
       8            ·5    „  2·5  „
       2           2·5    „  more „

With his apparatus for vertical motion Dr. Wagener also made
observations on the absolute vertical motion. This seldom reached ·02
mm. The greatest value was that observed for the destructive shock of
Feb. 22, 1880, which was ·56 mm.

By means of a number of instruments distributed at various localities
round Tokio, the chief of which were pendulums with friction pointers
to render them ‘_dead beat_,’ and with magnifying apparatus to show
the actual motion of the ground, the author arrived at results similar
to those obtained by Dr. Wagener—namely, that the earth’s maximum
horizontal motion at the time of a small earthquake was usually only
the fraction of a millimetre, and it seldom exceeded three or four
millimetres. When we get a motion of five or six millimetres, we
usually find that brick and stone chimneys have been shattered.

The results obtained for vertical motion were also very small. In Tokio
it is seldom that vertical motion can be detected, and when it is
recorded it is seldom more than a millimetre.

These results, which were put forward some years ago, have since
received confirmation by the use of a variety of instruments in the
hands of different observers.

Mallet, in his account of the Neapolitan earthquake of 1857,
approximated to the amplitude of an earth particle by observing the
width, at the level of the centre of gravity, of fissures formed
through and remaining in great masses of very inelastic masonry.

Taking stations situated on or very nearly on the same line passing
through the seismic vertical (_epicentrum_), Mallet observed the
amplitude increased as some function of the distance, as will be seen
from the following table:—

  +-------------------------+------+-------+-------+---------+-------+
  |          Station        |Polla |La Sala|Certosa|Tramutola|Sarconi|
  +-------------------------+------+-------+-------+---------+-------+
  | Distance from Seismic } |      |       |       |         |       |
  |  Vertical in          } | 3·45 | 11·60 | 16·50 |  20·60  | 26·7  |
  |  geographical miles   } |      |       |       |         |       |
  | Amplitude in inches     | 2·5  |  3·5  |  4·0  |   4·5   |  4·75 |
  +-------------------------+------+-------+-------+---------+-------+

The possibility of a law such as this having an existence for places at
a distance from the seismic vertical comparable with the vertical depth
of the centrum will be shown farther on.

With regard to the maximum displacement of an earth particle. Mallet
was of opinion that there was evidence to show that it had in some
cases been over one foot. M. Abella, in an earthquake which occurred in
the Philippines in 1881, made a rough observation of the motion of the
earth to a distance of about _two metres_. This, as might be expected,
was beyond the elastic limits of the material, and caused fissures to
be formed, which were seen to open and shut.

_Intensity of Earthquakes._—In speaking of the strength of an
earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’
&c. Although these expressions, accompanied by illustration of the
effects which an earth quake has produced, convey a general idea of
the strength of a shock as felt at some particular locality, our ideas
nevertheless wanting in definiteness; and if we endeavour to compare
one shock with another, as a whole, our want of exactness is augmented.
We have seen that Palmieri’s seismograph indicates intensity by a
certain number of degrees, which, to a certain extent, is a measure of
the violence of the motion as indicated at a particular locality. The
degrees, as before stated, refer to the height to which in consequence
of the shaking, a certain quantity of mercury was washed in a tube,
which is a function of the depth of mercury in the tube, and also of
the duration of the disturbance.

From this it seems possible that a very slow motion of small amplitude,
continuing over a sufficient period of time, might, if it agreed with
the period of the mercury, indicate an earthquake of many degrees of
intensity, whilst residents in the neighbourhood might not have noticed
the disturbance; and, on the other hand, a short but intense shock
creating considerable destruction might have been recorded as of only a
few degrees of intensity.

Although objections like these might be raised to such a method of
recording intensity, in practice it would appear that such results are
not pronounced, and the indications of the instrument usually give us
approximate indications of relative intensity.

In writing about the Neapolitan earthquake of 1857, Mallet says that
‘area alone affords no test of seismic energy.’

The area over which a shock is felt will depend not only upon the
initial force of the disturbance, but also upon the focal depth of
a shock, the form and position of that focus, the duration of the
disturbance, and the nature and arrangement of the materials which are
shaken.

From observations in Japan, it is clearly shown that massive mountain
ranges exert a considerable influence upon the extension of seismal
disturbances. On one side of a large range of mountains large cities
might be laid in ruins, whilst on the other side the disturbance
creating this destruction might not be noticed.

_Velocity and Acceleration of an Earth Particle._—We now pass on to
methods of determining the intensity of an earthquake which are less
arbitrary than those which have just been discussed. These methods have
already been discussed when speaking of artificial disturbances, where
it was shown that the intensity of an earthquake as measured by its
destructive effects greatly depended upon the suddenness with which the
backward and forward motions of the ground were commenced or ended.

Amongst the earlier investigators of seismic phenomena who observed
that there existed a connection between the distance to which bodies
had been projected during an earthquake and the suddenness or initial
velocity with which the ground had been moved beneath them, was
Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks
from his chimneys had, by the New England earthquake of 1755, been
thrown thirty feet. From this and the known height of the chimney, he
calculates that the bricks had been projected with an initial velocity
of twenty-one feet per second.[13]

The calculations made by Mallet respecting the maximum velocity of
an earth particle at the time of the Neapolitan earthquake in 1857
depended upon the overthrow, projection, and fracture of bodies.

The principles which guided him in making the calculations will be
understood from the following illustration.

[Illustration: Fig. 14.]

If a column, A B C D, receive a shock or be suddenly moved in the
direction of the arrow, the centre of gravity, G, of this column will
revolve round the edge, and tend to describe the path G O. If it passes
O, the column will fall. The work done in such a case as this is equal
to lifting the column through the height _o_ _h_.

If G A = _a_, the angle G A _h_ = φ, and the weight of the body = W,
then the above work equals

                           W_a_ (1 - cos φ).

This must equal the work acquired—that is to say, the kinetic energy of
rotation of the body, or

                                        W _w_^2 K^2
                     W_a_ (1 - cos φ) = ———————————.
                                           2 _g_

Where _w_ is the angular velocity of the body at starting, K the radius
of gyration round A, and _g_ the velocity acquired by a falling body in
one second. Whence

                    _w_^2 K^2 = 2 _ga_ (1 - cos φ),

but _w_, the angular velocity, is equal to the statical couple applied,
divided by the moment of inertia, or,

                                 V_a_ cos φ
                           _w_ = ——————————,
                                    K^2

squaring and substituting

                                  K^2   1 - cos φ
                     V^2 = 2_g_ × ——— × —————————,
                                  _a_    cos^2 φ

                                                            K^2
and since the length of the corresponding pendulum is _l_ = ———,
                                                            _a_

                                      1 - cos φ
                        V^2 = 2_gl_ × —————————.
                                       cos^2 φ

To apply this to any given case we must find the value of _l_ or of

K^2
———.
_a_

Mallet finds these values for the cube, solid and hollow rectangular
parallelopipeds, solid and hollow cylinders, &c. In these formulæ we
have a direct connection between the dimensions and form of a body and
the velocity with which the ground must move beneath it to cause its
overthrow.

[Illustration: FIG. 15.]

Not only is the case discussed for horizontal forces, but also for
forces acting obliquely. Similar reasonings are applied to the
productions of fractures in walls, but as there is uncertainty in our
knowledge of the co-efficient of force necessary to produce fracture
_through joints across_ beds of masonry, the deductions ought not to
be applied as the measures of velocity. Where the fractures occur at
the base or in horizontal planes, or in those of the continuous beds of
the masonry, or through homogeneous bodies, the uncertainty is not so
great, and for cases like these Mallet gives several illustrations. The
distance to which bodies had been projected, as, for example, ornaments
from the tops of pedestals, coping-stones from the edges of roofs, were
also used as means of determining the angle at which the shock had
emerged, or, if this be known, for determining the velocity.

Thus by a shock in the direction O C, a ball, A, on the top of a
pedestal would describe a trajectory to the point C. Let the angle
which O C makes with the horizon be _e_, the vertical height through
which the ball has fallen be _b_, and the horizontal distance of
projection be _a_; then

                                          _a_^2
                   _b_ = _a_ tan _e_ + ————————————,
                                       4H cos^2 _e_

H being the height due to the velocity of projection. Whence

                              ___________________
                        2H ± √4H(H + _b_) - _a_^2
              Tan _e_ = ——————————————————————————.
                                  _a_

                                _a_^2 _g_
                V^2 = ———————————————————————————————.
                      2 cos^2 _e_ (_b_ - _a_ tan _e_)

For the back motion or subnormal wave in the direction C O,

                              ___________________
                        2H ± √4H(H + _b_) - _a_^2
              Tan _e_ = ——————————————————————————.
                                  _a_

                                _a_^2 _g_
                V^2 = ———————————————————————————————.
                      2 cos^2 _e_ (_b_ + _a_ tan _e_)

A serious error which may enter into calculations of this description
when practically applied has been pointed out when speaking of columns
as seismometers. It was then shown that such bodies before being
overthrown may often be caused to rock, and therefore that their final
overthrow may not have any direct connection with the impulse of the
succeeding shock.

Another point to which attention must be drawn respecting the above
calculations is that if there was no friction or adherence between the
projected body and its pedestal, in consequence of its inertia it
would be left behind by the forward motion of the shock, and simply
drop at the foot of its support. In the case of frictional adherence it
would be carried forward by the velocity acquired before this adherence
was broken, and thrown in a direction _opposite_ to that given in the
figure—that is to say, in the direction of the shock.[14]

_The Absolute Intensity of the Force exerted by an Earthquake._—No
doubt it has occurred to many who have experienced an earthquake
that the power which gave birth to such a disturbance must have been
enormously great. The estimates which we shall make of the absolute
amount of energy represented by an earthquake cannot, on account of the
nature of the factors with which we deal, be regarded as accurate. They
may, however, be of assistance in forming estimates of quantities about
which we have at present no conception. One method of obtaining the
result we seek is that which was employed by Mallet in his calculations
respecting the Neapolitan earthquake. Although disbelieving in the
general increment of temperature as we descend in the earth at an
average rate of 1° F. for every fifty or sixty feet of descent, for
want of better means. Mallet assumes this law to be true, and, knowing
from a variety of observations the depth of various parts of the cavity
from which the disturbance sprang, he calculates the temperatures of
this cavity in various parts as due to its depth beneath the surface.
Next, it is assumed that steam was suddenly admitted into this cavity,
which might exert the greatest possible pressure due to the maximum
temperature. This was calculated as being about 684 atmospheres.

Next, he determined the column of limestone necessary to balance such a
pressure, which is about 8,550 feet in height. As the least thickness
of strata above this cavity was 16,700 feet, the pressure of 684
atmospheres was not sufficient to blow away its cover, but if suddenly
admitted or generated in the cavity it might have produced the wave of
impulse by the sudden compression of the walls of the cavity.

The pressure of 684 atmospheres is equivalent to about 4·58 tons on
the square inch, and, as the total area of the walls of the cavity is
calculated at twenty-seven square miles, the total accumulated pressure
would be more than 640,528 millions of tons. Mallet, however, shows
that it is probable that the temperature of the focal cavity was much
greater than that due to the hypogeal increment, and that therefore the
pressure may have been greater.

The capability of producing the earthquake impulse, however, depends
on the _suddenness_ with which the steam is flashed off. According
to the experiments of Boutigny and others, Mallet tells us that the
most sudden production of steam would take place at a temperature of
500°-550° C., which is but a few degrees below that calculated for the
mean focal depth.

Assuming the above calculated pressure to be true, and knowing the
co-efficient of compression of the materials on which it acted, the
volume of the wave at a given moment near the instant of starting—that
is, at the focus—can be calculated, and from this the wave amplitude on
reaching the surface may be deduced.

Proceeding backwards, if we have observed the wave amplitude,
calculated the depth of the focus, and know the co-efficient of
expansion, then the total compression may be calculated and the
temperature due to the pressure producing this may be arrived at. In
this way earthquakes may be used as a means of calculating subterranean
temperature at depths that can never be attained experimentally.

A method of proceeding which is probably more definite than that
adopted by Mallet would be the application of the method indicated when
speaking of the intensity of artificial disturbances.

If for a given earthquake the origin of which is known we have
determined by seismographs the mean acceleration of an earth particle
at two or more stations at different distances from that origin, we are
enabled to construct a curve of intensity the area between which and
its asymptotes was shown to be a measure of the total intensity of the
shock. Comparing this area with that of a unit disturbance produced,
say, by the explosion of a pound of dynamite, one may approximately
calculate in terms of this unit the initial intensity of the earthquake.

_Radiation of an Earthquake._—The tremors preceding the more violent
movements of an earthquake may be due, as Mallet has suggested,[15]
to the free surface waves reaching a distant point before the direct
vibrations.

The fact that earth vibrations produced by striking a blow on or near
the surface of the ground are wholly obliterated in reaching a cutting
or valley, there being no underground waves of distortion to crop up
on the opposite side of the valley, indicates that the disturbance is
one that travels on the surface; the same fact is illustrated when we
endeavour to transmit vibrations through the side of a hill into a
tunnel.

In the tunnel, although the distance may be small, no sensible effects
are produced, whilst the same disturbance may be recorded at a long
distance from its origin on the surface of the ground outside the
tunnel.

Lastly, we may refer to the experiences of miners underground.

Occasionally it has happened that miners when deep underground, as in
the Marienberg in the Saxon Erzgebirge, have felt shocks which have not
been noticed on the surface. These observations are rare, and it is
possible that they may be explained by the caving in of subterranean
excavations.

The usual experience is, that if a shock is felt underground it is also
felt on the surface, as for example in the lead mines in Derbyshire at
the time of the Lisbon disturbance (1755).

The most frequent observation, however, is that a shock may be felt on
the surface while it is not remarked by the miners beneath the surface,
as at Fahlun and Presburg in November, 1823.

At the Comstock Lode in Colorado about twelve years ago many
earthquakes were felt. On one particular day twenty-four were counted.
Superintendent Charles Foreman told the author when he visited Virginia
City in 1882, that special observations were made to determine whether
these shocks were felt as severely deep down in the mines as on the
surface, where they were on the verge of being destructive. The
universal testimony of many observers was that in most cases they were
not felt at all underground, and when a shock was felt it was extremely
feeble. At Takashima Colliery, in Japan, it is seldom that shocks are
felt underground.

The explanation of these latter observations appears to be either
that, in consequence of a smaller amplitude of motion in the solid
rocks beneath the surface as compared with the extent of motion on
the surface, the disturbances are passed by unnoticed, or else the
disturbance is, at a distance from its origin, practically confined to
the surface.

_Velocity of Propagation of an Earthquake._—Although many have written
upon earthquakes and have endeavoured to give to us the velocity with
which they were propagated, the subject is one about which we have as
yet but little exact information.

The importance of this branch of investigation is undoubtedly great.
By knowing the velocity with which an earthquake has travelled in
various directions we are assisted in determining the locality of
its origin; we may possibly make important deductions respecting the
nature of the medium through which it has passed; perhaps also we
may learn something regarding the intensity of the disturbance which
created the earthquake. In the Report of the British Association for
1851 Mallet gives the table on next page, in which are placed together
the approximate rates of transit of shocks of several earthquakes
which he discusses. Some of these, it will be observed, are records of
disturbances which must have passed through or across the bed of the
ocean.

In Mallet’s British Association Report for 1858, he gives data compiled
by Mr. David Milne[16] respecting the Lisbon earthquakes of 1755 and
1761, from which data the tables of velocities (p. 89) have been
calculated, omitting those which Mr. Mallet has marked as uncertain.

The distances are marked in degrees of seventy English miles each, and
the time is reduced to Lisbon time.

  +---------------------+-------+--------------------------+----------+
  |                     |Approx.|                          |          |
  |                     |rate in| Formation constituting   |          |
  | Occasion and Place  |feet   | Range on surface as far  |Authority |
  |                     | per   | as known or conjectured  |          |
  |                     |second |                          |          |
  +---------------------+-------+--------------------------+----------+
  |Rev. John Mitchell’s |1,760  |Sea bottom, probably on   | Mitchell |
  | guesses from the    |       | slates, secondary and    |          |
  | Lisbon earthquakes  |       | crystalline rocks        |          |
  |Von Humboldt’s       |1,760  |From observations in      | Humboldt |
  | guesses from South  |  to   | various South American   |          |
  | America             |2,464  | rocks in great part      |          |
  |                     |       | volcanic                 |          |
  |                     |       |                          |          |
  | _Lisbon Earthquake  |       |                          |          |
  |     of 1761._       |       |                          |          |
  |Lisbon to Corunna    |1,994  |Transition, carboniferous | ‘Annual  |
  |                     |       | and granitoid            |Register’ |
  |Lisbon to Cork       |5,228  |Transition, carboniferous |   „      |
  |                     |       | crystalline slates and   |          |
  |                     |       | granitoid, probably,     |          |
  |                     |       | under sea bottom         |          |
  |Lisbon to Santa Cruz |3,261  |The same with many        |   „      |
  |                     |       | alterations              |          |
  |                     |       |                          |          |
  |     _Antilles._     |       |                          |          |
  |Pointe à Pitre to    |6,586  |Probably volcanic rocks   |Stier and |
  |  Cayenne (doubtful) |       | under sea bottom         | Perrey’s |
  |                     |       |                          | memoran- |
  |                     |       |                          |dum, Dijon|
  |                     |       |                          |          |
  |      _India._       |       |                          |          |
  |Cutch to Calcutta,   |1,173  |Alluvial, secondary,      | ‘Royal   |
  |  1819               |       | granitoid and later      | Asiatic  |
  |                     |       | igneous rocks            | Journal’ |
  |India, Nepauls, and  |       |                          |          |
  | basin of the Ganges,|       |                          |          |
  | 1834:--             |       |                          |          |
  |Rungpur to Arrah     |2,314 }|Deep alluvia, with        | ‘Royal   |
  |Monghyr to Gorackpur |3,520 }| occasional transition,   | Asiatic  |
  |Rungpur to Monghyr   |  990 }| carboniferous, granitoid,| Journal’ |
  |Rungpur to Calcutta  |1,210 }| and later igneous rocks  |          |
  |Ships ‘Rambler’ and  |1,056  |Sea bottom resting on     |‘Nautical |
  | ‘Millwood,’ at sea, |       |  unknown rock            |Magazine’ |
  | 1851; between lat.  |       |                          |          |
  | 16° 30′ N.L., 54°   |       |                          |          |
  | 30′ W., and lat. 23°|       |                          |          |
  | 30′ N.L., 58° 0′ W. |       |                          |          |
  +---------------------+-------+--------------------------+----------+

              THE LISBON EARTHQUAKE ON NOVEMBER 1, 1755.

  +----------------------------+---------+--------+--------+
  |                            | Moment  |Distance|Velocity|
  |         Localities         |observed |  from  |in feet |
  |                            |of shock |presumed|  per   |
  |                            |         | origin | second |
  +----------------------------+---------+--------+--------+
  |                            |   h. m. |   °  ′ |        |
  | Presumed focus of shock,   | }  9 23 |    --  |   --   |
  |   lat. 30°, long. 11° W.   | }       |        |        |
  | A ship at sea in lat. 38°, | }  9 24 |   0 30 |  3,091 |
  |   long. 10° 47′ W.         | }       |        |        |
  | Colares                    |    9 30 |   1 30 |  1,325 |
  | Lisbon                     |    9 32 |   1 30 |  1,030 |
  | Oporto                     |    9 38 |   2 30 |  1,030 |
  | Ayamont                    |    9 50 |   4  0 |    916 |
  | Cadiz                      |    9 48 |   5  0 |  1,236 |
  | Tangier and Tetuan         |    9 46 |   5 30 |  1,478 |
  | Madrid                     |    9 43 |   6  0 |  1,855 |
  | Funchal                    |   10  1 |   8 30 |  1,382 |
  | Portsmouth                 |   10  3 |  12 30 |  1,431 |
  | Havre                      |   10 23 |  13  0 |  1,339 |
  | Reading                    |   10 27 |  13 30 |  1,304 |
  | Yarmouth                   |   10 42 |  15  0 |  1,174 |
  | Amsterdam                  |   10  6 |  17  0 |  2,444 |
  | Loch Ness                  |   10 42 |  18  0 |  1,409 |
  +----------------------------+---------+--------+--------+


             THE LISBON EARTHQUAKE OF MARCH 31, 1761.[17]

  +----------------------------+---------+--------+--------+
  |                            | Moment  |Distance|Velocity|
  |         Localities         |observed |  from  |in feet |
  |                            |of shock |presumed|  per   |
  |                            |         | origin | second |
  +----------------------------+---------+--------+--------+
  |                            |   h. m. |   °  ′ |        |
  | Presumed focus, lat. 43°,  | } 11 51 |    --  |  --    |
  |   long. 11° W.             | }       |        |        |
  | Ship at sea in lat. 43°,   | }       |        |        |
  |   many leagues from coast  | } 11 52 |   0 30 |  3,091 |
  |   of Portugal              | }       |        |        |
  | Ship in lat. 44° 8′ and    | } 11 54 |   1 45 |  3,607 |
  |   about 80 leagues from    | }       |        |        |
  |   coast                    | }       |        |        |
  | Corunna                    |   11 51 |   2 30 |  2,576 |
  | Ship lat. 44° 8′ and 80    | }       |        |        |
  |   leagues NNW. of Cape     | } 11 58 |   3 30 |  3,091 |
  |   Finisterre               | }       |        |        |
  | Lisbon                     |   noon  |   4 30 |  3,091 |
  | Madeira                    |   12  6 |  10  0 |  4,122 |
  | Cork                       |   12 11 |   9 30 |  2,937 |
  +----------------------------+---------+--------+--------+

These tables, owing to the nature of the materials which Mallet had at
his disposal, are but rude approximations to the truth. Two interesting
facts are, however, observable: the first being that the velocities
for the earthquake of 1761 are much higher than those obtained for the
earthquake of 1755; and, secondly, that in both cases the velocities as
determined from the observations of ships at sea closely approximate
to each other, in all cases being nearly the same as that with which a
sound wave would travel through water.

The great differences in transit velocity obtained for different
earthquakes is a point worthy of attention.

Seebach’s velocity is a _true_ transit velocity, and its determination
is dependent on the assumption that the shock radiated from the
_centrum_ and not from the _epicentrum_, Seebach’s method is explained
when speaking about the determination of origins.

Some interesting observations on the velocity with which the earthquake
of October 7, 1874, was propagated, are given by M. S. di Rossi.[18]

One assumption is that the disturbance radiated from an origin
to surrounding points of observation, whilst another is that the
disturbance followed natural fractures, the direction of which is
derived from the crest of certain mountain ranges. These velocities are
as follows, Maradi being at or near the origin of the disturbance:—

  +-----------------------------+-----------------------------------+
  | Velocity in feet per second |  Velocity in feet per second by   |
  |   with direct radiation     | propagation along mountain chains |
  +-----------------------------+-----------------------------------+
  | Modigliana       820        | By the Valley of Marenzo    1,080 |
  | Bologna          656        |  „       „       Saveno     1,080 |
  | Forli            874        |  „       „       Montone    1,080 |
  | Modena           518        |  „       „       Panaro    {1,080 |
  |                             |                            {  984 |
  | Firenze          273        |  „       „       Sieve        540 |
  | Compiobbi        328        |  „       „         „          540 |
  +-----------------------------+-----------------------------------+

Another set of interesting results are those of P. Serpieri on the
earthquake of March 12, 1873. The curious manner in which this shock
radiated is described in the chapter on the Geographical Distribution
of Earthquakes (see p. 231). Two large areas appear to have been
almost simultaneously struck, so that, there being no time for elastic
yielding, the velocities calculated between places situated on either
of the areas are exceedingly great.[19]

  From Ragusa to  Venice the velocity was 2,734 feet per second
   „   Spoleto     „            „         4,101   „        „
   „   Perugia to Orvieto       „           601   „        „
   „      „     „ Ancona        „         1,640   „        „
   „      „     „ Rome          „    {    1,640   „        „
                                     {or, 2,186

The following are examples of approximate earthquake velocities which
have been determined in Japan.

_The Tokio Earthquake of October 25, 1881._—From records respecting
this earthquake it appears to have been felt over the whole of Yezo
and the northern and eastern coast of Nipon, a little farther south
than Tokio. It was severest at Nemuro and Hakodate, and at the former
place a little damage was done. From these facts, together with the
indications of instruments recording direction of movement and a
general inspection of the time records, it seems that the disturbance
must have originated beneath the sea on the east coast of Yezo at a
very long distance to the north-east of Tokio, from which place it
passed in a practically direct line on to Yokohama.

As the disturbance was felt at Yokohama twenty-one seconds later than
at Tokio, and the distances between these two places is about sixteen
geographical miles, for this portion of its course the disturbance
must have travelled at a rate of at least 4,300 _feet per second_. If
we assume that the shock, after having reached Hakodate, travelled on
at the same rate as it did between Tokio and Yokohama in order to reach
Saporo, where the shaking was felt eighteen seconds after Hakodate,
it must have had about thirteen geographical miles to travel after
Hakodate was shaken before Saporo felt its effect.

Drawing from Hakodate a tangent to the eastern side of a circle of
thirteen miles radius described round Saporo, the origin of the
disturbance must be on the line bisecting this tangent at right angles.
As it also lies on a line drawn through Tokio and Yokohama, it lies in
a position about 41 N. lat. and 144° 15′ E. long., which is a position
somewhat nearer to Nemuro than Hakodate, as we should anticipate. If
this be taken as approximately indicating the origin, then the shock,
after reaching Hakodate from the Hakodate _homoseist_, travelled about
218 miles to reach Tokio in 128 seconds, which gives a _velocity of
10,219 feet per second_.

The method here followed is equivalent to that of the hyperbola and one
direction (see p. 204). The hyperbola is described on the assumption
that the velocity deduced from the time taken to travel between Tokio
and Yokohama is correct, and also that the earth waves travelled with
approximately the same velocity in the vicinity of Saporo as near
Tokio. The probability, however, is that they travelled more quickly.
If this be so, then the origin is thrown somewhat to the south-east and
the velocity between the Hakodate homoseist and Tokio reduced. Thus,
if the velocity in the Saporo district be double that observed in the
Tokio district, the origin is shifted about twenty-eight miles to the
south-west, and the last-mentioned velocity is reduced to about 9,000
feet per second.

If we work by the method of circles, and assume the velocity to have
been constant in all directions, then this velocity must have been
about 6,000 feet per second. If we assume that the indications of
direction obtained from seismographs and other sources give to us by
this intersection a proper origin, the velocity in some directions may
have been as much as 17,000 feet per second.

An origin thus determined, or even if determined by the method of
circles, is in discord with the fact that places like Nemuro, in the
north-east of Yezo, were nearer to the origin than any of the other
places which have been mentioned.

The conclusion which we are therefore led to with regard to this shock,
assuming of course that the time observations are tolerably correct,
is that the velocity of propagation was variable, being greater when
measured between points near to the origin than between points at a
distance. The velocities estimated vary between 4,000 and 9,000 feet
per second.

In the case of the earthquake which has just been discussed, we have
an example of a disturbance which must have passed between Tokio and
Yokohama in what was almost a straight line from the origin. As this
direction ought to give the maximum time of transit if all earthquakes
are propagated with the same velocity, the following table is given of
the interval between the time of observation of several shocks at these
two stations:—

       FROM YOKOHAMA TO TOKIO.         FROM TOKIO TO YOKOHAMA.

  1880 December 20th    36 seconds | 1882 October 25th   21 seconds
  1881 January 7th   14–31    „    | 1883 February 6th   23    „
   „   March  8th       60    „    |  „   March 1        53    „
   „     „   17th.      66    „    |  „     „   „        63    „
   „   November 15th    31    „    |  „          8th     27    „
  1882 February 16th    22    „    |  „     „   11th     26    „

As these are observations which have been made with the assistance of a
telegraphic signal daily employed to correct and rate the clocks from
which the observations were obtained, they may be regarded as being
tolerably, correct.

The disturbance of February 6, the two shocks of March 1, appear, like
that of October 25, to have passed in almost a direct line from an
origin in the N.N.E, through Tokio on to Yokohama. Their velocities of
propagation as calculated from the above intervals are approximately
3,900, 1,900, and 1,400 feet per second. The shock of February 16
appears to have had its origin near to a point in Yedo Bay about eight
miles east of Yokohama. Assuming this to be the case, the shock between
the Yokohama homoseist and Tokio travelled at the rate of 2,454 feet
per second, but between the Tokio homoseist and Chiba at the rate
of 750 feet per second; that is to say, the velocity of propagation
rapidly decreased as the disturbance spread outwards.

At Yokohama it was recorded at 5.31.54, at Tokio at 5.32.16, and at
Chiba at 5.33.48. These times are given in Tokio mean time.

The shock of March 11, which was recorded at Tokio at 7.51.22 P.M.
and at Yokohama at 7.51.33 P.M., appears, from the indications of
instruments which were exceptionally definite in their records, to have
originated in the N.E. corner of Yedo Bay, about nineteen miles S.S.W.
from Chiba. This shock was rather severe, fracturing several chimneys.
From the Tokio homoseist it appears to have travelled on to Yokohama at
the rate of about 2,200 feet per second. Assuming these observations
to be _approximately_ accurate, if we take them with the records of
previous observers they lead us to the following conclusions:—

1. Different earthquakes, although they may travel across the same
country, have very variable velocities, varying between several
hundreds and several thousands of feet per second.

2. The same earthquake travels more quickly across districts near to
its origin than it does across districts which are far removed.

3. The greater the intensity of the shock the greater is the velocity.




                              CHAPTER VI.

            EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.

  The destruction of buildings is not irregular—Cracks in
    buildings—Buildings in Tokio—Relation of destruction to
    earthquake motion—Measurement of relative motion of parts of a
    building shaken by an earthquake—Prevention of cracks—Direction
    of cracks—The pitch of roofs—Relative position of openings in a
    wall—The last house in a row—The swing of buildings—Principle of
    relative vibrational periods.


The subject of this chapter is, from a practical point of view, one
of the most important with which a seismologist has to deal. We
cannot prevent the occurrence of earthquakes, and unless we avoid
earthquake-shaken regions, we have not the means of escaping from
them. What we can do, however, is in some degree to protect ourselves.
By studying the effects produced by earthquakes upon buildings of
different construction and variously situated, we are taught how to
avoid or at least to mitigate calamities which, in certain regions of
the world, are continually repeated. The subject is an extensive one,
and what is here said about it must be regarded only as a contribution
to the work of future writers who may give it the attention it
deservedly requires.

_The Destruction produced by Earthquakes is not irregular._—If we
were suddenly placed amongst the ruins of a large city which had
been shattered by an earthquake, it is doubtful whether we should at
once recognise any law as to the relative position of the masses of
_débris_ and the general destruction with which we are surrounded.
The results of observation have, however, shown us that, amongst the
apparently chaotic ruin produced by earthquakes, there is in many cases
more or less law governing the position of bodies which have fallen,
the direction and position of cracks in walls, and the various other
phenomena which result from such destructive disturbances.

Mallet, at the commencement of his first volume, describing the
Neapolitan earthquake of 1857, discusses the general effect produced by
various shocks upon differently constructed buildings. First he shows
that, if we have a rectangular building, the walls at right angles to
the shock will be more likely to be overthrown than those which are
parallel to it. Experience teaches a similar lesson. Thus Darwin, when
speaking of the earthquake at Concepcion in 1835,[20] tells us that
the town was built in the usual Spanish fashion, with all the streets
running at right angles to each other. One set ranged S.W. by W. and
N.E. by E. and the other N.W. by N. and S.E. by S. The walls in the
former direction certainly stood better than those in the latter. The
undulations came from the S.W.

In Caraccas it is said that every house has its _laga securo_, or safe
side, where the inhabitants place their fragile property. This _laga
securo_ is the north side, and it was chosen because about two out of
every three destructive shocks traversed the city from west to east,
so that the walls in these sides of a building have been stricken
broadside on.[21]

_Cracks in Buildings._—Results like the above come from destructive
earthquakes rather than from movements such as those we have to deal
with ordinarily. When a building is subjected to a slight movement
it is assumed that the walls at right angles to the direction of the
shock move backwards and forwards as a whole, and there is little or
no tendency for them to be fractured at their weaker parts, these
weaker parts being those over the various openings. The walls, however,
which are parallel to the direction of the movement are, so to speak,
extended and contracted along their length, and in consequence they may
be expected to give way over the various openings. This tendency for
extension and contraction of a wall along its length may be supposed,
for instance, to be due to the different portions of a wall, owing to
differences in dimensions and elasticity, having different periods of
natural vibration, or possibly for two portions of a long line of wall
to be simultaneously affected by portions of waves in different phases.

As an illustration of the giving way of a building in the manner here
suggested we may take the case of a large brick structure which was
recently being erected in Tokio. This building, at the time of the
earthquake, was only some fourteen or fifteen feet above the surface
of the ground. The length of the building stretched from N.W. to S.E.,
and it was intersected by many walls at right angles to this direction.
Through all the walls of this building there were many arched openings.
In the central part of the transverse walls, which walls were fully
five feet in thickness, the arches which joined them together were 4
feet 4 inches in thickness. The arches therefore formed a comparatively
lightly constructed link between heavy masses of brickwork.

On March 3, 1879, at 4.43 P.M., an earthquake was felt throughout
Tokio, the strength of which, as judged by our feelings, was above that
of an average shock. As registered by one of Palmieri’s instruments, it
had a direction S.S.W. to N.N.E. and an intensity of 11°. On the same
day there were several smaller shocks having the same direction, and
these were succeeded by others on the 9th of the month.

Immediately after these shakings it was discovered that almost every
arch in the internal walls of the building here referred to had been
cracked across the crown in a direction about N. 40° W. All the
other arches of the building, of which there were a great number in
walls at right angles to the direction of the shock, were found not
to have sustained any injury. To this statement, however, there was
one exception, which was subsequently proved to have been due to a
settlement taking place.

After examining these cracks the only cause to which they could be
attributed was the series of shakings which they had just experienced.
It seemed as if the heavy walls right and left of the arches had been
in vibration without synchronism in their periods, and as a consequence
the arches which connected them had been torn asunder.

Although the time at which the cracks were formed and the peculiar
positions in which they were only to be found pointed distinctly to
their origin, to be certain that they were not due to settlement of the
foundations, horizontal lines were ruled upon the brickwork and from
time to time subsequently observed.

The points to which the various cracks extended were also marked and
observed. Beneath the walls as foundations there were beds of concrete
about three feet thick and about ten feet in width. These had been
under the pressure of the partially built walls for two years before
the arches had been put in. As these foundations were unusually strong,
being intended to carry so very much greater weight than that to which
they had been subjected, if any settlement had been detected it would
have been a matter of surprise.

Some weeks after the formation of these cracks it was observed that
they gradually closed. This was probably due to the gradual falling
inwards of the two broken portions of the arch, their position when
open being one of instability.

[Illustration: FIG. 16.—Cracks in a corner house, Belluno, June 29,
1873 (Bittner).]

Had this building been more complete at the time of the shock, and
the heavy walls been tied together at higher points, although the
archways would have been points of weakness, it is quite possible
that fracture would not have taken place. This illustration shows us
that when a building is shaken in a definite direction there will be
some rule as to the positions in which fractures occur. As another
example, we may take the observations of Alexander Bittner upon the
buildings of Belluno after the shock of June 29, 1873 (see Beiträge
zur Kenntniss des Erdbebens von Belluno am 29 Juni, 1873, p. 40. Von
Alexander Bittner. Aus dem LXIX. Bande der Sitzungsb. der K. Akad. der
Wissensch., II. Abth., April-Heft. Jahrg. 1874).

Speaking generally, he remarks that ‘Houses similarly situated have
suffered in corresponding walls and corners in a similar manner. In
Belluno there is a certain kind of damage which is repeated everywhere,
making a peculiar system of splits in the S.W. and N.E. corners of the
houses.’ This is well shown in the accompanying sketch, which evidently
illustrates the effect of a shock oblique to the direction of two walls
at right angles to each other.

[Illustration: FIG. 17.—Brick buildings in Tokio, showing
fractures.]

_Buildings in Tokio._—For the purpose of finding out what has been the
effect produced by earthquakes upon the buildings of Tokio, and at
the same time for ascertaining whether blocks of buildings ranging in
different directions suffered to the same extent, the author examined,
in company with Mr. Josiah Conder, a large number of foreign-built
houses in the district of the Ginza. The chief reason for choosing
this was because it was the only district where a large number of
_similar_ buildings could be found. By examining houses or buildings of
different constructions, the effects produced upon them by earthquakes
are very often likely to show so many differences that it becomes
almost an impossibility to determine what the general effect has
been—unsymmetrical construction involving unsymmetrical ruin.

A number of similarly constructed buildings in one locality may be
regarded as a number of seismographs, the effect upon any one of them
being judged of by the average of the general effect which has been
produced upon the whole. The general form of two of these houses
which were examined is shown in fig. 17. In this figure the general
character of the fractures which have been produced can also be seen.
The houses are built of brick, and are in many cases faced with a thin
coat of white plaster. Projecting from the level of the upper floor
there is a balcony fronted by a low balustrade. This is supported by
small beams which at their outer extremity are carried on a row of
cylindrical columns. This forms a covered way in front of each row of
houses. The roofs are covered with thick tiles. It will be observed
that the arches of the upper windows spring _sharply_ from their
abutments, and at their crown they carry a heavy key-stone. The lower
openings, which have a span of 9 feet, have evidently been constructed
in imitation of the open front of an ordinary Japanese house. These
archways curve out _gently_ from their abutments. The outside walls
have a thickness of 13½ inches.

The results obtained from a careful examination of 174 houses in
streets running N.E. and 156 houses in streets running N.W., all of
these houses being similar, were as follows:—

1. In the upper windows nearly all the cracks ran from the springing,
which formed an angle with the abutment.

2. In the lower arches, which _curved_ into the abutments, not a single
crack was observed at the springway. The cracks in these arches were
near the crown, where beams projected to carry the balcony. In many
instances the cracks proceeded from such beams, even if there were
no arch beneath. That cracks should occur in peculiar positions, as
is here indicated, is shown in the illustrations which accompany the
accounts of many earthquakes.

3. The houses which were most cracked were in the streets running
parallel to the direction in which the greater number and most powerful
set of shocks cross the city.

The results showed that, in order to avoid the effects of small shocks,
all walls containing principal openings should be placed as nearly
as possible at right angles to the direction in which the shocks of
the districts usually travel. The blank walls, or those containing
unimportant openings, would then be parallel to the direction of the
shocks—that is, presuming our building to be made up of two sets of
walls at right angles to each other.

Another point of importance would be to build archways _curving_ into
the supporting buttresses; the archways over doors and windows which we
find in earthquake countries do not appear to be in any way different
from those which are built in countries free from earthquakes. In
the one country these structures have simply to withstand vertical
pressures applied statically; in the other, they have to withstand more
or less horizontal stresses, applied suddenly.

_Relation of Destruction to Earthquake Motion._—The relations which
exist between the overturning and projection of bodies and the motion
of the ground have already been discussed. It may be interesting
to call attention to the fact that in the formulæ showing three
relationships, it was the _shape_ rather than the _weight_ of a body
which determined whether it should be overturned or projected by a
motion at its base.

As an interesting proof that light bodies may be overturned as easily
as heavy ones. Mallet refers to the overturning of several large
haystacks as one of the results of the Neapolitan earthquake.

If masses of material are displaced or fractured, then Mallet remarks
                                       _____
that the maximum velocity will exceed √2_gh_, where _h_ is the
amplitude of the wave. Should the maximum velocity be less than this
quantity, the masses which are acted upon will be simply raised and
lowered, and there will be no relative displacements even if the
emergence of the wave be nearly or quite vertical.

When we get a vertical wave acting upon an irregular mass of masonry,
the heavier portions of the masonry, by their inertia, tend to descend
relatively to the remaining portions, and in this way vertical fissures
will be produced. For this reason it would not be advisable to use
heavy materials above archways, heavy roofs, or heavy floors. The
vertical fissures, Mallet remarks, would have their widest opening at
the base.

In considering cases of fracture produced by earthquake motion, it
must be remembered that these are due to stresses applied _suddenly_,
and that if the same amount of stress had been _slowly_ applied to a
building, fractures might not have occurred.

If a disturbance is horizontal, and has a direction parallel to the
length of a wall, the wall is carried forward at its foundations. This
motion is opposed by the inertia of the upper portion of the wall and
the various loads it carries. The wall being elastic, distortion takes
place, and cracks, which are widest at the top, will be formed. In a
uniform wall the two most prominent fissures ought to be near the ends.

If the horizontal backward and forward movement has a direction
oblique to the plane of the wall, the wall will be either overthrown,
fractured, or have a triangular fragment thrown off towards the origin
from the end last reached.

Should the wave emerge steeply, diagonal fissures at right angles to
the direction of transit will be formed, or else triangular pieces will
be projected.

The accompanying figures are reduced from Mallet’s ‘Account of the
Neapolitan Earthquake of 1857.’

[Illustration: FIG. 18.—Cathedral Church, Potenza (Mallet).]

Taking _a_ _b_ as the general direction of the fractures in fig. 18,
then _c_ _d_ will represent the direction in which the shock emerged,
which is at an angle of 23°·20′ to the horizon. It might be argued that
the direction of these fractures was due to the direction in which
surface undulations had travelled, or to the relative strengths and
proportions of different portions of the building. The directions of
cracks in a building are undoubtedly due to a complexity of causes, but
for buildings situated in the region of shock the impulsive effect of
the shock is probably the most important function to be considered.
The method of applying the directions of emergence, deduced from
observations on fractures, to determine the origin of a disturbance
will be referred to in Chapter X.

[Illustration: FIG. 19.—The Cathedral, Paterno (Mallet).
Neapolitan Earthquake of 1857.]

Mallet observed that, although two ends of a building might be nearly
the same, the fissures and joints do not occur at equal distances from
the ends, nor are they equally opened.

The end where the joints are the most opened is that which was first
acted upon, and this phenomenon may be sufficiently well pronounced
to indicate the direction in which we must look to find the origin
of a disturbance. Amongst possible explanations for this disposition
of fractures in a wall. Mallet suggests that they may be due to real
differences in the two semiphases of the wave of shock, the second
semiphase being described with a somewhat slower velocity than the
first. This, it will be observed, is contrary to the indications of
seismographs.

Fig. 19, of the cathedral at Paterno, shows the effect of a subnormal
shock striking a wall obliquely and projecting one of its corners.


           MEASUREMENTS OF THE RELATIVE MOTION OF PARTS OF A
                BUILDING AT THE TIME OF AN EARTHQUAKE.

In 1880 a series of observations was made in Tokio to determine whether
at the time of an earthquake the various parts of the arched openings
which we see in many buildings synchronised in their vibrations, or,
for want of synchronism, were caused to approach and recede from each
other. The arches experimented on were heavy brick arches forming the
two corridors of the Imperial College of Engineering. The direction of
one set of these corridors is N. 40° E. and that of the other N. 50° W.

The thickness of the walls in which these arches are placed is 1 ft.
11 in. They are built of Japanese bricks bound together with ordinary
lime. The span of the arches is 8 ft. 3 in., and the height of the arch
from the springing-line to the crown 4 ft. 1 in. The height of the
abutments is 7 ft. 1½ in. The voussoirs of the arch are formed of a
light grey soft volcanic rock, and on their faces show a depth of 12
inches. The width of the intermediate columns between the arches is 4
ft. 6⅞ in.

To determine whether at the time of an earthquake there was any
variation in the dimensions of these arches, a light stiff deal
rod, about 2 in. by ½ in. in cross section, was placed across the
springing-line of the arch. One end of this was firmly fixed to the
top of one abutment by means of a spike; on the other end, which was
to indicate any horizontal movement if the abutments approached each
other, there was fixed a pointer made out of a piece of steel wire.
This rested on a piece of smoked glass fixed to the ledge on which
the loose end of the rod was resting. If the abutments approached or
receded from each other a line would be drawn measuring the extent of
the motion. As a further indication of motion, a second smoked glass
plate was fixed on the transverse rod, which plate was marked on by a
pointer attached to a vertical rod hanging down from the crown of the
arch.

As a general result of these experiments it may be said that the
portions of the building which were examined usually either did not
move at all, or else they practically synchronised in their movements.
When they did move, the extent of motion was small, and the small
differences in movement which were observed were in every probability
far within the elastic limits of the structure.

_Observations on Cracks._—To determine whether the walls of a building
which have once been cracked, when subjected to a series of shocks,
similar to those which they experienced before being cracked, still
continued to give way, the extremities of a considerable number of
cracks in the N.E. end of the museum buildings of the Engineering
College were marked with pencil. Although since the time of marking
there had been many severe shocks, these cracks did not visibly extend.
These marks were made on the outside wall of the building. On the
inside, one of these same cracks showed itself as a fissure about ¼
inch in width. Across this crack a horizontal steel wire pointer was
placed. One end of this wire was fixed in the wall; the other end,
which was pointed, rested on the surface of a smoked glass plate placed
on the other side of the crack. After small earthquakes there was no
indication of motion having taken place, but after a shock on February
21, as indicated by a line upon the smoked glass plate, it was seen
that the sides of the crack had approached and receded from each other
through a distance of about 1/16 inch.

By similar contrivances placed on cracks in a neighbouring building
exactly similar results were obtained, namely, that during small
earthquakes the two sides of the crack had retained their relative
positions, but at the time of a large shock this position had been
changed.

In this building it was also observed that the cracks in many instances
increased their length.

By attaching levers to the end of the pointers to multiply any motion
that might take place, no doubt the indications would be more frequent
and more definite. It would also be easier to note the relative
distances of motion in two directions, namely, how far the cracks
had closed and how far they had opened. As to whether motion would
occur or not, much would no doubt depend upon the direction of the
earthquake.

_Prevention of Fractures._—One conclusion which may perhaps be drawn
from these observations is, that a cracked building at the time of an
earthquake shows a certain amount of flexibility. Whether a building
which had been designed with cracks or joints between those parts
which were likely to have different periods of vibration would be more
stable, so far as earthquake shakings are concerned, than a similar
building put up in an ordinary manner, is a matter to be decided by
experiment. Certainly some of the cracks which have been examined
indicate that if they had not existed, the strain upon the portion of
the building where they occur would have been extremely great.

_Direction of Cracks._—In looking at the cracks produced by small
earthquakes it is interesting to note the manner of their extension.
The basements of the buildings which have been most carefully examined
are, for a height of two or three feet, built of large rectangular
blocks of a greyish-coloured volcanic rock. In these parts the cracks
pass in and out between the joints of the stone, indicating that
the stones have evidently been stronger than the mortar which bound
them together, and as a consequence the latter had to give way.
Above this basement when the cracks enter the brickwork, they no
longer exclusively confine themselves to the joints, but run in an
irregular line through all they meet with, sometimes across the bricks
and sometimes through the mortar joints. In places where they have
traversed the brickwork, we can say that the mortar has been stronger
than the bricks. This traversing of the bricks rather than the joints
is, I think, the general rule for the direction of the cracks in the
brickwork of Tokio buildings.

_The Pitch of Roofs._—From observation of the effects produced by
earthquakes, it appears to us that the houses which lost the greater
number of tiles appear to be those with the steepest pitch, and those
where the tiles were simply laid upon the roof and not in any manner
fastened down. It would seem that destruction of this sort might to a
great extent be obviated by giving the roofs a less inclination and
fixing the tiles with nails. It was also noticed that the greatest
disturbance amongst the tiles was upon the ridges of the roofs.
Destruction of this sort might be overcome by giving especial attention
to these portions during the construction of the roof.

[Illustration:  FIG. 20.      FIG.  21.]

_Relative Position of Openings in Walls._—From what has been said about
the fractures in the buildings of Tokio it will have been seen that,
with but few exceptions, they have all taken place above openings
like doorways and windows. If architecture demands that openings like
arches should be placed one above another in heavy walls of this kind,
as in fig. 17, there will be lines of weakness running through the
openings parallel to the dotted lines. As arches are only intended
to resist vertical thrusts, special construction must be adopted to
make them strong enough to resist horizontal pulls. For instance, a
flat arch would offer more resistance to horizontal pulls than an
arch put together with ordinary voussoirs, there being in the former
case more friction to prevent the component parts sliding over each
other. Or again, above each arch an iron girder or wooden lintel might
be inserted in the brick or stone arch. It was suggested to me by my
colleague, Mr. Perry, that the best form calculated to give a wall
uniform strength, would be to build it so that the openings of each
tier would occupy alternate positions, that is to say, along lines
parallel to the struts and ties of a girder. In this way we should have
our materials so arranged that they would offer the same resistance to
horizontal as to vertical movements. Such a wall is shown in fig. 20:
the dotted lines running through the openings, and all similar lines
parallel to the former, representing lines of weakness. If we compare
this with fig. 21, we shall see that in the case of a horizontal
movement _a_ _b_ or of a vertical movement _c_ _d_, we should rather
expect to find fractures in a house built like fig. 21 than in one
built like fig. 20. If, however, these two buildings were shaken by a
shock which had an angle of emergence of about 45° in the direction _e_
_f_, the effects might be reversed. Usually, however, and always in a
town like Tokio which is visited by shocks originating at a distance,
the movements are practically horizontal ones, and, therefore,
buildings erected on the principles illustrated by fig. 20 should
be much superior, so far as resisting earthquakes is concerned, to
buildings constructed in the ordinary manner, as in fig. 21. Fractures
following a vertical line of weakness are shown in the accompanying
drawing, fig. 22, of the Church of St. Augustin, at Manilla, shattered
by the earthquakes of 1880.

_The last House in a Row._—When an earthquake shock enters a line
of buildings, and proceeds in a direction coincident with that of
the buildings, we should expect that the last of these houses, being
unsupported on one side, would be in the position of the last person in
Tyndall’s row of boys. From this it would seem that the end house in a
row would show the greatest tendency to fly away from its neighbours.
If the last house stood upon the edge of a deep canal or a cliff,
there would be a layer of ground, equal in thickness to the depth of
the canal or to the height of the cliff, as the case may be, which
would also be in a position to be thrown forward. The effect which is
sometimes produced upon an end building is shown in fig. 23, which is
taken from the photograph of a house shattered in 1868 at San Francisco.

[Illustration: FIG. 22.—Church of St. Augustin, Manilla.
Earthquakes of July 18–20, 1880.]

_The Swing of Buildings._—The distance through which buildings are
moved at the time of an earthquake depends partly on their construction
and partly on the extent, nature, and duration of the movement
communicated to them at their foundations. By violent shocks buildings
may be completely overthrown. In the case of small earthquakes, the
upper portion of a house may frequently move through a much greater
distance than the ground at its foundation. For instance, during the
Yokohama earthquake of February, 1880, when the maximum amplitude of
the earth’s motion was probably under ¾ of an inch, from the slow swing
of long Japanese pictures, from three to six feet in length, which
oscillated backwards and forwards on the wall, it is very probable
that the extent through which the upper portion of houses moved was
very considerable. In some instances these pictures seem to have swung
as much as two feet, and from the manner in which they swung they
evidently synchronised with the natural swing of the house.

[Illustration: FIG. 23.—Webber House, San Francisco. Oct. 21, 1868.]

From this it would seem that such a house must have rocked from side to
side one foot out of its normal perpendicular position. That the motion
was great is testified by nearly all who tried to stand at the time of
the shock, it having been impossible to walk steadily across the floor
of a room in an upper story. The houses here referred to are either
those which are purely Japanese, or else those which are framed of wood
and built on European models, a class of building which is very common
in Tokio and Yokohama.

Perry and Ayrton calculated the period of a complete natural vibration
of different structures. For a square house whose outer and inner
sections were respectively 30 and 26 feet, and whose height was 30
feet, the period calculated would be about ·06 second.

At the time of the above earthquake many houses seem to have moved like
inverted pendulums. On the morning after the shock my neighbour, who
was living upstairs in a tall wooden house with a tile roof, told me
that he endeavoured to count the vibrations, and was of the impression
that to make a complete swing it took about 2 seconds.

Assuming now that the distance through which the top of a wooden house
moved was about 1 foot, and the number of vibrations which it made per
second was about ·5, then the greatest velocity of a point on the top
of such a house must have been about 6 feet per second.

Mallet, who made observations upon the vibrations of various
structures, tells us that Salisbury spire moves to and fro in a gale
more than 3 inches. A well-constructed brick and mortar wall, 40 feet
high and 1 foot 6 inches thick, was observed to vibrate in a gale 2
feet transversely before it fell.

An octagonal chimney with a heavy granite capping, 160 feet high, was
observed instrumentally to vibrate at the top nearly 5 inches.[22]

At the time of a severe earthquake it does not seem impossible but that
a building may be swung completely over. The accompanying illustration,
fig. 24, taken from a photograph,[23] apparently indicates a movement
of description.

[Illustration: FIG. 24.—Stud Mill at Haywards, California. Oct. 21,
1868.]

_Principle of relative Vibrational Period._—If a lath or thin pole
loaded at one end with a weight fixed to the ground, so as to stand
vertically, be shaken by an earthquake it will be caused to rock to
and fro like an inverted pendulum. The period of its swing will be
chiefly dependent on its dimensions, its elasticity, and its load. In
a building we have to consider the vibration of a number of parts,
the periods of which, if they were independent of each other, would be
different. On account of this difference in period, whilst one portion
of a building is endeavouring to move towards the right, another
is pulling towards the left, and, in consequence, either the bonds
which join them or else they themselves are strained or broken. This
was strikingly illustrated by many of the chimneys in the houses at
Yokohama, which by the earthquake of February 20, 1880, were shorn off
just above the roof. The chimneys were shafts of brick, and probably
had a slower period of vibration than the roof through which they
passed, this latter vibrating with the main portion of the house, which
was framed of wood.

A particularly instructive example of this kind which came under my
notice is roughly sketched in fig. 25.

[Illustration: FIG. 25.]

This is a chimney standing alone, which, for the sake of support, was
strapped by an iron band to an adjoining building. It would seem that
at the time of the shock, the building moving one way and the chimney
another, the swing of the heavy building gave the chimney a sharp jerk
and cut it off. The upper portion, being then loose upon the lower
part, rotated under the influence of the oscillations in manner similar
to that in which gravestones are rotated.

Mallet made observations similar to these in Italy. He tells us that a
buttress may often not have time to transmit its stability to a wall.
The wall and the buttress have different periods of vibration, and
therefore they exert impulsive actions on each other. Effects like
these were strikingly observable in many of the rural Italian churches
where the belfry tower is built into one of the quoins of the main
rectangular building.

Not only have we to consider the relative vibrations of the various
parts of a building amongst themselves, but we have to consider the
relation of the natural vibrations of any one of them or the vibration
of the building as a whole, with regard to the earth, the vibrations of
which it must be remembered are not strictly periodic.

Some of the more important results dependent upon the principle of
‘relative vibrational periods’ may be understood from the following
experiments:—

[Illustration: FIG. 26.]

In fig. 26 A, B, and C are three flat springs made out of strips of
bamboo, and loaded at the top with pieces of lead. At the bottom they
are fixed into a piece of board D E, and the whole rests on a table F
G. The legs of this table being slightly loose, by placing the fingers
on the top of it, a quick short backward and forward movement can be
produced. The weights on A and B are the same, but they are larger than
the weight on C. Consequently the periods of A and B are the same, but
different to the period of C. The dimensions of these springs are as
follows: height, 18 inches; A and B each carry weights equal to 320
grammes, and they make one vibration per second; C has a weight of 199
grammes, and makes 0·75 vibrations per second.

_First Experiment._—It will be found that by giving the table a gentle
backward and forward movement, the extent of which movement may be so
small that it will be difficult to detect it with the eye, either A
and B may be made to oscillate violently whilst C remains still; or
_vice versâ_, C may be caused to oscillate whilst A and B remain still.
In the one case the period of shaking will have been synchronous with
the natural period of A and B, whilst in the latter it will have been
synchronous with that of C. This would seem to show us that if the
natural period of vibration of a house, or of parts of it, at any time
agree with the period of the shock, it may be readily thrown into a
state of oscillation which will be dangerous for its safety.

_Second Experiment._—Bind A and B together with a strip of paper
pasted between them. (The paper used was three-eighths of an inch
broad and would carry a weight of nearly three pounds.) If the table
be now shaken as before, A and B will always have similar movements,
and tend to remain at the same distance apart, and as a consequence
the strip of paper will not be broken. From this experiment it would
seem that so long as the different portions of a building have almost
the same periods of vibration, there will be little or no strain upon
the tie-rods or whatever contrivance may be used in connecting the
different parts.

_Third Experiment._—Join A and C, or B and C with a strip of paper in a
manner similar to the last experiment. If the table be now shaken with
a period approximating either to that of A and B, or with that of C,
the paper will be suddenly snapped.

This indicates that if we have different portions of a building of
such heights and thicknesses that their natural periods of vibration
are different, the strain upon the portions which connect such parts
is enormous, and it would seem, as a consequence, that either the
vibrators themselves, or else their connections, must, of a necessity,
give way. This was very forcibly illustrated in the Yokohama earthquake
of February 1880 by the knocking over of chimneys. The particular case
of the chimneys is, however, better illustrated by the next experiment.

_Fourth Experiment._—Take a little block of wood three-quarters of an
inch square and about one inch high, and place it on the top of A, B,
or C. It will be found that, although the spring on which it stands
is caused to swing backwards and forwards through a distance of three
inches, the little block will retain its position.

This little block we may regard as the upper part of a chimney standing
on a vibrating stack, and we see that, so long as this upper portion is
light, it has no tendency to fall.

_Fifth Experiment._—Repeat the fourth experiment, having first placed a
small leaden cap on the top of the block representing the chimney. (The
cap used only weighed a few grammes.) When vibration commences it will
be found that the block quickly falls. This would seem to indicate that
chimneys with heavy tops are more likely to fall than light ones.

_Sixth Experiment._—Bind A and B together with a strip of paper and
stand the little block upon the top of either. It will be found that
the block will stand as in the fourth experiment.

_Seventh Experiment._—Bind A and C, or B and C together, and place
the block upon the top of either of them. When vibration commences,
although the paper may not be broken, the little block will quickly
fall.

_Eighth Experiment._—Take two pencils or pieces of glass tube and
place them under the board D E. If the table F G be now shaken in the
direction D E, it will be found that the springs will not vibrate.

In a similar manner if a house or portion of a house were carried on
balls or rollers, as has already been suggested, it would seem that the
house might be saved from much vibration.

_Ninth Experiment._—Set any of the springs in violent vibration by
gently shaking D E instead of the table, and then suddenly cease the
actuating motion. It will be observed that at the moment of cessation
the board and the springs will have a sudden and very decided motion
of translation in the same direction as that in which the springs were
last moving, and although the springs were at the time swinging through
a considerable arc, all motion will suddenly cease.

This shows, that if a house is in a state of vibration the strain at
the foundations must be very great.

It would not be difficult to devise other experiments to illustrate
other phenomena connected with the principle of relative vibrational
periods, but these may perhaps be sufficient to show to those who have
not considered this matter its great importance in the construction
of buildings. Perhaps the greater portion of what is here said may by
many be regarded as self-evident truisms hardly worth the trouble of
demonstration. Their importance, however, seems to be so great that I
hope that their discussion has not been altogether out of place.

I may remark that in the rebuilding of chimneys in Yokohama the
principles here enunciated were taken advantage of by allowing the
chimneys to pass freely through the roofs without coming in contact
with any of the main timbers.

In putting up buildings to resist the effects of an earthquake, besides
the idea of making everything strong because the earthquake is strong,
there are several principles which, like the one just enunciated, might
advantageously be followed which as yet appear to have received but
little attention.




                             CHAPTER VII.

             EFFECTS PRODUCED UPON BUILDINGS (continued).

  Types of buildings used in earthquake countries—In Japan, in Italy,
    in South America, in Caraccas—Typical houses for earthquake
    countries—Destruction due to the nature of underlying rocks—The
    swing of mountains—Want of support on the face of hills—Earthquake
    shadows—Destruction due to the interference of waves—Earthquake
    bridges—Examples of earthquake effects—Protection of
    buildings—General conclusions.


_Types of buildings used in earthquake countries._—In Japan there
are excellent opportunities of studying various types of buildings.
The Japanese types, of course, form the majority of the buildings.
The ordinary Japanese house consists of a light framework of 4 or
5 inch scantling, built together without struts or ties, all the
timbers crossing each other at right angles. The spaces are filled
in with wattle-work of bamboo, and this is plastered over with mud.
This construction stands on the top of a row of boulders or of square
stones, driven into the surface soil to a distance varying from a few
inches to a foot. The whole arrangement is so light that it is not an
uncommon thing to see a large house rolled along from one position to
another on wooden rollers. In buildings such as these after a series of
small earthquake shocks, we could hardly expect to find more fractures
than in a wicker basket.

The larger buildings, such as temples and pagodas, are also built of
timber. These are built up of such a multitude of pieces and framed
together in such an intricate manner that they also are capable of
yielding in all directions. The European buildings are, of course,
made of brick and stone with mortar joints. Some of these, as the
buildings of the Ginza in Tokio, are not designed for great strength.
On the other hand, others have thick and massive walls and are equal in
strength to those we find in Europe.

The third type of buildings are those which are built in blocks; and
these blocks being bound together with iron rods traversing the walls
in various directions are especially designed to withstand earthquakes.
A system somewhat similar to this has been patented in America, and
examples of these so-called earthquake-proof buildings are to be found
in San Francisco.

Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted
especial attention to them says that,[24] ‘to imagine that slight
buildings, such as are seen here (i.e. in Japan), are the best
calculated to withstand an earthquake shock is an error of the most
palpable kind.’ After describing the construction of a Japanese house
in pretty much the same terms as we have used, he says ‘that with its
unnecessarily heavy roof and weak framework it is a structure of all
others the worst adapted to withstand a heavy shock.’ He tells us,
further, that these views are sustained by the truest principles of
mechanics. In order to render buildings to some extent proof against
earthquakes, some of the heavy roofs in Tokio have been so constructed
that they are capable of sliding on the walls. Mr. Brunton mentions
a design for a house, the upper part of which is to rest on balls,
which roll on inverted cups fixed on the lower part of the building,
which is to be firmly embedded in the earth. A similar design was,
at the suggestion of Mallet, used to support the tables carrying the
apparatus of some of the lighthouses erected in Japan by Mr. Brunton.
The very existence of these designs seems to indicate that the ordinary
European house, however solidly and strongly it may be built, is not
sufficient to meet the conditions imposed upon it. What is required,
is something that will give way—an approximation to the timber frame
of a Japanese house, so strongly condemned by Mr. Brunton and others.
The crucial test of the value of the Japanese structure, as compared
with the modern buildings of brick and stone, is undoubtedly to be
found by an appeal to the buildings themselves. So far as my own
experience has gone, I must say that I have never seen any signs in
the Japanese timber buildings which could be attributed to the effects
of earthquakes, and His Excellency Yamao Yozo, Vice Minister of Public
Works, who has made the study of the buildings of Japan a speciality,
told me that none of the temples and palaces, although many of them
are several centuries old, and although they have been shaken by small
earthquakes and also by many severe ones, show any signs of having
suffered. The greatest damage wrought by large earthquakes appears to
have resulted from the influx of large waves or from fires. In every
case where an earthquake has been accompanied by great destruction,
by consulting the books describing the same, it can be seen, from the
illustrations in these books portraying conflagrations, that this
destruction was chiefly due to fire. When we remember that nearly
all Japanese houses are constructed of materials that are readily
inflammable, it is not hard to imagine how destruction of this kind
has come about. To a Japanese, living as he does in a house which has
been compared to a tinder-box, fire is one of his greatest enemies,
and in a city like Tokio it is not at all uncommon to see during the
winter months many fires which sweep away from 100 to 500 houses. In
one winter I was a spectator of three fires, each of which was said to
have destroyed upwards of 10,000 houses.

Although it would appear that the smaller earthquakes of Japan produce
no visible effect upon the native buildings, it is nevertheless
probable that small effects may have been produced, the observation
of which is rendered difficult by the nature of the structure. If we
look at buildings of foreign construction, by which are meant buildings
of brick and stone, the picture before us is quite different, and
everywhere the effects of earthquakes are palpable even to the most
casual observer. Of these effects numerous examples have already been
given. Not only are these buildings damaged by the cracking of walls
and the overturning of chimneys, but they also appear to be affected
internally. For instance, in the timbers of the roof of the museum
attached to the Imperial College of Engineering in Tokio, there are a
number of diagonal faces acting as struts or ties intended to prevent
more or less horizontal movements taking place. Those which are rigidly
joined together with bolts and angle irons have apparently suffered
from their rigidity, being twisted and bent into various forms. The
buildings in Tokio, which are strongly put together, being especially
designed to withstand earthquakes, appear to have suffered but little.
I know only one example which at the time of the severe shock of 1880
had several of its chimneys damaged.

The ordinary houses in Italy, though built of stone and mortar, are
but poorly put together, and, as Mallet has remarked, are in no way
adapted to withstand the frightful shakings to which they are subjected
from time to time.

In the large towns, like Naples, Rome, and Florence, where happily
earthquakes are of rare occurrence, although the building may be
better than that found in the country, the height of the houses and
the narrowness of the streets are sufficient to create a shudder, when
we think of the possibility of the occurrence of a moderately severe
earthquake.

In South America, although many buildings are built with brick
and stone, the ordinary houses, and even the larger edifices, are
specially built to withstand earthquakes. In Mr. James Douglas’s
account of a ‘Journey Along the West Coast of South America,’ we read
the following[25]: ‘The characteristic building material of Guayaquil
is bamboo, which grows to many inches in thickness, and which, when
cut partially through longitudinally at distances of an inch or so,
and once quite through, can be opened out into fine elastic boards of
serviceable width. Houses, and even churches, of a certain primitive
beauty are built of such reeds, so bound together with cords that few
nails enter into the construction, and which, therefore, yield so
readily to the contortions of the earth during an earthquake as to be
comparatively safe.’

Here we have a house, which, so far as earthquakes are concerned, is
an exaggerated example of the principles which are followed in the
construction of an ordinary Japanese dwelling.

Another plan adopted in South America can be gathered from the same
author’s writings upon Lima, about which he says, ‘To build high houses
would be to erect structures for the first earthquake to make sport of,
and, therefore, in order to obtain space, safety, and comfort, the
houses of the wealthy surround court after court, filled with flowers,
and cooled with fountains, connected one with another with wide
passages which give a vista from garden to garden.’

History would indicate that houses of this type have been arrived at as
the results of experience, for it is said that when the inhabitants of
South America first saw the Spaniards building tall houses, they told
them they were building their own sepulchres.[26]

In Jamaica, we find that even as early as 1692 experience had taught
the Spaniards to construct low houses, which withstood shakings better
than the tall ones.[27]

In Caraccas, which has been called the city of earthquakes, it is
said that the earthquakes cause an average yearly damage amounting to
the equivalent of a _per capita_ tax of four dollars. To reduce this
impost to a minimum much attention is paid to construction. ‘Projecting
basement corners (giving the house a slightly pyramidal appearance)
have been found better than absolutely perpendicular walls; mortised
corner-stones and roof beams have saved many lives when the central
walls have split from top to bottom; vaults and key-stone arches, no
matter how massive, are more perilous than common wooden lintels, and
there are not many isolated buildings in the city. In many streets
broad iron girders, riveted to the wall, about a foot above the house
door, run from house to house along the front of an entire square.
Turret-like brick chimneys, with iron top ornaments, would expose the
architect to the vengeance of an excited mob; the roofs are flat,
or flat terraced; the chimney flues terminate near the eaves in a
perforated lid.’[28]

_Typical houses for earthquake countries._—From what has now been said
about the different buildings found in earthquake countries, it will be
seen that if we wish to put up a building able to withstand a severe
shaking, we have before us structures of two types. One of these types
may be compared with a steel box, which, even were it rolled down a
high mountain, would suffer but little damage; and the other, with a
wicker basket, which would equally withstand so severe a test. Both
of these types may be, to some extent, protected by placing them upon
a loose foundation, so that but little momentum enters them at their
base. One suggestion is to place a building upon iron balls. Another
method would be to place them upon two sets of rollers, one set resting
upon the other set at right angles. The Japanese, we have seen, place
their houses on round stones. The solid type of building is expensive,
and can only be approached partially, whilst the latter is cheap, and
can be approached closely. In the case of a solid building it would be
a more difficult matter to support it upon a movable foundation than in
the case of a light framework. Such a building is usually firmly fixed
on the ground, and consequently at the time of an earthquake, as has
already been shown by experiment, must be subjected to stresses which
are very great. In consequence also of the greater weight of the solid
structure, more momentum will enter it at its base than in the case of
the light structure. Also, we must remember that the rigidity favours
the transmission of momentum, and with rigid walls we are likely to
have ornaments, coping-stones, and the comparatively freer portions
forming the upper part of a building displaced; whilst, with flexible
walls absorbing momentum in the friction of their various parts, such
disturbances would not be so likely. Mr. T. Ronaldson, referring to
this, says, that in 1868, at San Francisco, the ornamental stone work
in stone and cement buildings was thrown from its position, whilst
similar ornaments in neighbouring brick buildings stood.

To reduce the top weight of a building, hollow bricks might be
employed. To render a building more homogeneous and elastic, the
thickness of bricks might be reduced. Inasmuch as the elasticity
of brick and timber are so different, the two ought to be employed
separately. For internal decorations plaster mouldings might be
replaced by _papier mâché_ and _carton-pierre_, the elastic yielding of
which is comparatively great.[29] Houses, whether of brick and stone,
or of timber, ought to be broad and low, and the streets three or four
times as wide as the houses. The flatter the roofs the better.

One of the safest houses for an earthquake country would probably be a
one-storied strongly framed timber house, with a light flattish roof
made of shingles or sheet-iron, the whole resting on a quantity of
small cast-iron balls carried on flat plates bedded in the foundations.
The chimneys might be made of sheet-iron carried through holes free of
the roof. The ornamentation ought to be of light materials.

At the time of severe earthquakes many persons seek refuge from their
houses by leaving them. In this case accidents frequently happen from
the falling of bricks and tiles. Others rush to the doorways and
stand beneath the lintels. Persons with whom the author has conversed
have suggested that strongly constructed tables and bedsteads in
their rooms would give protection. To see persons darting beneath
tables and bedsteads would undoubtedly give rise to humiliating and
ludicrous exhibitions. This latter idea is not without a value, and
most certainly, if applied in houses of the type described, would be
valuable.

The great danger of fire may partially be obviated by: the use of
‘earthquake lamps,’ which are so constructed that before they overturn
they are extinguished. It is said that in South America some of the
inhabitants are ready at any moment to seek refuge in the streets,
and they have coats prepared, stocked with provisions and; other
necessaries, which, if occasion demands, will enable them to spend the
night in the open air. These coats, called ‘earthquake coats,’ might
also, with properly constructed houses, be rendered unnecessary.

_Destruction due to the nature of the underlying rocks._—That the
nature of the ground on which a building stands is intimately related
with the severity of the blow it receives is a fact which has often
been demonstrated.

One cause of destruction is due to placing a building on foundations
which are capable of receiving the full effects of a shock, and
transmitting it to the buildings standing on them.

For instance, the reason why a soft bed might possibly make a good
foundation, is, as has been pointed out by Messrs. Perry and Ayrton,
because the time of transmission of momentum is increased; in fact, the
soft bed is very like a piece of wood interposed between a nail and
the blows of a hammer—it lengthens the duration of impact. For this
reason we are told that a quaking bog will make a good foundation. When
a shock enters loose materials its waves will be more crowded, and it
is possible that a line of buildings may rest on more than one wave
during a shock. There are many examples on record of the stability of
buildings which rested on beds of particular material at the time of
destructive earthquakes. As the observations which have been made
by various writers on this subject appear to point in a contrary
direction, I give the following examples:—

In the great Jamaica earthquake of 1692, the portions of Port
Royal which remained standing were situated on a compact limestone
foundation; whilst those on sand and gravel were destroyed (‘Geological
Observer,’ p. 426). Again, on p. 148 of the same work, we read,
‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe,
the destroying effects of this earthquake were confined to the tertiary
strata, and were most violent on the blue clay, on which the lower part
of the city is constructed. Not a building on the secondary limestone
or on the basalt was injured.’

In the great earthquakes of Messina, those portions of the town
situated on alluvium, near the sea, were destroyed, whilst the high
parts of the town, on granite, did not suffer so much. Similar
observations were made in Calabria, when districts consisting of
gravel, sand, and clay became, by the shaking, almost unrecognisable,
whilst the surrounding hills of slate and granite were but little
altered. At San Francisco, in 1868, the chief destruction was in the
alluvium and made ground.

At Talcahuano, in 1835, the only houses which escaped were the
buildings standing on rocky ground; all those resting on sandy soil
were destroyed.

From the results of observations like these, it would seem the harder
rocks form better foundations than the softer ones. The explanation of
this, in many cases, appears to lie in the fact that the soft strata
were in a state of unstable equilibrium, and by shaking, they were
caused to settle. Observations like the following, however, point out
another reason why soft strata may sometimes afford a bad foundation.

‘Humboldt observed that the Cordilleras, composed of gneiss and
mica-slate, and the country immediately at their foot, were more shaken
than the plains.’[30]

‘Some writers have asserted that the wave-like movements (of the
Calabrian earthquake in 1783) which were propagated through recent
strata from west to east, became very violent when they reached the
point of junction with the granite, as if a reaction was produced when
the undulatory movement of the soft strata was suddenly arrested by the
more solid rocks.’

Dolomieu when speaking of this earthquake says, the usual effect ‘was
to disconnect from the sides of the Apennines all those masses (of sand
and clay) which either had not sufficient bases for their bulk, or
which were supported only by lateral adherence.’

These intensified actions taking place at and near to lines of
junction between dissimilar strata is probably due to the phenomena of
reflection and refraction.

When referring to the question as to whether buildings situated on
loose materials suffered more or less than those on solid rocks,
Mallet, in his description of the Neapolitan earthquake of 1857,
remarks: ‘We have in this earthquake, towns such as Saponara and
Viggiano, situated upon solid limestone, totally prostrated; and we
have others such as Montemarro, to a great extent based upon loose
clays, totally levelled. We have examples of almost complete immunity
in places on plains of deep clay as that of Viscolione, and in places
on solid limestone, like Castelluccio, or perched on mountain tops like
Petina.’[31]

After reading the above, we see that the probable reason why, in
several cases, beds of soft materials have not made good foundations,
consists in the fact that they have either been of small extent or else
have been observed only in the neighbourhood of lines which divided
them from other formations, which lines are always those of great
disturbances.

At the end of his description of the Neapolitan earthquake of 1857,
Mallet says that more buildings were destroyed on the rock than on the
loose clay. This, however, he remarks, is hardly a fact from which we
can draw any valuable deductions, because it so happened that more
buildings were constructed on the hills than on the loose ground.[1]

Professor D. S. Martin, writing on the earthquake of New England in
1874, remarks that in Long Island the shock was felt where there was
gneiss between the drift. Around portions to the east the observations
were few and far between. He also remarks that generally the shocks
were felt more strongly and frequently on rocky than on soft ground.[32]

From these examples, it would appear that the hard ground, which
usually means the hills, forms a better foundation than the softer
ground, which is usually to be found in the valleys and plains. Other
examples, however, point to a different conclusion. For instance, a
civil engineer, writing about the New Zealand earthquake of 1855, when
all the brick buildings in Wellington were overthrown, says that ‘it
was most violent on the sides of the hills at those places, and least
so in the centre of the alluvial plains.’[33]

In this example it must be noticed that the soft alluvium here referred
to was of large extent, and not loose material resting on the flanks of
rocks, from which it was likely to be shaken down, as in most of the
previous examples.

The results of my own observations on this subject point as much in one
direction as in the other. In Tokio, from instrumental observations
upon the slopes and tops of hills, the disturbance appears to be
very much less than it is in the plains. Thus, at my house, situated
on the slope of a hill about 100 feet in height, for the earthquake
of March 11, 1882, I obtained a maximum amplitude of motion of from
three to four millimètres only, whilst Professor Ewing, with a similar
instrument, situated on the level ground at about a mile distant,
found a motion of fully seven millimètres. This calculation has been
confirmed by observations on other earthquakes. Thus, for instance, in
the destructive earthquake of 1855, when a large portion of Tokio was
devastated, it was a fact, remarked by many, that the disturbance was
most severe on the low ground and in the valleys, whilst on the hills
the shock had been comparatively weak. As another illustration, I may
mention that within three-quarters of a mile from my house in Tokio
there is a prince’s residence which has so great a reputation for the
severity of the shakings it receives that its marketable value has been
considerably depreciated, and it is now untenanted.

In Hakodadi, which is a town situated very similarly to Gibraltar,
partly built on the slope of a high rocky mountain and partly on a
level plain, from which the mountain rises, the rule is similar to
that for Tokio, namely, that the low, flat ground is shaken more
severely than the high ground. At Yokohama, sixteen miles south-west
from Tokio, the rule is reversed, as was very clearly demonstrated
by the earthquake of February 1880, when almost every house upon the
high ground lost its chimney, whilst on the low ground there was
scarcely any damage done; the only places on the low ground which
suffered were those near to the base of the hills. The evidence as to
the relative value of hard ground as compared with soft ground, for
the foundation of a building, is very conflicting. Sometimes the hard
ground has proved the better foundation and sometimes the softer, and
the superiority of one over the other depends, no doubt, upon a variety
of local circumstances.

These latter observations open up the inquiry as to the extent to which
the intensity of an earthquake may be modified by the topography of the
disturbed area.

_The swing of mountains._—If an earthquake wave is passing through
ground the surface of which is level, so long as this ground is
homogeneous, as the wave travels further and further we should expect
its energy to become less and less, until, finally, it would insensibly
die out. If, however, we have standing upon this plain a mountain,
judging from Mallet’s remarks, this mountain would be set in a state
of vibration much in the same way as a house is set in vibration,
and it would tend to oscillate backward and forward with a period of
vibration dependent upon the nature of its materials, size, and form.
The upper portion of this mountain would, in consequence, swing through
a greater arc than the lower portion, and buildings situated on the top
of it would swing to and fro through a greater arc than those which
were situated near its foot. This explanation why buildings situated
on the top of a mountain should suffer more than those situated on
a plain, is one which was offered by Mallet when writing of the
Neapolitan earthquake. He tells us that towns on hills are ‘rocked as
on the top of masts,’ and if we accept this explanation it would, in
fact, be one reason why the houses situated on the Bluff at Yokohama
suffered more than those situated in the settlement. This explanation
is given on account of the great authority it claims as a consequence
of its source. It is not clear how the statement can be supported,
as different portions of the mountain receive momentum in opposite
directions at the same time.

_Want of support on the faces of hills._—When a wave of elastic
compression is propagated through a medium, we see that the energy of
motion is being continually transmitted from particle to particle of
that medium. A particle, in moving forwards, meets with an elastic
resistance of the particles towards which it moves, but, overcoming
these resistances, it causes these latter particles to move, and in
turn to transmit the energy to others further on. So long as the medium
in which this transfer of energy is continuous, each particle has a
limit to its extent of motion, dependent on the nature of the medium.
When, however, the medium, which we will suppose to be the earth, is
not continuous, but suddenly terminates with a cliff or scarp, the
particles adjacent to this cliff or scarp, having no resistance offered
to their forward motion, are shot forward, and, consequently, the
ground here is subjected to more extensive vibrations than at those
places where it was continuous. This may be illustrated by a row of
marbles lying in a horizontal groove; a single marble rolled against
one end of this row will give a concussion which will run through the
chain, like the bumping of an engine against a row of railway cars, and
as a result, the marble at the opposite end of the row, being without
support, will fly off. Tyndall illustrates the same thing with his
well known row of boys, each one standing with his arms stretched out
and his hands resting upon the shoulders of the boy before him. A push
being given to the boy at the back, the effect is to transmit a push
to the first boy, who, being unsupported, flies forward.

In the case of some earthquakes, most disastrous results have occurred
which seem only to admit of an explanation such as this. A remarkable
instance of this kind occurred when the great earthquake of 1857 ‘swept
along the Alps from Geneva to the east-north-east, and its crest
reached the edge of the deep glen between Zermatt and Visp. Then the
upper part of the wave-movement, a thousand or two thousand feet in
depth from the surface, came to an end; the forward pulsation acted
like the breaker of the sea, and heavy falls of rock encumbered the
western side of the valley.’

_Earthquake shadows._—If a mountain stands upon a plain through which
an elastic wave is passing, which is almost horizontal, the mountain
is, so to speak, in the _shadow_ of such a wave. If we only consider
the normal motion of this wave, we see that the only motion which the
mountain can obtain will be a wave of elastic distortion produced
by a shearing force along the plain of the base. Should, however,
the wave approach the mountain from below, and emerge into it at a
certain angle, only the portion of the mountain on the side from
which the wave advanced could remain in shadow, whilst the portion
on the opposite side would be thrown into a state of compression and
extension. Portions in shadow, however, would be subject to waves
of elastic distortion. In a manner similar to this we may imagine
that certain portions of the bluff, so far as the advancing wave was
concerned, were in shadow, and thus saved from the immediate influence
of the direct shock. A hypothetical case of such a shadow is shown in
the accompanying section, illustrating the contour of the ground at
Yokohama. The situation which might be in the shadow of one shock,
however, it is quite possible might not be in that of another. We
must also remember that a place in shadow for a direct shock might be
affected by reflected waves, and also by the transverse vibrations of
the direct shock. These effects are over and above the effects produced
by the waves of elastic distortion just referred to. It might be asked
whether whole countries, like England, which are but seldom shaken, are
in shadow.

[Illustration: FIG. 27.—Hypothetical section at Yokohama.]

_Destruction due to the interference of waves._—Referring to the
section of the ground at Yokohama (Fig. 27), it will be seen that
both the settlement and the bluff stand upon beds of gravel capping
horizontal beds of grey tuff. The gravel of that portion of the
settlement on the seaboard originally formed the line of a shingle
beach. That portion of the settlement back from the sea stands upon
ground which was originally marshy. In the central portions of the
settlement this bed of gravel is very thick, perhaps 100 feet or so,
but as you near the edge of the bluff it probably becomes thinner,
until it finally dies out upon the flanks of the scarps.

On the top of the bluff, the beds of gravel will, in every probability,
be generally thinner than they are upon the lower level. The beds of
tuff, which is a soft grey-coloured clay-like rock, produced by the
solidification of volcanic mud, appear, when walking on the seaboard,
to be horizontally stratified. If there is a dip inland, it is in all
probability very slight. Here and there the beds slightly faulted.
Taken as a whole we may consider these beds as being tolerably
homogeneous, and an earthquake in passing through them would meet
with but little reflection or refraction. At the junction of these
beds with the overlying gravels, both reflection and refraction would
comparatively be very great.

On entering the gravel, as the wave would be passing into a less
elastic medium, the direction of the wave would be bent towards the
perpendicular to the line of junction, and the angle of emergence at
the surface would consequently be augmented. At the surface certain
reflection would also take place, but the chief reflections would be
those at the junction of the tuff and the alluvium.

Under the settlement it is probable that all the reflections which
took place would be single. Thus wave fronts like A_{1} advancing in
a direction parallel to the line _a__{1}; would be reflected in a
direction _a__{2} and give rise to a series of reflected waves A_{2}.
These are shown by thicker lines. Similarly all the neighbouring waves
to the right and left of A_{1} would give rise to a series of reflected
waves. If the lines drawn representing wave fronts are districts of
compression, then, where two of the lines cross each other, there would
be double energy in producing compression. Similarly, districts of
rarefaction might accord, and, again, compression of one wave might
meet with the rarefaction of another and a neutralisation of effect
take place. A diagram illustrating concurrence and interference of this
description is given in Le Conte’s ‘Elements of Geology,’ p. 115. The
interference which has been spoken of, however, is not the greatest
which would occur. The greatest would probably be beneath the bluff and
the scarps which run down to join the level ground below. This would be
the case because it is a probability that there might not only be cases
of interference of single reflected waves, but also of waves which
had been not only twice but perhaps thrice reflected. For example, a
wave like B_{1} (which is parallel to A_{1} of the first supposition),
advancing in a direction parallel to _b__{1} might be reflected along
the line _b__{2} giving rise to waves like B_{2}, which in turn might
be reflected along _b__{3} giving rise to waves like B_{3}. The number
of districts where there would be concurrence and interference would,
in consequence of the number of times waves might be reflected, be
augmented. Here the violence of the shock would, at certain points, be
considerably increased, but as a general result energy must be lost,
so that even if some of the reflected waves found their way into the
portion we have regarded as being in shadow, their intensity would not
be so great as if they had entered it directly.

The shaking down of loose materials from the sides of hills may be
partially explained on the assumption of an increased disturbance due
to interference.

_Earthquake bridges._—In certain parts of South America there appear
to exist tracts of ground which are practically exempt from earthquake
shocks, whilst the whole country around is sometimes violently shaken.
It would seem as if the shock passes beneath such a district as water
passes beneath a bridge, and for this reason these districts have been
christened ‘bridges.’

This phenomenon appears to depend upon the nature of the underlying
soil. When an elastic wave passes from one bed of rock to another of a
different character, a certain portion of the wave is reflected, while
the remainder of it is transmitted and refracted, and ‘bridges’ we
may conceive of as occurring where the phenomenon of total reflection
occurs.

In the instances given of soft materials having proved good
foundations, it was assumed that they had chiefly acted as absorbers
of momentum. They have also acted as reflecting surfaces, and where no
effects have been felt by those residing on them, this may have been
the result of total reflection, and the soft beds thus have played the
part of bridges.

Fuchs gives an example taken from the records of the Syrian earthquake
of 1837, where not only neighbouring villages suffered differently, but
even neighbouring houses. In one case a house was entirely destroyed,
whilst in the next house nothing was felt.

In Japan, at a place called Choshi, about 55 miles east of the capital,
earthquakes are but seldom felt, although the surrounding districts may
be severely shaken.

From descriptions of this place it would appear that there is a large
basaltic boss rising in the midst of alluvial strata. The immunity from
earthquakes in this district has probably given rise to the myth of
the Kanam rock, which is a stone supposed to rest upon the head of a
monstrous catfish (Namadzu), which by its writhings causes the shakings
so often felt in this part of the world.[34]

Prof. D. S. Martin, writing on the earthquake of New England in 1874,
says that it was felt at four points; it was felt in the heart of
Brooklyn all within a circle of half a mile across; ‘and this fact
would suggest that a ridge of rock perhaps approaches the surface at
that point, though none is known to appear.’[35]

The subject of special districts, which are more or less protected from
severe shakings, will be again referred to, and it will be seen that
after a seismic survey has been made even of a country like Japan,
where there are on the average at least two earthquakes per day, it
is possible to choose a place to build in as free from earthquakes as
Great Britain.

_General examples of earthquake effects._—The following examples of
earthquake effects are drawn from Mallet’s account of the Neapolitan
earthquake of 1857.

At a town called Polla there was great destruction. Judging from
the fissures in the parts that remained standing it seemed that the
emergence of the shock had been more vertical in the upper part of the
town than in the lower, proving that whatever had been the angle below,
the hill had itself vibrated, which, being horizontal, had modified the
angle of the fissures.

Diano suffered but little, partly because it was well built, and partly
on account of its situation, which was such that before the shock
reached it the disturbance had to pass from beds of clay into nearly
vertically placed beds of limestone. Also a great portion of the shock
was cut off by the Vallone del Raccio to the north and north-west of
the town. Here the effects of the partial extinction of the wave on the
‘free outlaying stratum’ were visible in the masses of projected rock.

Castellucio did not suffer because its well buttressed knoll was end
on to the direction of shock, and on account of a barrier of vertical
breccia beds protecting it upon the east.

Pertosa stands on a mound. The destruction was least in the southern
part of the town. From the relation of the beds of breccia on which the
town stands, and the direction of the wave path, it is evident that the
southern part of the town received the force of the shock through a
greater thickness of the breccia beds than the other parts did.

Petina, standing on a level limestone spur jutting out from a mountain
slope, suffered nothing, whilst Anletta five miles to the south-west,
and Pertosa six miles distant, were in great part prostrated. (1) The
terrace did not vibrate, and (2) between Petina and Anletta there is
almost 6,000 feet of piled up limestone, so that any shock emergent at
a steep angle had to pass up transversely through these beds.

_Protection of buildings._—In addition to giving proper construction
to our buildings, choosing proper foundations and positions for them,
something might possibly be done to ward off the destructive effects of
an earthquake. We read that the Temple of Diana at Ephesus was built on
the edge of a marsh, in order to ward off the effect of earthquakes.
Pliny tells us that the Capitol of Rome was saved by the Catacombs,
and Elisée Reclus[36] says that the Romans and Hellenes found out that
caverns, wells, and quarries retarded the disturbance of the earth,
and protected edifices in their neighbourhood. The tower of Capua was
saved by its numerous wells. Vivenzis asserts that in building the
Capitol the Romans sunk wells to weaken the effects of terrestrial
oscillations. Humboldt relates the same of the inhabitants of San
Domingo.

Quito is said to receive protection from the numerous cañons in the
neighbourhood, whilst Lactacunga, fifteen miles distant, has often been
destroyed.

Similarly, it is extremely probable that many portions of Tokio have
from time to time been protected more or less from the severe shocks of
earthquakes by the numerous moats and deep canals which intersect it.

Although we are not prepared to say how far artificial openings of this
description are effectual in warding off the shocks of earthquakes,
from theoretical considerations, and from the fact that their use has
been discovered by persons who, in all probability, were without the
means of making theoretical deductions, the suggestions which they
offer are worthy of attention.

_General conclusions._—The following are a few of the more important
results which may be drawn from the preceding chapter:—

1. In choosing a site for a house find out by the experience of others
or experimental investigation the localities which are least disturbed.
In some cases this will be upon the hills, in others in the valleys and
on the plains.

2. A wide open plain is less likely to be disturbed than a position on
a hill.

3. Avoid loose materials resting on harder strata.

4. If the shakings are definite in direction, place the blank walls
parallel to such directions, and the walls with many openings in them
at right angles to such directions.

5. Avoid the edges of scarps or bluffs, both above and below.

6. So arrange the openings in a wall, that for horizontal stresses the
wall shall be of equal strength for all sections at right angles.

7. Place lintels over flat arches of brick or stone.

8. To withstand destructive shocks either rigidly follow one or other
of the two systems of constructing an earthquake-proof building. The
light building on loose foundations is the cheaper and probably the
better.

9. Let all portions of a building have their natural periods of
vibration nearly equal.

10. If it is a necessity that one portion of a building should have a
very different period of vibration to the remainder, as for instance a
brick chimney in a wooden house, it would seem advisable either to let
these two portions be sufficiently free to have an independent motion,
or else they must be bound together with great strength.

11. Avoid heavy topped roofs and chimneys. If the foundations were free
the roof might be heavy.

12. In brick or stone work use good cement.

13. Let archways curve into their abutments.

14. Let roofs have a low pitch, and the tiles, especially those upon
the ridges, be well secured.




                             CHAPTER VIII.

                    EFFECTS OF EARTHQUAKES ON LAND,

  1. Cracks and fissures—Materials discharged from
    fissures—Explanation of fissure phenomena. 2. Disturbances in
    lakes, rivers, springs, wells, fumaroles, &c.—Explanation of
    these latter phenomena. 3. Permanent displacement of ground—On
    coast lines—Level tracts—Among mountains—Explanation of these
    movements.


_Cracks and fissures formed in the ground._—Almost all large
earthquakes have produced cracks in the ground. The cracks which were
found in the ground at Yokohama (February 22, 1880) were about two or
three inches wide, and from twenty to forty yards in length. They could
be best seen as lines along a road running near the upper edge of some
cliffs which overlook the sea at that place. The reason that cracks
should have occurred in such a position rather than in others was
probably owing to the greater motion at such a place, due to the face
of the cliff being unsupported, and there being no resistance opposed
to its forward motion. It often happens that earthquake cracks are many
feet in width. At the Calabrian earthquake of 1783, one or two of the
crevasses which were formed were more than 100 feet in width and 200
feet in depth. Their lengths varied from half a mile to a mile.[37]
Besides these large cracks, many smaller ones of one or two feet in
breadth and of great length were formed. In the large fissures many
houses were engulfed. Subsequent excavations showed that by the closing
of the fissures these had been jammed together to form one compact
mass. These cracks are usually more or less parallel, and at the same
time parallel to some topographical feature, like a range of mountains.
For example, the cracks which were formed by the Mississippi earthquake
of 1812 ran from north-east to south-west parallel to the Alleghanies.
By succeeding shocks these crevasses are sometimes closed and sometimes
opened still wider. Their permanency will also depend upon the nature
of the materials in which they are made.

During an earthquake large cracks may suddenly open and shut.

During the convulsions of 1692 which destroyed Port Royal, it is said
that many of the fissures which were formed, opened and shut. In some
of these, people were entirely swallowed up and buried. In others they
were trapped by the middle, and even by the neck, where if not killed
instantaneously they perished slowly. Subsequently their projecting
parts formed food for dogs.[38]

The earthquake which, July 18, 1880, shook the Philippines caused many
fissures to be found, which in some places were so numerous that the
ground was broken up into steps. Near to the village of San Antonio the
soil was so disturbed that the surface of a field of sugar-canes was
so altered that in some cases the top of one row of full grown plants
was on a level with the roots of the next. Into one such fissure a boat
disappeared, and into another, a child.

Subsequently the child was excavated, and its body, which was found a
short distance below the surface, was completely crushed.[39]

At the time of the Riobamba earthquake, not only were men engulfed,
but animals, like mules, also sank into the fissures which were formed.

The fissures which were formed at the time of the Owen’s Valley
earthquake in 1872 extended for miles nearly parallel to the
neighbouring Sierras. In some places the ground between the fissures
sank twenty or thirty feet, and at one place about three miles east of
Independence, a portion of the road was carried eighteen feet to the
south by a fissure.[40]

Speaking generally, it may be said that all large earthquakes are
accompanied by the formation of fissures. The Japanese have a saying
that at the time of a large earthquake persons must run to a bamboo
grove.

The object of this is to escape the danger of being engulfed in
fissures, the ground beneath a bamboo grove being so netted together
with fine roots that it is almost impossible for it to be rent open.

_Materials discharged from fissures._—Together with the opening of
cracks in the earth it often has happened that water, mud, vapours,
gases, and other materials, have been ejected.

At the time of the Mississippi earthquake water, mixed with sand and
mud, was thrown out with such violence that it spurted above the tops
of the highest trees. In Italy such phenomena have often been repeated.

From the fissures which were formed in 1692 at the time of the
earthquakes in Sicily, water issued which in some instances was
salt.[41]

By the Cachar earthquake (January 10, 1869) numerous fissures were
formed parallel to the banks of a river, from this water and mud were
ejected. Dr. Oldham, who describes this earthquake, says that the
first shot of dry mud or sand was mistaken for smoke or steam. The
water was foul, and hotter than surface water at the time, but only
slightly so; and the sulphurous smell was nothing more than you would
perceive in stirring up the mud at the bottom of any stagnant pool
which had lain undisturbed for some time.[42]

In 1755, when Tauris was destroyed, boiling water issued from the
cracks which were formed. Similar phenomena were witnessed at a place
eight miles from La Banca in Mexico, in the year 1820. Part of this hot
water was pure and part was muddy.

Sometimes the water which has been ejected has been so muddy that the
mud has been collected to form small hills. This was the case at the
time of the Riobamba earthquake. The mud in this case consisted partly
of coal, fragments of augite, and shells of infusoria.

At the time of the Jamaica earthquake men who had fallen into crevices
were in some cases thrown out again by issuing water.

Sometimes, as has already been mentioned, vapour, gases, and even
flames issue from fissures. Vapour of sulphur appears to be exceedingly
common. Kluge says that many fish were killed in consequence of the
sulphurous vapours which rose in the sea near to the coast of New
Zealand in 1855.

On December 14, 1797, an insupportable smell of sulphur was observed to
have accompanied the earthquake which at that time shook Cumana, which
was greatest when the disturbance was greatest.

Sulphurous fumes which were combustible were belched out of the
earth at the time of the Jamaica earthquake in 1692. The smell which
accompanied this was so powerful that it caused a general sickness
which swept away about 3,000 persons.[43]

From the fissures formed at Concepcion in 1835, water, which was black
and fœtid, issued.[44]

The earthquakes of New England in 1727 were accompanied by the
formation of fissures, from which sand and water boiled out in
sufficient quantity to form a quagmire. In some places ash and sulphur
are said to have been ejected.

At one house the stink of sulphur accompanying the earthquake was so
great that the family could not bear to remain in doors.[45]

Emanations of gas sometimes appear to have burst out from submarine
sources.

Thus the earthquake at Lima, in March, 1865, was accompanied with great
agitation of the water and an odour of sulphuretted and carburetted
hydrogen. This former gas was developed to such an extent that the
white paint of the U.S. ship ‘Lancaster’ was blackened.[46] With the
smell, flames have sometimes been observed, as, for instance, at the
time of the Lisbon earthquake.

At the time of the earthquakes of 1811 and 1813, in the Mississippi
valley, steam and smoke issued from some of the fissures which were
formed.

Instances are recorded where stones have been shot up from fissures
unaccompanied by water, as, for instance, at the earthquake of Pasto
(January, 1834). It is imagined that the propelling power must have
been the sudden expansion of escaping gases.

It has been suggested that flames seen above fissures might perhaps
be due to the burning of materials like sulphur. Mr. D. Forbes, who
examined the effects of the earthquakes of Mendoza, which were felt for
a distance of 1,200 miles, says that where the hard rock came to the
surface there were no traces of fissures, these being entirely confined
to the alluvium. The rumours of fire and smoke having appeared at some
of the fissures were without foundation, the presumed smoke being
nothing but dust.[47]

In addition to flames lights appear often to have been observed, the
origin of which cannot be easily explained.

The earthquake of November 22, 1751, at Genoa is said to have been
accompanied by a light like that of a prodigious fire which seemed to
arise out of the ground.[48]

_Explanation of fissure phenomena._—The manner in which fissures are
formed has already been explained when referring to the want of support
in the face of hills (page 136).

Similar remarks may be applied to the banks of rivers and all
depressions, whether natural or artificial, which have a steep slope.
At such places the wave of shock emerges on a free surface, which,
being unsupported in the direction of its motion, tends to tear itself
away from the material behind, and form a fissure parallel to the face
of the free surface. The distance of the fissure from the face of the
free surface will, theoretically, be equal to half the amplitude of the
wave of motion, one half tending to move forwards, and the other half
backwards. The reason that water and other materials rush forth from
fissures has been explained by Schüler as being due to cracks having
been opened through impervious strata, which, before the earthquake,
by their continuity prevented the rising of subterranean water under
hydrostatic pressure.[49]

Kluge explains the coming up of the waters as being due to the same
causes which he considers may be the origin of disturbances in the sea.

The most reasonable explanations of the eruption of water, mud, sand,
and gas through fissures are those given by Oldham and Mallet in their
account of the Cachar earthquake.

In the case of a horizontal shock passing through a bed of ooze or
water-bearing strata, the elastic wave will tend to pack up the water
during the forward motion to such an extent that it will flow or
spout up through any aperture communicating with the surface. By the
repetition of these movements causing ejections, sand or mud cones,
like those produced by a volcanic eruption, may be formed, and by a
similar action water may be shot violently up out of wells, as was the
case in Jamaica in 1692.

If an emergent wave acts through a water-bearing bed upon a
superincumbent layer of impervious material, this upper layer is,
during the upward motion, by its inertia suddenly pressed down upon the
latter.

This pressure is equal to that which would raise the upper layer to
a height equal to the amplitude of the motion of an earth particle,
and with a velocity at least equal to the mean velocity of the earth
particle resolved in the vertical direction.

For a moment the water-bearing strata receive an enormous squeeze, and
the water or mud starts up through any crevice which may be formed
leading to the surface.

From this we see that liquids may rise far beyond the level due to
hydrostatic pressure.[50]

Volger has attributed the origin of lights or flames appearing above
fissures to the friction which must take place between various rocky
materials at the time when the fissures are opened. As confirmatory of
this he refers to instances where similar phenomena have been observed
at the time of landslips. At the time of these landslips the heat
developed by friction has been sufficiently intense to convert water
into steam, the tension of which threw mud and earth into the air like
the explosion of a mine.[51]

The gas eruptions which occasionally take place with earthquakes are
probably due to the opening of fissures communicating with reservoirs
or strata charged with products of natural distillation, or chemical
action, which previously had accumulated beneath impervious strata. Of
the existence of such gases we have abundant evidence. In coal mines we
have fire damp which escapes in increased quantities with a lowering of
the barometrical pressure. In volcanic regions we have many examples of
natural springs of carbon dioxide.

These various gases sometimes escape in quantity, or erupt without
the occurrence of earthquakes. Rossi mentions an instance where a few
years ago quantities of fish were killed by the eruption of gas in the
Tiber, near Rome. Another instance is one which occurred at Follonica
on April 6, 1874. On the morning of that day many of the streets and
roads were covered with the dead bodies of rats and mice. It seemed as
if it had rained rats. From the facts that the bodies of the creatures
seemed healthy, that the destruction had happened suddenly, and not
come on gradually like an epidemic, it was supposed that they had been
destroyed by an emanation of carbon dioxide. The fact that many of them
lay in long lines suggested the idea that they had been endeavouring
to escape at the time of the eruption.[52] If we can suppose sudden
developments of gas like this to have occasionally accompanied
earthquakes, we may sometimes have the means of accounting for the
sickness which has been felt.

_Disturbances in lakes._—It has often been observed that, at the time
of large earthquakes, lakes have been thrown into violent agitation,
and their waters have been raised or lowered. At the time of the great
Lisbon earthquake, not only were the waters of European lakes thrown
into a state of oscillation, but similar effects were produced in the
great lakes of North America. In some instances, as in the case of
small ponds, these movements may be produced by the horizontal backward
and forward motion of the ground. At other times they are probably due
to an actual tipping of a portion of their basins. Movements like these
latter will be again referred to in the chapter on Earth Pulsations.
On January 27, 1856, there was a shock of earthquake at Bailyborough,
Ireland, which occasioned an adjacent lough to overflow its banks
and rush into the town with great impetuosity. In returning it swept
away two men, leaving behind a great quantity of pike and eels of a
prodigious growth.[53]

_Disturbances in rivers._—Just as lakes have been disturbed, so also
have there been sudden disturbances in rivers. Sometimes these have
overflowed their banks, whilst at other times they have been suddenly
dried up. In certain cases the reason that a portion of a river should
have become dry has been very apparent, as, for instance, at the time
of the Zenkoji earthquake in Japan in 1847, when the Shikuma-gawa
became partly dry in consequence of the large masses of earth which
had been shaken down from overhanging cliffs damming a portion of
its course, and thus forming, first, lakes, and subsequently, new
water-courses. As another example, out of the many which might be
quoted, may be mentioned the sudden drying up of the river Aboat,
a tributary of the Magat, in the Philippine Islands, on July 27,
1881, shortly after a severe shock of earthquake. The water of this
river ceased to flow for two hours, after which it reappeared with
considerable increase of volume and of a reddish colour. Signor E. A.
Casariego, who describes this, remarks that the phenomenon could easily
be explained through the slipping down of the steep banks in narrow
parts of its upper valley, by which means its flow had been obstructed
until the water had time to accumulate and pass over or demolish the
obstruction.

After the earthquake of Belluno (June 29, 1873), the torrent Tesa,
which is ordinarily limpid, became very muddy.[54] Similar phenomena
have been observed even in Britain, as, for instance, in 1787, when, at
the time of a shock which was felt in Glasgow, there was a temporary
stoppage in the waters of the Clyde. Again, in 1110, there was a
dreadful earthquake at Shrewsbury and Nottingham, and the Trent became
so low at Nottingham that people walked over it.

The earthquake of 1158, which was felt in many parts of England, was
accompanied by the drying up of the Thames, which was so low that it
could be crossed on foot even at London.[55]

Facts analogous to these are mentioned in the accounts of many large
earthquakes. Sometimes rivers only come muddy or change their colour.
In an account of the Lisbon earthquake we read that some of the rivers
near Neufchâtel suddenly became muddy.[56]

At other times large waves are formed. Thus the earthquake of Kansas
(April 24, 1867) apparently created a disturbance in the rivers at
Manhattan, which rolled in a heavy wave from the north to the south
bank.[57]

Sometimes curious phenomena have happened with regard to rivers without
the occurrence of earthquakes. Thus, for instance, on November 27,
1838, there was a simultaneous stoppage of the Teviot, Clyde, and Nith.

In these rivers similar phenomena have been observed in previous years.

Again, on January 1, 1755, there was a sudden sinking of the river
Frooyd, near Pontypool. This appears to have been due to the water
sinking into chasms which were suddenly opened.[58]

_Effects produced in springs, wells, fumaroles, &c._—Springs also are
often affected by earthquakes. Sometimes the character of their waters
change; those which were pure become muddy, whilst those which were hot
have their temperature altered.

Sometimes springs have been dried up, whilst at other times new springs
have been formed.

This latter was the case in New England (October 27, 1727). In some
places springs were formed, whilst at other places they were either
entirely or partly dried up.[59]

At and near Lisbon, in 1755, some fountains became muddy, others
decreased, others increased, and others dried up. At Montreux, Aigle,
and other places, springs became turbid.

The baths at Toplitz, in Bohemia, which were discovered in
A.D. 762, were seriously affected by the same earthquake.
Previous to the earthquake it is said that they had always given a
constant supply of hot water. At this time, however, the chief spring
sent up vast quantities of water and ran over. One hour before this
it had grown turbid and flowed muddy. After this it stopped for about
one minute, but recommenced to flow with prodigious violence, driving
before it considerable quantities of reddish ochre. Finally, it settled
back to its original clear state and flowed as before.[60]

In 1855, at the earthquake of Wallis, many new springs burst forth, and
some of these in Nicolai Thale were so rich in iron that they quickly
formed a deposit of ochre.

At the time of the Belluno earthquake (June 29, 1873), a hot spring, La
Vena d’Oro, suddenly became red.[61]

The following examples of like changes are taken from the writings of
Fuchs.[62]

In 1738 the hot springs of St. Euphema rose considerably in their
temperature.

During the earthquake of October, 1848, the hot springs of Ardebil,
which usually had a temperature of from 44° to 46° C., rose so high
that their temperature was sufficient to cause scalding.

At the time of the earthquake of Wallis, in 1855, the temperature of
hot springs rose 7°, and the quantity of water increased three times.

During the earthquake of 1835 in Chili, the springs of Cauquenes fell
from 118° to 92° F. Subsequently, however, they again rose.

Fumaroles are similarly disturbed. Thus, at the time of the earthquakes
of Martinique (September, 1875), the fumaroles there showed an abnormal
activity.[63]

Wells often appear to be acted upon in the same manner as springs.

At the time of the California earthquake (April, 1855), the level of
the water in certain wells was raised ten to twelve feet.

A consequence of the earthquake at Neufchâtel, in 1749, was to fill
some of the wells with mud.[64] At Constantinople, on September 2,
1754, wells became dry.[65]

_Explanation of the above phenomena._—That the water in springs and
wells should be caused to rise at the time of an earthquake, admits of
explanation on the supposition of compressions taking place similar
to those which cause the rise of water in fissures. That the water in
wells and springs should be rendered turbid, is partly explained on the
supposition of more or less dislocation taking place in the earthy or
rocky cavities in which they are contained or through which they flow.

At the time of a large earthquake it is extremely probable that there
is a general disturbance in the lines of circulation of subterranean
waters and gases throughout the shaken area. By these disturbances, new
waters may be brought to the surface, two or more lines of circulation
may be united, and the flow of a spring or supply of a well be
augmented. Fissures, through which waters reached the surface, may be
closed, wells may become dry, or springs may cease to flow, hot springs
may have their temperature lowered by the additions of cold water
from another source, and, in a similar manner, waters may be altered
in their mineralisation. An important point to be remembered in this
consideration is the mutual dependence of various underground water
supplies, and the area over which any given supply may circulate. A
well in the higher part of Lincoln Heath is said to be governed by the
river Trent, which is ten miles distant; when the river rises the well
rises in proportion, and when the river falls the water in the well
falls.[66]

The change which is usually observed in hot springs is, that before or
with earthquakes they increase in temperature, but afterwards sink back
to their normal state. This increase in temperature may possibly be due
to communication being opened with new or deeper centres of volcanic
activity, or a temporarily increased rate of flow.

That the water issuing from newly formed fissures or springs should
be hot, might be explained on the supposition of its arising from a
considerable depth, or from some volcanic centre. It might also be
attributed to the heat developed by friction at the opening of the
fissures. These changes which earthquakes produce upon the underground
circulation of waters are phenomena deserving especial attention.
Although we know much about the circulation of surface water, it is but
little that we yet know about the movement of the streams hidden from
view, from which these surface waters have their sources. Earthquakes
may be regarded as gigantic experiments on the circulatory system of
the earth, which, if properly interpreted, may yield information of
scientific and utilitarian value.

The sudden elevations, depressions, or lateral shifting of large tracts
of country at the time of destructive earthquakes are phenomena
with which all students of geology are familiar. In most cases these
displacements have been permanent; and evidences of many of the
movements which occurred within the memory of man, remain as witnesses
of the terrible convulsions with which they were accompanied.

_Movements on coast lines and level tracts._—At the time of the
great earthquake of Concepcion, on February 20, 1835, much of the
neighbouring coast line was suddenly elevated four or five feet
above sea level. This, however, subsequently sank until it was only
two feet. A rocky flat, off the island of Santa Maria, was lifted
above high-water mark, and left covered with ‘gaping and putrefying
mussel-shells, still attached to the bed on which they had lived.’ The
northern end of the island itself was raised ten feet and the southern
extremity eight feet.[67]

By the earthquake of 1839, the island of Lemus, in the Chonos
Archipelago, was suddenly elevated eight feet.[68]

Of movements like these, especially along the western shores of South
America, Darwin, who paid so much attention to this subject, has given
many examples. In 1822, the shore near Valparaiso was suddenly lifted
up, and Darwin tells us that he heard it confidently asserted ‘that a
sentinel on duty, immediately after the shock, saw a part of a fort
which previously was not within the line of his vision, and this would
indicate that the uplifting was not vertical.’[69] That the large areas
of land should be shifted permanently in horizontal directions, as well
as vertically, we should anticipate from the observations which we are
able to make upon large fissures which are caused by earthquakes.

Another remarkable example of sudden movement in the rocky crust is
that which took place during the earthquakes of 1811–12 in the valley
of the Mississippi, near to the mouth of the Ohio, which was convulsed
to such a degree, that lakes, twenty miles in extent, were formed
in the course of an hour. This country, which is called the ‘sunk
country,’ extends some seventy to eighty miles north and south, and
thirty miles east and west.[70]

In the ‘Gentleman’s Magazine’ we read of the little territory of
Causa Nova, in Calabria, being sunk twenty-nine feet into the earth
by an earthquake, without throwing down a house. The inhabitants,
being warned by a noise, escaped into the fields, and only five were
killed.[71]

Other examples of these permanent dislocations of strata are to be
found in almost every text-book on geology.

_Geological changes produced._—Passing over the accounts of earth
movements which are more or less fictitious, and confining our
attention to the well authenticated facts, we see at once the important
part which earthquakes have played as agents working geological
changes. Even in the nineteenth century long tracts of coast, as in
Chili and New Zealand, have been raised, whilst other areas, like the
Delta of the Indus, have been sunk. Sir H. Bartle Frere, speaking about
the disturbance which took place in his latter region in 1819, remarks
that all the canals drawn from the Fullalee River ceased to run for
about three days, probably indicating a general upheaval of the lower
part of the canal. In consequence of the earthquakes in former times
it is not unlikely that water-courses have ceased to flow, water has
decreased in wells, and districts have been depopulated.[72]

Sometimes these changes have taken place gradually and sometimes with
violence. Mountains have been toppled over, valleys have been filled,
cities have been submerged or buried.

With the records of these convulsions before us, we see that seismic
energy yet exhibits a terrible activity in changing the features of the
globe.

_Reason of these movements._—To formulate a single reason for these
catastrophes would be difficult. Where they are of the nature of
landslips, or materials have been dislodged from mountain sides, the
cause is evidently the sudden movement of this ground acting upon
strata not held together in a sufficiently stable condition. A similar
explanation may be given for the sudden elevations or depressions of
strata in a district removed from the centre where the disturbance had
its origin. The seismic effort exhibits itself in a certain area round
its origin as a sudden push, and by this push, strata are fractured and
caused to move relatively to each other.

At or near to the origin of an earthquake it might be argued that it
was the sudden falling of rocky strata towards a position of stable
equilibrium that caused the shaking, and in such a case the movements
referred to may be regarded as the cause rather than the effect of an
earthquake.

A subject closely connected with the sudden dislocation of strata,
is the production of secondary or consequent earthquakes, due to the
disturbance of ground in a critical state (see p. 248).




                              CHAPTER IX.

                      DISTURBANCES IN THE OCEAN.

  Sea vibrations—Cause of vibratory blows—Sea waves: Preceding
    earthquakes; Succeeding earthquakes—Magnitude of waves—Waves
    as recorded in countries distant from the origin—Records on
    tide gauges—Waves without earthquakes—Cause of waves—Phenomena
    difficult of explanation—Velocity of propagation—Depth of the
    ocean—Examples of calculations—Comparison of velocities of
    earthquake waves with velocities which ought to exist from the
    known depth of the ocean.


_Sea vibrations._—Whilst residing in Japan I have had many
opportunities of conversing with persons who had experienced
earthquakes when on board ships, and it has often happened that these
same earthquakes have been recorded on the shore. For example, at the
time of every moderately severe earthquake which has shaken Yokohama,
the same disturbance has been felt on board the ships lying in the
adjoining harbour. In some cases the effect had been as if the ship
was grounding; in others, as if a number of sharp jerks were being
given to the cable. The effect produced upon a man-of-war lying in the
Yokohama harbour on the evening of March 11, 1881, was described to me
as a ‘violent irresistible shaking.’ Vessels eighty miles at sea have
recorded and timed shocks which were felt like sudden blows. These were
accompanied by a noise described as a ‘dull rattle like thunder.’

In none of the cases here quoted was any disturbance of the water
observed.

The great earthquake of Lisbon was felt by vessels on the Atlantic,
fifty miles away from shore.

On February 10, 1716, the vessels in the harbour of New Pisco were so
violently shaken that both ropes and masts were broken, and yet no
motion in the water was observed. Some have described these shocks like
those which would be produced by the sudden dropping of large masses of
ballast in the hold of the vessel. Other cases are known where rigging
was damaged, and even cannon have been jerked up and down from the
decks on which they rested.

_Cause of vibratory blows._—From the rattling sound which has
accompanied some of these submarine shocks, many of which, it may be
remarked, have never been recorded as earthquakes upon neighbouring
shores, it does not seem improbable that they may have been the result
of the sudden condensation of volumes of steam produced by submarine
volcanic eruptions.

As confirmatory of this supposition we have the fact, that many of
the marine disturbances which might be called ‘sea-quakes,’ have been
observed in places which are close to, or in the line of, volcanic
vents. Thus, M. Daussy, who has paid special attention to this subject,
has collected evidence to show that a large number of shocks have been
felt by vessels in that portion of the Atlantic between Cape Palamas,
on the west coast of Africa, and Cape St. Roque, on the east coast of
South America.[73]

Some of the vessels only felt shocks and tremblings, but others saw
smoke, and some even collected floating ashes. In considering the
submarine shocks of this particular area, we must bear in mind that it
lies in the line of Iceland, the west coast of Scotland, the Azores,
Canaries, Cape de Verd Islands, St. Helena, and other places, all of
which, if not at present in volcanic activity, shew evidence of having
been so within recent times. The connection between volcanic action and
earthquakes will be again referred to.

_Sea waves._—Although in the above-mentioned instances sea waves have
not been noticed, it is by no means uncommon to find that destructive
earthquakes have been accompanied by waves of an enormous size, which,
if the earthquake has originated beneath the sea, have, subsequently
to the shaking, rolled in upon the land, to create more devastation
than the actual earthquake. It may, however, be mentioned that a few
exceptional cases exist when it is said that the sea wave has preceded
the earthquake, as, for example, at Smyrna, on September 8, 1852.

Again, at the earthquake in St. Thomas, in 1868, it is said that the
water receded shortly _before_ the first shock. When it returned, after
the second shock, it was sufficient to throw the U.S. ship ‘Monagahela’
high and dry.[74]

Another American ship, the ‘Wateree,’ was also lost in 1868 by being
swept a quarter of a mile inland by the sea wave which inundated
Arequippa.

Much of the great destruction which occurred at the time of the great
Lisbon earthquake was due to a series of great sea waves, thirty to
sixty feet higher than the highest tide, which swamped the town. These
came in about an hour after the town had been shattered by the motion
of the ground.

The first motion in the waters was their withdrawal, which was
sufficient to completely uncover the bar at the mouth of the Tagus.
At Cadiz, the first wave, which was the greatest, is said to have been
sixty feet in height. Fortunately the devastating effect which this
would have produced was partially warded off by cliffs.

At the time of the Jamaica earthquake (1692) the sea drew back for a
distance of a mile.

In South America sea waves are common accompaniments of large
earthquakes, and they are regarded with more fear than the actual
earthquakes.

On October 28, 1724, Lima was destroyed, and on the evening of that day
the sea rose in a wave eighty feet over Callao. Out of twenty-three
ships in the harbour, nineteen were sunk, and four others were carried
far inland. The first movement which is usually observed is a drawing
back of the waters, and this is so well known to precede the inrush of
large waves, that many of the inhabitants in South America have used it
as a timely warning to escape towards the hills, and save themselves
from the terrible reaction which, on more than one occasion, has so
quickly followed.

At Caldera, near to Copiapo, on May 9, 1877, which was the time when
Iquique was devastated, the first motion which was observed in the sea
was that it silently drew back for over 200 feet, after which it rose
as a wave over five feet high. At some places the water came in as
waves from twenty to eighty feet in height.

At Talcahuano, on the coast of Chili, in 1835, there was a repetition
of the phenomena which accompanied the destruction of Penco in 1730
and 1751. About forty minutes after the first shock, the sea suddenly
retired. Soon afterwards, however, it returned in a wave twenty feet
high, the reflex of which swept everything towards the sea. These
phenomena were repeated three times.[75]

When Callao and Lima were destroyed, in 1746, the sea first drew back,
then came in as waves, four or five minutes after the earthquake.
Altogether, between October 28 and February 24, 451 shocks were
counted. At one time the sea came in eighty feet above its usual
level. One account says that the large waves came in forty-one and a
half hours after the first shock, and seventeen and a half hours after
comparative tranquillity had prevailed.[76]

At the eruption of Monte Nuovo, near Naples, in September 27, 1538, the
water drew back forty feet, so that the whole gulf of Baja became dry.

In 1696, at the time of the Catanian earthquake, the sea is said to
have gone back 2,000 fathoms. Instances are recorded where the sea has
receded several miles.

The time taken for the flowing back of the sea is usually very
different. Sometimes it has only been five or six minutes, whilst at
other times over half an hour, and there are records where the time is
said to have been still longer.

Thus, at the earthquake of Santa (June 17, 1678), the sea is stated to
have gone as far back as the eye could reach, and did not rise again
for twenty-four hours, when it flooded everything.

In 1690 at Pisco the sea went back two miles, and did not return for
three hours. When it returns it does so with violence, and examples of
the heights to which it may reach have been given. The greatest sea
wave yet recorded, according to Fuchs, is one which, on October 6,
1737, broke on the coast of Lupatka, 210 feet in height.

There are, however, cases known where the sea has returned as gradually
as it went out. Thus, on December 4, 1854, when Acapulco was
destroyed, the sea is said to have returned as gently as it went out.

When sea waves have travelled long distances from their origin, as,
for instance, whenever a South American wave crosses the Pacific to
Japan, the phenomena which are observed are like those which were
observed at Acapulco; the sea falls and rises, at intervals of from ten
minutes to half an hour, to heights of from six to ten feet, without
the slightest appearance of a wave. Its phenomenon is like that of an
unusually high tide, which repeats itself several times per hour. Even
if we watch distant rocks with a telescope, although the surface of the
ocean may be as smooth as the surface of a mirror, there is not the
slightest visible evidence of what is popularly called a wave. The sea
being once set in motion it continues to move as waves of oscillation
for a considerable time. In 1877, as observed in Japan, the motion
continued for nearly a whole day. The period and amplitude of the rise
and fall were variable, usually it quickly reached a maximum, and then
died out gradually. As observed in a self-recording tide gauge at San
Francisco, the disturbance lasted for about four days. A diagram of
this is here given. In its general appearance it is very similar to the
records of other earthquake waves. The large waves represent the usual
six hours rise and fall of the tides; usually these are fairly smooth
curves. Superimposed on the large waves are the smaller zigzag curves
of the earthquake disturbance, lasting with greater or less intensity
for several days. As these curves are drawn to scale—horizontally for
hours, and vertically one fifth inch to the foot, to show the extent of
the rise and fall—they will be easily understood.

Sometimes, as in the present example, the first movement in the
waters is that of an incoming wave. In many instances, however, this
observation may be due to the slow and more gentle phenomena of the
previous drawing out of the water, which in a steep waste, or when the
water is rough, would be difficult to observe, not having been remarked.

The distance to which these sea waves have extended has usually been
exceedingly great.

[Illustration: FIG. 28.—Record of Tide Gauge at Port Point, San
Francisco. Showing Earthquake Waves of May 1877.]

The sea wave of the Iquique earthquake of May 9, 1877, like many of its
predecessors, was felt across the basin of the whole Pacific, from New
Zealand in the south, to Japan and Kamschatka in the north. And but for
the intervention of the Eurasian and American continents would have
made itself appreciable over the surface of the whole of our globe. At
places on the South American coast, it has been stated that the height
of the waves varied from twenty to eighty feet. At the Samoa Islands
the heights varied from six to twelve feet. In New Zealand the sea rose
and fell from three to twenty feet. In Australia the heights to which
the water oscillated were similar to those observed in New Zealand. In
Japan it rose and fell from five to ten feet. In this latter country,
the phenomena of sea waves which follow a destructive earthquake on the
South American coast are so well known that old residents have written
to the local papers announcing the probability of such occurrences
having taken place some twenty-five hours previously in South America.
In this way news of great calamities has been anticipated, details of
which only arrived some weeks subsequently. Just as the destructive
earthquakes of South America have announced themselves in Japan, in
a like manner the destructive earthquakes of Japan have announced
themselves upon the tide gauges of California. Similarly, but not so
frequently, disturbances shake the other oceans of the world.

For example, the great earthquake of Lisbon propagated waves to the
coasts of America, taking on their journey nine and a half hours.

_Sea waves without earthquakes._—Sometimes we get great sea waves
like abnormal tides occurring without any account of contemporaneous
earthquakes. Although earthquakes have not been recorded, these ill
understood phenomena are usually attributed to such movements.

Several examples of these are given by Mallet. Thus, at 10 A.M. on
March 2, 1856, the sea rose and fell for a considerable distance
at many places on the coast of Yorkshire. At Whitby, the tide was
described ebbing and flowing six times per hour, and this to such a
distance that a vessel entering the harbour was alternately afloat and
aground.

In 1761, on July 17, a similar phenomena was observed at the same place.

A like occurrence took place at Kilmore, in the county of Wexford, on
September 16, 1864, when the water ebbed and flowed seven times in the
course of two hours and a half. These tides, which appear to have taken
about five minutes to rise and five minutes to fall, were seen by an
observer approaching from the west as six distinct ridges of water. The
general character of the phenomena appears to have been very similar to
that which was produced at the same place by the Lisbon earthquake of
1755; and the opinion of those who saw and wrote about their occurrence
was that it was due to an earthquake disturbance. Such phenomena are
not uncommon on the Wexford coast, where they are popularly known as
‘death waves,’ probably in consequence of the lives which have been
lost by these sudden inundations.

They have also been observed in other parts of Ireland, the north-east
coast of England, and in many parts of the globe. They will be again
referred to under the head of earth pulsations.

_Cause of sea waves._—Mallet, who in his report to the British
Association in 1858, writes upon this last-mentioned occurrence at
considerable length, whilst admitting that many may have originated
from earthquakes, he thinks it scarcely probable that an earthquake
blow, sufficiently powerful to have produced waves like those observed
at Kilmore, should not have been felt generally throughout the south of
Ireland. He, therefore, suggests that sometimes waves like the above
might be produced by an underwater slippage of the material forming
the face of a submarine bank, the slope of which by degradation and
deposition, produced by currents, had reached an angle beyond the
limits of repose of the material of which it was formed. Mallet does
not insist upon the existence of these submarine landslips, but only
suggests their existence as a means of explaining certain abnormal sea
waves which do not appear to have been accompanied by earthquakes.

In the generality of cases sea waves are accompanied by earthquakes,
but it may often happen that the connection between the two is
difficult to clearly establish. One simple explanation for the origin
of waves occurring with earthquakes, is, that in consequence of the
earthquake a large volume of water suddenly finds its way into cavities
which have been opened, the disturbance produced by the inrush giving
rise to waves.

A second explanation is, that the land along a shore is caused by an
earthquake to oscillate upwards, the water running off to regain its
level. A supposition like this is negatived by the fact that these
disturbances are felt far away from the chief disturbance, on small
islands. Also, it may be added, that the whole disturbance appears to
approach the land from the sea, and not in the opposite direction.
Thus, in the earthquake of Oahu (February 18, 1871), it was remarked
that the shock was first felt by the ships farthest from the land.[77]

Another suggestion is that the waves are due to a sudden heaving up
of the bottom of the ocean. If this lifting took place slowly, then
the first result would be that the water situated over the centres of
disturbance would flow away radially in all directions from above the
area of disturbance.

If, however, the submarine upheaval took place with great rapidity, say
by the sudden evolution of a large volume of steam developed by the
entry of water into a volcanic vent, as the water was heaped above the
disturbed area, water might run in radially towards this spot.

Supposing a primary wave to be formed in the ocean by any such causes,
then the falling of this will cause a second wave to be formed,
existing as a ring round the first one. The combined action of the
first and second wave will form a third one, and so the disturbance,
starting from a point, will radiate in broadening circles. During the
up and down motion of these waves, the energy which is imparted to any
particle of water will, on account of the work which it has to do in
displacing its neighbours, by frictional resistance, gradually grow
less and less, until it finally dies away. The waves which are the
result of this motion will also grow less and less.

If a series of sea waves were produced by a single disturbance, we see
that these will be of unequal magnitude. Now, for small waves, the
velocity with which they travel depends upon the square root of their
lengths; but with large waves, like earthquake waves, the velocity
depends upon the square root of the depth of water, and these latter
travel more quickly than the former.

If, therefore, we have a series of disturbances of unequal magnitude
producing sea waves, which, from the series of shocks which have
been felt upon shores subsequently invaded by waves, seems in all
probability often to have been the case, it is not unlikely that the
waves of an early disturbance may be overtaken and interfered with by a
series which followed.

These considerations help us to understand the appearance of the
records on our tide gauges, and also the phenomena observed by those
who have recorded tidal waves as they swept inwards upon the land. For
instance, we understand the reason why sea waves, as observed at places
at different distances from the origin of a disturbance, should be of
different heights. We also see an explanation for the fact that small
waves should sometimes appear to be interpolated between large ones,
and that these should occur at varying intervals.

The fact that whenever a wave is produced, a certain quantity of water
must be drawn from the level which surrounds it, in order that it
should be formed, explains the phenomena that the sea is often observed
first to draw back. Out in the open ocean it is drawn from the hollow
between two waves. As has been pointed out by Darwin, it is like the
drawing of the water from the shore of a river by a passing steamer.

The difference in the height of waves, as observed at places lying
close to each other, is probably due to the configuration of the coast,
the interference of outlying islands, reefs, &c.—causes which would
produce similar effects in the height of tide.

As a wave approaches shallow water it gradually increases in height,
its front slope becomes steep, and its rear slope gentle, until finally
it topples over and breaks. This increasing in height of waves is
no doubt connected with the destruction of Talcahuano and Callao,
which are situated at the head of shallow bays. Valparaiso, which is
on the edge of deep water, has never been overwhelmed.[78] Another
case tending to produce anomalies in the character of waves would be
their reflection and mutual interference, the reflections due to the
configuration of the ocean bed and coast lines.

The complete phenomena which may accompany a violent submarine
disturbance are as follows:—

By the initial impulse of explosion or lifting of the ground, a ‘great
sea wave’ is generated, which travels shorewards with a velocity
dependent upon its size and the depth of the ocean. At the same
instant, a ‘sound wave’ may be produced in the air, which travels at a
quicker rate than the ‘great sea wave.’ A third wave which is produced,
is an ‘earth wave,’ which will reach the shore with a velocity
dependent on the intensity of the impulse and the elasticity of the
rocks through which it is propagated. This latter, which travels the
fastest, may carry on its back a small ‘forced sea wave.’ On reaching
the shore and passing inland, this ‘earth wave’ will cause a slight
recession of the water as the ‘forced sea wave’ slips from its back.

As these ‘forced sea waves’ travel they will give blows to ships
beneath which they may pass, being transmitted from the bottom of the
ocean to the bottom of the ships like sound waves in water. At the
time of small earthquakes, produced, for example, by the explosion of
small quantities of water entering volcanic fissures, or by the sudden
condensation of steam from such a fissure entering the ocean, aqueous
sound waves are produced, which cause the rattling and vibrating jars
so often noticed on board ships.

_Phenomena difficult of explanation._—Although we can in this way
explain the origin and phenomena of sea waves, we must remember, as
Kluge has pointed out, that it is not the simple backward and forward
movement of the ground which produces sea waves, and that the majority
of earthquakes which have occurred in volcanic coasts have been
unaccompanied by such phenomena. Out of 15,000 earthquakes observed on
coast lines, only 124 were accompanied by sea waves.[79] Out of 1,098
earthquakes catalogued by Perrey for the west coast of South America,
only nineteen are said to have been accompanied by movements in the
waters. According to the ‘Geographical Magazine’ (August 1877, p. 207),
it would seem that out of seventy-one severe earthquakes which have
occurred since the year 1500 upon the South American coast many have
been accompanied by sea waves. Darwin also remarks, when speaking of
South America, that almost every large earthquake has been accompanied
by considerable agitation in the neighbouring sea.[80]

On April 2, 1851, when many towns in Chili were destroyed, the sea was
not disturbed. At the time of the great earthquake of New Zealand (June
23, 1855), although all the shocks came from the sea, yet there was no
flood. The small shock of February 14, however, was accompanied by a
motion in the sea.

To these examples, which have been chiefly drawn from the writings of
Fuchs, must be added the fact that the greater number of disturbances
which are felt in the north-eastern part of Japan, although they
emanate from beneath the sea, do not produce any visible sea waves.
They are, however, sufficient to cause a vibratory motion on board
ships situated near their origin.

Another point referred to by Fuchs, as difficult of explanation,
is, that the water, when it draws back, often does so with extreme
slowness, and farther, in some instances, it has not returned to its
original level. That the sea might be drawn back for a period of
fifteen or thirty minutes is intelligible, when we consider the great
length of the waves which are formed. Cases where it has retired for
several hours or days, and when its original level is altered, appear
only to be explicable on the assumption of more or less permanent
changes in the levels of the ground. For example, in the earthquake
of 1855 which shook New Zealand, the whole southern portion of the
northern island was raised several feet.

These sudden alterations in the levels of coast lines have already
been referred to.

Other points which are difficult to understand are the occurrence
of disturbances in the sea at the time of feeble earthquakes, and
with earthquakes occurring in distant places. As examples of such
occurrences, Fuchs quotes the following: ‘On May 16, 1850, at 4.28
A.M., an earthquake took place in Pesth, and at 7.30 a motion was
observed in the sea at Livorno. Again, at the time of the earthquake
of December 19, 1850, which shook Heliopolis, a flood suddenly came
in upon Cherbourg.’ May not these phenomena be the result of an earth
pulsation, which produced an earthquake at one point, and a sea wave at
another?

Equally difficult to understand are the observations when the
disturbance in the sea has occurred several hours after an earthquake;
as, for instance, at Batavia, in 1852, when there was an interval of
two hours; and to this must be added the observations where the motion
of the sea has preceded that of the earthquake—as, for instance, in
1852, at Smyrna. Whilst recognising the fact that it is possible to
suggest explanations for many of these anomalies, we must also bear
in mind that they are, generally speaking, exceptional, and, in some
instances, may possibly be due to errors in observations.

_Velocity of propagation of sea waves, and depth of the ocean._—It
has long been known to physical science that the velocity with which
a given wave is propagated along a trough of uniform depth, holds a
relation to the depth of the trough.

If _v_ is the velocity of the wave, and _h_ the depth of the trough,
this relation may be expressed as follows:—

                           _v_^2          (_v_)
                     _h_ = ————— or _h_ = (———)^2
                            _g_           (_k_)

                  Where _g_ = 32·19 and _k_ = 5·671.

It will be observed that these two formulæ (the first of which is
known as Russell’s formula, and the second as Airy’s) are practically
identical.

The apparent difference is in the average value assigned to the
constant.

For large waves such as we have to deal with, it would be necessary,
if we were desirous of great accuracy, to increase the value of _h_
by some small fraction of itself. We might also make allowance for
the different values of _g_, according to our position on the earth’s
surface. With these formulæ at our disposal it is an easy matter, after
having determined the velocity with which a wave was propagated, to
determine the average depth of the area over which it was transmitted.

In making certain earthquake investigations the reverse problem is
sometimes useful—namely, determining the velocity with which a sea wave
has advanced upon a shown line, from a knowledge of the depth of the
water in which it has been propagated.

Calculations of the average depths of the Pacific, dependent on the
velocity with which earthquake waves have been propagated, have been
made by many investigators.

In most cases, however, in consequence of having assumed the wave to
have originated on a coast line, when the evidence clearly showed it
to have originated some distance out at sea, the calculations which
have been made are open to criticism. The average depths which I
obtained for various lines across the Pacific appear to be somewhat
less than the average depth as given by actual soundings. We must,
however, remember that the common error in actual soundings is that
they are usually too great, it being difficult in deep-sea sounding
to determine when the lead actually reaches the bottom. Until oceans
have been more thoroughly surveyed with the improved forms of sounding
apparatus, we shall not be able to verify the truth of the results
which have been given to us by earthquake waves.


                EXAMPLES OF CALCULATIONS ON SEA WAVES.

1. _The wave of 1854._—This wave originated near Japan, and it was
recorded on tide gauges at San Francisco, San Diego, and Astoria.

On December 23, at 9.15. A.M., a strong shock was felt at Simoda in
Japan, which, at 10 o’clock, was followed by a large wave thirty feet
in height. The rising and falling of the water continued until noon.
Half an hour after, the movement became more violent than before. At
2.15 P.M. this agitation decreased, and at 3 P.M. it was comparatively
slow. Altogether there were five large waves.

On December 23 and 25, unusual waves were recorded upon the
self-registering tide gauges at San Francisco, San Diego, and Astoria.

At San Francisco three sets of waves were observed. The average time of
oscillation of one of the first set was thirty-five minutes, whilst one
of the second and third sets was almost thirty-one minutes.

At San Diego three series of waves were also shown, but with average
times of oscillation of from four to two minutes shorter than the waves
at San Francisco.

The San Francisco waves appear to indicate a recurrence of the same
phenomena.

The record at San Diego shows what was probably the effect of a
series of impulses, the heights increasing to the third wave, then
diminishing, then once more renewed, after which it died away.

The result of calculations based on these data were:—

 +---------------+------------+------------+--------+--------------+
 |               |  Distance  |    Time    |Velocity|   Depth of   |
 |               |geographical|     of     |in feet |   ocean in   |
 |               |   miles    |transmission|per sec.|   fathoms    |
 +---- ----------+------------+------------+--------+--------------+
 |               |            |  h.  m.    |        |              |
 | Simoda to San |    4917    |  12  13    |  545   |     2100     |
 |   Diego       |            |            |        |              |
 | Simoda to San |    4527    |  12  39    |  528   | 2500 or 2230 |
 |   Francisco   |            |            |        |              |
 +---------------+------------+------------+--------+--------------+

The difference for the depths in the San Francisco path depends whether
the length of the waves is reckoned at 210 or 217 miles. The length of
the waves on the San Diego path were 186 or 192 miles.[81]

_The wave of 1868._—On August 11, 1868, a sea wave ruined many cities
on the South American coast, and 25,000 lives were lost. This wave,
like all the others, travelled the length and breadth of the Pacific.

In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A.,
who very kindly gave me the following account:

On August 15, at 10.30 A.M., a series of bores or tidal waves
commenced, and lasted until 3 P.M. In ten minutes there was a
difference in the sea level of ten feet, the water rising above high
water and falling below low water mark with great rapidity. The
ordinary tide is only two and a half to three feet. The disturbance
producing these waves originated between Iquique and Arica, in about
lat. 18.28 S. at about 5 P.M. on August 13. In Greenwich time this
would be about 13h. 9m. 40s. August 13. The arrival of the wave at
Hakodate in Greenwich time would be about 14h. 7m. 6s. August 14:
that is to say, the wave took about 24h. 57m. to travel about 8,700
miles, which gives us an average rate of about 511 feet per second.
These waves were felt all over the Pacific. At the Chatham Islands they
rushed in with such violence that whole settlements were destroyed. At
the Sandwich Islands the sea oscillated at intervals of ten minutes for
three days.

Comparing this wave with the one of 1877 we see that one reached
Hakodate with a velocity of 511 feet per second, whilst the other
travelled the same distance at 512 feet per second.

An account of this earthquake wave has been given by F. von Hochstetter
(‘Über das Erdbeben in Peru am 13. August 1868 und die dadurch
veranlassten Fluthwellen im Pacifischen Ocean,’ Sitzungsberichte
der Kaiserl. Akademie der Wissenschaften, Wien 58. Bd., 2. Abth.
1868). From an epitome of this paper given in ‘Petermann’s Geograph.
Mittheil.’ 1869, p. 222, I have drawn up the following table of the
more important results obtained by F. von Hochstetter.

The wave is assumed to have originated near Arica.

 +----------------+------------+----------+----------+----------+
 |                |  Distance  |   Time   | Velocity | Depth of |
 |                |  sea miles | taken by | in feet  | ocean in |
 |                | from Arica |   wave   |per second|   feet   |
 +----------------+------------+----------+----------+----------+
 |                |            |  h.  m.  |          |          |
 |Valdivia        |   1,420    |   5   0  |    479   |   7,140  |
 |Chatham Islands |   5,520    |  15  19  |    608   |  11,472  |
 |Lyttleton       |   6,120    |  19  18  |    533   |   8,838  |
 |Newcastle       |   7,380    |  22  28  |    538   |   9,006  |
 |Apia (Samoa)    |   5,760    |  16   2  |    604   |  11,346  |
 |Rapa            |   4,057    |  11  11  |    611   |  11,598  |
 |Hilo            |   5,400    |  14  25  |    555   |   9,568  |
 |Honolulu        |   5,580    |  12  37  |    746   |  17,292  |
 +----------------+------------+----------+----------+----------+

Calculations on the same disturbance are also given by J. E.
Hilgard.[82]

Assuming the origin of the wave to have been at Arica, his results are
as follows:

 +----------------+------------+------------+----------+----------+
 |                |            |            | Nautical |          |
 |                |  Distance  |  Time of   |   miles  |Mean depth|
 |                |from Africa |transmission| per hour | of ocean |
 +----------------+------------+------------+----------+----------+
 |                |   miles    |   h.  m.   |          |   feet   |
 |San Diego       |   4,030    |   10  55   |    369   |  12,100  |
 |Fort Point      |   4,480    |   12  56   |    348   |  10,800  |
 |Astoria         |   5,000    |   18  51   |    265   |   6,200  |
 |Kodiak          |   6,200    |   22  00   |    282   |   7,000  |
 |Rapa            |   4,057    |   10  54   |    372   |  12,200  |
 |Chatham Islands |   5,520    |   15  01   |    368   |  12,100  |
 |Hawaii          |   5,460    |   14  10   |    385   |  13,200  |
 |Honolulu        |   5,580    |   12  18   |    454   |  18,500  |
 |Samoa           |   5,760    |   15  38   |    368   |  12,100  |
 |Lyttelton       |   6,120    |   19  01   |    322   |   9,200  |
 |Newcastle       |   9,380    |   22  10   |    332   |   9,800  |
 |Sydney          |   7,440    |   23  41   |    314   |   8,800  |
 +----------------+------------+------------+----------+----------+

_The wave of 1877._—Two sets of calculations have been made upon the
wave of 1877 by Dr. E. Geinitz of Rostock.[83]

The following table is taken from Dr. Geinitz’s second paper, in which
there are several modifications of his first results. The origin of the
disturbance is assumed to have been near Iquique.

 +---------------------+--------+----------+-------+--------+--------+
 |                     |Distance|          |       |Velocity|  Mean  |
 |     Observation     |  from  | Arrival  | Time  |in feet |depth of|
 |      stations       |Iquique | of wave  | taken |  per   | ocean  |
 |                     | geol.  |          |by wave|second  |  in    |
 |                     | miles  |          |       |        |fathoms |
 +---------------------+--------+----------+-------+--------+--------+
 |                     |        | h. m.    | h. m. |        |        |
 |Taiohāc (Marquesa    |        |          |       |        |        |
 |  Islands)           |  4,086 | 8 40 A.M.| 12 15 |  563·8 |  1,647 |
 |Apia (Samoa)         |  5,740 |12  0  „  | 15 30 |  610·4 |  1,930 |
 |Hilo (Sandwich       |  5,526 |10 24  „  | 14  0 |  667·9 |  2,310 |
 |  Islands)           |        |          |       |        |        |
 |Kahuliu     „        |  5,628 |10 30  „  | 14  5 |  675·2 |  2,361 |
 |Honolulu    „        |  5,712 |10 50  „  | 14 25 |  669·7 |  2,319 |
 |Wellington (New      |  5,657 | 2 40 P.M.| 18 15 |  524·2 |  1,430 |
 |  Zealand)           |        |          |       |        |        |
 |Lyttelton    „       |  5,641 | 2 48  „  | 18 23 |  519·8 |  1,400 |
 |Newcastle (Australia)|  6,800 | 2 32  „  | 18  7 |  633·0 |  2,075 |
 |Sydney         „     |  6,782 | 2 35  „  | 18 10 |  631·4 |  2,065 |
 |Kamieshi (Japan)     |  8,790 | 7 20  „  | 22 55 |  649·0 |  2,182 |
 |Hakodate    „        |  8,760 | 9 25  „  | 25  0 |  592·5 |  1,818 |
 |Kadsusa     „        |  8,939 | 9 50  „  | 25 15 |  604·9 |  1,895 |
 +---------------------+--------+----------+-------+--------+--------+

The mean depths represent a mean of two sets of calculations, one
made with the aid of Airy’s formula, and the other by Scott-Russell’s
formula. The result of my own investigation about this disturbance, the
origin of which, by several methods of calculation, is shown to have
been beneath the ocean, near 71° 5′ west long., and 21° 22′ south lat.,
are given on next page.

Dr. Geinitz considers that his calculated depths of the ocean and those
obtained by actual soundings are in accordance, a result which is
diametrically opposed to that which I have obtained.

This difference between my calculations and those of Dr. Geinitz,
Hochstetter, and others, chiefly rests on the origin we have assigned
for the sea waves. Dr. Geinitz, for instance, although he says that
the origin of the 1877 earthquake was not on the continent but to the
west in the ocean, bases all his calculations on the assumption that
the _centrum_ was at or near to Iquique, and the time at which that
city was disturbed was the time at which the waves commenced to spread
across the ocean. This time is 8.25 P.M. At this time, however, it
appears that the waves must have been more than double the distance
between the true origin and Iquique, from Iquique on their way towards
the opposite side of the Pacific. Introducing this element into the
various calculations which have been made respecting the depth of the
Pacific Ocean as derived from observations on earthquake waves—which
element, insomuch as the waves appear to have come in to inundate the
land some time after the shock, needs to be introduced—we reduce the
velocity of transit of the earthquake wave and, consequently, the
resultant depths of the ocean.

 Key:
   A Distance from the origin in miles (calculated in great circles)
   B Velocity in feet per second
 +-----------+---------+---------+-----+-----+---+-------+--------+---------+
 |           |         | Arrival |Time |     |   |Depth  |        |Interval |
 |           |         | of wave |taken|     |   |of the | Height | between |
 |           |Longitude|   in    | by  | A   | B |ocean  |  of    |waves in |
 |           |         |Greenwich|wave |     |   |in feet| waves  | minutes |
 |           |         |mean time|     |     |   |       |        |         |
 +-----------+---------+---------+-----+-----+---+-------+--------+---------+
 |           |  °  ′   |day h. m.|h. m.|     |   |       |        |         |
 |Origin     |         |         |     |     |   |       |        |         |
 |  of wave  | 71  5 W.| 9 12  59|     |     |   |       |        |         |
 |San        |         |               |     |   |       |        |         |
 |  Francisco|122 32   |10  2  28|13 29|4,578|498| 7,721 |  9 in. |         |
 |Callao     | 77 15   | 9 17   9| 4 10|  658|231| 1,657 |        |         |
 |Iquique    | 70 14½  | 9 13  21| 0 22|   87|348| 3,770 | 20 ft. |22       |
 |Cobija     | 70 21   | 9 13  19| 0 20|   80|352| 3,857 | 30 „   |         |
 |Mejillones | 70 35   | 9 13  27| 0 28|  108|339| 3,587 | 35 „   |15 or 45 |
 |Chanaral   | 71 34   | 9 15  26| 2 27|  455|272| 2,309 |        |10       |
 |Coquimbo   | 71 24   | 9 15  15| 2 16|  508|328| 3,363 |        |30       |
 |Valparaiso | 71 38   | 9 16  16| 3 17|  695|310| 3,000 |        |         |
 |Concepcion | 73  5   | 9 16  52| 3 53|  928|350| 3,824 |        |12 to 15 |
 |Honolulu   |157 55   |10  3  52|14 53|5,694|561| 9,807 |34 to   |25       |
 |           |         |         |     |     |   |       |  54 ft.|         |
 |Hilo       |155  3   |10  3   5|14  6|5,506|563|10,217 |30 or   |{3 or 15 |
 |           |         |         |     |     |   |       |   8  „ |{18 or 27|
 |Kahuliu    |156 43   |10  3  12|14 13|5,611|579|10,437 |        |         |
 |Samoa      |171 41 W.|10  3  57|14 58|5,773|566| 9,972 | 12 ft. |10       |
 |Taurauga   |176 11 E.|10  8  15|19 16|5,615|427| 5,697 |        |         |
 |Wellington |174 30   |10  7  22|18 23|5,574|445| 6,168 | 11 „   |10       |
 |Akaroa     |172 59   |10  7  28|18 29|5,542|440| 6,031 |        |         |
 |Lyttelton  |172 45   |10  7  29|18 30|5,558|441| 6,055 |        |         |
 |Kameishi   |140 50   |10 12  37|23 38|8,844|549| 9,378 |  6 „   |15       |
 |Hakodate   |140 50   |10 14   7|25  8|8,778|512| 8,169 |  7 „   |20       |
 +-----------+---------+---------+-----+-----+---+-------+--------+---------+

In Dr. Geinitz’s paper there are also some slight differences in the
times at which the earthquake phenomena were observed at various
localities. These, however, are but of minor importance. At the end
of the paper by Dr. Geinitz two interesting tide gauge records are
introduced, one from Sydney and the other from Newcastle. These appear
to show a marked difference in the periods of the sea waves at these
two places.[84]

_Comparison of velocities of wave-transit which have been actually
observed, with velocities which ought to exist from what we know of
the depth of the Pacific by actual soundings._—From a chart given in
‘Petermann’s Geograph. Mittheilungen,’ Band xxiii. p. 164, 1877, it is
possible to draw approximate sections on lines in various directions
across the bed of the Pacific.

From the origin of the shock to Japan (Kameishi) the line would be as
follows:—

  about 7,441 miles       15,000  feet deep
        1,100   „         18,000      „
          160   „         27,000      „
           80   „         12,000      „
           60   „          6,000      „

On account of the Tuscarora and Belkap Deeps this would be the most
irregular line over which the wave had to travel.

From the origin to New Zealand (Wellington) the line would be

  about 5,274 miles       15,000 feet deep.
    „     300  „          12,000   „

From the origin to Samoa the line would be

  about 5,773 miles       15,000 feet deep.

From the origin to the Sandwich Islands (Honolulu) the line would be

  almost 6,634 miles      15,000 feet deep
  and       60   „        12,000     „

By Scott-Russell’s rule, or, what is almost identically the same, by
Airy’s general formula, we can calculate how long it would take such
waves as we have been speaking about to travel over the different
portions of each of these lines, and by adding these times together we
obtain the time taken to travel across any one line. I have made these
calculations, but as I get in every case answers which are too small, I
think it unnecessary to give them.

The actual times taken to travel the distances just referred to were,

  To Japan (Kameishi)               23 hr. 38 min.
   „ New Zealand (Wellington)       18  „  23  „
   „ Samoa                          14  „  58  „
   „ Sandwich Islands (Honolulu)    14  „  53  „

From San Francisco to Simoda the line is almost 3,567 miles, 3,000
fathoms deep, 840 miles, 2,500 fathoms deep, and 120 miles 1,000
fathoms deep. This gives an average depth of about 2,854 fathoms. Bache
calculated the depth at 2,500 fathoms.

If we are to consider that, because the sea wave at Simoda came in
some time after the land shock had been felt, the origin of this
earthquake, instead of being at Simoda, was some distance out at sea,
this calculated depth would be reduced.




                              CHAPTER X.

                 DETERMINATION OF EARTHQUAKE ORIGINS.

  Approximate determination of an Origin—Earthquake-hunting
    in Japan—Determinations by direction of motion—Direction
    indicated by destruction of buildings—Direction determined by
    rotation—Cause of rotation—The use of time observations—Errors
    in such observations—Origin determined by the method of
    straight lines—The method of circles, the method of hyperbolas,
    the method of co-ordinates—Haughton’s method—Difference in time
    between sound, earth, and water waves—Method of Seebach.


One of the most practical problems which can be suggested to the
seismologist is the determination of the district or districts in any
given country from which earthquake disturbances originate. With a
map of a country before us, shaded with tints of different intensity
to indicate the relative frequency of seismic disturbance in various
districts, we at once see the localities where we might dwell with
the least disturbance, and those we should seek if we wish to make
observational seismology a study. Before erecting observatories for
the systematic investigation of earthquakes in a country, it would
be necessary for us, in some way or other, to examine the proposed
country to find out the most suitable district. The special problem
of determining _approximately_ the origin or origins of a set of
earthquakes would be given to us. Having made this preliminary
investigation, the next point is, by means of observatories so
arranged that they could always work in conjunction with each other,
to determine the origins more accurately. By knowing the origin from
which a set of shocks spring we know the general direction in which
we may expect the most violent disturbances, and we can arrange our
seismometers accordingly.

_Approximate determination of origins._—In 1880 I obtained a tolerably
fair idea of the distribution of seismic energy throughout Japan, by
compiling the facts obtained from some hundreds of communications
received from various parts of the country respecting the number of
earthquakes that had been felt.

The communications were replies to letters sent to various residents
in the country and to a large number of public officers. By taking
these records, in conjunction with the records made by instruments, it
was ascertained that in Japan alone there were certainly 1,200 shocks
felt during the year, that is to say, three or four shocks per day.
The greater number of these shocks were felt along the eastern coast,
commencing at Tokio, in the south, and going northwards to the end of
the main island. These shocks were seldom felt on the west coast. It
appeared as if the central range of mountains formed a barrier to their
progress. Similarly, ranges of mountains to the south-west of Tokio
prevented the shocks from travelling southwards. Proceeding in this way
the conclusion was arrived at that the west coast, the southern part
of Japan, and the islands of Shikoku and Kiushiu, had their own local
earthquakes.

_Earthquake-hunting._—These preliminary enquiries having shown that
the northern part of Japan was a better district for seismological
observations than the southern half, the next step was to subject the
northern half to a closer analysis. This analysis was commenced by
sending to all the important towns, from thirty to one hundred miles
distant from Tokio, bundles of postcards. These were entrusted to the
local government offices with a request that each week one of these
cards would be returned to Tokio stating the number of shocks felt.
In this way it was quickly discovered that the majority of shakings
emanated from the north and east, and seldom, if ever, passed a heavy
range of mountains to the south. The barricade of postcards was then
extended farther northwards, with the result of surrounding the origin
of certain shocks amongst the mountains, whilst others were traced to
the sea shore. By systematically pursuing earthquakes it was seen that
many shocks had their origin beneath the sea—they shook all the places
on the north-east coast, but it was seldom that they crossed through
the mountains, forming the backbone of the island, to disturb the
places on the west coast.

The actual results obtained in three months by this method of working
are shown in the accompanying map, which embraces the northern half of
the main island of Nipon and part of Yezo. The shaded portion of the
map indicates the mountainous districts, which are traversed by ranges
varying in height from about 2,000 and 7,000 feet. The dotted lines
show the boundaries of the more important groups of earthquakes which
were recorded.

I. is the western boundary of earthquakes, which at places to the
eastward are usually felt somewhat severely. Some of these have been
felt the most severely at or near Hakodadi, whilst farther south their
effects have been weak. Occasionally the greatest effect has been near
to Kameishi. Sometimes these earthquakes terminate along the western
boundaries of III. or IV., not being able to pass the high range of
mountains which separate the plain of Musashi from Kofu.

[Illustration: FIG. 29.—Northern Japan. Mountainous districts shaded
with oblique lines.]

II. is the boundary of a shock confined to the plain which surrounds
Kofu. These earthquakes are evidently quite local. Many of the
disturbances have evidently originated beneath the ocean, having come
in upon the land in the direction of the arrows A or B.

III. This line indicates the boundary of a group of shocks which are
often experienced in Tokio. These may come in the directions D, E,
or F. It is probable that some of them originate to the eastward of
Yokohama, on or near to the opposite peninsula.

IV. V. and VI. The earthquakes bounded by these lines probably
originate in the directions C or D.

VII. The earthquakes bounded by this line probably come from the
direction E.

VIII. This line gives us the boundary of earthquakes which may come
from the direction B.

The above boundaries sometimes do not extend so far to the westward as
they are shown. At other times, groups like V. and VI. extend farther
to the south-west. These earthquake boundaries, which so clearly show
the effects of high mountains in preventing the extension of motion,
have been drawn up, not from single earthquakes, but from a large
series of earthquakes which have been plotted upon blank maps, and are
now bound together to form an atlas. To give an idea of the material
upon which I have been working, I may state that between March 1 and
March 10, 1882, I received records of no less than thirty-four distinct
shocks felt in districts between Hakodate and Tokio, and for each of
these it is quite possible to draw a map. In addition to the boundaries
of disturbances given in the accompanying map, other boundaries might
be drawn for shocks which were more local in their character. The
groups which contained the greatest number of shocks are III., IV.,
V., VI., and VII. By work of this description it was found that a very
important group of earthquakes might be studied by a line of stations
commencing at Saporo in the north, passing through Hakodate, down the
east coast of the main island, to Tokio or Yokohama in the south. A
further aid to the study of this group, together with the study of
an important local group, might be effected with the help of a few
additional stations properly distributed on the plain of Musashi, which
surrounds Tokio. With this example before us it will be recognised that
the choice of sites for a connected set of seismological observatories
will often be more or less a special problem. If earthquake stations
were to be placed in different directions around Tokio without
preliminary investigation, it is quite possible that some of them might
be so situated that they would seldom if ever work in conjunction with
the remaining observatories, and therefore be of but little value.
And this remark must equally apply to districts in other portions
of the globe. The method is crude, and, so far as actual earthquake
origins are concerned, it only yields results which are approximate.
The crudeness and the want of absoluteness in the results is, however,
more than counterbalanced by the certainty with which we are enabled
to express ourselves with regard to such results as are obtained. Even
when working with the best instruments we have at our command, unless
we are employing some elaborate system, this method of working gives a
most valuable check upon our instrumental records, and enables us to
interpret them with greater confidence.

_Determination of earthquake origins from the direction of motion._—If
we assume that an earthquake is propagated from a centre as a series
of waves, in which normal vibrations are conspicuous, and obtain at
two localities, not in the same straight line with the origin, and
sufficiently far separated from each other, the direction of movement
of these normal motions, by drawing lines parallel to these directions
through our two stations, the lines would intersect at a point above
the required origin. If instead of two points we had three, or, better
still, a large number, the results we should obtain ought to be
still more certain. Unfortunately, it seems that earthquakes seldom
originate from a given point, and, further, normal motions are not
always (sufficiently) prominent. Sometimes, as has already been shown
in the chapters on earthquake motion, they may be non-existent. It is
probable, however, that difficulties of this sort are more usually
associated with non-destructive earthquakes. Mallet regards the
destructive effects of an earthquake as almost solely due to normal
motions. If this be true, for destructive earthquakes, the problem is
shorn of many of its difficulties. In cases where normal vibrations
are not prominent, where we have only transverse vibrations, motions
due to the interference of normal or transverse motions, or directions
of motions due to the topographical or geological nature through which
the disturbance has passed, the determination of the origin of an
earthquake by observations on the direction in which the ground has
been moving appears to be a problem which is practically without a
solution. We will, therefore, only consider the determination of the
origin of those earthquakes which have predominating directions in
their movements, which directions we will consider as normal ones.
The question which is, then, before us, is the determination of the
direction of these normal movements. First of all we may take the
evidence of our senses. In exceptional cases these have given results
which closely approximate to the truth, but in the majority of cases
such results are not to be relied upon, as the inhabitants of a town
will, for the same shock, give directions corresponding to all points
of the compass. Much, no doubt, depends upon the situation of the
observer, and much, perhaps, upon his temperament. If he is sitting
in a room alone, and is accustomed to making observations on an
earthquake, on feeling the earthquake, if he concentrates his attention
on the direction in which he is being moved, his observations may be
of value. If, however, he is not so situated, and his attention is
not thus concentrated, his opinions, unless the motion has been very
decided in its character, are usually of but little worth.

_Direction determined from destruction of buildings._—When an observer
first sees a town that has been partially shattered by an earthquake,
all appears to be confusion, and it is difficult to imagine that in
such apparent chaos we are able to discover laws. If, however, we take
a general view of this destruction and compare together similarly
built buildings, it is possible to discover that similar and similarly
situated structures have suffered in a similar manner. By carefully
analysing the destruction we are enabled to infer the direction in
which the destroying forces have acted. It was chiefly by observing the
cracks in buildings, and the direction in which bodies were overthrown
or projected, that Mallet determined the origin of the Neapolitan
earthquake. From the observations given in Chapter VII. it would appear
that, with destructive earthquakes, walls which are transverse to the
direction of motion are most likely to be overturned, whilst, with
small earthquakes, these walls are the least liable to be fractured.

From a critical examination of the _general_ nature of the damage
done on the buildings of a town, earthquake observers have shown that
the direction of a shock may often be approximately determined. The
direction in which a body having a regular form like a prismatic
gravestone or a cylindrical column is overturned sometimes gives the
means by which we can determine the direction from which a movement
came.

_The rotation of bodies._—It has often been observed that almost all
large earthquakes have caused objects like tombstones, obelisks,
chimneys, &c., to rotate.

One of the most natural and at the same time most simple explanations
is to suppose that during the shock there had been a twisting, or
backward and forward screw-like motion in the ground. Amongst the
Italians and the Mexicans earthquakes producing an effect like this are
spoken of as ‘vorticosi.’ In the Calabrian earthquake, not only were
bodies like obelisks twisted on their bases, but straight rows of trees
seem to have been left in interrupted zigzags. These latter phenomena
have been explained upon the assumption of the interference of direct
waves and reflected waves, the consequence of which being that points
in close proximity might be caused to move in opposite directions.
Reflections such as these would be most likely to occur near to the
junction of strata of different elasticity, and it may be remarked that
it is often near such places that much twisting has been observed.

Another way in which it is possible for twisting to have taken place
would be by the interference of the normal and transverse waves which
probably always exist in an earthquake shock, or by the meeting of
the parts of the normal wave itself, one having travelled in a direct
line from the origin, whilst the other, travelling through variable
material, has had its direction changed.

Mallet, however, has shown that the rotation may have been in many
cases brought about without the supposition of any actual twisting
motion of the earth—a simple backward and forward motion being quite
sufficient. If one block of stone rests upon another, and the two are
shaken backwards and forwards in a straight line, and if the vertical
through the centre of gravity of the upper block does not coincide with
the point where there is the greatest friction between the blocks,
rotation must take place. If the vertical through the centre of gravity
falls on one side of the centre of friction, the rotation would be in
one direction, whilst, if on the other side, the rotation would be in
the opposite direction.

Although the above explanation is simple, and also in many cases
probably true, it hardly appears sufficient to account for all the
phenomena which have been observed.

Thus, for instance, if the stones in the Yokohama cemetery, at the time
of the earthquake of 1880, had been twisted in consequence of the cause
suggested by Mallet, we should most certainly have found that some
stones had turned in one direction whilst others had been twisted in
another. By a careful examination of the rotated stones, I found that
every stone—the stones being in parallel lines—had _revolved in the
same direction_, namely in a direction opposite to that of the hands of
a watch.

As it would seem highly improbable that the centre of greatest friction
in all these stones of different sizes and shapes should have been at
the same side of their centres of gravity, an effect like this could
only be explained by the conjoint action of two successive shocks, the
direction of one being transverse to the other.

Although fully recognising the sufficiency of two transverse shocks
to produce the effects which have been observed in Yokohama, I will
offer what appears to me to be the true explanation of this phenomenon:
it was first suggested by my colleague, Mr. Gray, and appears to be
simpler than any with which I am acquainted.

[Illustration: FIG. 30.]

If any columnar-like object, for example a prism which the basal
section is represented by A B C D (see fig. 30), receives a shock at
right angles to B C, there will be a tendency for the inertia of the
body to cause it to overturn on the edge B C. If the shock were at
right angles to D C, the tendency would be to overturn on the edge D C.
If the shock were in the direction of the diagonal C A, the tendency
would be to overturn on the point C. Let us, however, now suppose the
impulse to be in some direction like E G, where G is the centre of
gravity of the body. For simplicity we may imagine the overturning
effect to be an impulse given through G in an opposite direction—that
is, in the direction G E. This force will tend to tip or make the body
bear heavily on C, and at the same time to whirl round C as an axis,
the direction of turn being in the direction of the hands of a watch.
If, however, the direction of impulse had been E′ G, then, although the
turning would still have been round C, the direction would have been
_opposite_ to that of the hands of a watch.

To put these statements in another form, imagine G E′ to be resolved
into two components, one of them along G C and the other at right
angles, G F. Here the component of the direction G C tends to make the
body tip on C, whilst the other component along G F causes revolution.
Similarly G E may be resolved into its two components G C and G F′, the
latter being the one tending to cause revolution.

From this we see that if a body has a rectangular section, so long
as it is acted upon by a shock which is parallel to its sides or to
its diagonals, there ought not to be any revolution. If we divide
our section A B C D up into eight divisions by lines through these
directions, we shall see that any shock the direction of which passes
through any of the octants which are shaded will cause a _positive_
revolution in the body—that is to say, a revolution corresponding in
its direction to that of the movements of the hands of a watch; whilst
if its direction passes through any of the remaining octants the
revolution will be _negative_, or opposite to that of the hands of a
watch. From the direction in which any given stone has turned, we can
therefore give two sets of limits between one of which the shock must
have come.

Further, it will be observed that the tendency of the turning is to
bring a stone, like the one we are discussing, broadside on to the
shock; therefore, if a stone with a rectangular cross section has
turned sufficiently, the direction of a shock will be parallel to one
of its faces, but if it has not turned sufficiently it will be more
nearly parallel to its faces in their new position than it was to its
faces when in their original position.

If a stone receives a shock nearly parallel with its diagonal, on
account of its instability it may turn either positively or negatively
according as the friction on its base or some irregularity of surface
bearing most influence. Similarly, if a stone receives a shock parallel
to one of its faces, the twisting may be either positive or negative,
but the probability is that it would only turn slightly; whereas in the
former case, where the shock was nearly parallel to a diagonal, the
turning would probably be great.

_Determination of direction from instruments._—When speaking about
earthquakes it was shown, as the result of many observations, that
the same earthquake in the space of a few seconds, although it may
sometimes have only one direction of motion, very often has many
directions of motion. In certain cases, therefore, our records, if we
assume the most permanent motions to be normal ones, give definite and
valuable results. In other cases it is necessary to carefully analyse
the records, comparing those taken at one station with those taken at
another.

One remarkable fact which has been pointed out in reference to
artificial earthquakes produced by exploding charges of gunpowder or
dynamite, and also with regard to certain earthquakes, is that the
greatest motion of the ground is _inwards_, towards the point from
which the disturbance originated. Should this prove the rule, it gives
a means of determining, not only the direction of earthquake, but the
side from which it came.

_Determination of earthquake origins by time observations._—The times
at which an earthquake was felt at a number of stations are among the
most important observations which can be made for the determination of
an earthquake origin. The methods of making time observations, and the
difficulties which have to be overcome, have already been described.
When determining the direction from which a shock has originated, or
determining the origin of the shock by means of time observations,
it has been usual to assume that the velocity of propagation of the
shock has been uniform from the origin. The errors involved in this
assumption appear to as follows:—

1. We know from observations on artificial earthquakes that the
velocity of propagation is greater between stations near to the origin
of the shock than it is between more remote stations; and also the
velocity of propagation varies with the initial force which produced
the disturbance. If our points of observation are sufficiently close
together as compared with their distance from the origin of the
disturbance, it is probable that errors of this description are small
and will not make material differences in the general results.

2. We have reasons for believing that the transit velocity of an
earthquake is dependent on the nature of the rocks through which it
is propagated. Errors which arise from causes of this description
will, however, be practically eliminated if our observation points are
situated on an area sufficiently large, so that the distribution of the
causes tending to alter the velocity of a shock balance each other. It
must be remarked, that causes of this description may also produce an
alteration in the direction of our shock.

Other errors which may sometimes enter into our results, when
determining the origin of shocks by means of observations on
velocities, are the assumptions that the disturbance has travelled
along the surface from the _epicentrum_ and not in a direct line from
the _centrum_. Again, it is assumed that the origin is a point, whereas
it may possibly be a cavity or a fissure. Lastly, if we desire extreme
accuracy, we must make due allowance for the sphericity of the earth
and the differences of elevation of the observing stations.

I. _The method of straight lines._—Given a number of pairs of points
A_{0}, A_{1}, B_{0}, B_{1}, C_{0}, C_{1}, &c., at each of which the
shock was felt simultaneously, to determine the origin.

Theoretically if we bisect the line which joins A_{0} and A_{1} by a
line at right angles to A_{0}, A_{1}, and similarly bisect the lines
B_{0}, B_{1}, C_{0}, C_{1}, all these bisecting lines _a__{0}, _a__{1},
_b__{0}, _b__{1}, _c__{0}, _c__{1}, &c., ought to intersect in a point,
which point will be the _epicentrum_ or the point above the origin.

This method will fail, first, if A_{0}, A_{1}, B_{0}, B_{0}, C_{0},
C_{1} form a continuous straight line, or if they form a series of
parallel lines.

Hopkins gives a method based on a principle similar to the one which
is here employed—namely, given that a shock arrives simultaneously
at _three_ points to determine, the centre. In this case, the
relative positions of the three points, where the time of arrival was
simultaneous, must be accurately known, and these three points must
not lie in a straight line, or the method will fail. For practical
application the problem must be restricted to the case of three points
which do not lie nearly in the same straight line.

II. _The method of circles._—Given the times _t__{0}, _t__{1}, _t__{2},
&c., at which a shock arrived at a number of places A_{0}, A_{1},
A_{2}, &c., to determine the position from which the shock originated.

Suppose A_{0} to be the place which the shock reached first, and that
it reached A_{1}, A_{2}, A_{3}, &c., successively afterwards.

  Let     _t__{1} - _t__{0} = _a_
          _t__{2} - _t__{0} = _b_
          _t__{3} - _t__{0} = _c_, &c.

With A_{1}, A_{2}, A_{3}, &c. as centres, describe circles with radii
proportional to the known qualities _a_, _b_, _c_, &c., and also a
circle which passes through A_{0} and touches these circles. The centre
of the last circle will be the _epicentrum_. The radii proportional to
_a_, _b_, _c_, &c. may be represented by the quantities _ax_, _bx_,
_cx_, &c., where _x_ is the velocity of propagation of the shock.

It will be observed that the times at which the shock arrived at three
places might alone be sufficient. If, instead of taking the times of
arrival of a shock, the arrival of a sea wave be taken, the result
would be a closer approximate to the absolute truth.

It will be observed that this method is not a direct one, but is
one of trial. If, however, an imaginary case be taken, and three
given points of observation, A_{0}, A_{1}, A_{2}, be plotted on a
piece of paper, it will be found that it is not a difficult matter to
determine two numbers proportional to _a_ and _b_ which will allow
you to draw two circles so that they may be touched by a third circle
drawn through A_{0}. This problem has practically been applied in the
case of the arrival of a sea wave at a number of places on the South
American coast, at the time of the earthquake of May 9, 1877. This is
illustrated as follows. The places which were chosen were Huanillos,
Tocopilla, Cobija, Iquique, Mejillones.

In the following table the first column gives the times at which the
sea wave arrived at each of these places in Iquique time; in the second
column the difference between these times and the time at which it
reached Huanillos is given; in the third column the distances through
which a sea wave, propagated at the rate of 350 feet per second, could
travel during the intervals noted in the second column is given.

 +------------+----------+----------+------------+
 |            | Arrival  |Time after| Distance at|
 |            |  of sea  |arrival at|350 feet per|
 |            |   wave   | Huanillos|   second   |
 +------------+----------+----------+------------+
 |            |  h. m.   |  minutes |   miles    |
 | Huanillos  |  8  30   |     0    |     0      |
 | Tocopilla  |  8  32   |     2    |     8      |
 | Cobija     |  8  38   |     8    |    32      |
 | Iquique    |  8  40   |    10    |    40      |
 | Mejillones |  8  46   |    16    |    64      |
 +------------+----------+----------+------------+

The distances marked in the third column are used as radii of the
circles drawn round the places to which they respectively refer.

The centre of the circle drawn to touch the circles of the first
column, and at the same time to pass through Huanillos, is marked C.

The position from which the shock originated appears therefore to have
occurred very near to a place lying in Long. 7° 15′ W. and Lat. 21° 22′
S.

[Illustration: FIG. 31.]

The actual operations which were gone through in making the
accompanying map were as follows. First, the places with which we had
to deal were represented on a map in orthographical projection, the
centre of projection being near to the centre of the map. This was
done so that the measurements which were made upon the map might be
more correct than those we should obtain from an ordinary chart where
this portion of the world was not the centre of projection. Next, a
number was taken as equal to the velocity with which the sea wave had
travelled. The first velocity taken was about 400 feet per second—this
being about the velocity with which, theoretically, it must have
travelled in an ocean having a depth equal to that indicated upon the
charts—also it seemed to have travelled at this rate from the various
times of arrival as recorded at places along the coast. Circles were
then drawn round Tocopilla, Cobija, Iquique, and Mejillones with radii
equal to 2, 8, 10, and 15, each multiplied by (60 × 400). It was then
seen _by trial_ that it was impossible to draw a single circle which
should touch four circles and also pass through Huanillos. These four
circles were, in fact, too large. Four new but smaller circles, which
are shown in the map, were next drawn, their radii being respectively
equal to the numbers 2, 8, 10, and 16, each multiplied by (60 × 350),
and it was found that a circle, with a centre C, could be drawn which
would practically touch the four circles, and at the same time would
pass through Huanillos.

III. _The method of hyperbolas._—The method which I call that of
hyperbolas is only another form of the method of circles. It is,
however, useful in special cases, as, for instance, where we have the
times of arrival of earthquakes at only two stations. Between Tokio
and Yokohama, at which places I frequently obtain tolerably accurate
time records, the method has been applied on several occasions with
advantage. In the preceding example let us suppose that the only time
records which we had were for Huanillos and Mejillones, and that the
wave was felt at the latter place sixteen minutes or 960 seconds
after it was experienced at the former. Calling these places H and M
respectively, round M draw a circle equal to the 960 multiplied by
the velocity with which the wave was propagated. It is then evident
that the origin of this disturbance must be the centre of a circle
which passes through H and touches the circle drawn round M. Join H
M, cutting the circle round M in Y. Bisect Y H in V. It is evident
that V is one possible origin for the disturbance. Next, from M, in
the direction of H, draw any line M Z P; join Z H; bisect Z H at right
angles by the line O P N. Because PH = PZ, it is evident that P is
a second possible origin. Proceeding in this way a series of points
lying to the right and left of V on the curve R V T may be found, and
we may therefore say that the origin lies somewhere in the curve R
V T. By increasing or decreasing our velocity we vary the position
of the curve R V T, and, instead of a line on which our origin may
be, we obtain a band. As the assumed velocity increases, the circle
round M becomes larger, and has its limit when it passes through H,
where the two arms of the curve R V T will close together and form a
prolongation of the line M Y H as the assumed velocity diminishes.
The circle round M becomes smaller until it coincides with the point
M. At such a moment the curve R V T opens out to form a straight line
bisecting M H at right angles. The curve R V T is a hyperbola with a
vertex V and foci H and M. Inasmuch as PM - PH = a constant quantity.
If we have the time given at which the shock or wave arrived at a third
station as at Iquique, it is evident that a second hyperbola R′ V′ T′
might be drawn with Iquique and Huanillos as foci, and that the mutual
intersection of these two hyperbolas with a third hyperbola, having for
its foci Iquique and Mejillones, would give the origin of the wave.
The obtaining of a mutual intersection would depend on the assumed
velocity, and the accuracy of the result, like that of the method
of circles, would depend upon the trials we made. The method here
enunciated may be carried farther by describing hyperboloids instead
of hyperbolas, the mutual intersection of which surfaces would, in the
case of an earth wave, give the actual origin or _centrum_ rather than
the point above the origin or _epicentrum_.

IV. _The method of co-ordinates._—Given the times at which a shock
arrived at five or more places, the position of which we have marked
upon a map, or chart, to determine the position on the map of the
centre of the shock, its depth, and the velocity of propagation.

Commencing with the place which was last reached by the shock, call
these places _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let the
times taken to reach these places from the origin be respectively _t_,
_t__{1}, _t__{2}, _t__{3}, and _t__{4}.

Through _p_ draw rectangular co-ordinates, and with a scale measure the
co-ordinates of _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let these
respectively be _a__{1}, _b__{1}; _a__{2}, _b__{2}; _a__{3}, _b__{3};
_a__{4}, _b__{4}. Then if _x_, _y_, and _z_ be the co-ordinates of the
origin of the shock, _d_, _d__{1}, _d__{2}, _d__{3}, and _d__{4}, the
respective distances of _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4}
from this origin, and _v_ the velocity of the shock, we have

  1. _x_^2 + _y_^2 + _z_^2 = _d_^2 = _v_^2 _t_^2
  2. (_a__{1} - _x_)^2 + (_b__{1} - _y_)^2 + _z_^2 = _v_^2 _t__{1}^2
  3. (_a__{2} - _x_)^2 + (_b__{2} - _y_)^2 + _z_^2 = _v_^2 _t__{2}^2
  4. (_a__{3} - _x_)^2 + (_b__{3} - _y_)^2 + _z_^2 = _v_^2 _t__{3}^2
  5. (_a__{4} - _x_)^2 + (_b__{4} - _y_)^2 + _z_^2 = _v_^2 _t__{4}^2

Because we know the actual times at which the waves arrived at the
places _p_, _p__{1}, _p__{2}, _p__{3}, _p__{4}, we know the values
_t_—_t__{1}, _t_—_t__{2}, _t_—_t__{3}, _t_—_t__{4}. Call these
respectively _m_, _p_, _q_, and _r_. Suppose _t_ known, then

                          _t__{1} = _t_ - _m_
                          _t__{2} = _t_ - _p_
                          _t__{3} = _t_ - _q_
                          _t__{4} = _t_ - _r_.

Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5,
we obtain,

  _a__{1}^2 + _b__{1}^2 - 2_a__{1} _x_ - 2_b__{1} _y_
      = _v_^2 (_t__{1}^2 - _t_^2) = _v_^2 (_m_^2 - 2_t_ _m_)

  _a__{2}^2 + _b__{2}^2 - 2_a__{2} _x_ - 2_b__{2} _y_
      = _v_^2 (_t__{2}^2 - _t_^2) = _v_^2 (_p_^2 - 2_t_ _p_)

  _a__{3}^3 + _b__{3}^2 - 2_a__{3} _x_ - 2_b__{3} _y_
      = _v_^2 (_t__{3}^2 - _t_^2) = _v_^2 (_q_^2 - 2_t_ _q_)

  _a__{4}^2 + _b__{4}^2 - 2_a__{4} _x_ - 2_b__{4} _y_
      = _v_^2 (_t__{4}^2 - _t_^2) = _v_^2 (_r_^2 - 2_t_ _r_)

Now let _v_^2 = _u_, and 2_v_^2 _t_ = _w_.

Then

  1. 2_a__{1} _x_ + 2_b__{1} _y_ + _u_ _m_^2 - _n_ _m_
      = _a__{1}^2 + _b__{1}^2

  2. 2_a__{2} _x_ + 2_b__{2} _y_ + _u_ _p_^2 - _n_ _p_
      = _a__{2}^2 + _b__{2}^2

  3. 2_a__{3} _x_ + 2_b__{3} _y_ + _u_ _q_^2 - _n_ _q_
      = _a__{3}^2 + _b__{3}^2

  4. 2_a__{4} _x_ + 2_b__{4} _y_ + _u_ _r_^2 - _n_ _r_
      = _a__{4}^2 + _b__{4}^2

We have here four simple equations containing the four unknown
quantities _x_, _y_, _u_, and _w_.

_x_ and _y_ determine the origin of the shock. The absolute velocity
_v_ equals √ _u_. From _v_ and _w_ we obtain _t_. Substituting
_x_, _y_, _v_, and _t_ in the first equation we obtain _z_.

We have here assumed that the points of observation have all about the
same elevation above sea level.

If it is thought necessary to take these elevations into account, a
sixth equation may be introduced.

If the propagation of the wave is considered as a horizontal one, as
would be done when calculating the position of the _epicentrum_ or
point above the origin, by means of the times of arrival of a sea wave,
the ordinate _z_ of the first five equations would be omitted. Working
in this way the resulting four equations, viz.

  2_a__{1} _x_ + 2_b__{1} _y_ + _u__m_^2 - _w__m_^2 = _a__{1}^2 + _b__{1}^2
              &c.                      &c.              &c.

remained unchanged.

Applying this method to the same example as that used as illustration
for the two previous methods, we obtain for the co-ordinates of
Mejillones, Iquique, Cobija, Tocopilla, and Huanillos, measured in
geographical miles, and the times in Iquique time at which the wave
reached each, as given in the following table; _ox_ and _oy_ being,
drawn through Mejillones.

 +------------+--------------------------------+---------------+
 |            |         Co-ordinates           |Time of arrival|
 +------------+----------------+---------------+---------------+
 |            |      OX        |      OY       |  h.  m.       |
 | Mejillones | _a_     or   0 | _b_     or  0 |  8  46 p. m.  |
 | Iquique    | _a_{1}_ or 150 | _b_{1}_ or 96 |  8  40   „    |
 | Cobija     | _a_{2}_ or  36 | _b_{2}_ or 14 |  8  38   „    |
 | Tocopilla  | _a_{3}_ or  66 | _b_{3}_ or 31 |  8  32   „    |
 | Huanillos  | _a_{4}_ or 102 | _b_{4}_ or 58 |  8  30   „    |
 +------------+----------------+---------------+---------------+

From this data we find the co-ordinates _x_ and _y_ of this origin to
be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per
second.

Measuring these ordinates upon the map, we obtain a centre lying very
near Long. 71° 5′ W. and Lat. 21° 22′ S., a position which is very near
to that which has already been obtained by other methods.

If instead of Huanillos we substitute the ordinates and time of arrival
of the sea wave for Pabalon de Pica, another point for the origin will
be obtained lying farther out at sea. To obtain the best result, the
method to be taken will evidently be, first to reject those places
at which it seems likely that some mistake has been made with the
time observations, and then with the remaining places to form as many
equations as possible, and from these to obtain a mean value. This
is a long and tedious process, and as the time observations of this
particular earthquake are probably one and all more or less inaccurate,
it is hardly worth while to follow the investigation farther.

In this example, as in the preceding ones, it will be observed that
it has been sea waves that have been dealt with, rather than earth
vibrations. It is evident, however, that these latter vibrations may be
dealt with in a similar manner.

In these determinations it will also have been observed that it
is assumed that the disturbance has radiated from a centre, and,
therefore, approached the various stations in different directions.
If we assume that we have three stations very near to each other as
compared with their distances from the origin, so that we can assume
that the wave fronts at the various stations were parallel, the
determination of the direction in which the wave advanced appears to be
much simplified. The determination of the direction in which a wave has
passed across three stations was first given by Professor Haughton.

_Haughton’s method._—Given, the time of an earthquake shock at three
places, to determine its horizontal velocity and coseismal line.

The solution of this is contained in the formula

                       _a_ (_t_{2}_ - _t_{1}_) sin β
    tan φ = ——————————————————————————————————————————————————————.
            _c_ (_t_{3}_ - _t_{2}_) + _a_ (_t_{2}_ - _t_{1}_ cos β

When A, B, and C are three stations at which a shock is observed at
the times _t_{1}_, _t_{2}_, and _t_{3}_; _a_, _b_, and _c_ are the
distances between A, B, and C, and φ is the angle made by the coseismal
lines _x_ A _x_, _y_ B _y_, and the line A B, which are assumed to be
parallel.

This I applied in the case of the Iquique earthquake, but owing to the
smallness of the angles between the three stations A, B, and C, the
result was unsatisfactory. The problem ought to be restricted, first,
to places which are a long distance away from a centre, and, secondly,
to places which are not nearly in a straight line. This problem may be
solved more readily by geometrical methods. Plot the three stations A,
B, and C on a map, join the two stations between which there was the
greatest difference in the time observation. Let these, for example,
be A and C. Divide the line A C at point D, so that A D : D C as the
interval between the shock felt at A and B is to the interval between
the shock felt at B and C. The line B D will be parallel to the
direction in which the wave advanced.

_The difference in time of the arrival of two disturbances._—In the
various calculations which have been made to determine an origin
based on the assumption of a known or of a constant velocity, we have
only dealt with a single wave, which may have been a disturbance in
the earth or in the water. A factor which has not yet been employed
in this investigation is the difference in time between the arrival
of two disturbances; one propagated, for instance, through the
earth, and the other, for example, through the ocean. The difference
in the times of the arrival of two waves of this description is a
quantity which is so often recorded that it is well not to pass it
by unnoticed. To the waves mentioned we might also add sound waves,
which so frequently accompany destructive earthquakes, and, in some
localities, as, for instance, in Kameishi, in North Japan, are also
commonly associated with earthquakes of but small intensity. It was by
observing the difference in time between the shaking and the sound in
different localities that Signor Abella was enabled to come to definite
conclusions regarding the origin of the disturbances which affected the
province of Neuva Viscoya in the Philippines, in 1881; the places where
the interval of time was short, or the places where the two phenomena
were almost simultaneous, being, in all probability, nearer to the
origin than when the intervals were comparatively large. I myself
applied the method with considerable success when seeking for the
origin of the Iquique earthquake of 1877. The assumptions made in that
particular instance were, first, that the velocity of the disturbance
through the earth was known, and, secondly, that the velocity with
which a sea wave was propagated was also known.

A method similar to the above was first suggested by Hopkins. It
depended on the differences of velocity with which normal and
transversal waves are propagated.[85]

_Seebach’s method._—To determine the true velocity of an earthquake,
the time of the first shock, and the depth of the centre.

[Illustration: FIG. 32.]

Let the straight line M, _m__{1}, _m__{2}, _m__{3} represent the
surface of the earth shaken by an earthquake. For small earthquakes, to
consider the surface of the earth as a plane will not lead to serious
errors.

If an earthquake originates at C, then to reach the surface at M it
traverses a distance _h_ in the time _t_. To reach the surface at M_{1}
it traverses a distance _h_ + _x__{1} in a time _t__{2}. If _v_ equals
the velocity of propagation,

                          _h_             _h_ + _x__{1}
               then _t_ = ———, _t_{1}_ = —————————————,
                          _v_                 _v_

                               _h_ + _x__{2}
                     _t_{2}_ = —————————————, &c.
                                    _v_

Seebach now says that _if we have given the position of_ M _or
epicentrum of the shock_, and draw through it rectangular axes like M
_m_{3}_ and M T_{3}, and lay down on M _m_{3}_ in miles the distances
from M of the various stations which have been shaken, and in equal
divisions for minutes lay down on M T_{3} the differences of time at
which M, _m_{1}_, _m_{2}_, &c. were shaken, then M_{1} T_{1}, M_{2}
T_{2}, &c. are the co-ordinates of points on an hyperbola. The degree
of exactness with which this hyperbola is in any given case constructed
is a check upon the accuracy of the time observations and the position
of the _epicentrum_. The apex of the hyperbola is the _epicentrum_.

The intersection of the asymptote with the ordinate axis is the time
point of the first shock, which, because the scale for time and
for space were taken as equal, gives the absolute position of the
_centrum_. This intersection is shown by dotted lines. Knowing the
position of the _centrum_, we can directly read from our diagram how
far the disturbance has been propagated in a given time.




                              CHAPTER XI.

                  THE DEPTH OF AN EARTHQUAKE CENTRUM.

  The depth of an earthquake centrum—Greatest possible depth of an
    earthquake—Form of the focal cavity.


_Depth of centrum._—The first calculations of the depth at which an
earthquake originated were those made by Mallet for the Neapolitan
earthquake of 1857. These were made on the assumption that the earth
wave radiated in straight lines from the origin, and, therefore, at
points at different distances from the _epicentrum_ it had different
angles of emergence. These angles of emergence were chiefly calculated
from the inclination of fissures produced in certain buildings, which
were assumed to be at right angles to the direction of the normal
motion. If we have determined the _epicentrum_ of an earthquake and
the muzoseismal circle, and make either the assumption that the angle
of emergence in this circle has been 45° or 54° 44′ 9″ (see page
54), it is evidently an easy matter by geometrical construction to
determine the depth of the _centrum_. Höfer followed this method when
investigating the earthquake of Belluno.

Other methods of calculation which have been employed are based on
time observations, as, for instance, the method of Seebach, the method
of co-ordinates, the method of hyperboloids or spheres (see pages
200–212).

By means of a number of lines parallel to twenty-six angles of
emergence, drawn in towards the seismic vertical, Mallet found that
twenty-three of these intersected at a depth of 7⅛ geographical miles.
The maximum depth was 8⅛ geographical miles, and the minimum depth 2¾
geographical miles.

The mean depth was taken at a depth of 5¾ geographical miles where,
within a range of 12,000 feet, eighteen of the wave paths intersected
the seismic vertical.

The point where these wave paths start thickest is at a depth not
greater than three geographical miles, and this is considered to be the
vertical depth of the focal cavity itself.

For the Yokohama earthquake of 1880, from the indications of
seismometers, and by other means, certain angles of emergence were
obtained, leading to the conclusion that the depth of origin of that
earthquake might be between 1½ and 5 miles.

Possibly, perhaps, the earthquake may have originated from a fissure
the vertical dimensions of which was comprised between these depths.

A source of error in a calculation of this description is that the
vertical motions may have been a component of transverse motions or
perhaps due to the slope of surface waves.

The following table of the depths at which certain earthquakes have
originated has been compiled from the writings of several observers.

 +---------------------------------+-----------------------------+
 |                                 |           In feet           |
 +----------------+----------------+---------+---------+---------+
 |                |                | Minimum |   Mean  | Maximum |
 | Rhineland      | 1846 (Schmidt) |         | 127,309 |         |
 | Sillien        | 1858 (Schmidt) |         |  86,173 |         |
 | Middle Germany | 1872 (Seebach) |  47,225 |  58,912 |  70,841 |
 | Herzogenrath   | 1873 (Lasaulx) |  16,553 |  36,516 |  56,477 |
 | Neapolitan     | 1857 (Mallet)  |  16,705 |  34,930 |  49,359 |
 | Yokohama       | 1880 (Milne)   |   7,920 |  17,260 |  26,400 |
 +----------------+----------------+---------+---------+---------+

A table similar to this has been compiled by Lasaulx.[86]

With the exception of the determination for the two last disturbances
these calculations have been made with the assistance of the method
of Seebach, which depends, amongst other things, on the assumptions
of exact time determinations, direct transmission of waves from the
centrum, and a constant velocity of propagation.

Admitting that our observations of time are practically accurate, it
appears that the other assumptions may often lead to errors of such
magnitude that our results may be of but little value.

From what has been said respecting the velocity with which earth
disturbances are propagated, it seems that these velocities may vary
between large limits, being greatest nearest to the origin.

If we refer to Seebach’s method, we shall see that a condition of this
kind would tend to make the differences in time between various places,
as we recede from the _epicentrum_, greater than that required for the
construction of the hyperbola. The curve which is obtained would, in
consequence, have branches steeper than that of the hyperbola, and the
resultant depth, obtained by the intersection of the asymptotes of this
curve with the seismic vertical, indicates an origin which may be much
too great.

Another point worthy of attention, which is common to the method
of Mallet as well as to that of Seebach, is the question whether
the shock radiates directly from the origin, or is propagated from
the origin more or less vertically to the surface, and then spreads
horizontally. We know that earthquakes, both natural and artificial,
may be propagated as undulations on the surface of the ground, and
that the vertical motion of the latter, as testified by the records
of well-constructed instruments, has no practical connection with the
depth from which the disturbance originated.

In cases like these, the direction of cracks in buildings, and other
phenomena usually accredited to a normal radiation, may in reality be
due to changes in inclination of the surface on which the disturbed
objects rested. When our points of observation are at a distance from
the _epicentrum_ of the disturbance which, as compared with the depth
of the same, is not great, calculations or observations based on the
assumption of a direct radiation of the disturbance may possibly lead
to results which are tolerably correct. The calculations of Mallet
for the Neapolitan earthquake appear to have been made under such
conditions.

For smaller earthquakes, and for places at a distance from the seismic
vertical of a destructive earthquake, the results which are deduced
from the observations on shattered buildings, and all observations
based upon the assumption of direct radiation, we must accept with
caution.

Another error which may enter into calculations of this description
is one which has been discussed by Mallet at some length. This is the
effect which the form and the position of the focal cavity may have
upon the transmission of waves.

Should the impulse originate from a point or spherical cavity, then
we might, in a homogeneous medium perhaps, regard the isoseismals as
concentric circles, and expect to find that equal effects had been
produced at equal distances from the _epicentrum_. Should, however,
this cavity be a fissure, it is evident that even in a homogeneous
medium the inclination of the plane of such a cavity will have
considerable effect upon the form of the waves which would radiate from
its two walls.

For example, let it be assumed that the first impulse of an earthquake
is due to the sudden formation of a fissure, rent open from its centre,
and that the waves leave the walls at all points normal to its surface.
Then, as Mallet points out, it is evident that the disturbance will
spread out in ellipsoidal waves, the greatest axis of which will be
perpendicular to the plane of the fissure.

By taking a number of cases of fissures lying in various directions
and drawing the ellipsoidal waves which would result from an elastic
pressure, like that of steam suddenly admitted into such cavities, the
differences in effect which would be simultaneously produced by these
waves on reaching the surface can be readily understood. The following
example of an investigation on this subject will serve as an example to
illustrate the general nature of the many other cases which might be
taken.

[Illustration: FIG. 33.]

Let a disturbance simultaneously originate from all points of the
fissure _f_ _f_. This will spread outwards in ellipsoidal shells to
the surface of the earth _e_ _e_. The major axis of these ellipsoidal
shells will be the direction of greatest effect. In the direction _c_
_d_ the waves will plunge into the earth, and places to the right side
of the fissure will, to use an expression due to Stokes, when speaking
of analogous phenomena connected with sound, be in _earthquake shadow_.
The same expression has been employed, somewhat differently, when
speaking of the effects produced on buildings.

For places, like _s_ and _p_, situated at equal distances from the
seismic vertical, it is evident that the intensity of the shock will be
different, and also its time of arrival. It will also be observed that
the isoseismals will take the form of ovals or distorted ellipses, the
larger or fuller end of which being to the left of the fissure.

Other cases, like those just given, which are discussed by Mallet in
his account of the Neapolitan earthquake, are where the fissure forms
the division between materials of different elasticities. In the hard
and more elastic material the waves will be more crowded, the velocity
of a wave particle will be greater, and the transit will be quicker
than in the less elastic medium.

The result is that the distance of equal effect from the seismic
vertical will be greatest in the direction of the more compressible
material.

Unless these considerations are kept carefully before the mind when
investigating the depth and, we may add, the position and form of the
centrum of an earthquake, serious errors may arise.

_Greatest depth of an earthquake origin._—A curious but instructive
calculation which Mallet made was a determination of the greatest
possible depth at which an earthquake may occur. This calculation is
based upon the idea that the impulsive effect of an earthquake has an
intimate relationship with the height of neighbouring volcanoes, the
column of lava supported on a volcanic cone being a measure of the
internal pressure tending to rupture the adjacent crust of the earth.

Mitchell, in 1700, virtually propounded this idea, when he suggested that
the velocity of propagation of an earthquake was related to the height
of such a column.[87]

Mallet showed that there was probably considerable truth in such a
supposition by appealing to the results of actual observation. The
pressure gauge of the Neapolitan district would be Vesuvius, the height
of which has in round numbers varied between 3,500 to 4,000 feet. One
of the most destructive earthquakes in this district—namely, the one of
1857—projected bodies with an initial velocity of about fifteen feet
per second. The Riobamba earthquake, which projected bodies with an
initial velocity of eighty feet per second, appears to have been the
most violent earthquake, so far as its impulsive effort is concerned,
of which we have any record. It occurred amongst the Andes, where there
are volcanoes from 16,000 to 21,000 feet in height.

Comparing these two earthquakes together, we see that the Riobamba
shock had a destructive power 5·33 times that of the Neapolitan shock,
and we also see that the Riobamba volcanoes were about 5·33 times
higher than Vesuvius. The accordance in these quantities is certainly
interesting, and tends to substantiate the idea that volcanoes are
barometrical-like pressure gauges of a district.

Carrying the argument still further. Mallet says that if the depth of
origin of earthquakes were the same, then the _area of disturbance_
would, for like formations and configuration of surface, be a measure
of the earthquake effort, and also some function of the velocity of the
wave. From this we may generally infer ‘that earthquakes, like that of
Lisbon, which have a _very great area_ of sensible disturbance, have
also a very deep seismal focus, and also the greatest depth of seismal
focus within our planet is probably not greater than that ascertained
for this Neapolitan earthquake, multiplied by the ratio that the
velocity of the Riobamba wave bears to that of its wave, or, what is
the same thing, by the ratio of the altitudes of the volcanoes of the
Andes to that of Vesuvius.’

Now, as the depth of the Neapolitan shock may be taken at 34,930
feet, the greatest probable depth of origin of any earthquake impulse
occurring in our planet is limited to 5·333 × 4,930 feet, or 30·64
geographical miles.

Ingenious as this argument is, we can hardly admit it without certain
qualifications.

First, we are called upon to admit the identity of the originating
cause of the volcano and the earthquake—as to what may be the
originating cause of earthquakes we have yet to refer, but certainly in
the case of particular earthquakes, as, for instance, those which occur
in countries like Scotland, Scandinavia, and portions of Siberia, the
direct connection between these phenomena are not at first sight very
apparent.

Secondly, even if we admit the identity of the origin of these
phenomena, it is not difficult to imagine that the fluid pressure
brought to bear upon certain portions of the crust of the earth may
possibly in many instances be infinitely greater than that indicated
by the height of the column of liquid lava in the throat of a volcano,
the true height of which we are unable to obtain. Further, in certain
instances such a column only appears to be a measure of the pressure
upon the crust of the earth in the immediate vicinity of the cone.

Thus, in the Sandwich Islands, we have lava standing in the throat
of the volcano of Mauna Loa 10,000 feet higher than it stands in the
crater Kilauea, only twenty miles distant. That these columns should be
measures of the same pressure, originating in a general subterranean
liquid layer with which they are connected, is a supposition difficult
to satisfactorily substantiate.

Another measure of the impulsive efforts which subterranean forces
may exert upon the crust above them is evidently the height to which
volcanoes eject materials. Cotopaxi is said to have hurled a 200-ton
block of stone nine miles. Sir W. Hamilton tells us that in 1779
Vesuvius shot up a column of ashes 10,000 feet in height; and Judd
tells us that this same mountain in 1872 threw up vapours and rock
fragments to the enormous height of 20,000 feet. This would indicate an
initial velocity of 1,131 feet per second.

Notwithstanding Mallet’s calculation that thirty miles is the limiting
depth for the origin of an earthquake, the origin of the Owen’s
Valley earthquake of March 1872 was estimated as being at least fifty
miles.[88]

_Form of the focal cavity._—Among the various problems which are put
before those who study the physics of the interior of our earth it
would at first sight appear that there was none more difficult than the
attempt to determine the form of the cavity, if it be a cavity, from
which an earthquake originates. Almost all investigators of seismology
have recognised that the birthplace of an earthquake is not a point,
and have made suggestions about its general nature. The ordinary
supposition is that the earthquake originates from a fissure, and if
the focus of a disturbance could be laid bare to us it would have the
appearance of a fault such as we so often see exposed on the faces of
cliffs.

A strong argument, tending to demonstrate that some of the shakings
which are felt in Japan are due to the production of such fissures,
is the fact that the vibrations which are recorded are transverse to
a line joining the point of observation and the district from which,
by time observations, we know the shock to have originated. The most
probable explanation of this phenomena appears to be that one mass of
rock has been sliding across another mass, giving rise to shearing
strains, and producing waves of distortion.

The first seismologist who attacked the problem of finding out the
dimensions and position of such a fissure was Mallet, when working on
the Neapolitan earthquake of 1857. The reasons that the origin should,
in the first place, have been a fissure, rather than any other form of
cavity, was that such a supposition seemed to be _a priori_ the most
probable, and, further, that it afforded a better explanation of the
various phenomena which were observed, than that obtained from any
other assumption.

The method on which Mallet worked to determine the form and position of
the assumed fissure, which method was subsequently more or less closely
followed by other investigators, was as follows:—

From an observation of the various phenomena produced upon the surface
of the disturbed area, a map of isoseismals was constructed. These were
seen, as has been the case with many earthquakes, not to distribute
themselves in circles round the _epicentrum_, but as distorted oval or
elliptical figures, the major axes of which roughly coincided with each
other. Further, the _epicentrum_, did not lie in the centre of these
ovals, but was near to the narrow end where they converged.

This at once showed, if the reasoning respecting the manner in which
waves are propagated from an inclined fissure be correct, that the
fissure was at right angles to the major axis of the curves, dipping
from their narrow end downwards, in the direction of their larger
widespread ends.

The next weapon which Mallet employed to attack this problem was the
sound which was heard at different points round about the focus. These
sounds appear to have been of the nature of sudden explosive reports
accompanied by rushing, rolling sounds. The form of the area in which
these sounds were heard was closely similar to that of the first two
isoseismals. Except in the central area of great disturbance, no sound
was heard to accompany the shock.

Those at the northern and southern extremity of the sound area all
described what they heard as a ‘low, grating, heavy, sighing rush, of
twenty to sixty seconds’ duration.’ Those in the middle and towards the
east and west boundaries of this area described a sound of the same
tone, but shorter and more abrupt, and accompanied with more rumbling.

The nature of the arguments which were followed in discussing the sound
observations will be found in the chapter relating to these phenomena.

A portion of the argument which it is difficult to follow relates to
the maximum rate at which it can be supposed possible for a fissure to
be rent in rocks, which rate depends on the density and elasticity of
these rocks and other constant factors.

Next it was observed that the paths of the waves drawn on the surface,
although generally intersecting in a point, did not do so absolutely,
but along a line passing through the main focus some 7½ miles in
length. This, coupled with the observations of sounds, led to the
supposition that the centre of disturbance, considered horizontally,
originated along a curved line passing through the chief focus and the
various intersections of the wave paths.

The last phenomena brought forward to assist in the solution of this
interesting problem were a study of the tremulous movements that
preceded and followed the shock, and their relation to the sound
phenomena.

If the earthquake originated by the formation of a fissure, after the
rending has gone on for a certain time the focal cavity is enlarged
to a certain extent, and the great shock takes place. This would be
followed by concluding tremulous waves. A succession of phenomena like
those accompanied the shock about which Mallet writes.

By observations such as these, coupled with what has been said about
the maximum and mean depths of the focal cavity, Mallet came to the
conclusion that the focal cavity was a fissure, the rending open of
which produced the earthquake. The vertical dimensions of this cavity
were not more than 5·3 miles, but were probably limited to three miles.

From the intersection of the wave paths upon the surface and the
observed emergences, this fissure followed horizontally a curve of
double flexure, about nine geographical miles in length. The area of
this fissure was twenty-seven geographical miles. The time of rending
it open in Apennine limestone would be about 7½ seconds, which
should be the same as the period during which tremors were felt. The
time actually recorded was six or eight seconds.

Briefly, this is, then, the line of reasoning which was followed by
Mallet in an investigation the results of which are as interesting
as they are startling. Since the line of investigation has been
opened, and the existence of new problems has been indicated, other
investigators, although not exactly following Mallet’s method in
all their details, have, when endeavouring to attain the same ends,
employed similar weapons.

Thus, for example, Seebach, when determining the depth and nature of
the origin of the earthquake of Middle Germany, reasoned somewhat as
follows:—

Had the origin been more or less of a spherical cavity, then the region
of most violent disturbance upon the surface would, according to a
theorem we have already mentioned, have been upon or near a circle of
about 8·8 miles in radius round the _epicentrum_. This region, however,
was found by observation to lie along a curved band about forty miles
in length, altogether on one side of the _epicentrum_.

To explain this anomaly Seebach followed Mallet, and assumed that the
origin was not a spherical cavity, but a fissure.

The depth and strike of this fissure was determined by the observation
that the area of greatest disturbance was along a curved line lying
radial to the _epicentrum_. Such a condition it was assumed indicated
that the fissure of origin must be inclined towards this area of
greatest disturbance. A line was then drawn from this area to the
_centrum_. A second line at right angles to this one gave the dip of
the fissure.

Höfer, when working on the earthquake of Belluno, came to the
conclusion that the disturbance originated from two faults meeting
each other at an angle of 60°. In this determination he was chiefly
influenced by the form of a certain homoseist which was of the form of
an elongated ellipse met on one side by a second ellipse, the principal
axes of the two ellipses giving the strike of the two faults.




                             CHAPTER XII.

            DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.

  General distribution of earthquakes—Occurrence along lines—Examples
    of distribution—Italian earthquake of 1873—In Tokio—Extension
    of earthquake boundaries—Seismic energy in relation to
    geological time; to historical time—Relative frequency of
    earthquakes—Synchronism of earthquakes—Secondary earthquakes.


_General distribution of earthquakes._—The records of earthquakes
collected by various seismologists lead us to the conclusion that
at some time or other every country and every ocean in the world
has experienced seismic disturbances. In some countries earthquakes
are felt daily, and from what will be said in the chapter on earth
pulsations it is not unlikely that every large earthquake might with
proper instrumental appliances be recorded at any point on the land
surfaces of our globe. The area over which any given earthquake extends
is indeterminate. The area over which an earthquake is sensible is
sometimes very great. The Lisbon shock of 1755 is estimated as having
been sensible over an area of 3,300 miles long and 2,700 miles wide,
but in the form of tremors and pulsations it may have shaken the whole
globe.

The regions in which earthquakes are frequent are indicated in the
accompanying map, which, to a great extent, is a reproduction of a map
drawn by Mallet. The regions coloured with the darkest tint are those
where great earthquakes are the most frequent. The actual number of
earthquakes which have been felt in the differently coloured areas are
given, when speaking of the relation of seismic energy to season.

When looking at this chart it must be remembered that if we were
to make a detailed map of any one of the different countries where
earthquakes are frequent, we should find in it all the differences that
we observe in the general chart. For instance, one portion of Japan,
where perhaps sixty shocks are felt per year, would be coloured with
a dark tint, whilst other portions of the same country, where there
is only one slight shaking felt every few years, would be left almost
uncoloured. The black dots indicating the position of volcanic vents
are even more general in their signification than the tinted areas.
Professor Haughton gives for the world a list of 407 volcanoes, 225 of
which are active. These numbers are the same as those given by A. von
Humboldt. Of the active volcanoes 172 are on the margin of the Pacific,
and of the total number eight are in Japan. From my own observations
in Japan independently of the Kurile Islands, I have enumerated
fifty-three volcanoes which are either active or have been active
within a recent period. In a few years’ time this list will probably be
increased. I mention this fact to show how very imperfect our knowledge
is respecting the number of volcanic vents existing on our globe. If we
were in a position to indicate the volcanoes which had been in eruption
during the last 4,000 years, the probability is that they would number
several thousands rather than four or five hundred.

An inspection of the map shows that earthquakes chiefly occur in
volcanic and mountainous regions. The most earthquake-shaken regions of
the world form the boundaries of the Pacific ocean. It may be remarked
that these boundaries slope beneath the neighbouring ocean at a much
steeper angle than the boundaries of countries where earthquakes occur
but seldom. The coasts of South America, Kamschatka, the Kuriles,
Japan, and the Sandwich Islands, for example, have slopes beneath the
Pacific from one in twenty to one in thirty. The coasts of Australia,
Scandinavia, and the eastern parts of South America, where earthquakes
are practically unknown, have slopes from one in fifty to one in two
hundred and fifty. Many earthquakes have taken place in mid-ocean. In
the Atlantic Ocean M. Perrey has given about 140 instances of such
occurrences.

The majority of the earthquakes which shake Japan appear to have their
origin in the neighbouring ocean. If we could draw a map of earthquake
origins, it is probable that the greater number of the marks indicating
these origins would be found to be suboceanic and along lines parallel
to the shores of continents and islands which rise steeply from the
bed of deep oceans. In countries like Switzerland and India, our marks
would hold a relationship to the mountains of these countries.[89]
Looking at the broad features of the globe, we see on its surface
many vast depressions. Some of these saucer-like hollows form land
surfaces, as in central Asia. The majority of these, however, are
occupied by the oceans. Active volcanoes chiefly occur near the rim of
the hollows which have the steepest slopes. The majority of earthquakes
probably have their origin on or near the bottom of these slopes. To
these, however, there are exceptions, as for instance the earthquakes
in the Alps, in the hills of Scotland, and the shakings which are
occasionally felt in countries like Egypt. The earthquakes which shake
the borders of the Pacific have their origins in, and their effects are
almost exclusively felt on, the sides of the bounding ridge facing this
ocean. In Japan it is the eastern sides of the islands which suffer,
the western side being almost as free from these convulsions as England.

Similar remarks may be made about the eastern side of South America,
especially the southern portion of the continent. At Buenos Ayres, for
example, there has been no disturbance since Mendoza was destroyed,
some twenty years ago. In British Guiana slight shocks are occasionally
felt in the low delta which forms the settled portion of the colony,
but they are extremely rare.

_Disturbances in lines or zones._—It has often been observed that
disturbances are propagated along the length of mountains or valleys,
and it is but seldom that earthquakes cross them transversely. Thus the
valleys of the Rhone, the Rhine, and the Danube are lines along which
disturbances travel.

The major axes of the elliptical areas of disturbances which have
shaken India have a general direction parallel to the valley of the
Ganges along the flanks of the Himalayas.

The disturbances which have shaken London appear to have been chiefly
east and west, or along the valley of the Thames. In South America the
line of disturbance is along the western sides of the Andes. Another
line is along the northern coast of the continent through Andalusia
and Caraccas towards the Antilles and Trinidad. The shocks of the
Pyrenees are chiefly felt along the southern side of these mountains.
In the middle and on the northern side they are but seldom felt. This
propagation in lines or zones may in certain cases be apparent rather
than real. Thus the north and south ranges of mountains in Japan are
mountains almost simultaneously shaken along their eastern flanks,
giving the impression that an earthquake had originated simultaneously
from a fissure parallel to this line, or else, starting at one end,
had run down their lengths. Time observations have, however, shown
that such disturbances had their origin at some distance in the ocean,
and, travelling inwards, had reached all points on the flanks of these
mountains almost simultaneously. The same explanation will probably
hold for the so-called linear disturbances of western South America.

All earthquake disturbances have probably a tendency to radiate from
their source, and are only prevented from doing so by meeting with
heavy mountainous districts, which by their mass and structure absorb
the energy communicated to them. Much energy is also lost by emergence
on the open flanks of a range of mountains. Rather than say that
high mountains often bound the extension of an earthquake, or that
earthquakes appear to run along the flanks of such mountains, we might
say that earthquakes have boundaries parallel to the strike of the
rocks in a given district, that such a direction is the one in which
the propagation is the easier.

Rossi is of opinion that volcanic fractures play an important part in
governing the distribution of seismic disturbances. When a volcano is
formed, a series of starlike fractures are formed round the central
crater. Secondary craters may indicate the line of these fissures. The
mountains about Rome are regarded as typical of this radial structure.
The more distant the secondary craters are from the centre of the
system, the smaller will they be, and also the younger. If two fissures
intersect we get a larger crater at the junction. Earthquakes are
propagated along the direction of these fissures, whilst the rising
and falling of these lips throw off transverse waves. Rossi adduces
observations which appear to meet with explanation on such suppositions.

Suess, who has written upon the earthquakes of lower Austria, shows how
the majority of the disturbances have had their origin along certain
lines which form a break in the continuity of the Alps. One line runs
north-east from Bruck towards Vienna. Near Wiener Neustadt, where the
greatest number and heaviest shocks have occurred, this line is met by
a north-north-west line crossing the Danube and following the valley
of the river Kamp.[90] Hoeffer has drawn similar lines from the head
of the Adriatic, one set running north-north-east to intersect near
Litschau, and the other north-north-west to intersect near Frankfort in
the valley of the Rhine.[91]

_Examples of distribution._—A curious example of the distribution of
seismic movement is that of the earthquake of March 12, 1873, worked
out by Professor P. A. Serpieri. This earthquake appears to have been
simultaneously felt on the Dalmatian coast and in central Italy, in a
region lying north-east from Rome and south-east from Florence. In both
of these areas the motion was from south-east to north-west. The shock
then radiated from the central Italian regions, so that at places on
the western shore of the Adriatic it was felt after it had been felt on
the Dalmatian coast.

Many explanations might be offered for this peculiar distribution
of seismic activity. Possibly the shock originated at a great depth
beneath the bed of the southern part of the Adriatic, and by following
existing lines of weakness simultaneously reached the surface of the
earth in central Italy and Dalmatia.

In Tokio, which is built partly on a flat plain, partly in valleys
denuded from a low tableland, and partly on the spurs of the tableland
itself, the distribution of earthquakes is a subject yet requiring
attention. Sometimes it has happened that persons in one house have
been sufficiently alarmed to escape into the open air, whilst others,
not more than a mile distant, have not been aware that the city had
been shaken.

[Illustration: FIG. 34.
  Areas almost simultaneously struck from S.E. to N.W. [graphic]

  Subsequent radial disturbance [graphic]]

_Extension of earthquake boundaries._—Natural obstructions which may be
sufficient to retard small earthquakes may in certain instances not be
found sufficient to retard the larger disturbances. Thus the shocks of
Calabria are usually only felt on the western side of the Apennines,
but instances have occurred when they have crossed this barrier. In
1801 the earthquake of Cumana crossed a branch of the coast range.

Sometimes earthquake boundaries give way, and countries which they
sheltered subsequently become exposed to all disturbances. The true
explanation of this is probably in a shifting of the centre of seismic
activity. Thus up to December 14, 1797, although Cumana was often
devastated, the peninsula of Araya was not hurt. On this date Araya
commenced to suffer, and has continued to suffer ever since.

Fuchs gives an example of the movement of a seismic centre in the case
of the Calabrian earthquake. The first shock commenced near Oppiedo,
the second shock commenced four or five miles farther to the north, and
the third shock had its origin five or six miles still farther, near to
Girifalco.




                             CHAPTER XIII.

          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).


_Seismic energy in relation to geological time._—If we admit that
seismic energy is only a form of volcanic energy, it must also be
admitted that any cause tending to produce a general decrease in the
amount of the latter will also produce an alteration in the amount of
the former.

The nebular hypothesis of Laplace tells us that the solar system is the
result of the whirling of a heated gaseous mass, which as it cooled
continually contracted and consequently whirls the faster. With this
hypothesis before us, we understand why all the planets and their
satellites have a similarity in the directions of their movements, why
they revolve nearly in the same plane, in orbits nearly circular, why
some have a flattened figure and are surrounded by rings or belts, why
the exterior planets should have a greater velocity of rotation, a
greater number of satellites, and a less density as compared with the
interior planets, the similarity of the elements in meteoric stones,
the sun, the stars, and those found upon our earth, and lastly why
there should be an increase in temperature as we descend into our
earth.[92] This increase in temperature as we descend into the earth
as deduced from many observations appears to be about 1° F. for every
fifty or sixty feet of descent.

To explain this and other kindred phenomena it is assumed that the
earth was once very much hotter than it is at present, and to reach its
present stage it has been gradually cooling. As the laws of cooling are
perfectly known, to calculate how many years it must have taken a body
like our earth to cool down to its present temperature is a definite
problem. Sir William Thomson, starting with the temperature of 7,000°
F., when all the rocks of the earth must have been molten and a skin or
crust upon the surface, such as is so quickly produced upon the surface
of molten lava, finds by calculation that the time taken to reach the
present temperature must have been about one hundred million years.
Into this period he and other physicists desire to compress the history
of all the stratified deposits. Geologists find this period too short.
Others seeking to reconcile the views of physicists and geologists
endeavour to show that the various agencies engaged in degrading rocks
and accumulating sediments in former ages are not to be judged of by
the agencies we now see around us; in former times they were more
active. At one period the elastic tides in the earth may have been so
great that they resulted in the fracturing off from our planet its
satellite the moon, and subsequently the moon, acting on the waters of
the earth, may, even as late as 150,000 years ago, have produced every
three hours tides 150 feet in height.

Whatever may be the value of the figures here quoted, reasonings like
these bring us to the conclusions that the various agencies which we
now know to be acting upon our earth were once far more potent than
they are at present, and if the moon, as a producer of elastic tides,
has any influence upon the occurrence of earthquakes, it must have had
a much greater influence in bygone times.

We might speak similarly with regard to the internal heat of the earth.

From the present heat gradient of our globe it is possible to calculate
how much heat flows from the earth every year.

This is equivalent to a quantity which would raise a layer of water ·67
centimetres thick, covering the whole of our globe, from a temperature
of 0° to 100° C.

Similarly, we might calculate the quantity of heat which would be lost
when the average heat gradient, instead of being 1° F. for fifty feet
of descent, was 1° F. for twenty-five feet of descent.

We might also calculate how many years ago it was since such a gradient
existed.

The general result which we should arrive at would be that in past
ages the loss of heat was more rapid than it is at present. Now the
contraction of a body as it cools is for low temperatures proportional
to its loss of heat, and this law is also probably true for contraction
as it takes place from high temperatures.

Contraction being more rapid, it is probable that phenomena like
elevations and depressions would be more rapid than they are at
present, and generally all changes due to plutonic action, as has
already been pointed out by Sir William Thomson, must have been more
active.

We have, therefore, every reason to imagine that earthquakes which
belong to the category of phenomena here referred to were also numerous
and occurred on a grander scale during the earlier stages of the
world’s history than they do at present, and seismic and volcanic
energy, when considered in reference to long periods of time, is
probably a decreasing energy.

In making this statement we must not overlook the fact that in
geological time, as testified by the records of our rocks, volcanic
action, and with it probably seismic action, has been continually
shifting, first appearing in one area and then in another, and even
in the same area we have evidence to show that these have periods
of activity and repose successively succeeding each other. Thus in
Britain, during the Palæozoic times, we have many evidences of an
intense volcanic activity. During the Mesozoic or Secondary period
volcanic energy appears to have subsided, to wake up with renewed
vigour in the Cainozoic or Tertiary period.

During this latter period it is not at all improbable that Scotland
was in past times as remarkable for its earthquakes as Japan is at the
present day.

Later on it will also be shown that earthquakes are concomitant
phenomena, with those elevatory processes which we have reason to
believe are slowly going on in certain portions of the earth’s crust.
If, therefore, we are able by the examination of the rocks which
constitute the accessible portions of our globe to determine which
periods were characterised by elevation, we may assume that such
periods were also periods of seismic activity.

Amongst these periods we have those in which various mountain ranges
appeared. Thus the Grampians, and the mountains of Scandinavia, were
probably produced before the deposition of the Old Red sandstone. The
Urals were upheaved prior to Permian times. The chief upheaval in the
Alps took place after Eocene times. The Rigi and other sub-Alpine
mountains were formed after the deposition of the Miocene beds. About
this same time the Himalayas were upheaved.[93]

The earthquakes which from time to time shake those newer mountains
apparently indicate that the process of mountain-making is hardly ended.

_Seismic energy in relation to historical time._—The distribution
of seismic energy with regard to historical time is a subject which
has been very carefully examined by Mallet, who collected together
a catalogue of between six and seven thousand earthquakes, embraced
between the periods B.C. 1606 and A.D. 1850. The earthquake of B.C.
1606 was on the occasion of the delivery of the law at Mount Sinai.
Between B.C. 1604 and B.C. 1586 an earthquake probably occurred in
Arabia, when Korah, Dathan, and Abiram were swallowed up. Another
biblical record is that of B.C. 1566, when the walls of Jericho were
overthrown.

The earliest records from China is in B.C. 595; in Japan B.C. 285; in
India A.D. 894.

By using the number of earthquakes which have been recorded in each
century as ordinates, Mallet constructed a curve, which apparently
shows a continual increase in seismic energy, especially during recent
times. This, Mallet remarks, contradicts all the analogies of the
physics of the globe, and points out that the rapid increase in the
number of earthquakes in latter years is chiefly due to the greater
number of records which have been made, and the increase of the area
of observation. No doubt many of the records made by the ancients have
been lost.

If we compare Mallet’s records, as he invites us to do, with the
great outlines of human progress, we see that the two increase
simultaneously, and we come to the conclusion that, taken as a whole,
during the historical period the seismic activity of the world has been
tolerably constant.

These conclusions, based on the evidence at our command, are not to
be confuted. If, however, instead of considering the seismic energy of
the whole world, we consider the seismic energy of particular areas,
it seems reasonable to expect that in certain instances sometimes a
decrease and sometimes an increase in this energy might be discovered,
especially, perhaps, in areas which are highly volcanic.

In France we know that volcanic activity ceased at a period closely
bordering on historical times, and it is not unlikely that seismic
activity may have ceased at a corresponding time.

In a country like Japan, it is possible that in one district seismic
energy may be on the increase, whilst in another upon the decrease.

In a country like England, it is probable that the seismic state is
constant, and, whatever changes may be now occurring, they are taking
place at so slow a rate that, even if our records of the historical
period were complete, we could hardly be expected to find these changes
sufficiently marked to be observable.

For purposes of reference, and also for examining the present question,
the table, page 240, has been compiled. The earthquakes given are
chiefly those which have been recorded in histories as being more or
less destructive.

In the second column of this table will be seen the number of
earthquakes which have occurred in Japan during each century, the
centuries being marked in the first column. In columns 3 to 18
inclusive are given the number of earthquakes which have occurred
during different centuries in the various countries and districts
mentioned at the head of each column. These latter, which are taken
from the writings of Mallet, are given for the sake of comparison with
the Japanese earthquakes. If we commence with the seventh century in
the column for Japan, we see that a great increase in the number of
earthquakes, as we come towards the present time, is not so observable
as it is in the other columns.

 Key:
    1 Centuries
    2 Japan
    3 Scandinavia and Iceland
    4 British Isles and Northern Isles
    5 Spanish Peninsula
    6 France, Belgium, Holland
    7 Rhine Basin
    8 Switzerland and Rhine Basin
    9 Danube Basin
   10 Italy, Sicily, Sardinia, and Malta
   11 Supplemental table for Italy, Sardinia, and Malta
   12 Turco-Hellenic Territory, Syria, Ægean Isles, and Levant
   13 United States and Canada
   14 Mexico and Central America
   15 Antilles
   16 Cuba
   17 Chili and La Plata Basin
   18 Northern Zone of Asia
   19 Approximate Intensity in the Kioto District of Japan
 +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+
 |   1  |2 | 3 | 4 |5 | 6 |7 | 8 | 9 | 10|11| 12|13|14| 15|16| 17|18|19|
 +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+
 |I.    | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
 |II.   |--| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
 |III.  | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--|
 |IV.   |--| --| --|--| --|--| --| --|  6|--| 23|--|--| --|--| --|--|--|
 |V.    | 1| --| --|--|  1|--| --| --|  5|--| 19|--|--| --|--| --|--|--|
 |VI.   | 1| --| --|--|  6|--| --|}  |  3|--| 27|--|--| --|--| --|--|--|
 |VII.  |12| --| --|--| --|--| --|}  |  1|--|  8|--|--| --|--| --|--|15|
 |VIII. |11| --| --|--| --|--| --|}  |  2| 1| 12|--|--| --|--| --|--|17|
 |IX.   |40| --| --|--| 21|--| 19|}  |  6|--|  7|--|--| --|--| --|--|60|
 |X.    |17| --| --|--|  2|--|  2|}19|  3| 3|  5|--|--| --|--| --|--|24|
 |XI.   |20| --|  8| 3| 16|--|  9|}  |  7| 5| 18|--|--| --|--| --|--|28|
 |XII.  |18| --| 11| 4| 12|--|  8|}  | 18|22| 23|--|--| --|--| --|--|20|
 |XIII. |16|}  | 15| 3|  9|--|  3|}  | 15|26| 13|--|--| --|--| --|--|16|
 |XIV.  |19|}  |  4| 8| 21|--| 18|}  | 20|51|  8|--|--| --|--| --|--|25|
 |XV.   |36|}28|  1| 4| 14|--| 12|}  | 18|47| 11|--|--| --|--| --|--|29|
 |XVI.  |17|}  |  8|10| 61|10| 52| 35| 32| 5| 22|--| 6|  1| 4|  5|--|17|
 |XVII. |26|}  | 14|10| 91|29|120| 31|121| 9| 53|10| 7| 16| 4|  9|--|11|
 |XVIII.|31|111| 63|93|237|71|141| 88|438|20|124|88|24| 85| 2| 10|32| 8|
 |XIX.  |27|113|110|85|211|81|173|145|390|88|194|51|30|145|50|170|57| 8|
 +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+

The explanation for this probably lies in the fact that Japan has
practised civilised arts for a longer period than many of the European
and other countries mentioned in the table.

In Japan, no doubt, the records of later years have been more perfect
than they were in early times, but this, although so potent an effacer
of what was probably the true state of natural phenomena in the case of
Europe, has not quite obliterated the truth in Japan; for instead of an
apparent increase of seismic energy since early times it shows a slight
decrease.

To draw up a table of earthquakes such as the one which has just been
given, and then, after the inspection of it, draw conclusions as to
whether there has been an increase or decrease in seismic energy, is,
however, hardly a just method of reasoning. The earthquakes, taken as
they are, for the whole of Japan, represent a collection of places some
of which are 1,000 miles apart. When we consider that many earthquakes
which occurred at one end of this line were never felt at the other
end, in order to justly estimate the periodicity of seismic phenomena
it would seem that we ought either to take some particular seismic area
or else the whole world.

The particular area which has been taken is that of Kioto in Central
Japan, and the earthquakes which have been felt there are enumerated in
the table.

In order to show the variation in seismic activity of this district a
curve has been plotted, fig. 35, with ordinates equal to the values
given for the Kioto earthquakes during succeeding centuries. The upper
points of these ascending and descending lines are joined by a free
curve. The lower points are similarly joined. The points of bisection
of ordinates drawn between these two curves are taken as points in a
curve to show the true secular change in seismic energy.

[Illustration: FIG. 35.—Curve of Seismic Intensity for Kioto.]

By looking at this wavy line it will be seen that the intervals between
maxima and minima are closer together in early times than they are
later on.

Thus, between the eighth century and the ninth century, points of
maximum and minimum seismic efforts occurred at times a century apart,
whilst later on, from the eleventh to the fifteenth century, they were
at intervals of 300 years apart.

By inspecting either the wavy line or the resultant curve, it will be
seen that since the ninth century down to the present time there has
been a decided decrease in seismic energy. From the ninth century down
to the fifteenth century this decrease is represented by a regular
curve. At this point, however, the decrease becomes slightly more
rapid, and is represented by a second curve. If, instead of calculating
ordinates for my curve, in which intensity has been considered, simply
the number of earthquakes are counted, a similar result is obtained.
From this it appears that the rate at which seismic energy decreased
during the last 500 years was about the same as that at which it
decreased during the 500 years previous to this period.

If the lists for the Italian and Turco-Hellenic districts could be
similarly analysed, and the earthquakes of any particular district
picked out from the others, it is very probable that a similar decrease
or alteration in seismic energy might be observed.

Provided that we have at our disposal records of the various
earthquakes which have occurred in any given district during a
sufficiently long period of time, one conclusion that we may expect
to arrive at is that we shall be able to trace some variation in
the seismic activity of that district. For the Kioto area, it has
been shown that there is a diminution in seismic activity, In other
districts, however, there may possibly be an increase.[94]

_Relative frequency of earthquakes._—A question which is of great
interest to those who dwell in shaken districts is as to how often
disturbances may be expected to occur.

From a general examination of this question, considering the
earthquakes of the whole world. Mallet arrived at the following
conclusions:—

1. While the smallest or minimum paroxysmal intervals may be a year
or two, the average interval is from five to ten years of comparative
repose.

2. The shorter intervals are in connection with periods of fewer
earthquakes—not always with those of least intensity, but usually so.

3. The alternations of paroxysm and of repose appear to follow no
absolute law deducible from these curves.

4. Two marked periods of extreme paroxysm are observable in each
century, one greater than the other—that of greatest number and
intensity occurring about the middle of each century, the other towards
the end of each.

The form of the curves which Mallet has drawn seem to indicate that
seismic energy came in sudden bursts, and then subsided, gradually
gathering strength for another exhibition. This is continually seen
in the shocks experienced in various seismic areas—a large shock, or
the maximum of the activity dying out by repeated small shocks on
succeeding days.

Mr. I. Hattori, writing on the large earthquakes of Japan, remarks that
on the average there has been one large earthquake every ten years.
They, however, occur in groups, as shown in the following table.

 +------+---------------+----------+
 |No. of|               |          |
 |shocks|    Period     | Interval |
 +------+---------------+----------+
 |  6   | A.D. 827–836  | 10 years |
 |  6   |  „   880–890  | 10   „   |
 |  4   |  „  1040–1043 |  4   „   |
 |  5   |  „  1493–1507 |  5   „   |
 |  4   |  „  1510–1513 |  4   „   |
 |  5   |  „  1645–1650 |  6   „   |
 |  5   |  „  1662–1664 |  3   „   |
 |  4   |  „  1853–1856 |  4   „   |
 +------+---------------+----------+

Dr. E. Naumann, who has also written on the earthquakes in Japan,
remarks that if periods of seismic activity do not occur every 490
years, there is a repetition of the cycle after 980 years, but there
is much variability. A period of 68 years is very marked. On the
average, large earthquakes have occurred every 5·9 years. Fuchs
gives some interesting examples of the repetition of earthquakes at
definite intervals, of which the following are examples. Sometimes
earthquakes appear to have repeated themselves after 100 years. One
remarkable example of this is that of Lima, on June 17, 1578, which was
repeated on the same day in the year 1678. In Copiapo it is believed
that earthquakes occur every twenty-three years, and examples of such
repetitions are found in the years 1773, 1796, and 1819. In Canada,
near to Quebec, earthquakes lasting forty days are said to occur every
twenty-five years. The plateau of Ardebil is said to be regularly
shaken by earthquakes every two years.

A. Caldcleugh, writing on the earthquake of Chili, in 1835,[95] remarks
that the Spaniards first had the idea that a great earthquake occurs
every century. Afterwards they thought the period was every fifty
years. As a matter of fact, however, there were large earthquakes in
1812, at Caracas; in 1818, at Copiapo; in 1822, at Santiago; in 1827,
at Bogota; in 1828, at Lima; in 1829, at Santiago; and in 1832, at
Huasco.

The average period of seismic disturbances in any country probably
depends upon the subterranean volcanic activity of that country.
When the activity is great the large earthquakes may occur at short
intervals; but when the activity is small, as in England, shocks of
moderate intensity may not be felt more than once or twice per century.
A general idea of the relative frequency of the large earthquakes in
various parts of the world may be easily obtained by an inspection of
the table on page 240.

Between the years 1850 and 1857 Kluge found that in the world there had
been 4,620 earthquakes, which is, upon the average, nearly two per day.
This estimate of the frequency of earthquakes of sufficient intensity
to be recorded without the aid of instruments is, however, much below
the truth. In Japan alone there probably occurs, as a daily average, a
number at least equal to that which has been just given for the whole
world. Boussingault considered that, in the Andes, earthquakes were
occurring every instant of time.[96]

To state definitely how many earthquakes are felt in the world on the
average every day is, from the data which we have at our command, an
impossibility. Perhaps there may be ten, perhaps there may be 100. The
question is one which remains to be decided by statistics which have
yet to be compiled.

After a large earthquake, smaller shocks usually occur at short
intervals. At first the succession of disturbances are separated from
each other by perhaps only a few minutes or hours. Later on, the
intensity of these shocks usually decreases, and the intervals between
them become greater and greater, until, finally, after perhaps a few
months, the seismic activity of the area assumes a quiescent state.

The great earthquake which overtook Concepcion on February 20, 1835,
was followed by a succession of shocks like those just referred to,
there being registered, between the date of the destructive shock and
March 4, 300 smaller disturbances.

During the twenty-four hours succeeding the destruction of Lima
(October 28, 1746), 200 shocks were counted, and up to the 24th of
February in the following year 451 shocks were felt.

At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter
hours.

Similar examples might be taken from the description of almost
all destructive earthquakes of which we have records. For a large
earthquake to occur, and not to be accompanied by a train of succeeding
earthquakes, is exceptional. Sometimes we find that a large number of
small earthquakes have occurred without a large one being felt. Seismic
storms of this description have happened, even in England—for instance,
in the year 1750, which appears to have been a year of earthquakes for
many portions of the globe.

In this year, which is known as the ‘earthquake year,’ shocks were felt
in England as follows: On March 14, in Surrey; March 18, in south-west
of England; April 2, at Chester; June 7, at Norwich; August 23, in
Lincolnshire; September 30, Northamptonshire.

_Synchronism of earthquakes._—One of the first writers who drew
attention to the fact that two shocks of earthquakes have been felt
simultaneously at distant places was David Milne, who published a list
of these occurrences.[97]

In two instances, February and March 1750, shocks were simultaneously
felt in England and Italy. In September 1833 shocks appear to have been
simultaneously felt in England and Peru. These and many other similar
examples are discussed by Mallet, who thinks with Milne that these
coincidences are in every probability matters of accident. According to
Fuchs, Calabria and Sicily appear often to have had earthquakes at the
same time, as for instance in 1169, 1535, 1638, when the town Euphemia
sank, and in the years 1770, 1776, 1780, and 1783.

A remarkable example of coincidence occurred on November 16, 1827, when
a terrible earthquake was felt in Columbia, and at the same time a
shock occurred on the Ochotsk plains, nearly antipodal to each other.

Kluge also gives a large number of instances of simultaneous
earthquakes; thus, on January 23, 1855, on the same day that
Wellington, New Zealand, so severely suffered, there was a heavy
earthquake in the Siebengeberge, and also in North America. To this
might be added the fact that the last destructive earthquake in Japan
occurred within a few days of this time.

Sometimes neighbouring countries where earthquakes are common are
equally remarkable by their utter want of synchronism. For example,
Southern Italy and Syria are said never to be shaken simultaneously.

_Secondary earthquakes._—Although it is possible that the simultaneous
occurrence of earthquakes in distant regions may sometimes be a
matter of chance, it must also be remarked that the shaking produced
by one earthquake may be sufficient to cause ground which is in a
critical state to give way, and thus the first disturbance becomes the
originator of a second earthquake. Admitting that an earthquake, as
it radiates from its centre, may act in such a manner, we see that a
feeble disturbance might be the ultimate cause in the production of a
destructive earthquake, just as the disturbance of a stone upon the
face of a scarp might, by its impact upon other stones, cause many tons
of material to be dislodged.

It is also easy to conceive how the seismic activity of two districts
may be dependent upon each other. Inasmuch as these secondary shocks
are direct effects of primary disturbances, they might have been
treated in a previous chapter.

As examples of consequent or secondary earthquakes Fuchs tells us that
when small earthquakes take place in Constantinople and Asia Minor,
earthquakes are felt in Bukharest, Galazy, and Kronstadt.

The great Lisbon earthquake also appears to have given rise to several
consequent disturbances. One was in Derbyshire, occurring at 11 a.m.
It was sufficiently violent to cause plaster to fall from the sides
of a room and a chasm to open on the surface of the ground. Some
miners working underground were so alarmed that they endeavoured to
escape to the surface. During twenty minutes there were three distinct
disturbances.

Another shock was felt at Cork.[98]

Although these disturbances own a consequence of the Lisbon earthquake
they might properly perhaps be attributed to the pulsations produced by
the shock at Lisbon, which spread through England and other countries
without being felt.

The shocks which men felt in New Zealand and New South Wales in 1868
were probably secondary shocks, due to the disturbance at Arequipa and
other places on the South American coast.

These so-called secondary earthquakes, although in many instances
they may be due to earth pulsations produced by earthquake, or to the
immediate sensible shaking of a large earthquake, may perhaps, in
certain instances, be attributed to some widespread disturbance beneath
the crust of the earth. The occurrence of periods where all earthquake
countries suffer, unusual disturbances indicate the probability of such
underground phenomena.




                             CHAPTER XIV.

          DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_).

  The occurrence of earthquakes in relation to the position of the
    heavenly bodies—Earthquakes and the moon—Earthquakes and the sun;
    and the seasons; the months—Planets and meteors—Hours at which
    earthquakes are frequent—Earthquakes and sun spots—Earthquakes
    and the aurora.


_The position of the heavenly bodies and the occurrence of
earthquakes._—Since the earliest times, in searching for the cause of
various natural phenomena, man has turned his energies towards the
heavens. One of the earliest observations was the connection that
exists between the season of the year and the motions of the heavenly
bodies. Tides were seen to be influenced by the moon. In later times
it has been discovered that periods of maximum magnetic disturbances
occur every ten or eleven years with the sun spots, and Herr Kreil, of
Vienna, tells us our satellite, the moon, has also an influence upon
the magnet.

From day to day we see the bond connecting our planet with the sun, the
moon, and other heavenly bodies which are outside us gradually becoming
closer.

Inasmuch as many phenomena, like the motion of the tides, the rise and
fall of the barometer, fluctuations in temperature, are all more or
less directly connected with the relative position of our planet with
regard to the sun and moon, any coincidence between the phases of
these bodies and the occurrence of earthquakes more or less involves a
time relationship with the other phenomena resultant on lunar and solar
influences.

_Earthquakes and the position of the moon._—Many earthquake
investigators have attempted to show the connection between earthquakes
and the phases of the moon.

The first and most successful worker in this branch of seismology was
Professor Alexis Perrey, of Dijon, who, after many years of arduous
labour in tabulating and examining catalogues of earthquakes, showed
that earthquakes were more likely to occur at the following periods
than at others.

1. They are more frequent at new or full moon (syzygies) than at half
moon (quadratures).

2. They are more frequent when the moon is nearest the earth (perigee)
than when she is farthest off (apogee).

3. They are more frequent when the moon is on the meridian than when
she is on the horizon.

These results were obtained by Perrey after analysing his catalogues
by three different and independent methods, and they were confirmed
by the report of a committee appointed by the Academy of Sciences. It
must, however, be remarked that in several instances anomalies occur,
and also that the difference between the number of earthquakes at any
two periods is not a very large one. Thus, for instance, the annual
catalogues compiled by Perrey from 1844 to 1847, the earthquakes in
perigee are to those in apogee as 47 : 39. Between the years 1843 and
1872 Perrey finds that 3,290 shocks occurred at the moon’s perigee, and
3,015 at the apogee.[99]

Between 1761 and 1800 earthquakes occurred as follows:—

  In Perigee       526
  Apogee           465

The following table shows the results which enabled Perrey to deduce
his first law.

Dividing the period of lunation into quarters, with the time of
syzygies and quadratures as the centres of these quarters, he found
that the earthquakes were distributed as follows.

 +-----------+--------+----------+-------------+---------------+
 |           |        |          |             | Difference in |
 |           | Totals | Syzygies | Quadratures | favour of the |
 |           |        |          |             |   Syzygies    |
 +-----------+--------+----------+-------------+---------------+
 | 1843–1847 |  1,604 |   850·48 |    753·52   |     69·96     |
 | 1848–1852 |  2,049 | 1,053·53 |    995·47   |     58·06     |
 | 1853–1857 |  3,018 | 1,534·13 |  1,483·87   |     50·26     |
 | 1858–1862 |  3,140 | 1,602·99 |  1,537·41   |     65·98     |
 | 1863–1867 |  2,845 | 1,463·42 |  1,381·58   |     81·84     |
 | 1868–1872 |  4,593 | 2,333·48 |  2,259·52   |     73·96     |
 | 1843–1872 | 17,249 | 8,838·03 |  8,410·97   |    427·06     |
 +-----------+--------+----------+-------------+---------------+

The reported earthquakes between 1751 and 1843 are shown to conform
with the same rule.[100] Julius Schmidt, astronomer at Athens, found
for the earthquakes of Eastern Europe and adjacent countries for the
years 1776 to 1873 that there were more earthquakes when the moon was
in perigee. Other maxima were at new moon, and two days after the first
quarter. There was a diminution at full moon, and a minimum on the day
of the last quarter. As one example of results which are antagonistic
to the general results obtained by Perrey may be quoted the results of
an examination by Professor W. S. Chaplin of the earthquake recorded at
the meteorological observatory in Tokio. The list of earthquakes, 143
in number, extending over a period of three years, was recorded by one
of Palmieri’s instruments. The results were as follows:—

1. There have been maxima of earthquakes when the moon was two and
nine hours east and seven hours west. At the upper transit there is a
minimum.

2. Considering the moon’s position with regard to the sun, at
conjunction there were 32, at opposition 37, and at quadrature 74. East
of the meridian the maximum was at least four hours.

3. When the moon was north of the equator these were 68, when south 82.

4. A maximum of earthquakes seven and eleven days after the moon’s
perigee. The fact that these results were obtained for the earthquakes
of a special small seismic area renders them more interesting.[101]

_Frequency of earthquakes in relation to the position of the sun._—The
question as to whether there is a connection between the frequency of
earthquakes and the relative position of the sun is to a great extent
identical with the question as to the relative frequency of earthquakes
in the various seasons. It is a subject which we find referred to
by writers in the earliest ages. Pliny and Aristotle thought that
earthquakes occurred chiefly in spring and autumn. In later times it
has been a subject which has been most carefully considered by Merian,
von Hoff, Perrey, Mallet, Volger, Kluge, and others who have devoted
attention to seismology. In a résumé of the earthquakes of Europe, and
of the adjacent parts of Asia and Africa, from A.D. 306–1843, Mallet
gives the following results:—

 +------------------+------------------------+----------------------+
 |                  | For Nineteenth Century | For the whole period |
 +------------------+------------------------+----------------------+
 | Winter Solstice  |   177 } Solstices      | 253 }  Solstices     |
 | Spring Equinox   |   151 } }  306         | 170 } }  403         |
 | Summer Solstice  |   129 } } Equinoxes    | 150 } } Equinoxes    |
 | Autumnal Equinox |   164   }   315        | 159   }   329        |
 +------------------+------------------------+----------------------|

The above periods were called by Perrey _critical epochs_, because
as a general result of his researches he found that at such periods
there was a greater frequency of earthquakes. Fuchs, quoting from
Kluge’s tables, extending from 1850–1857, tells us that the recorded
earthquakes occurred as follows:—

  In the Northern Hemisphere—
      Equinoxes                =1324=
      Solstices                 1202
  In the Southern Hemisphere—
      Equinoxes                 =301=
      Solstices                  261

Earthquakes are, therefore, more frequent at the equinoxes, and this
especially at the autumnal equinox. In the northern hemisphere, at
the solstices, the greater number of shocks occur about the winter
solstices, whilst in the southern hemisphere, about the summer
solstices.

Exceptions, however, are found in Central America and the West Indies,
in the Caucasus, and the Ægean Sea.

The idea that earthquakes had a periodicity dependent upon the position
of the heavenly bodies is by no means confined to Europe. In a Japanese
work called ‘Jishin Setsu’ (an opinion about earthquakes) by a priest
called Tensho, it is stated that the relative positions and movements
of the twenty-eight constellations with respect to the moon cause
earthquakes. This Tensho asserts after careful calculation, and Falb
tells us that all future earthquakes can be predicted.

In the Kuriles and Kamschatka, Sicily, and in parts of South America,
it is said that the equinoxes are regarded as dangerous seasons.

_Frequency of earthquakes in relation to the seasons and months._—What
is here said respecting the relative frequency of earthquakes at the
different seasons and months is little more than an extension and
critical examination of the results which have been given respecting
the frequency of earthquakes in regard to the position of the sun.

That there is a difference between the number of earthquakes which are
felt at one season of the year as compared with those felt at another
is a fact which, as seismoscopic observations are extended, is becoming
more and more recognised.

Some of the more important results which were arrived at by Mallet from
5,879 observations made in the northern hemisphere, and 223 in the
southern hemisphere, may be expressed as follows:—

 +---------------------+------------------------+-------------------+
 |                     |        Maxima          |       Minima      |
 +---------------------+------------------------+-------------------+
 | Northern Hemisphere | January, also a slight | May, June, and    |
 |                     |   rise in August and   |   July            |
 |                     |   October              |                   |
 | Southern Hemisphere | November, also May     | March, extending  |
 |                     |   and June             |   over one month, |
 |                     |                        |   also August     |
 +---------------------+------------------------+-------------------+

Julius Schmidt, of Athens, who so carefully examined the earthquakes of
eastern Europe, came to the following conclusions:—

For the earthquakes between 1200 and 1873, a maximum on September 26
and January 17; a minimum on December 3 and June 13.

For the earthquakes between 1873 and 1874, a maximum on March 1 and
October 1; a minimum on July 7 and December 15.

For all the earthquakes of eastern Europe, a maximum on January 3; a
minimum on July 8, or there was a maximum at perihelion and aphelion.

When the months are grouped together according to the seasons, spring,
summer, autumn, and winter, we find that in the northern hemisphere the
minimum is in summer and the maximum in winter, whilst in the southern
hemisphere (giving the proper months corresponding to its seasons) we
find two maxima, one at the commencement of winter, and the other at
midsummer, whilst the minima are in spring and autumn.

[Illustration: FIG. 36.—Curves of Monthly Seismic Intensity (Mallet).]

In the following table the difference in the number of earthquakes felt
at different seasons is given more in detail.

In examining this table, we must remember that for countries like Peru,
Chili, and New Zealand, lying in the southern hemisphere, the records
given for the months April to September correspond to the winter months
of those countries. The Roman numerals indicate the centuries between
which the records date.

 +----------+----------------------------------------+-------+---------+
 |          |                                        |October|  April  |
 |          |                                        |  to   |   to    |
 |          |                                        | March |September|
 +----------+----------------------------------------+-------+---------+
 |         {| 1. Scandinavia and Iceland, xii–xix    | =129= |    91   |
 |         {| 2. British and Northern Isles, xi–xix  | =123= |    94   |
 |         {| 3. Belgium, France, and Holland, iv–xix| =395= |   272   |
 |         {| 4. Rhone Basin, xvi–xix                | =115= |    69   |
 |         {| 5. Switzerland and Rhine Basin, ix–xix | =327= |   205   |
 |         {| 6. Danube Basin, v–xix                 | =147= |   128   |
 |         {| 7. Spanish Peninsula, xi–xiv           | =114= |    87   |
 |         {| 8. Italy, Sicily, Sardinia, and Malta, |       |         |
 |Northern {|      iv–xix                            |  650  |   581   |
 | Regions {| 9. Turco-Hellenic Territory, Syria,    |       |         |
 |         {|    Ægean Isles, and Levant, iv–xix     |  214  |  =222=  |
 |         {|10. Northern Zone of Asia, xviii–xix    |  =46= |    36   |
 |         {|11. Japan (Tokio area), 1872–1880       |       |         |
 |         {|    (small earthquakes)                 | =213= |   157   |
 |         {|12. Japan B.C. 295-A.D. 1872 (large     |       |         |
 |         {|    earthquakes)                        |  165  |  =188=  |
 |         {|13. Algeria and Northern Africa         |  =26= |    20   |
 |         {|14. United States and Canada, xvii–xix  |  =86= |    48   |
 |        { |15. Java, Sumatra, and neighbouring     |       |         |
 |        { |      Islands, 1873–4–7–8               | =194= |   182   |
 |Central { |16. Mexico and Central America, xvi–xix |   26  |    26   |
 |Regions { |17. West Indies (Mallet), xvi–xix       |  108  |  =114=  |
 |        { |18. West Indies, xvi–xix                |  296  |  =343=  |
 |        { |19. Cuba, xvi–xix                       |  =28= |    23   |
 |         {|20. Chili, and La Plata Basin, xvi–xix  |   89  |    89   |
 |Southern {|21. Peru, Columbia, Basin of Amazons,   |       |         |
 |Regions  {|      xvi–xix                           |  506  |  =541=  |
 |         {|22. New Zealand, 1869–1879              |  166  |  =176=  |
 +----------+----------------------------------------+-------+---------+

Neglecting those records which show as many earthquakes for the winter
months as for the summer months, we see at a glance that generally
the greater number of shocks have happened during the colder seasons.
In the southern hemisphere, so far as the records go, this is not
true. In the northern regions, out of fourteen examples there are
two exceptions. In the central regions there are two cases where the
greatest number of earthquakes have been recorded in the winter months,
and two cases where the greatest number have been recorded for the
summer.

Altogether, out of twenty-two examples, there are only six exceptions
to the rule. These exceptions altogether occur among records many of
which are ancient, and are, therefore, more open to error than lists
which have been compiled in modern times.

Because small earthquakes are seldom noticed by persons out in the
open air, it might be expected that the number of earthquakes observed
in warm countries at one portion of the year would be equal to those
observed in any other season. Such an argument, however, would hardly
apply to most of the records which are quoted, as they refer to
destructive disturbances.

If, however, we take the records made in tropical countries from the
table just given, we see that in such countries there have been almost
as many observations of earthquakes at one season as at any other.

Another fact which might be adduced against the rule that the
greater number of earthquakes occur during the winter months would
be the comparison of a table of earthquakes recorded previous to the
nineteenth century. By doing this we see that for certain countries the
winter rule is inverted, and that the greater number of shocks are felt
during the summer.

Notwithstanding these objections to Perrey’s conclusions, the balance
of evidence is in favour of his general result, and we may conclude
that during the colder portions of the year we may expect more shakings
than during the warmer portions. Comparing the number of earthquakes of
winter and autumn to those of summer and spring, they are to each other
in the proportion of 4 : 3.

A fairer way to examine this question, and to determine what is
probably the present state of seismic activity in our globe, would
be only to consider the earthquakes which have taken place in
comparatively recent times, laying especial stress upon those
observations which have been made with the assistance of automatic
instruments, or those which have been collected by persons interested
in these investigations.

For this purpose the following table, showing the distribution of
earthquakes in different countries during the nineteenth century, has
been compiled.

The arrangement is mensual. Where the number of earthquakes in any
month is above the average, the number is printed in large type; where
below the average, in small type.


      EARTHQUAKES OF THE NINETEENTH CENTURY, CHIEFLY FROM PERREY.

 Key:
   Jan January
   Feb February
   Mar March
   Apr April
   May May
   Jun June
   Jul July
   Aug August
   Sep September
   Oct October
   Nov November
   Dec December
   Ave Average per month
 +------------------------------------------------------------------------------+
 |                         |Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec|Ave |
 |-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----|
 |Scandinavia and Iceland  | 17| 11| 11|  7|  7|  6|  8|  8| 10| 10| 11|  6| 9·3|
 |British Isles and        |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  Northern Isles         |  9|  9| 10|  7|  8|  6|  5| 11| 12|  8| 11| 12| 9  |
 |France, Belgium, Holland | 27| 17| 21| 13| 13|  8| 15| 17| 15| 17| 21| 25|17  |
 |Basin of the Rhone       | 12| 12|  8|  3|  3|  2|  2|  4|  6|  6|  8| 14| 6·6|
 |Basin of the Rhine and   |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  Switzerland            | 15| 17| 13| 12| 11|  6| 12| 11| 10| 17| 24| 25|14  |
 |Basin of the Danube      | 14| 15|  9|  8| 12|  8| 16| 11| 11| 16| 10| 12|11·8|
 |Spanish Peninsula        | 10|  5|  6|  7|  4|  6| 10|  5|  9| 11|  7|  5| 7  |
 |Italian Peninsula,       |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  Sicily, Sardinia,      |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  and Malta              | 44| 44| 48| 43| 40| 34| 41| 46| 27| 45| 26| 39|39  |
 |Turco-Hellenic Territory,|   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  Syria, Ægean Islands,  |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  and Levant             | 22| 20| 10| 10| 16| 15| 14| 22| 14| 17| 12| 14|16  |
 |Northern Zone of Asia    |  4|  6|  6|  4|  4|  3|  5|  7|  6|  3|  4|  5| 4·7|
 |1876–1881, Japan (Tokio  |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  area)                  | 39| 41| 41| 30| 33| 30| 27| 21| 10| 28| 34| 43|31·4|
 |Japan (large earthquakes)| --|  5|  3|  3|  1| --|  5|  4| --| --|  1|  5| 2  |
 |Algeria and North’rn     |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  Africa                 |  5|  2|  6|  7|  3|  2|  2|  5|  1|  4|  8|  1| 3·8|
 |United States and Canada |  4|  4|  3|  3|  3| --|  4|  6|  3|  2|  7|  5| 3·8|
 |Java, Sumatra, &c.,      |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  1873–4–5–7 and 9       | 35| 30| 38| 33| 22| 36| 27| 40| 24| 35| 30| 26|31  |
 |Mexico and Central       |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  America                |  3|  2|  2|  2|  6|  2|  2|  1|  1|  3|  2|  3| 2·5|
 |Antilles                 |  9|  8| 19| 12| 12| 10|  9| 16| 12| 10| 13| 12|11·8|
 |Cuba                     |  4|  3|  2|  3|  3|  4|  5|  2|  6|  5|  6|  4| 4  |
 |Chili and La Plata       | 14| 10| 14|  8| 19| 11| 16| 15| 16|  9| 27|  8|13·9|
 |Peru, Columbia, Basins   |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |  of Amazons, xvi–xix    | 92| 83| 92| 27|106| 79| 94| 93| 97| 77| 72| 90|87  |
 |New Zealand, 1869–79     | 31| 27| 37| 23| 22| 31| 27| 36| 37| 21| 27| 23|28·5|
 |    Jan. 1850, Dec. 1857 |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |Northern Hemisphere      |153|162|143|161|126|124|141|156|154|171|151|168|150 |
 |Southern Hemisphere      | 75| 43| 61| 66| 46| 42| 53| 39| 54| 55| 57| 46|53  |
 |      1821–1830          |   |   |   |   |   |   |   |   |   |   |   |   |    |
 |Northern Hemisphere      | 31| 36| 31| 29| 33| 33| 20| 31| 24| 41| 26| 34|30  |
 |Southern Hemisphere      |  2| --|  1|  1|  3|  1|  3|  2|  3|  2|  1|  1| 1·6|
 +-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----+

A glance at this table shows that for most countries in the northern
hemisphere the rule that there are generally more earthquakes during
the winter months—that is, from October to March—holds good. For
countries which lie comparatively near to the Equator, and also for
those countries in the southern hemisphere, the rule is not so clear.
When examining this table it must be remembered that it does not enable
us to judge of the relative frequency of earthquakes in different
countries, inasmuch as the periods over which the records were taken
are different in different cases.

To the above table might be added the records of P. Merian, who
examined the earthquakes felt in Basle up to 1831. As a result he
found that during the winter months eighty shocks had been felt,
whilst during the summer only forty. Taking the records for the two
hemispheres from 1850–1857, compiled by Kluge,[102] in the northern
hemisphere we have in the months between October and March 948 shocks
against 862 in the remainder of the year. In the same months in the
southern hemisphere we have for the corresponding periods the numbers
337 and 300, and thus both hemispheres would appear to follow the same
rule. If, however, we examine the table we see that the two seasons
are not so pronounced for the southern hemisphere as they are for
the northern, and that there may be two or three periods of maximum
disturbance as has been previously indicated.

_Earthquakes and the planets and meteors._—Just as the moon and the sun
may exert an attractive influence upon the earth and cause earthquakes
to predominate at certain seasons rather than at others, several
investigators of seismic phenomena have thought that the planets might
act in a similar manner.

M. J. Delauney, from a study of Perrey’s tables of earthquakes from
1750–1842, found two groups of maxima each with a period of about
twelve years, one commencing in 1759 and the other in 1756. Two
other groups with twenty-eight year periods respectively commence in
1756 and 1773. These groups coincide with the times when Jupiter and
Saturn reach the mean longitudes of 265° and 135°. From this Delauney
concludes that earthquakes have a maximum when the planets are in the
mean longitudes just mentioned.

The increased number of earthquakes, especially in November, are
attributed to the passage of the earth through swarms of meteors, and
in like manner supposes the influence of Jupiter and Saturn to be due
to their passing through meteor streams situated in mean longitudes
135° and 265°.

As a consequence of this he predicts an increase of earthquakes in the
years 1886, 1891, 1898, 1900, &c.[103]

Dr. E. Naumann, who critically examined the large earthquakes of Japan,
showed that there was an approximate coincidence between many of the
disturbances and the thirty-three year period of meteoric showers.[104]

Humboldt states that a great shower of meteors was seen at Quito before
the great earthquake of Riobamba (Feb. 4, 1797). The earthquakes of
1766 and 1799 at Cumana are also said to have been accompanied with
meteoric showers. Mallet gives a list of large earthquakes which
occurred at the times when meteors were observed.[105]

_The hours at which earthquakes are most frequent._—From the
examination of a catalogue of over 2,000 earthquakes which occurred in
various parts of the world between the years 1850 and 1857, made by
Kluge, it is found that both for the northern and southern hemispheres
the observations which were made during the night generally exceed
those which were made during the day.

 +---------------------------+-----------------------+
 |                           | Number of Earthquakes |
 +---------------------------+-----------------------+
 |                           |   Day   |    Night    |
 |In the Northern Hemisphere |   938   |    1592     |
 |In the Southern Hemisphere |   292   |     357     |
 +---------------------------+---------+-------------+

In the northern hemisphere the greatest number were observed between
10 P.M. and 12 P.M. (360 shocks), and the fewest between 12 and 2 P.M.
(139 shocks). In the southern hemisphere, the greatest number were
observed at night between 12 and 1, and the smallest number between 1
and 2 and 4 and 5 in the afternoon.[106] These distinctions, however,
are less distinctly marked as we approach the Equator. Schmidt found
for the earthquakes of the Orient between 1774 and 1873, that shocks
had been most frequent about half-past two A.M., and less frequent
about 1 P.M. With regard to these conclusions, which have been reached
with much labour, we might be inclined to think that they are partially
to be explained on the supposition that more observations are made
during the night than during the day—the personal experience of
residents in an earthquake country being, that many earthquakes which
occur during the day are passed by unnoticed, whilst those which occur
during the night are recorded by thousands of observers. Such a view is
certainly confirmed by the instrumental records obtained in Japan. From
1872 to 1880 inclusive there were 261 shocks recorded, 132 of which
occurred between the hours of 6 P.M. and 6 A.M.

_Earthquakes and sun spots._—Of late years considerable attention
has been drawn to a coincidence between the occurrence of sun spots,
magnetic disturbances, rainfall, and other natural phenomena.

These periods of sun spots occur about every eleven years, and appear
to be coincident with the periodical return of the planet Jupiter. In
Japan, Dr. E. Naumann sought for a coincidence between these periods of
sun spots and earthquakes, but without any marked results.

Schmidt, who carefully compared his lists of earthquakes with the
appearance of sun spots, came to the conclusion that there was no
marked coincidence. The occurrence of earthquakes had sometimes
synchronised with sun spots, whilst at other times there had been a
maximum of sun spots and no earthquakes.

M. R. Wolf[107] apparently considers that earthquakes, like volcanic
eruptions and the appearance of the aurora, are coincident with sun
spots.

Kluge, however, came to the conclusion that when there are few sun
spots, earthquakes, like volcanic eruptions and magnetic disturbances,
have been at a maximum.

M. A. Poey, who examined a catalogue of the earthquakes of Mexico
and the Antilles, extending from 1634 to 1870, shows by a table that
earthquakes have come in groups, first at the maxima and then at the
minima period of sun spots. Out of thirty-eight groups, seventeen
being at the maximum and seventeen at the minimum, the remaining four
are exceptions to the rule, being between the maximum and minimum.
Phenomena which are dependent upon heat occur with the minima of sun
spots, and those dependent upon cold with the maxima.[108]

_Earthquakes and the aurora._—The possible connection between
earthquakes and the aurora is a subject which has attracted some
attention. Boué has especially made a careful examination of this
subject.[109]

He comes to the conclusion that if we compare the monthly periods of
earthquake frequency and the aurora there is an agreement between
the two. Comparing Perrey’s tables of earthquakes from the fourth to
the nineteenth century, with tables of the aurora, one-third of both
phenomena have occurred, not only in the same day, but often at the
same hour. Between 1834 and 1847, 457 earthquakes are given and 351
notices of the aurora.

Out of these:—
    48 occur on the same day,
     5 occur in the same hour,
    30 approximate to the same time.

The nearer together that these phenomena have occurred the stronger
have they been.

Professor M. S. di Rossi brings forward many examples where there
has been a coincidence between the appearance of the aurora and
earthquakes. On 139 nights out of 211 days the aurora was seen in
some parts of Italy, and ninety-three times earthquakes were felt. On
forty-six occasions earthquakes and aurora took place together.[110] In
considering the probability of a connection existing between these two
phenomena, we must bear in mind that the aurora is at no great height
above the surface of our earth, and, further, that it can be partially
imitated. The fact that in earthquake countries, like Japan, the aurora
is practically never seen, would indicate that we can neither regard
this imperfectly understood phenomenon either as an effect or cause
of earthquakes. That earthquakes and the appearance of the aurora in
certain countries should not sometimes coincide is an impossibility.

Dr. Stukeley, who, it must be remembered, attempted to correlate
the phenomena of earthquakes and electricity, when writing of the
disturbances which shook England in 1849 and 1850, says that the
weather had been unusually warm, the aurora borealis frequent and of
unusually bright colours, whilst the whole year was remarkable for its
fire-balls, lightnings, and corruscations.[111]

The aurora was observed before the commencement of the Maestricht
earthquakes in 1751[112]; whilst at the time of the shock flashes of
light like lightning were observed in the sky.

Glimmering lights were seen in the sky before the New England
earthquakes (Nov. 18, 1755), and again, before the disturbances which
occurred in the same region in 1727, peculiar flashes of light were
seen.

Preceding the Sicilian earthquake of 1692 strange lights were seen
in the sky. Ignis fatui have also been observed with earthquakes. At
the time of auroral displays Bertelli has observed microseismical
disturbances, and M. S. di Rossi, who has made similar observations,
thinks that there is an intimate connection between the aurora and
earthquakes; the aurora either occurring in a period of earthquakes, or
else taking the place of earthquakes.




                              CHAPTER XV.

 BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE
                           AND EARTHQUAKES.


_Changes in the barometer and earthquakes._—Mallet, who collected
together a number of examples of earthquakes which have occurred with
a fall of the barometer, and a number which have happened with a rise,
concludes that there are as many instances of the one as of the other.
At the great earthquake of Calabria, in 1783, the barometer was very
low. The earthquake of the Rhine (February 23, 1828) was preceded by
a gradual fall of the barometer, which reached its lowest point upon
that day. After the earthquake the barometer again rose. The earthquake
of February 22, 1880, in Japan, was accompanied by exactly similar
phenomena. Caldcleugh, who observed the heavy shocks in Chili (February
20, 1835), noticed that on February 17 and 18 the barometer fell 5/10
inches. Similar phenomena were observed before the succeeding smaller
shocks. After the shocks the barometer again rose. Principal Dawson,
speaking of the earthquakes of Canada, observes that some of the shocks
have been accompanied with a low barometer.

P. Merian, who examined the connection between the Swiss earthquakes
and atmospheric pressure, found that out of twenty-two earthquakes
observed in Basle between 1755 and 1836, thirteen of these were
local shocks, of which eight were accompanied with sudden changes of
pressure. Of the remaining nine, which were only felt slightly in
Basle, no change in atmospheric pressure was observed. Of thirty-six
earthquakes which, between 1826 and 1836, were felt in Switzerland,
thirty were chiefly confined to Switzerland, and ten of these occurred
with a low or falling barometer.

Humboldt is of opinion that earthquakes only occur with changes in
barometric pressure in those countries where earthquakes are few; and
he gives examples where the regular variations of the barometer have
gone on without interruption at the time of earthquakes.

Frederick Hoffmann, who examined fifty-seven earthquakes which occurred
at Palermo between 1788 and 1838, came to the following result:—

  The barometer was sinking                 in 20 cases
      „          „  rising                  in 16   „
      „          „  at a minimum            in  7   „
      „          „    „  maximum            in  3   „
      „          „  undetermined            in 11   „[113]

The observations of M. S. di Rossi apparently show that the earthquakes
in Italy chiefly occur with a barometrical depression and with sudden
jumps in atmospheric pressure.

Schmidt, who examined the earthquakes of the Orient, which occurred
between 1858 and 1873, says that they were rare with a high barometer,
but numerous when the barometer was low.

From an examination of a table of 396 earthquakes (May 8, 1875-Dec.
1881) felt in Tokio, furnished to me by Mr. Arai Ikunosuke, the
director of the meteorological department, I obtained the following
results:—

  The barometer was rising                  in  169 cases
      „          „  falling                 in  154   „
      „          „  steady                  in   73   „
      „          „  below the monthly mean  in  189   „
      „          „  above        „          in  192   „

From this it would appear that in Japan at least the movements of the
barometer do not show any marked connection with the occurrence of
earthquakes.

When considering this question we must remember the marked effects
which a lowering of the barometer produces upon certain volcanoes and
solfataras. The volumes of steam emitted from Stromboli and from some
of the solfataras in Tuscany hold a marked connection with atmospheric
pressure as the quantity of fire damp given off from coal seams—these
being greatest when the barometer is low. At certain changes of
the weather it is said that the volcano of Vulture, near Melfi,
emits noises. These phenomena at once place volcanic phenomena and
barometrical pressure in direct relationship.

_Changes in temperature._—If, with an earthquake, it should happen
that there is a change in the height of the barometer, we should
naturally expect that this might be accompanied with the changes in the
temperature, in the wind, and in other atmospheric phenomena which are
more or less connected with the height of the barometer.

Many times it has been observed that after an earthquake there has been
a sudden fall in the temperature. Such was the case with the Yokohama
earthquakes of 1880.

Cotte endeavours to show that the earthquakes of Lisbon produced a
change upon the temperature of all Europe. In the year which followed
this earthquake storms were more common than usual.

Kluge has collected together a large number of examples when there has
been a fall of temperature at the time of an earthquake.[114]

At Kiachta, in Siberia, at the time of the earthquake of December
27, 1856, the thermometer fell from 12° to 25° R. We must, however,
remember that there are many cases known where the thermometer rose.

M. S. di Rossi remarks that we have the highest records of temperature
in the years richest in earthquakes. Thus, in 1873, at the time of the
earthquakes in Central and Northern Italy, an abnormal high temperature
was remarked. Japanese writers have remarked upon the unusual heat
which has shaken their countries. The temperature of subterranean
waters have been known to increase before earthquakes.




                             CHAPTER XVI.

              RELATION OF SEISMIC TO VOLCANIC PHENOMENA.

  Want of synchronism between earthquakes and volcanic
    eruptions—Synchronism between earthquakes and volcanic
    eruptions—Conclusion.


_Connection between earthquakes and volcanic eruptions._—Insomuch as
it is a recognised fact that regions which are characterised by their
seismic activity are chiefly those which are also characterised by
the number of their volcanoes, it is generally assumed that these
two phenomena have an intimate relation. The residents in a volcanic
country, when seeking for the origin of an earthquake, invariably turn
towards the volcanoes which surround them. If a neighbouring volcano
is in a state of activity, it is often regarded as a safeguard against
seismic convulsions, in other cases it is looked upon as being the
cause of such disturbances. In certain instances both of these views
have apparently been corroborated. When we consider that an earthquake
and a volcanic eruption may both be the result of some great internal
convulsion, and that first one and then the other may take place in the
same neighbourhood, it is natural to expect that when these internal
forces have expended themselves in the production of one of these
phenomena, it is not so likely that they should exhibit themselves in
the other. The inhabitants of Sicily and Naples, we are told, regard
eruptions of Etna and Vesuvius as safeguards against earthquakes. A
similar belief is to be found in portions of South America with regard
to the volcanoes for which that country is so celebrated.

From an examination of the records of the large earthquakes and the
volcanic eruptions which have taken place in Japan during the last
2,000 years, Dr. Naumann found that there was often an approximate
coincidence between the times of the occurrence of these phenomena,
suggesting the idea that the efforts which had been sufficient to
establish the volcano had at the same time been sufficient to shake the
ground.

Of destructive earthquakes which have occurred at the time of volcanic
eruptions, and of examples when these phenomena have occurred at widely
separated intervals, the records are extremely numerous.

_Want of synchronism between earthquakes and volcanic eruptions._—Many
of the great earthquakes of South America do not appear to have been
connected with volcanic eruptions.

The great earthquakes of the world, like those of Calabria and Lisbon,
which took place in regions which are not volcanic, have not, Fuchs
tells us, taken place in conjunction with volcanic outbursts.

In Japan, as in the Sandwich Islands and in many other parts of the
globe, the small earthquakes which occur almost daily do not appear to
show any marked connection with volcanic disturbances.

In 1881, during the eruption of Natustake, a volcano lying about a
hundred miles north of Tokio, there was neither an increase nor a
decrease in the earthquakes which were felt in Tokio. Similar remarks
apply to the state of seismic activity of 1876–77, when Oshima, a
volcanic island about seventy miles to the south of Tokio, was in
eruption. In the Sandwich Islands Mauna Loa seems to have its eruptions
independently of the disturbances which shake these islands.[115]

_Synchronism of earthquakes and volcanic eruptions._—Although many
examples like the above may be quoted, which apparently show an utter
want of connection between earthquakes and volcanoes, we must not
overlook that class of earthquakes which almost invariably accompany
all great volcanic disturbances. In fact the sudden explosions which
take place at volcanic foci, as, for instance, at the commencement
of an eruption, are enumerated as one of the causes which produce
earthquakes. Earthquakes like these usually continue until the pressure
of the steam and lava have found for themselves an opening. As compared
with the total number of earthquakes which are recorded, they form but
an insignificant portion.

The direct connection which exists between these phenomena has, no
doubt, done very much to spread the popular belief that all earthquakes
may be connected with volcanic eruptions. As examples where this
connection has existed we might quote from almost all the volcanic
countries in the world.

Thus, Fuchs tells us that on October 6, 1737, almost the whole of
Kamschatka and the Kurile Islands were disturbed by movements which
were simultaneous with the outbreak of the great volcano Klutschenskja
of North Kamschatka.

One of the earliest records of a severe earthquake and a volcanic
eruption occurring simultaneously is found in the accounts of the
destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo
in the neighbourhood of Pozzuoli was accompanied with a dreadful
earthquake.[116]

In 1868 the earthquake of Arequipa was accompanied by the opening of
the volcano Misti, on its north side. The distance to the volcano is
about fourteen miles.

At the time of the eruptions of Kilauea in 1789 the ground shook and
rocked so that persons could not stand.

The first eruption of the volcano Irasu, in Costa Rica (1783), was
accompanied by violent earthquakes.[117] The smoke and flames which
are said to have issued from the side of Mount Fojo at the time of the
Lisbon earthquake are regarded by some as having been volcanic. Others
thought that the phenomena, rather than being on the side of Fojo,
which showed no traces of volcanic action, had taken place in the ocean.

At the time of the great earthquake at Concepcion (1835), whilst the
waves were coming in, two great submarine eruptions were observed. One,
behind the Isle of Quiriquina, appeared like a column of smoke. The
other, in the bay of San Vicente, appeared to form a whirlpool. The
sea-water became black, and had a sulphurous smell, there being a vast
eruption of gas in bubbles. Many fish were killed.[118]

With this same earthquake, near to Juan Fernandez, about one mile from
the shore, the sea appeared to boil, and a high column of smoke was
thrown into the air. At night flames were seen.

In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a
volcano at the foot of which Mendoza is situated burst into eruption.

The earthquake of 1822 at Valdivia was accompanied by eruptions of the
neighbouring mountains, which only lasted a few minutes.

At the time of the Leghorn shocks (January 16–27, 1742) some fishermen
observed a part of the sea to rage violently, to raise itself to a
great height, and then rush landwards.[119]

In 1797, when Riobamba was destroyed, the neighbouring volcanoes were
not affected, but Mount Pasto, 120 miles distant, suddenly ceased to
throw out its usual column of water.

On the night of December 10, 1874, a strong shock was felt in New
England, whilst at 4.45 A.M. on December 11 a shock was felt in the Pic
du Midi, in the Pyrenees. In the middle of December there were volcanic
outbursts in Iceland.[120]

It is possible that these occurrences might be the results of some
widespread disturbance beneath the crust of the earth, or perhaps even
of widely extended earth pulsations. The probability, however, is that
these coincidences are accidental. When we remember that in a small
area like the northern half of Japan alone there are periods when there
are at least two shocks per day on the average, it is impossible for
these coincidences not to exist. Less frequently coincidences between
the larger disturbances must occur. Over and above these accidental
coincidences, it would appear that in the world’s history periods
have occurred when earthquakes were unusually frequent, and at such
times distant countries have suffered simultaneously. This approximate
coincidence in period, which has been referred to when speaking of the
distribution of destructive earthquakes in historical time, does not
imply an exact synchronism in the single shocks.

Small earthquakes, or, more properly speaking, local tremblings, are a
necessary accompaniment of almost all volcanic eruptions. Tremors of
this description are seldom, however, felt beyond the crater, or at the
most upon the flanks of the mountain where the eruption is going on.

They are due to the explosive action of steam bursting through the
molten lava.

_Volcanic eruption succeeding earthquakes._—Sometimes it has happened
that an earthquake, or a series of earthquakes, have terminated with
the formation of volcanic vents.

As an example of a volcanic outburst terminating a seismic disturbance,
may be mentioned the appearance of a new volcano in the centre of
Lake Ilopango, as a sequel to the shocks which had disturbed that
neighbourhood in 1879.[121]

In 1750 there were continuous shakings lasting over three months at
Manilla. These terminated with an eruption of a small island in the
middle of a neighbouring lake. Three days after the commencement of
this eruption, four other small islands rose in the same lake.[122]

Antonio d’Ulloa, when speaking of the Andes, remarks that after a
volcanic eruption the shocks cease.[123]

_Conclusion._—Looking at this question generally, insomuch as the
greatest number of volcanic eruptions appear, according to Fuchs,
to have taken place in summer, whilst the greatest number of its
earthquakes have apparently taken place in winter, it would seem that
the two phenomena are without any direct connection, unless it be that
both are different effects of a common cause.

Regarded in this manner, an earthquake may be looked upon as an
uncompleted effort to establish a volcano. To use the words of Mallet,
‘The forces of explosion and impulse are the same in both; they differ
only in degree of energy, or on the varying sorts and degrees of
resistance opposed to them.’[124]

Although we have many examples of earthquakes having occurred without
volcanic eruptions, and, on the other hand, of volcanic eruptions
without earthquakes, volcanoes may still be regarded as ‘safety-valves
of the earth’s crust,’ which, by giving relief to internal stresses,
guard us against the effects of earthquakes.

That many earthquakes are felt at Copiapo is attributed to the fact
that in the neighbouring mountains there are no volcanic vents.

We must not, however, overrate the protective influence of volcanoes.
In the Sandwich Islands we see the columns of liquid lava in
neighbouring mountains standing at different heights, indicating a
want of subterranean connection between these vents. In consequence
of this it would seem that enormous pressures might be generated in
the neighbourhood of one of these mountains without finding relief at
the other. When we have conditions like these, it would seem that the
eruption of a volcano may have little or no influence in protecting
neighbouring districts.

This may possibly be the explanation of the fact that in 1835
Concepcion was destroyed, notwithstanding there being an unusual
activity in the volcanic vents of the neighbouring mountains.




                             CHAPTER XVII.

                       THE CAUSE OF EARTHQUAKES.

  Modern views respecting the cause of earthquakes—Earthquakes due
    to faulting—To explosions of steam—To volcanic evisceration—To
    chemical degradation—Attractive influence of the heavenly
    bodies—The effect of oceanic tides—Variation in atmospheric
    pressure—Fluctuation in temperature—Winds and earthquakes—Rain
    and earthquakes—Conclusion.


As the results of modern inquiries respecting the cause of earthquakes,
we see many investigators chiefly attributing these phenomena to
special causes. A few attribute them to several causes. It seems to us
that they might be attributed to very many causes which often act in
a complex manner. The primary causes are telluric heats, solar heat,
and variations in gravitating influences. These may be the principal,
and sometimes the immediate, cause of an earthquake. The secondary
causes are those dependent upon the primary causes, such as expansions
and contractions of the earth’s crust, variations in temperature,
barometrical pressure, rain, wind, the attractive influences of the
sun and moon in producing tides in the ocean or the earth’s crust,
variations in the distribution of stress upon the earth’s surface
caused by processes of degradation, the alterations in the position of
isogeothermal surfaces, &c.

The part which may be played by these various causes in the production
of oscillations, pulsations, and tremors will be referred to.

_Earthquakes consequent on faulting._—In the chapter on Earth
Oscillations, the causes producing the phenomena of elevation and
depression are briefly indicated.

By the variations in stress accompanying elevations and depressions,
cracks are produced. Inasmuch as compression would crush the rocks
constituting the earth’s crust, we must conclude with Captain Dutton
that these cracks are formed by tension. By elevation, the upper rigid
crust of the earth is stretched, and fissures are produced. The sudden
formation of these fissures or faults gives rise to earthquakes, and
perhaps also to volcanic vents. That earthquake and volcanic regions
are situated on areas where there is evidence of rapid elevation is
strikingly illustrated round the shores of the Pacific.

Lasaulx considered that the earthquake of Herzogenrath was more
or less intimately connected with the great mountain fissure—the
_Feldbiss_—which crosses the coal region of the Wurm.[125] The sudden
elevation or sinking of large areas at the time of an earthquake may be
a consequence of these dislocations.

It has already been pointed out that the earthquake region of Japan
is the one where we have evidence of recent and rapid elevation. That
certain earthquakes of this region may possibly be the result of
faulting we have the evidence of our senses and of our instruments. The
sudden blows and jolts which are sometimes felt are indicative of the
sliding of one mass of rock across another.

Should the ground be simply torn asunder, this tearing would give rise
to a series of waves of distortion, vibrating in directions parallel
to the plane of the fissure. Supposing this motion to be propagated
to a number of surrounding stations, it would be recorded at each of
these as having the same direction. To those situated on a line forming
a continuation of the strike of the fissure, the vibrations would
advance so to speak _end on_, whilst to those stations lying in a line
perpendicular to the strike of the fissure, the motion would advance
_broadside on_.

Motions like these latter have been recorded in Tokio, where
earthquakes which from time observations were known to have come from
the faulted and rising region to the south have been registered as a
series of east and west motions, or vibrations transverse to this line
of propagation.

It must, however, be here mentioned that the registration of only
transverse motion may possibly be due to the extinction of normal
motion, although this is not generally regarded as probable.

It would therefore appear that certain earthquakes and faults are
closely related phenomena, the former being an immediate effect of
the latter. Faults are due to earth oscillations, and to a variety of
causes producing disturbances in the equilibrium of the earth’s crust;
the principal cause of all these phenomena being alterations in the
distribution of heat, and the attractive force of gravity.

_Earthquakes consequent on the explosion of steam._—Humboldt regarded
volcanoes and earthquakes as the results of a common cause, which he
formulated as ‘the reaction of the fiery interior of the earth upon
its rigid crust.’ Certain investigators, who have endeavoured to
reduce Humboldt’s explanation to definite limits, have suggested that
earthquakes may be due to sudden outbursts of steam beneath the crust
of the earth, and its final escape through cracks and fissures.

Admitting that steam may accumulate by separating out from the cooling
interior of our globe, its sudden explosion might be brought about by
its own expansive force, or by the movements in the bubbling mass from
which it originated.

Others, however, rather than regard the steam as being a primeval
constituent of the earth’s interior, imagine it arises from the gradual
percolation of water from the surface of the earth down to volcanic
foci, into which it is admitted against opposing pressures, by virtue
of capillary action.

Mallet, in his account of the Neapolitan earthquake, shows that the
whole of the observed phenomena can be accounted for by the admission
of steam into a fissure, which by the expansive force exerted on its
walls was rent open. Just as at the Geysers we hear the thud and
feel the trembling produced by the sudden evolution and condensation
of steam, so may steam by its sudden evolution and condensation in
the ground beneath us give rise to a series of shocks of varying
intensity, accompanied by intermediate vibratory motions—that is to
say, a motion which, as judged of by our feelings, is not unlike many
earthquakes. Often it may happen that the result of the explosion may
be the production of a fault, or at least a fissure; and thus in the
resulting movements we may have a variety of vibrations, some being
those of compression and distortion, produced by the blow of the
explosion, and others being those of distortion alone, produced by the
shearing action which may have taken place by the opening of the fault.
Sometimes one set of these vibrations may be prominent, and sometimes
the other. Thus, when we say that an earthquake has shown evidence by
the nature of its vibrations that it was produced by a fault, this
by no means precludes the possibility that an explosion of steam may
also have been connected with the production of the disturbance.
Mallet threw out the suggestion that the opening of fissures beneath
the ocean might admit water to volcanic foci. During the time that the
water was in the spheroidal state, the preliminary tremors, so common
to many earthquakes, would be produced. These would be followed by the
explosion, or series of explosions, constituting the shock or shocks of
the earthquakes.

The chief reasons for believing that the earthquakes of North-Eastern
Japan are partly due to explosive efforts are:—

1. That the greater number of disturbances, perhaps ninety per cent.,
originate beneath the sea, where we may imagine that the ground,
under the superincumbent hydrostatic pressure, is continuously being
saturated with moisture.

2. Many of the diagrams show that the prominent vibrations, of which
there are usually from one to three, in a given disturbance have the
same character as those produced by an explosive like dynamite, the
greatest and probably the most rapid motions being inwards towards the
origin.

It may here be remarked that a very large proportion of the destructive
earthquakes of the world have originated beneath the sea, as has often
been testified by the succeeding sea waves. Also, it must be observed,
that earthquake countries, like volcanic countries, are chiefly those
which have a coast line sloping at a steep angle beneath the sea—that
is to say, earthquakes are frequent along coasts bordered by deep water.

The earthquakes which occur at volcanic foci constitute another class
of disturbances which may be accredited to the explosive efforts of
steam.

_Earthquakes due to volcanic evisceration._—By the ejection of ashes
and lava from volcanic vents, there is an extensive evisceration of
the neighbouring ground. When we look at a volcano like Fujiyama,
13,000 feet in height, and at least fifty miles in circumference, and
remember that the mass of cinders and slag of which it is composed
came from beneath the area on which it rests, the point to be wondered
at is, that earthquakes, consequent on the collapse of subterranean
hollows, are not more frequent than they are. At the time of a single
eruption of a volcano, the quantity of lava ejected amounts to many
thousand millions of cubic feet. In 1783 the quantity of lava ejected
from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in
magnitude the bulk of Mont Blanc.’[126] Admitting that hollow spaces
are the results of these eruptions, and that in consequence of this
evisceration the ground is rendered unstable, the instability being
increased by the additional load placed above the eviscerated area, it
would seem that from time to time earthquakes are inevitable.

Facts, however, teach us that volcanoes act as safety valves, and
that, as a rule, at or shortly after an eruption, earthquakes cease.
The relationship of earthquakes to volcanic eruptions would therefore
indicate, notwithstanding the arguments put forward to show that an
area loaded by a volcano has in consequence of the evisceration and the
load a quaquaversal dip, that evisceration does not take place beneath
volcanoes as is usually supposed, and we may conclude that it is but
few earthquakes which have an origin due to these causes.

_Earthquakes and evisceration by chemical degradation._—A powerful
agent, which tends to the formation of subterranean hollows, is
chemical degradation. The effects of this have been often measured by
quantitative analysis of the solid materials which are daily carried
away by many of our springs. In limestone districts this is very great.
Prof. Ramsay estimates that the mineral matter discharged annually by
the hot springs of Bath is equivalent in bulk to a column 140 feet in
height and 9 feet in diameter. At San Filippo, in Tuscany, the solid
matter discharged from the springs has formed a hill a mile and a
quarter long, a third of a mile broad, and 250 feet in thickness.[127]
Many other examples of subterranean chemical degradation will be found
in text-books of geology.

By this chemical action large cavernous hollows are produced. Beneath
a volcano it is probable that liquid material immediately takes the
place of that which is ejected, and that hollows are not formed as in
the case of chemical degradation. If a cavern becomes too large, it
eventually collapses.

Of the falling in of large excavations we have examples in large mines.
As a consequence, not only is a trembling produced, but also a noise,
which is so like that produced by certain earthquakes that the South
American miners have but one word, ‘bramido,’ to express both.[128]

Boussingault, who was an advocate for the theory that many earthquakes
are produced by the sinking of the ground, calls attention to the fact
that we have evidences of the subsidence of great mountains, like
the Andes, the districts around which are so well known for their
earthquakes. Capac Urcu is one of these mountains which legends tell us
has decreased in height.

The variation in the height of mountains is a subject which deserves
attention. That mountains may possibly be hollow, we have the
remarkable results attained by Captain Herschel, who found that the
attractive force of gravity in the neighbourhood of the Himalayas was
not so great as it ought to have been had these mountains been solid.
The Rev. O. Fisher gives another explanation of this phenomenon.
Palmieri considers that the terrible earthquake which devastated
Casamicciola (1881) was due to the hot springs having gradually eaten
out cavernous spaces beneath the town. The extremely local character of
this shock was certainly favourable to such a view.

The earthquake which, in 1840, caused Mount Cernans, in the Jura, to
fall, is also attributed to the solvent action of waters in undermining
its foundations. This undermining action was in great measure probably
due to a large spring, which, twenty-five years previously, had
disappeared, and which subsequently may possibly have been slowly
disintegrating the foundations of the mountain. Earthquakes of this
order would be principally confined to districts where there are rocks
which are more or less soluble, as, for instance, rock salt, gypsum,
and limestone.

_Earthquakes and the attractive influences of the heavenly bodies._—The
most important attractions exercised upon our planet are those due to
the sun and moon. To these influences we owe the tides in our ocean,
and possibly elastic tides in the earth’s crust. Some theorists would
also insist upon liquid tides in the fluid interior of our earth. The
nature of the earth’s interior is, however, a question on which there
is a diversity of opinion.

One doctrine, which, until recent years, received much support, was
that the interior of the earth was a reservoir of molten matter
contained within a thin crust. Hopkins showed that the least
possible thickness of such a crust must be from 800 or 1,000 miles,
otherwise the motions of precession and nutation would be subject to
interference.

M. Delauney objected to the views of Hopkins, on the supposition that
the fluid interior of the earth had a certain viscosity.

Sir William Thomson arrives at the conclusion that the earth on the
whole must be more rigid than a continuous solid globe of glass. Mr.
George H. Darwin’s investigations on the bodily tides of viscous or
semi-elastic spheroids tend to strengthen the arguments of Sir William
Thomson.

Some philosophers hold the view that the central portion of the earth,
although intensely hot, is solid by pressure, whilst the outer crust is
solid by cooling. Between the two there is a shell of liquid or viscous
molten matter.

Another argument is, that although the interior of the globe may be
solid, it is only retained in that condition by an immense pressure, on
the relief of which it is liquefied—it is potentially liquid.

As these views, and the arguments for and against them, are to be
found in all modern text-books of geology, we will at once proceed
to consider the effect of solar and lunar attractive influences in
producing earthquakes upon a globe which is either solid, partially
solid, or which has an interior wholly liquid.

_Effect of the attractive influences of the sun and moon._—In 1854
M. F. Zantedeschi put forward the view—that it is probable there is
a continual tendency of the earth to protuberance in the direction
of the radii vectores of the two luminaries which attract it. In
consequence of these protuberances, pendulums ought at one time to
swing more slowly than at others. Zantedeschi remarks that the periods
of earthquakes appear to confirm such a view, insomuch as they occur
more often at the syzygies, or epoch of the spring tides, than at neap
tides—an observation found in the works of Georges Baglivi (1703) and
Joseph Toaldo (1770).[129]

Prof. Perrey, of Dijon, who did so much for seismology, held the view
that the preponderance in the number of earthquakes felt at particular
seasons was possibly due to the attractive influence of the sun and
moon producing a tide in the fluid interior of the earth, which, acting
on the solid crust, produced fractures.

Rudolf Falb, whose writings have of late years attracted considerable
attention, brings forward views which may be regarded as amplifications
of those suggested by Perrey.

According to Falb, the inner portion of the earth must be regarded as
fluid. In the crust above this fluid reservoir are cracks and channels,
into which, by the attraction of the moon and sun, the fluid is drawn.
On entering these cracks cooling takes place, together with explosions
of gas and subterranean volcanic disturbances. The attractions
producing the internal tides required by Falb are chiefly dependent
upon the following factors:—

1. The nearness and distance of the sun from the earth (January 1 and
July 1).

2. The position of the moon with regard to the earth, which in every
twenty-seven days is once near and once distant.

3. The phases of the moon—whether full or new moon (syzygies), or
whether first or last quarter (quadratures).

4. The equinoxes, the position of the sun in the equator, and the
relative position of the earth.

5. The position of the moon relative to the equator.

6. The concurrence of the ‘centrifugal force’ of the earth with the
last quarter of the moon.

7. The entrance of the moon on the ecliptic—the so-called nodes.

Assuming that earthquakes are wholly consequent on these attractions,
it at once becomes possible to predict their occurrence. This Falb
does, and when his predictions have been fulfilled he has certainly
gained notoriety.

He commenced by the predictions of great storms. In 1873 he predicted
the destructive earthquake of Belluno, which earned for himself a
eulogistic poem, which he has republished in his ‘Gedanken und Studien
über Vulkanismus.’ After this, in 1874, he predicted the eruption of
Etna. He also explained why, in B.C. 4000, there should have been a
great flood, and for A.D. 6400 he predicts a repetition of such an
occurrence.

When we approach the question of the extent to which the attraction
of the sun and moon may influence the production of earthquakes, a
question which we have to answer is, whether it is likely that the
attractive power of the moon is so great that it could draw up the
crust the earth beyond its elastic limits. We know what it can do with
water. It can lift up a hemispherical shell 8,000 miles in diameter
about two or three feet higher at its crown than it lifts the earth.
Even supposing the solid crust to be lifted 100 times the apparent
rise of the tide, is it likely that a hemispherical arch 8,000 miles
in diameter when it is raised 200 feet at its crown could by any
possibility suffer fracture? If an arch is 12,000 miles in length, all
that we here ask is, whether the materials which compose the arch are
sufficiently elastic to allow themselves to be so far stretched that
the crown may be raised 200 feet. The result which we should arrive
at is apparently so obvious that actual calculation seems hardly
necessary. If we regard the earth as being solid, the question resolves
itself into the inquiry as to whether a column of rock, which is equal
in length to the diameter of the earth, or about 8,000 miles, can be
elongated 200 feet without a fracture. This is equivalent to asking
whether a piece of rock one yard in length can be stretched one seventy
thousandth of a foot. Considering that this is a quantity which is
scarcely appreciable under the most powerful of our microscopes, we
must also regard this as a question which it is hardly necessary to
enter into calculations about before giving it an answer. To vary
the method of treating such a question, may we not ask what is the
utmost limit to which it would be possible to raise up or stretch the
crust of the earth without danger of a fracture? Thus, for instance,
to what extent might a column of rock be elongated without danger
of its being broken? From what we know of the tenacity of materials
like brick and their moduli of elasticity, it would seem possible to
stretch a bar of rock 8,000 miles in length for approximately half
a mile before expecting it to break. As to whether there is a wave,
the height of which is equal to half this quantity, running round our
earth as successive portions of its surface pass beneath the attracting
influences of the sun and moon, is a phenomenon which, if it exists,
would probably long ago have met with a practical demonstration.

The deformation which a solid globe or spherical shell would
experience under the attractive influences of the sun and moon has
been investigated by Lamé, Thomson, Darwin, and other physicists and
mathematicians.

A conclusion that we are led to as one result of these valuable
investigations is, that if the interior of the earth be fluid, and
covered with a thin shell, then enormous elastic tides must be
produced. A consequent phenomenon, dependent on the existence of these
tides, would be a marked regularity in the occurrence of earthquakes.
As this marked regularity does not exist, we must conclude that
earthquakes are not due to the attractive influences of the sun and
moon acting upon the thin crust of the earth covering a fluid interior.
The periodicity of earthquakes corroborates the conclusions of Sir
William Thomson, who remarks that if the earth were not extremely rigid
the enormous elastic tides which must result would be sufficient to
lift the waters of the ocean up and down so that the oceanic tide would
be obliterated.

Assuming that the earth has the rigidity assigned to it by mathematical
and physical investigators, we nevertheless have travelling round our
earth, following the attractions of the moon and sun, a tidal stress.
This stress, imposed upon an area in a critical state, may cause it to
give way, and thus be the origin of an earthquake. Earthquakes ought
therefore to be more numerous when these stresses are the greatest.

The periods of maximum stress or greatest pull exerted by the moon and
sun will occur when these bodies are nearest to our planet—that is, in
perigee and perihelion, and again when they are acting in conjunction
or at the syzygies. That earthquakes are _slightly_ more numerous
at these particular periods than at others is a strong reason for
believing that the attractions of the moon and sun enter into the list
of causes producing these phenomena.

Had there been a strongly marked distinction in the number of
earthquakes occurring at these particular seasons as compared with
others, we might have attributed earthquakes to the existence of
elastic tides of a sensible magnitude. As the facts stand, it appears
that the maximum pulls exerted by the moon and sun are only sufficient
to cause a slight preponderance in the number of earthquakes felt at
particular seasons, and therefore that these pulls only result in
earthquakes when the distorting effort has been exerted on an area
which, by volcanic evisceration, the pressure of included gases, and
other causes, is on the verge of yielding.

_Earthquakes and the tides._—If we assume that earthquakes are in many
cases due to the overloading of an area and its consequent fracture,
such loading may occur by the rising of the tide. A belief that the
earthquakes of Japan were attributable to the tides may be found in the
diary of Richard Cocks under the date November 7, 1618, who remarks:—

‘And, as we retorned, about ten aclock, hapned a greate earthquake,
which caused many people to run out of their howses. And about the lyke
hower the night following hapned an other, this countrey being much
subject to them. And that which is comunely markd, they allwais hapen
at a hie water (or full sea); so it is thought it chauseth per reason
is much wind blowen into hollow caves under ground at a loe water, and
the sea flowing in after, and stoping the passage out, causeth these
earthquakes, to fynd passage or vent for the wind shut up.’[130]

Although we may not acquiesce in Cock’s views respecting the imprisoned
wind, it would seem that a comparison of the occurrence of earthquakes
and the state of the tide would be a legitimate research. Inasmuch
as the stresses which are brought to bear upon an area by the rising
of the tide are so very much greater than those due to barometrical
changes, it is not unlikely that a marked connection would be found.
But it must be remembered that because researches, so far as they have
gone, tend to show that earth movements are more frequent when an area
is relieved of a load, it is not unlikely that the greatest number of
earthquakes may be found to occur at low water. Prof. W. S. Chaplin
attempted to make this investigation in Japan, but not being able to
obtain the necessary information respecting the tides, was compelled to
relinquish this interesting work.

Every foot of rise in a tide is equivalent to a load being placed on
the area over which the tide takes place of sixty-two pounds to the
square foot. This load is not evenly distributed, but stops abruptly at
a coast line. Lastly, it may be observed that many coast lines are not
simultaneously subjected to stresses consequent upon this load. Japan,
for instance, may be regarded as an arch placed horizontally. The
area near the crown of this arch is loaded by the tidal wave crossing
the Pacific before the areas near the abutment, and farther there is
a horizontal pressure at the crown which, if Japan were like a raft,
would tend, as the tide advanced, to straighten its bow-like form, but
as the wave passed its abutments to increase its curvature.

Prof. G. Darwin has calculated the amount of rise and fall of a shore
line due to tidal loads (see p. 336, ‘Earth Pulsations’). The result
of these calculations apparently indicates that these loads may have a
considerable influence upon the stability of an area in a more or less
critical condition.

Mr. J. Carruthers suggests that tidal action may hold a general but
indirect relationship to volcanic and seismic action by the retardation
it causes on the earth’s rotation. By this retardation the polar axis
tends to lengthen, and tensile stresses are induced, resulting in
fracture. The fluid interior of the earth, being no longer restrained,
would move polewards, and, leaving equatorial portions unsupported,
this would gradually collapse. The primary fractures would be north and
south, while the secondary fractures would be east and west.[131]

That the rise of the tide is accompanied by a greater percolation
of water to volcanic foci, which, in consequence, assume a greater
state of activity, is a theory which was advanced many years ago. To
determine how far tides may directly be connected with earthquakes, the
necessary records have yet to be examined.

_Variations in atmospheric pressure._—When we consider the immense
load which, by a sudden rise of the barometer, is placed upon the area
over which this rise takes place, it is not difficult to imagine that
this rise may occasionally be the final cause which makes the crust of
the earth to give way. A barometric rise of an inch is equivalent to a
load of about seventy-two pounds being put upon every square foot of
area over which this rise takes place. On the other hand, a fall in the
barometric column indicates that a load has been removed, and whatever
elastic effort may be exerted by subterranean forces in endeavouring to
escape, being met by less resistance, they may burst these bonds, and
an earthquake will result. For reasons such as these the final cause
of earthquakes has often been attributed to variations in atmospheric
pressure. In Japan there are practically as many earthquakes with a
high barometer as with a low one.

The extent to which barometric fluctuations have acted as final causes
in the production of earthquakes may be judged of by a comparison of
the times of barometric variation and the times at which earthquakes
have occurred.

Three important laws of barometric variation are the following:—

1. In the world generally the average barometric pressure is highest in
winter. (Exceptions occur near Iceland and in the North Pacific.)

2. The summer and winter monthly mean barometer differs least near the
equator and over the great oceans. They differ most over the great
continents and generally with increasing latitude.

3. The greatest number of barometrical fluctuations usually take place
in winter.

Inasmuch as there are generally more earthquakes in winter than in
summer, the first of these laws would indicate that this might be due
to the greater load which acts upon the crust of the earth at that
season. The second law would indicate that the distinction between
the winter and summer earthquakes ought to be most marked in high
latitudes, which, if we refer to the table on p. 257, we observe to be
borne out by the results of observation. The countries where there are
as many earthquakes in winter as in summer are chiefly those in low
latitudes. The number of these countries from which we have records
are, however, few.

Facts opposed to the idea that earthquakes may be caused by an increase
of barometric pressure are the results of observations like those of
Schmidt and Rossi, which show that earthquakes chiefly occur with a low
barometer.

Assuming that these latter observations will be found by future
investigators to be generally true, we must conclude that the relief
of atmospheric pressure has an influence upon the occurrence of
earthquakes. Such a conclusion would partially accord with the third
barometrical law, or the fact that there are more occasions on which we
get a low barometer during the winter months.

Other writers who have examined this question are Volger, Kluge,
Andrès, and Poly. The latter investigator sought a connection between
earthquakes and revolving storms, in the centres of which there is
usually an abnormal decrease of atmospheric pressure. If an area over
which such a sudden change in pressure took place was in a critical
state, it is not difficult to see that storms such as Poly refers to
might sometimes be accompanied by earthquakes.

_Fluctuations in temperature._—Inasmuch as fluctuations in temperature
are governed by the sun, it may at once be said that there is a
connection between earthquakes and readings of the thermometer.
Certainly earthquakes occur mostly during the cold months or in
winter. Similarly, as changes in temperature are so closely connected
with barometric fluctuations, and these are said to have a direct
influence upon the yielding of the earth’s crust, seismic phenomena are
indirectly linked to fluctuations in temperature. A rise in temperature
is usually accompanied by a fall in the barometer, and this in turn may
be a condition favourable for the occurrence of an earthquake.

If we regard solar heat as an agent causing expansions or contractions
in the earth’s crust, then fluctuations in temperature become an
immediate cause of earthquakes. The probability, however, is that
solar heat has little or no connection with the final cause producing
earthquakes, although at the same time coincidences between the
occurrence of earthquakes and unusual fluctuations in temperature may
from time to time be observed.

_Winds and earthquakes._—Although it may be admitted that high winds
exert enormous pressures upon mountain ranges, and might occasionally
give rise to stresses causing rocky masses in unstable equilibrium to
give way, the coincidences which have been established between the
occurrence of storms and earthquakes can usually only be regarded as
occurrences which have synchronised by chance.

Storms are usually accompanied with a barometric depression, and the
relation of diminutions in atmospheric pressure to earthquakes has been
discussed.

_Rain and earthquakes._—It has already been shown that earthquakes
have occasionally been found to coincide with rain and rainy seasons.
Whether the saturation of the ground with moisture or the percolation
of the same to volcanic foci may be a direct effect producing
earthquakes it is difficult to say. The probability, however, is
that, rain being dependent on phenomena like changes in temperature,
barometric fluctuations, and winds, we must regard it and the
earthquakes which happen to coincide with these precipitations of
moisture as congruent effects of more general causes.

_Conclusion._—Although it would be an easy matter to discuss the
relationship of earthquakes and other phenomena, we must conclude
that the primary cause of earthquakes is endogenous to our earth, and
that exogenous phenomena, like the attraction of the sun and moon and
barometric fluctuations, play but a small part in the actual production
of these phenomena, their greatest effect being to cause a slight
preponderance in the number of earthquakes at particular seasons. They
may, therefore, sometimes be regarded as final causes. The majority of
earthquakes are due to explosive efforts at volcanic foci. The greater
number of these explosions take place beneath the sea, and are probably
due to the admission of water through fissures to the heated rocks
beneath. A smaller number of earthquakes originate at actual volcanoes.
Some earthquakes are produced by the sudden fracture of rocky strata or
the production of faults. This may be attributable to stresses brought
about by elevatory pressure. Lastly, we have earthquakes due to the
collapse of underground excavations.




                            CHAPTER XVIII.

                      PREDICTION OF EARTHQUAKES.

  General nature of predictions—Prediction by the observation of
    unusual phenomena (alteration in the appearance and taste of
    springs; underground noises; preliminary tremors; earthquake
    prophets—warnings furnished by animals, &c.)—Earthquake warning.


_General nature of predictions._—Ever since seismology has been
studied, one of the chief aims of its students has been to discover
some means which would enable them to foretell the coming of an
earthquake, and the attempts which have been made by workers in various
countries to correlate these occurrences with other well-marked
phenomena may be regarded as attempts in this direction.

Ability to herald the approach of these calamities would unquestionably
be an inestimable boon to all who dwell in earthquake-shaken countries,
and the attempts which have been made both here and in other places are
extremely praiseworthy. In almost all countries where earthquakes are
of common occurrence these movements of the earth have been more or
less connected with certain phenomena which, in the popular mind, are
supposed to be associated with an approach of an earthquake.

If predictions were given in general terms, and they only referred
to time, inasmuch as on the average there are in the world several
shakings per day, we should always find that predictions were
verified. We might even go further and predict that on certain days
earthquakes would occur in certain countries, and still find that in
many instances our supposed power of foresight had not deceived us.
Thus, for instance, in Japan, where on the average there are probably
one or two shakings every day, if prognostications were never correct
there would be a violation of the laws of chance.

What is required from those who undertake to forewarn us of an
earthquake is an indication not only of the time at which the
disturbance will happen, but also an indication of the area in which
it is to occur. Those who dwell in an area where there are certain
well-defined periods during which seismic activity is at a maximum—if
ten or fourteen days should have passed without a shock—might, in many
instances, find that a prophecy that there would be an earthquake
within the next few days would prove itself correct. Also, if a severe
shock had taken place, a prophecy that there would be a second or
third smaller disturbance within a short period would also meet with
verification.

Certain persons with whom I am intimate appear to have persuaded
themselves that they can foretell the coming of an earthquake by the
sultry state of the atmosphere or a certain oppressiveness they feel,
and an instinctive feeling arises that an earthquake is at hand.

It is said that a few minutes before many of the shocks which shook
New England between 1827 and 1847 people could foretell the coming
disturbance by an alteration in their stomach.[132] No doubt many who
dwell in earthquake countries, and have been alarmed by earthquakes,
are at times subject to nervous expectancy.

The author has had such sensations himself, due, perhaps, to a
knowledge that it was the earthquake season, that there had been no
disturbance for some weeks, and a consequent increasing state of
nervous presentments. In consequence of this, not only has he carefully
prepared his instruments for the coming shock, but he has written and
telegraphed to friends to do the same.

Sometimes these guesses have proved correct. One remarkable instance
was a few hours prior to the severe shock of February 22, 1880, when
he communicated with his friends in Yokohama and asked them to see
that their instruments were in good order. Oftener, however, his
prognostications have been incorrect. The point in connection with
this subject which he wishes to be remarked is, that the instances
where earthquakes occurred shortly after the receipt of his letters
are carefully remembered, and often mentioned, but the instances in
which earthquakes did not occur appear to be entirely forgotten. He is
led to mention these facts because they appear to be an experimental
proof of what has taken place in bygone times, and what still takes
place, especially amongst savages—namely, that the record of that which
is remarkable survives, whilst that which is of every-day occurrence
quickly dies. Had the records of all prognostications been preserved,
the probability is that we should find that they had, in the majority
of cases, been incorrect, whilst it would have been but in very few
instances they had been fulfilled.

_Prediction by the observation of natural phenomena._—The above remarks
may perhaps help us to understand the prognostications of the ancient
philosophers about which Professor Antonio Favaro, of Padua, has
written.[133] Cicero in the ‘De Divinatione,’ speaking on this subject,
says that ‘God has not predicted so much as the divine intelligence of
man.’—‘Non Deus prævidet tantum, sed et divini in genii viri.’ Favaro
regards these predictions, however, as the result of observations of
nature which show it is possible that indications of coming earthquakes
had been announced by variations in the gas given out from subterranean
sources, the change in colour, taste, level, temperature of the water
in springs, &c.

In 1843 a bishop of Ischia forewarned his people of a conning
earthquake, and thus was instrumental in the saving of many lives.
Naturally, in an age of superstition, the bishop would be regarded as
a prophet, but Favaro considers that the prognostication was probably
due to a knowledge of premonitory signs as exhibited in changes in the
characters of mineral waters.

The shock of 1851, at Melfi, was in this way predicted by the Capuchin
fathers, who observed that a lake near their door became frothy and
turbulent.

Underground noises have led persons to the belief that an earthquake
was at hand. It was in this way that Viduari, a prisoner at Lima,
predicted the destruction of that city.

Before the earthquake of 1868, so severely felt at Iquique, the
inhabitants were terrified by loud subterranean noises.

That underground noises have preceded earthquakes by considerable
intervals appears to be a fact, but, at the same time, it must
be remembered that similar noises have often occurred without an
earthquake having taken place.

Farmers predicted the earthquake of St. Remo, in 1831, by underground
noises.

On the day before the earthquake which, in 1873, shook Mount Baldo,
the inhabitants of Puos, a village north of Lake Santa Croce, heard
underground noises.

Before the earthquakes which, in 1783, shook Calabria and Sicily, fish
are said to have appeared in great numbers on the coast of Sicily, and
the whirlpool of Charybdis assumed an unusual excited state.

It is said that Pherecydes predicted the earthquakes of Lacedemon and
Helm out, by the taste of the water in the very deep well at the castle
of Lovain.[134]

The writer of an article on the Lisbon earthquake says that ‘after the
24th I felt apprehensive, as I observed the same prognostics as on the
afternoon of October 31, that is, the weather was severe, the wind
northerly, a fog came from the sea, the water in a fountain ran of a
yellow clay colour, and’ he adds, ‘from midnight to the morning of the
25th I felt five shocks.’[135]

At the present time Rudolf Falb, following a theory based upon the
attractive influences of the sun and moon, tells us the time at which
we are to expect earthquakes.

That occasionally there are signs attendant on earthquakes, although
we cannot give them a physical explanation, we cannot doubt. Also we
know that in certain areas earthquakes are more likely to occur at
one season than at another. Should earthquakes be foretold with the
assistance of knowledge of this description, the predictions at once
become the result of the application of certain natural laws, and are
not to be regarded as predictions in the popularly accepted sense of
that term, any more than the arrival of a friend is predicted by the
previous receipt of a telegram announcing his coming.

Rather than accredit the ancients and those of more modern times who,
in consequence of their feelings, have recorded the coming of an
earthquake, with a knowledge of premonitory signs, we might in many
instances regard the records of those prognostications as the survival
of accidental guesses, and, as such, examples of the survival of the
useless.

The effect of accidental occurrences of this description upon an
uneducated mind, in engendering superstition, is a subject which has
often been dwelt upon, and the difficulty of eradicating the same—as
may be judged of by the following accident which came under the
observation of Mr. T. B. Lloyd and the author, in 1873, when travelling
in Newfoundland—will be easily appreciated.

At the time to which I refer, my companion was bringing a canoe down
the rapids below the Grand Pond in a country which is practically
uninhabited, and where an Indian trapper would perhaps be the only
person met with, and this not more than once a year. Whilst shooting
the rapids one of the Indians, Reuben Soulian, shot at a deer passing
up one bank of the river. That the deer had been hit was testified by a
trail of blood which bespattered the rocks. Subsequently several more
shots were fired, and it was believed by all that the deer was killed.
Soulian quickly followed the animal to the spot where it was supposed
to have fallen. Some time after he returned, having failed to find any
trace of the animal. He was greatly agitated, but eventually became
melancholy, saying that the sudden disappearance of the animal was a
sure sign that some of his relations had suddenly died. About two hours
afterwards Mr. Lloyd’s party met with a party of Indians coming up
the river, the first they had seen for four weeks, who told them that
Soulian’s sister had just died on the coast.

In the northern part of South America certain shocks are anticipated by
preliminary vibrations which cause a little bell attached to a T-shaped
frame (cruz sonante) to ring. There are, however, persons (trembloron)
who are supposed to be endowed with seismic foresight, whose verdicts
are much relied upon.

In Caraccas it is said that nearly every street in the river suburb has
an earthquake Cassandra or two. Some of these go so far not only as
to predict the coming seisms, but also the vicissitudes of particular
streets.[136] Earthquake prophets are, however, by no means confined to
the new world, and many examples of them may be found in the histories
of countries where earthquakes have been felt.

The story of the crazy lifeguardsman who prophesied an earthquake to
take place in London on April 4, 1691, is an example. The Rev. Sig.
Pasquel E. Perdini, writing on the earthquakes at Leghorn in 1742,
says that ‘a Milanese astrologer predicted this earthquake for January
27, by which “misfortune” the faith and credit given to the astrologer
gained him more reverence and honour than the prophets and holy
gospel.’ Before the time at which he predicted a second shock, people
removed away from Leghorn.

_Warnings furnished by animals._—A study of the warnings furnished by
animals is also interesting. Several of the natives in Caraccas possess
oracular quadrupeds, such as dogs, cats, and jerboas, which anticipate
coming dangers by their restlessness.

Before the catastrophe of 1812, at Caraccas, a Spanish stallion broke
out from its stable and escaped to the highlands, which was regarded
as the result of the prescience of a coming calamity. Before the
disturbances of 1822 and 1835, which shook Chili, immense flocks of sea
birds flew inland, as if they had been alarmed by the commencement of
some suboceanic disturbance. Before this last shock it is also related
that all the dogs escaped from the city of Talcahuano.

_Earthquake warning._—What has here been said respecting the prediction
of earthquakes is necessarily imperfect—many of the signs which are
popularly supposed to enable persons to foretell the coming of an
earthquake having already been mentioned in previous chapters. That
we shall yet be able to prepare ourselves against the coming of
earthquakes, by the applications of laws governing these disturbances,
is not an unreasonable hope.

With an electric circuit which is closed by a movement of the ground,
we are already in a position to warn the dwellers in surrounding
districts that a movement is approaching.

An earthquake which travelled at the rate of four seconds to the
mile might, if it were allowed to close a circuit which fired a gun
at a station fifteen miles distant, give the inhabitants at that
place a minute’s warning to leave their houses. The inhabitants of
Australia and the western shores of the Pacific might, by telegraphic
communication, receive eighteen to twenty-five hours’ warning of the
coming of destructive sea waves resulting from earthquakes in South
America.

Although warnings like these might have their value, that which is
chiefly required is to warn the dwellers at and near an earthquake
centre of coming disturbances.

What the results of the observations on earth tremors will lead to is
problematical.

Should microseismic observation enable us to say when and where the
minute movements of the soil will reach a head, a valuable contribution
to the insurance of human safety in earthquake regions will have been
attained.

As to whether the movements of tromometers are destined to become
barometric-like warnings of increased activity beneath the earth
crust, or whether they are only due to vibrations of the earth
crust produced by variations in atmospheric pressure, has yet to be
investigated.

Other phenomena which may probably forewarn us of the coming of an
earthquake are phenomena resultant on the stresses brought to bear upon
the rocky crust previous to its fracture, or phenomena due to changes
in the position and condition of heated materials beneath the earth’s
surface. Amongst these may be mentioned electrical disturbances, which
appear to be so closely related to seismic phenomena.

At the time of earthquakes telegraph lines have been disturbed, but
as to what may happen before an earthquake we have as yet but little
information. The subject of earthquake warning is of importance to many
countries, and is deserving of attention.

As our knowledge of earth movements, and their attendant phenomena,
increases, there is but little doubt that laws will gradually be
formulated, and in the future, as telluric disturbances increase, a
large black ball gradually ascending a staff may warn the inhabitants
on the land of a coming earthquake, with as much certainty as the ball
upon a pole at many seaports warns the mariner of coming storms.




                             CHAPTER XIX.

                            EARTH TREMORS.

  Artificially produced tremors—Observations of Kater, Denman,
    Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
    M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
    seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
    Count Malvasia, M. S. di Rossi—Instruments employed in
    Italy—Tromometers, microseismographs, microphones—Results
    obtained in Italy—in Japan—Cause of microseismic motion.


During the past few years considerable attention has been drawn towards
the study of small vibratory motions of the ground, which to the
unaided senses are usually passed by without recognition. These motions
are called _earth tremors_. Their discovery appears to have been due
to accident, and not to the results of inductive reasoning. No sooner
had philosophers contrived astronomical and other instruments for the
purpose of making refined measurements and observations than they at
once discovered that they had an enemy to contend against in the form
of microscopic earthquakes.

_Artificially produced tremors._—Artificial disturbances of this
description exist in all our towns, and near a railway line they are
perceptible with every passing train. Those who have used microscopes
of high power will readily appreciate how small a disturbance of
the ground is visible in the apparent movement of the object under
examination.

Captain Kater found that he could not perform his pendulum experiments
in London on account of the vibrations produced by the rolling of
carriages. Captain Denman, who made some observations on artificially
produced tremors, found that a goods train produced an effect 1,100
feet distant in marshy ground over sandstone. Vertically, however,
above a tunnel through the sandstone, the effects only extended 100
feet.

A remarkable example of the trouble which artificially produced earth
vibrations have occasioned those who make astronomical observations
occurred some twenty years ago at the Greenwich Observatory. When
determining the collimation error of the transit circle by means of the
reflexion of a star in a tray of mercury, it was found that on certain
nights the surface of the mercury was in such a state of trembling
that the observers were unable to complete their observations until
long after midnight. After obtaining a series of dates on which these
disturbances occurred, it was found that they coincided with public and
bank holidays, on which days crowds of the poorer classes of London
flocked to Greenwich Park, and there amused themselves with running and
rolling down the hill on which the observatory is situated. On these
occasions it was found that the disturbances in the mercury were such
that observations could not be made until two or three hours after the
crowds had been turned out of the neighbouring park.[137]

To obviate this difficulty Sir George Airy suspended his dish of
mercury in a system of india-rubber bands, and in this way succeeded in
eating the intruders up.

Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of
Venus expedition in New Zealand, in 1874, was troubled with vibrations
produced from a neighbouring railway. To escape the enemy he intrenched
his instruments by placing them in pits. With pits 3½ feet deep he
found himself sufficiently protected. The distance from the line was
about 400 yards, and the soil through which the disturbances were
propagated was a coarse pebbly gravel.[138]

Before the United States Naval Observatory was established at
Washington, Professor H. M. Paul was deputed to make a tremor survey
to discover stable ground. The results of these experiments were
exceedingly interesting. By watching the reflected image of a star in
a dish of mercury a passing train would be noticed at the distance of
a mile. Its approach could be detected by the trembling of the image
before its coming could be heard. At one point of observation the
disturbance appeared to be cut off by a ravine. The strata was gravel
and clay.[139]

These few examples of artificially produced tremors, to which many
more might be added, have been given because they teach us something
respecting their nature. Hitherto earth tremors have only been regarded
as intruders, which it was necessary to escape from or destroy. From
what has been said they appear to be a superficial disturbance which
is propagated to an enormous distance. This distance appears to
depend upon the propagating medium, upon the intensity of the initial
disturbance, and upon its duration. In the observation of these
artificial disturbances, which are accessible to every one, and which
hitherto have been so neglected, we have undoubtedly a fruitful source
of study.

_Natural tremors._—Next let us turn to those microscopical
disturbances of our soil which are due to natural causes. Thus far they
seem to have been recorded wherever instruments suitable for their
detection have been erected, and it is not improbable that they are
common to the surface of the whole globe.

Some of the more definite observations which have been made upon earth
tremors were those made in connection with experiments on the deviation
of the vertical due to the attractive influence of the moon and sun.

Professor Zöllner, who invented the horizontal pendulum which he used
in the attempt to measure the change in level due to lunar and solar
attraction, found his instruments so sensitive that the readings were
always changing.

The most interesting observations which were made upon small
disturbances of the soil were those of M. d’Abbadie, who carried on his
experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant
from the Atlantic, and 62 mètres above sea level. The soil was a loamy
rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height,
which was pierced down the centre by a vertical hole or well, which was
continued two mètres below the cone into the solid rock. At the bottom
of this hole or well a pool of mercury was formed which reflected
the image of cross wires placed at the top of the hole. These cross
wires and their reflection were observed by means of a microscope.
The observations consisted in noting the displacement and azimuth of
the reflected image relatively to the real image of the wires. After
allowing this structure five years to settle, M. d’Abbadie commenced
his observations. To find the surface of the mercury tranquil was
a rare occurrence. Sometimes the mercury appeared to be in violent
motion, although both the air and neighbouring sea were perfectly calm.
At times the reflected image would disappear as if the mercury had
been disturbed by a microscopic earthquake.

The relative positions of the images were in part governed by the state
of the tide. Altogether the movements were so strange that M. d’Abbadie
did not venture any speculations as to their cause, but he remarks
that the cause of the changes he observed were sometimes neither
astronomical nor thermometrical. These observations, the principal
object of which was to determine changes in level rather than earth
vibrations, were carried on between the years 1868 and 1872.[140]

_Observations at Cambridge._—Another instructive set of observations
were those which were made in the years 1880–1882, by George and Horace
Darwin, in the Cavendish Laboratory, at Cambridge. The main object
in these experiments was to determine the disturbing influence of
gravity produced by lunar attraction. The result which was obtained,
however, showed that the soil at Cambridge was in such an incessant
state of vibration that whatever pull the moon may have exerted upon
the instrument which was employed was masked by the magnitude of the
effects produced by the earth tremors, and the experiments had, in
consequence, to be abandoned.

The principle of this instrument was similar to one devised by Sir
William Thomson, and put up by him in his laboratory at Glasgow.
As erected by the brothers Darwin, at Cambridge, it was briefly as
follows: A pendulum, which was a massive cylinder of pure copper,
was hung by a copper wire, about four feet long, inside a hollow
cylindrical tube rising from a stone support. A small mirror was then
hung by two silk fibres, one of which was fastened to the bob and the
other to the stone basement. A ray of light sent from a lamp on to
the mirror was reflected to a scale seven feet distant, and by this
magnification any motion of the bob relatively to the stone support
was magnified 50,000 times. In several ways the apparatus was insulated
from all accidental disturbances. The spot of light was observed from
another room by means of a telescope. This instrument was so delicate
that even at the distance of sixteen feet the shifting of your weight
from one foot to the other caused the spot of light to run along the
scale. So sensitive was the instrument that, notwithstanding its being
cut off from the surrounding soil by a trench filled with water and
the whole instrument being immersed in water to damp out the small
vibrations, it would seem that the ground was in a constant state of
tremor; in fact, so persistent and irregular were these movements that
it seemed impossible to separate them from the perturbations due to the
attraction of the moon.[141]

As a result of observations like these, the world had gradually
forced upon it the fact that the ground on which we live is probably
everywhere in what is practically an incessant state of vibration.

This led those who were interested in the study of earth movements to
establish special apparatus for the purpose of recording these motions
with the hope of eventually discovering the laws by which they were
governed.

_Experiments in Japan._—The simpler forms of apparatus which have been
used in Japan may be described as delicate forms of seismoscopes,
which, in addition to recording earth tremors, also record the
occurrence of small earthquakes.

A simple contrivance which may be used for the purpose of recording
small earthquakes can be made with a small compass needle.

If a light, small sensitive compass needle be placed on a table, it
will be found that a small piece of iron like a nail may be pushed so
near to it that the needle assumes a position of extremely unstable
equilibrium. If the table now receives the slightest tap or shake this
condition is overcome, and the needle flies to the iron and there
remains. By making the support of the needle and the iron the poles of
an electric circuit it is possible to register the time at which motion
took place with considerable accuracy.

With crude apparatus like this a large number of small earth
disturbances have been recorded.

Another form of apparatus, employed in Japan, has been a delicately
constructed _circuit closer_. The motions of this instrument were
recorded by causing an electro-magnet to deflect a pencil which was
tracing a circle on a revolving dial. The revolving dial was a disc of
wood covered with paper fixed to the hour-hand axle of a common clock.

A third form of apparatus used in Japan consisted of a small piece of
sheet lead about the size of a threepenny piece suspended by a short
loop from a rigid support. Projecting from the lead a fine wire, about
two inches in length, passed freely through a hole in a metallic plate.
By the slightest motion of the support the small pendulum of lead was
set into a state of tremor and caused its pointer to come in contact
with one or other side of the hole in the metal plate and thus to close
an electric circuit.

A more refined kind of apparatus which has been employed in Japan was
similar to that used by the Darwins at Cambridge. This was so arranged
that any deflection of the mirror was permanent until the instrument
was reset, and in this way the maximum disturbance which had taken
place between each observation was recorded.

In addition to these and other contrivances, experiments were made with
microphones.

The microphones used were small doubly pointed pencils of carbon
about three centimetres long, saturated with mercury, and supported
vertically in pivot holes bored in other pieces of carbon, which were
the terminals of an electric circuit. These microphones were screwed
down on the top of stakes driven deeply into the ground. They were
covered with a glass shade thickly greased at its base. The stakes were
in the ground at the bottom of a small pit—about two feet square and
two feet deep—which was lined with a box. The box was covered with a
lid, and earth to the depth of nine inches or one foot. One of these
pits was in the middle of a lawn in the front of my house, and the
other was at the foot of a hill at the back of the house. The wires
from the microphone passed through the side of the box into a bamboo
tube and thence up to my dining-room and bed-room. In one of the
circuits there were three Daniell’s cells, a telephone, and a small
galvanometer. I used the galvanometer because I found that when there
was sufficient motion in the microphone to produce a sound in the
telephone a motion in the needle of a galvanometer was produced. If in
any case motion took place in the magnetic needle during my absence, it
was held deflected by a small piece of iron with which it was brought
into contact by the movement.

The sensitiveness of the arrangement may be judged of from the fact
that if a person walked on the grass within six feet of the microphone,
each step caused a creak in the telephone, and the needle of the
galvanometer was caused to swing and come in contact with the iron.
Dogs running on the grass had no effect. A small stone one or two
inches in diameter thrown from the house so that it fell near to the
microphone pit caused a sharp creak in the telephone and a movement in
the needle.

The nature of the records I received from this contrivance may be
judged of from the following extract from my papers.

                  h. m.
  1879. Nov. 12th 7  0 P.M. contact of needle
                  7  2  „   difficult to set the needle
                  7  3  „   needle swings and telephone creaks
                  7  4  „         „           „           „
                  7  5  „         „           „           „
                  7  6  „         „           „           „
                  7 10  „   3 more swings
                  7 11  „   again    „

Here I went out, took away the covering, and examined the microphone.
Nothing wrong was to be observed. All that I saw was one small ant. I
do not think that this could have caused the disturbance, because it
could not get near the instrument.

On the succeeding nights I experienced similar disturbances, and it
seemed as if they might possibly have been the prelude to several small
shocks which occurred about this time (November 15, 16, and 17). On
November 17, at 8 A.M., the needle was found in contact, and again at
5 P.M., and at 6 P.M. the shock of a small earthquake was felt _which
caused a rattling sound in the telephone for about one minute after the
motion had appeared to cease_. The needle swung considerably, but did
not come in contact.

The great objection to these observations is that it is possible
that the movements and sounds which I have recorded might, with the
exception of one case when the shaking was actually felt, possibly have
been produced by causes other than that of the movement of the ground.
To determine this I subsequently put up two distinct sets of apparatus
to determine whether the motions of each were synchronous. So far as
I went this appeared only to be sometimes the case:—but this is a
question difficult to determine, unless a recorder of time be added to
the apparatus.

The greatest objection to observations of this sort is that the
sensibility of the instrument is not constant. After a current has been
running for several days it is no longer sensible to slight shocks, it
appears as if its resistance had been increased. To overcome this it is
necessary to resharpen the carbon points and bore out the pivot holes
every three or four days. Farther, the battery varies. This might to
some extent be overcome by using a battery with large plates. These
two causes tend to reduce the sensitiveness of the galvanometer-like
recorder—the deflection of the needle gradually becoming less and less,
and therefore day by day needing a greater swing to bring it into
contact with the iron. For reasons such as these this instrument, to be
used successfully, appears to require considerable attention.

Another form of microphone employed by the author consisted of an
aluminium wire standing vertically on a metallic plate, its upper end
passing loosely through a hole in an aluminium wire standard.

The upper end of the vertical wire was loaded with lead. This
contrivance possesses all the sensitiveness of an ordinary microphone,
whilst, if it receives a sudden impulse, there is a sudden break in the
current, and the vertical wire is thrown from one side to the other of
the hole in the standard.

After many months of tiresome observation with instruments of this
description, and after eliminating all motions which might have been
produced by accidental causes, the general result obtained showed that
in Tokio there were movements of the soil to be detected every day,
and sometimes many times per day, which to ordinary persons were passed
by unnoticed.

_Work in Italy._—The most satisfactory observations which have been
made upon microseismic disturbances are those which have been made
during the last ten years in Italy. The father of systematical
microseismical research appears to have been Father Timoteo Bertelli,
of Florence.

In 1870 Father Bertelli suspended a pendulum in a cellar, and observed
it with a microscope. As the result of his observations it was
announced that he had perceived the earthquakes which shook Romagna,
although to the ordinary observer in Florence these shakings had not
been perceptible.

In 1873 Bertelli, by means of microscopes fixed in several azimuths,
made 5,500 observations on free pendulums. He also made observations on
reflections from the surface of mercury.[142]

One result of these observations was to show that microseismic motions
increased with a fall of the barometer. Similar observations were made
at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near
Rome. On January 14, 15, and February 25 these three observers at their
respective stations simultaneously observed great disturbances.

Similar investigations were made at Nice by M. le Baron Prost.

Although doubt was cast upon Bertelli’s observations they appear to
have been the origin of a series of microseismical observations, a
distinguished leader in which is Professor Rossi, who, in 1874, found
that large earthquakes were almost always preceded or accompanied with
microseismical storms. In 1878 Professor Rossi worked upon these small
disturbances with the assistance of the microphone and telephone, and
his first results were published by Professor Palmieri.

Many of Professor Rossi’s observations were made in the grotto of Roca
de Papa, 700 mètres high and eighteen mètres under the soil. Here over
6,000 observations were made by means of microscopes, on pendulums of
different lengths, suspended in tubes cut in the solid rock.

_Instruments employed in Italy._—It is impossible to describe in detail
the various forms of apparatus which have been used by the Italian
investigators. A description of one or two of the more important
instruments may not, however, be out of place, inasmuch as they will
assist the reader to understand the manner in which the various results
respecting the laws governing microseismic movements have been arrived
at.

The most important of these instruments is the _Normal Tromometer_ of
Bertelli and Rossi.

This consists of a pendulum 1½ metres long, carrying, by means of
a very fine wire, a weight of 100 grammes. To the base of the bob
a vertical stile is attached, and the whole is enclosed in a tube
terminated, at its base, by a glass prism of such a form that when
looked through horizontally the motion of the stile can be seen in
all azimuths. In front of this prism a microscope is placed. Inside
the microscope there is a micromatic scale, so arranged that it can
be turned to coincide with the apparent direction of oscillation of
the point of the stile. In this way not only can the amplitude of the
motion of the stile be measured, but also its azimuth. The extent of
vertical motion is measured by the up and down motion of the stile due
to the elasticity of the supporting wire. This instrument is shown in
the accompanying drawing.

[Illustration: FIG. 37.—Normal Tromometer.

B, bob of pendulum; P, prism; M, microscope; S, rim of scale.]

Another apparatus is the _Microseismograph_ of Professor Rossi. Here we
have an arrangement which gives automatic records of slight motions.
It consists of four pendulums, each about three feet long, suspended
so that they form the corners of a square platform. In the centre
of this platform a fifth, but rather longer, pendulum is suspended.
The four pendulums are each connected just above their bobs to the
central pendulum with loose silk threads. Fixed to the centre of each
of these threads, and held vertically by a light spring, is a needle,
so adjusted that each thread is depressed to form an obtuse angle of
about 155°. These needles form the terminals of an electric circuit,
the other termination of which is a small cup of mercury placed just
below the lower end of the needle. By a horizontal swing of one of the
pendulums this arrangement causes the needle to move vertically, but
with a slightly multiplied amplitude. By this motion the needle comes
in contact with the mercury, and an electro-magnet with a lever and
pencil is caused to make a mark on a band of paper moved by clockwork.
The five pendulums being of different lengths allows the apparatus to
respond ‘to seismic waves of different velocities.’[143]

Lastly, we have Professor Rossi’s _Microphone_. This consists of a
metallic swing arranged like the beam of a balance. By means of a
movable weight at one end of the beam this is so adjusted that it
falls down until it comes in contact with a metallic stop. This can be
so adjusted that a slight tap will cause the beam to slightly jump from
the stop. The beam and the stop form two poles of an electric circuit,
in which there is a telephone. The slightest motion in a _vertical_
direction causes a fluctuation in the current passing between the stop
and the beam, and a consequent noise is heard in the telephone.

With instruments analogous to these, observations have been made by
various observers in all portions of Italy, extending over a period
of ten years. Every precaution appears to have been taken to avoid
accidental disturbances, and the experiments have been repeated in a
variety of forms.

_Results obtained in Italy._—The results which from time to time have
been announced are of the greatest interest to those who study the
physics of the earth’s crust, and they appear to be leading to the
establishment of laws of scientific value.

It would seem that the soil of Italy is in incessant movement, there
being periods of excessive activity usually lasting about ten days.
Such periods are called seismic storms. These storms are separated by
periods of relative calm. These storms have their greater regularity
in winter, and sharp maximums are to be observed in spring and autumn.
In the midst of such a period or at its end there is usually an
earthquake. Usually these storms are closely related to barometric
depressions. To distinguish these movements from those which occur
under high pressure, the latter are called baro seismic movements, and
the former _vulcano seismic_ movements. The relation of these storms
to barometric fluctuation has been observed to have been very marked
during the time of a volcanic eruption.

At the commencement of a storm the motions are usually small, and one
storm, lasting two or three days, may be joined to another storm.
In such a case the action may be a local one. It has been observed
that a barometrical depression tended to bring a storm to a maximum,
whilst an increase of pressure would cause it to disappear. Sometimes
these actions are purely local, but at other times they may affect a
considerable tract of land.

If a number of pendulums of different length are observed at the same
place, there is a general similarity in their movements, but it is also
evident that the free period of the pendulum more or less disturbs
the character of the record. The greatest amplitude of motion in a
set of pendulums is not reached simultaneously by all the pendulums,
and at every disturbance the movement of one will predominate. From
this Rossi argues that the character of the microseismical motions is
not constant. Bertelli observed that the direction of oscillation of
the pendulums is different at different places, but each place will
have its particular direction dependent upon the direction of valleys
and chains of mountains in the neighbourhood. Rossi shows that the
directions of movement are perpendicular to the direction of lines of
faults, the lips of these fractures rising and falling, and producing
two sets of waves, one set parallel to the line of fracture, and the
other perpendicular to such a direction. These movements, according
to Bertelli, have no connection with the wind, rain, change of
temperature, and atmospheric electricity.

[Illustration: FIG. 38.]

The disturbances, as recorded at different towns, are not always
strictly synchronous, but succeed each other at short intervals. If,
however, we take monthly curves of the disturbances as recorded at
different towns in Italy, we see that these are similar in character.
The maximum of disturbance occurs about the winter solstice, and the
minimum about the summer solstice, and in this respect they exhibit
a perfect accordance with the curves drawn by Mallet to show the
periodicity of earthquakes. The accompanying curves taken from one
of Bertelli’s original memoirs not only show this general result but
also show the close accord there is between the results obtained at
different towns during successive months.

At Florence, before a period of earthquakes there is an increase in the
amplitude and frequency of vertical movements. These vertical movements
do not appear to coincide with the barometrical disturbances, but they
appear to be connected with the seismic disturbances.

They are usually accompanied with noises in the telephone, but as the
microphone is so constructed as to be more sensitive to vertical motion
than to horizontal motion, this is to be expected. This vertical motion
would appear to be a local action, inasmuch as the accompanying motions
of an earthquake which originates at a distance are horizontal.

Storms of microseismical motions appear to travel from point to point.

Sometimes a local earthquake is not noticed in the tromometer,
whilst one which occurred at a distance, although it may be small,
is distinctly observed. To explain this, Bertelli suggests the
existence of nodes. Similar conclusions were arrived at by Rossi when
experimenting on different portions of the sides of Vesuvius. Galli
noticed an augmentation in microseismic activity when the sun and moon
are near the meridian. Grablovitz found from Bertelli’s observations
a maximum two or three days before the syzygies, and a minimum
three days after these periods. He also found that the principal
large disturbances occurred in the middle of periods separating
the quadrature from the syzygies, the apogee from perigee, and the
lunistigi period from the nodes, whilst the smallest disturbances
happened in the middle of periods opposed to these.

P. C. Melzi says that the curves of microseismical motions,
earthquakes, lunar and solar motions, show a concordance with each
other.

With the microphone Rossi hears sounds which he describes as roarings,
explosions, occurring isolated or in volleys, metallic and bell-like
sounds, ticking, &c., which, he says, revealed natural telluric
phenomena. Sometimes these have been intolerably loud. At Vesuvius the
vertical shocks corresponded with a sound like volleys of musketry,
whilst the undulatory shocks gave the roaring. Some of these sounds
could be imitated artificially by rubbing together the conducting wires
in the same manner in which the rocks must rub against each other in
an earthquake. Other sounds were imitated by placing the microphone
on a vessel of boiling water, or by putting it on a marble slab and
scratching and tapping the under side of it.

These, then, are some of the more important results which have been
arrived at by the study of microseismic motions. One point which seems
worthy of attention is that they appear to be more law-abiding than
their more violent relations, the earthquakes, and as phenomena in
which natural laws are to be traced they are certainly deserving our
attention. As to whether they will ever become the means of forewarning
ourselves against earthquakes is yet problematical. Their systematic
study, however, will enable us to trace the progress of a microseismic
storm from point to point, and it is not impossible that we may yet
be enabled to foretell where the storm may reach its climax as an
earthquake. These, I believe, are the views of Professor di Rossi, who
is at the present time engaged in the establishment of a system of
microseismic observations throughout Italy.

Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer
was in a state of agitation, and similar disturbances were observed at
Livorno, Florence, and Bologna.

Since February 1883 I have observed a tromometer in Japan, and such
results as have been obtained accord with results obtained in Italy.
The increase in microseismical activity with a fall of the barometer
is very marked. Other peculiarities in the behaviour of the instrument
will be referred to under ‘Earth Pulsations.’

_Cause of microseismic movements._—As to the cause of tromometric
movements, we have a field for speculation. Possibly they may be
due to slight vibratory motions produced in the soil by the bending
and crackling of rocks produced by their rise upon the relief of
atmospheric pressure. If this were so we should expect similar
movements to be produced at the time of an increase of pressure. Rossi
suggests that they may be the result of an increased escape of vapour
from the molten materials beneath the crust of the earth, consequent
upon a relief of pressure. The similarity of some of the sounds which
are heard with the microphone to those produced by boiling water are
suggestive of this, and Rossi quotes instances when underground noises
like those which we should expect to hear from a boiling fluid have
been heard before earthquakes without the aid of microphones. One
instance was that of Viduari, a prisoner in Lima, who, two days before
the shock of 1824, repeatedly predicted the same in consequence of the
noises he heard.

A possible cause of disturbances of this order may be small but
sudden fluctuations in barometric pressure, which are visible during
a storm. During a small typhoon on September 15, 1881, when in the
Kurile Islands, I observed that the needle of an aneroid worked back
and forth with a period of from one to three seconds. This continued
for several hours. With every gust of wind the needle suddenly rose
and then immediately fell. At times it trembled. These movements were
observed in the open air. The extent of these sudden variations
was approximately from ·03 to ·05 inches. Beckoning an increase of
barometrical pressure of one hundredth of an inch as equivalent to a
load of twenty million pounds on the square mile, during this storm
there must have been the equivalent of loads of from 60 to 100 million
pounds to the square mile continually placed on and removed from a
considerable tract of the earth’s surface. If the period of application
of these stresses approximately coincide with the natural vibrational
period of the area affected, it would surely seem, especially when
we reflect upon the effect of an ordinary carriage, that tremors of
considerable magnitude ought to be produced.

An inspection of the following few observations taken from my note-book
for the same typhoon will suggest that even the large and slower
variations are capable of producing tremulous motions.

                        Time           Barometer
                        h.  m.          reading
                        12  5 P.M.       29·02
                        12 10  „         29·05
                        12 12  „         29·07
                        12 13  „         29·05
                        12 25  „         29·10
                        12 50  „         29·00
                         1 10  „         29·00
                         1 20  „         29·07




                              CHAPTER XX.

                           EARTH PULSATIONS.

  Definition of an earth pulsation—Indications of
    pendulums—Indications of levels—Other phenomena indicating
    the existence of earth pulsations—Disturbances in lakes and
    oceans—Phenomena resultant on earth pulsations—Cause of earth
    pulsations.


The object of the present chapter is to show that from time to time it
is very probable that slow but large wave-like undulations travel over
or disturb the surface of the globe.

These movements, which have escaped our attention on account of their
slowness in period, for want of another term I call earth pulsations.

The existence of movements such as these may be indicated to us by
changes in the level of bodies of water like seas and lakes, by the
movements of delicate levels, by the displacement of the bob of a
pendulum relatively to some point on the earth above which it hangs,
and by other phenomena which will be enumerated.

_Indication of pendulums._—Pendulums which have been suspended for the
purposes of seismometrical observations have, both by observers in
Italy and Japan, been seen to have moved a short distance out from, and
then back to, their normal position.

This motion has simply taken place on one side of their central
position, and is not due to a swing. The character of these records
is such that we might imagine the soil on which the support of the
pendulum had rested to have been slowly tilted, and slowly lowered.
They are the most marked on those pendulums provided with an index
writing a record of its motions on a smoked glass plate, which index is
so arranged that it gives a multiplied representation of the relative
motion between it and the earth. As motions of this sort might be
possibly due to the action of moisture in the soil tilting the support
of the pendulum, and to a variety of other accidental causes, we cannot
insist on them as being certain indications that there are slow tips in
the soil, but for the present allow them to remain as possible proof of
such phenomena.

Evidence of displacement of the vertical, which are more definite than
the above, are those made by Bertelli, Rossi, Count Malvasia, and other
Italian observers, who, whilst recording earth tremors, have spent so
much time in watching the vibrations of stiles of delicate pendulums
by means of microscopes. As a result of these observations we are
told that the point about which the stile of a pendulum oscillates
is variable. These displacements take place in various azimuths, and
they appear to be connected with changes of the barometer. I have made
similar observations in Japan.

From this, and from the fact that it is found that a number of
different pendulums differently situated on the same area give similar
evidence of these movements, it would hardly seem that these phenomena
could be attributed to causes like changes in temperature and moisture.
M. S. di Rossi lays stress on this point, especially in connection with
his microseismograph, where there are a number of pendulums of unequal
length which give indications of a like character. The direction in
which these tips of the soil take place—which phenomena are noticeable
in seismic as well as microseismic motions—Rossi states are related to
the direction of certain lines of faulting.

_Indications of levels._—Bubbles of delicate levels can be easily seen
to change their position with meteorological variations; but Rossi also
tells us that they change their position, sometimes not to return for
a long time, during a microseismic storm. Here again we have another
phenomenon pointing to the fact that microseismic disturbances are the
companions of slow alterations in level.

One of the most patient observers of levels has been M. Plantamour,
who commenced his observations in 1878, at Sécheron, on the Lake of
Geneva. He used two levels, one placed north and south, and the other
east and west. During the summer of 1878 the east end rose, but at the
end of September a depression set in. The diurnal movements had their
maximum and minimum at 6 and 7.45 A.M. and P.M. The total amplitude was
4·89″. The variations of the east and west level appeared to be due to
the temperature, but the movements of the north and south level were
dependent upon an unknown cause.

Between October 1, 1879, and September 30, 1880, the east end fell
rapidly, from the middle of November up to December 26, amounting to
88·71″. It then rose 6·55″ to January 5, and then fell again. On
January 28 it reached 89·95″, after which it rose.

Between October 4, 1879, and January 28, 1880, the movement was 95·8″,
against 28·08″ of the previous year.

These movements were not due alone to temperature. The north and south
level, which was not influenced by the cold of the winter, moved
4·56″. In the previous year 4·89″.[144]

From February 17 to June 5, 1883, the author observed in Tokio the
bubbles of two delicate levels, one placed north and south, and the
other east and west. They were placed under glass cases on the head of
a stone column. The column, which is inside a brick building, rests
on a concrete foundation, and is about ten years old. It is in no way
connected with the building. The temperature of the room has a daily
variation of about 1° Fahr.

In both these levels diurnal changes are very marked. Occasionally they
are enormously great. Thus, on March 25, the readings of the south end
of the north south level were as follows:—

                            Time           Readings.
                            h. m.
                      25th. 4 00 P.M.       104·5
                            4  5  „         103
                            4 10  „         102
                            4 25  „         101
                            4 30  „         100
                            4 40  „          98
                            4 42  „          99·5
                            4 45  „         100
                            4 50  „         101
                            4 55  „         101
                            5 00  „         100
                      26th. 7 00 A.M.       105

Usually this level moves through about three divisions per day.

From March 25 to May 4 it travelled from 98 to 127. Since then, to June
5, it has descended to 116. During this period the east west level
has been _comparatively_ quiet. One division of the north south level
equals about 2″ of arc.

Many of these changes may be due to changes in temperature, variations
in moisture, and other local actions. Some of them, however, are hardly
explicable on such assumptions. The fact that the general direction in
change of the vertical, as indicated by a tromometer standing on the
same column with the levels, showed that the change which was taking
place was rather in the column than in the instruments.

The fact also that at the time of a barometrical depression a
_pulse-like surge_ can be seen in the levels, having a period averaging
about three seconds and sometimes amounting to about one second of
arc, is a phenomenon hardly to be attributed to sudden fluctuations in
moisture or temperature, but indicates real changes in level.[145]

In addition to variation in the bubbles of levels which come on more or
less gradually, we have many recorded instances of _sudden_ alterations
taking place in these instruments.

Examples of what may have been a slow oscillating motion of the earth’s
crust are referred to by Mr. George Darwin in a Report to the British
Association in 1882.

One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged
in levelling the axis of a telescope, observed spontaneous oscillation
in the bulb of the level.

This was on May 10 (April 28), 1877. The complete period was about
20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen
minutes before this he observes that there had been a severe earthquake
at Iquique, the distance to which in a straight line was 10,600
kilomètres, and on an arc of a great circle, 12,500 kilomètres.
On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova
oscillations of 3″, seven minutes before which there had been an
earthquake at Malta. On April 4 (March 23), 1868, an agitation of the
level had been observed by Mr. Gromadzki, five minutes before which
there had been an earthquake in Turkestan. Similar observations had
been made twice before. These, however, had not been connected with any
earthquakes—at least, Mr. Darwin remarks—with certainty.

_Phenomena analogous to the pendulum and level observations._—As
examples of phenomena which are analogous to those made on pendulums
and levels, the following may be noticed. On March 20, 1881, at 9 P.M.
a watchmaker in Buenos Ayres observed that all his clocks oscillating
north and south suddenly began to increase their amplitude, until some
of them became twice as great as before. Similar observations were
made in all the other shops. No motion of the earth was detected.
Subsequently it was learnt that this corresponded with an earthquake in
Santiago and Mendoza.[146]

Another remarkable example illustrating the like phenomena is furnished
by the observations which were made on December 21, 1860, by means of a
barometer in San Francisco, which oscillated, with periods of rest, for
half an hour. No shock was felt, nor is it likely that it was a local
accident, as it could not be produced artificially. On the following
day, however, a violent earthquake was experienced at Santiago.[147]

At the time or shortly after the great Lisbon earthquake, curious
phenomena were observed in distant countries, which only appear to be
explicable on the assumption of the existence of earth pulsations.

Thus at Amsterdam and other towns, chandeliers in churches were
observed to swing. At Haarlem water was thrown over the sides of tubs,
and it is expressly mentioned that no motion was perceived in the
ground.

At the Hague a tallow chandler was surprised at the clashing noise
made by his candles, and this the more so because no motion was felt
underfoot.

_Unusual disturbances in bodies of water._—At the time of large
earthquakes it would appear that earth pulsations are produced, which
exhibit themselves in countries where the actual shaking of the
earthquake is not felt, by disturbances in bodies of waters like lakes
and seas.

Some remarkable examples of these disturbances are to be found in the
records of the great Lisbon earthquake. This earthquake, as a violent
movement of the ground, was chiefly felt in Spain, Portugal, northern
Italy, the south of France and Germany, northern Africa, Madeira, and
other Atlantic islands. In other countries further distant, as, for
instance, Great Britain, Holland, Scandinavia, and North America,
although the records are numerous, the only phenomena which were
particularly observed was the slow oscillations of the waters in lakes,
ponds, canals, &c. In some instances the observers especially remark
that there was no motion in the soil.

Pebley Dam, in Derbyshire, which is a large body of water covering
some thirty acres, commenced to oscillate from the south. A canal near
Godalming flowed eight feet over the walk on the north side.

Coniston Water, in Cumberland, which is about five miles long,
oscillated for about five minutes, rising a yard up its shores. Near
Durham a pond, forty yards long and ten broad, rose and fell about one
foot for six or seven minutes. There were four or five ebbs and flows
per minute.

Loch Lomond rose and fell through about two and a half feet every
five minutes, and all the other lochs in Scotland seem to have been
similarly agitated.

At Shirbrun Castle, in Oxfordshire, where the water in some moats and
ponds was very carefully observed, it was noticed that the floods
began gently, the velocity then increased, till at last with great
impetuosity they reached their full height. Here the water remained for
a little while, until the ebb commenced, at first gently, but finally
with great rapidity. At two extremities of a moat about 100 yards long,
it was found that the sinkings and risings were almost simultaneous.
The motions in a pond a short distance from the moat were also
observed, and it was found that the risings and sinkings of the two did
not agree.

During these motions there were several maxima.

These few examples of the motions of waters, without any record of the
motions of the ground, at the time of the Lisbon earthquake, must be
taken as examples of a very large number of similar observations of
which we have detailed accounts.

Like agitations, it must also be remembered, were perceived in North
America and in Scandinavia, and if the lakes of other distant countries
had been provided with sufficiently delicate apparatus, it is not
unlikely that similar disturbances would have been recorded.

Besides these movements in the waters of seas and lakes, at or about
the time of great earthquakes, we have records of like movements, which
take place as independent phenomena.

Thus we read that on October 22, 1755, the waters of Lake Ontario rose
and fell five and a half feet several times in the course of half an
hour.[148] On March 31, 1761, Loch Ness rose suddenly for the period
of three-quarters of an hour.[149]

As another example of the disturbance of water at the time of a great
earthquake in districts where the earthquake was not felt, may be
mentioned the swelling of the waters of the Marañon, in 1746, on the
night when Callao was overwhelmed.

Sudden variations in the level of the water have been many times
observed in the North American lakes. The changes in level which
sometimes take place in the Genfer and Boden lakes are supposed to have
some relation to the condition of the atmosphere. A rising and falling
of especial note took place on April 18, 1855.

In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’

From the observations and calculations of Prof. Forel it would seem
that the period of the ‘seiches’ depends upon the dimensions of the
lakes; the calculated periods dependent on the depths of the lakes
being approximately equal to the observed periods.[150]

W. T. Bingham, writing on the volcanoes of the Hawaiian Islands,
remarks that it is not unusual for the sea to be agitated by great and
unusual tides, and that such sea waves have not been attended with
volcanic eruptions or seismic disturbances. Thus in May 1819 the tide
rose and fell thirteen times. On November 7, 1837, there was an ebb and
flow of eight feet every twenty-eight minutes. Again, on May 17, 1841,
like phenomena, unaccompanied by any other unusual occurrences, were
recorded.[151]

Phenomena which may possibly hold a relationship to earth pulsations
are the periodical swellings of the ocean on the coast of Peru. Dr. C.
F. Winslow, who made a long period observation upon the coast of Peru,
found ‘the highest tides to prevail at Callao and Paita in December
and January,’ and ‘also a series of enormous waves or sea-swells to
be thrown from time to time upon the coast, varying from twenty-four
to twenty-seven hours in continuance, accompanied by unusual height
of the tide during the same period.’ During June and July the ocean
was unusually tranquil. These phenomena do not appear to be connected
with great atmospheric storms, nor do they hold any relation to the
prevailing wind. They increase with and accompany the swelling of the
tides, and occur generally, but not always, about full moon.

Sometimes they break suddenly upon the coast. ‘_They are annual and
constant in their periodicity._’

The periodical swellings are most noticeable between Tumbez 3° S.L. and
the Chincha Islands 14° S.L.

These oceanic phenomena synchronise with the periodic intensity of
earthquake phenomena in that part of the globe, and these with tidal
movements.[152]

_Other phenomena possibly attributable to earth pulsations._—If we
assume that earth pulsations have an existence, these many phenomena
which are otherwise difficult to understand meet with an explanation.
The curious effects which were produced in the springs at Toplitz at
the time of the Lisbon earthquake may have been due to a pulse-like
wave. The flow of the principal spring was greatly increased. Before
the increase it became turbid and at one time stopped. Subsequently it
became clear and flowed as usual, but the water was hotter and more
strongly mineralised. Sudden changes in the flow of underground waters
which from time to time are observed may be attributed to like causes.
Secondary earthquakes such as occurred after the Lisbon earthquake,
as for instance in Derbyshire, may have been produced by pulsations
disturbing the equilibrium of ground in a critical state.

The falling in of subterranean excavations is also possibly connected
with these phenomena.

_Possible causes of earth pulsations._—Mr. George Darwin, in a report
to the British Association (1882), has shown that movements of
considerable magnitude may occur in the earth’s crust in consequence of
fluctuations in barometrical pressure. (A rise of the barometer over an
area is equivalent to loading that area with a weight, in consequence
of which it is depressed. When the barometer falls, the load is removed
from the area, which, in virtue of its elasticity, rises to its
original position. This fall and rise of the ground completes a single
pulsation.)

On the assumption that the earth has a rigidity like steel, Mr. Darwin
calculates that if the barometer rises an inch over an area like
Australia, the load is sufficient to sink that continent two or three
inches.

The tides which twice a day load our shores cause the land to rise and
fall in a similar manner. On the shores of the Atlantic, Mr. Darwin
has calculated that this rise and fall of the land may be as much as 5
inches. By these risings and fallings of the land the inclination of
the surface is so altered that the stile of a plummet suspended from a
rigid support ought not always to hang over the same spot. There would
be a deflection of the vertical.

In short, calculations respecting the effects of loads of various
descriptions, which we know are by natural operations continually
being placed upon and removed from the surface of various areas of the
earth’s surface, indicate that slow pulsatory movements of the earth’s
surface must be taking place, causing variations in inclination of one
portion of the earth’s crust relatively to another.

Although it is possible that phenomena like the surging of levels may
be attributable to causes like these, we can hardly attribute the other
phenomena to such agencies.

Rather than seek an explanation from agencies exogenous to our earth,
we might perhaps with advantage appeal to the endogenous phenomena of
our planet. When the barometer falls, which we have shown corresponds
to an upward motion of the earth’s crust, we know, from the results of
experiments, that microseismic motions are particularly noticeable.

As a pictorial illustration of what this really means, we may imagine
ourselves to be residing on the loosely fitting lid of a large
cauldron, the relief of the external pressure over which increases the
activity of its internal ebullition—the jars attendant on which are
gradually propagated from their endogenous source to the exterior of
our planet. This travelling outwards would take place much in the same
way that the vibrations consequent to the rattle and jar of a large
factory slowly spread themselves farther and farther from the point
where they were produced.

Admitting an action of this description to take place, it would then
follow that this extra liberation of gaseous material beneath the
earth’s crust would result in an increased upward pressure from within,
and a tendency on the part of the earth’s crust to elevation. If we
accept this as an explanation of the increased activity of a tremor
indicator, then such an instrument may be regarded as a barometer,
measuring by its motions the variations in the internal pressure of our
planet.

The relief of external pressure and the increase of internal pressure,
it will be observed, both tend in the same direction—namely, to an
elevation of the earth’s crust.

This explanation of the increased activity of earth tremors, which
has also been suggested by M. S. di Rossi, is here only advanced as a
speculation, more probable perhaps than many others.

We know how a mass of sulphur which has been fused in the presence of
water in a closed boiler gives up in the form of steam the occluded
moisture upon the relief of pressure. In a similar manner we see steam
escaping from volcanic vents and cooling streams of lava. We also
know how gas escapes from the pores and cavities in a seam of coal
on the fall of the barometrical column. We also know that certain
wells increase the height of their column under like conditions. The
latter of these phenomena, resulting in an increase in the rate of
drainage of an area by its tendency to render such an area of less
weight, facilitates its rise. If we follow the views of Mr. Mallet in
considering that the pressures exerted on the crust of our earth may in
volcanic regions be roughly estimated by the height of a column of lava
in the volcanoes of such districts, we see that in the neighbourhood of
a volcano like Cotopaxi the upward pressures must be enormously great.
Further, the phenomena of earthquakes and volcanoes indicate that these
pressures are variable. Before a volcano bursts forth we should expect
that there would be in its vicinity an upward bulging of the crust, and
after its formation a fall. Further, it is not difficult to conjecture
other possible means by which such pressures may obtain relief.

Should these pressures then find relief without rupturing the
surface, it is not difficult to imagine them as the originators of
vast pulsations which may be recorded on the surface of the earth as
wave-like motions of slow period.

As an explanation of the strange movements observed on seas and lakes,
Kluge brings forward the following strange and remarkable theory. The
oxygen of the air is magnetic, whilst water is diamagnetic and the
earth magnetic: we have, therefore, in our seas and lakes a diamagnetic
body lying between and being, consequently, repelled from two magnetic
bodies. By variation in temperature, the balance of repulsions
exerted by the air and the earth is destroyed. Thus, by an elevation
of temperature the air expands and flows away from the heated area,
where, in consequence, there is less oxygen. The result of this is,
that the repulsion of the air upon the waters is less than that of the
earth upon the waters, and the waters are in consequence raised up. By
a falling of temperature the waters may be depressed, and by either
of these actions waves may be produced without the intervention of
earthquakes or earth pulsations.

The more definite kinds of information which we have to bring forward,
tending to prove the existence of earth pulsations, too slow in period
to be experienced by ordinary observers, are those which appear to be
resultant phenomena of great earthquakes.

The phenomena that we are certain of in connection with earth
vibrations, whether these vibrations are produced artificially by
explosions of dynamite in bore-holes, or whether they are produced
naturally by earthquakes, are, first, that a disturbance as it dies out
at a given point often shows in the diagrams obtained by seismographs a
decrease in period; and, secondly, a similar decrease in the period of
the disturbance takes place as the disturbance spreads.

As examples of these actions I will quote the following.

The diagram of the disturbance of March 1, 1882, taken at Yokohama,
shows that the vibrations at the commencement of the disturbance had a
period of about three per second, near the middle of the disturbance
the period is about 1·1, whilst near the end the period has decreased
to ·46. That is to say, the backward and forward motion of the ground
at the commencement of the earthquake was six times as great as it was
near the end, when to make one complete oscillation it took between two
and three seconds. Probably the period became still less, but was not
recorded owing to the insensibility of the instruments to such slow
motions.[153]

We have not yet the means of comparing together diagrams of two or more
earthquakes, one having been taken near to the origin, and the other
at a distance. The only comparisons which I have been enabled to make
have been those of diagrams taken of the same earthquake, one in Tokio
and the other in Yokohama. As this base is only sixteen miles, and the
earthquake may have originated at a distance of several hundreds of
miles, comparisons like these can be of but little value.

Other diagrams illustrating the same point are those obtained at three
stations in a straight line, but at different distances from the
origin of a disturbance produced by exploding a charge of dynamite in
a bore-hole. A simple inspection of these diagrams shows that at the
near station the disturbance consisted of backward and forward motions,
which, as compared with the same disturbance as recorded at a more
distant station, were very rapid. Further, by examining the diagram
of the motions, say, at the near station, it is clearly evident that
the period of the backward and forward motion rapidly decreased as the
motion died out.

These illustrations are given as examples out of a large series of
other records, all showing like results.

An observation which confirms the records obtained from seismographs
respecting the increase in period of an earthquake as it dies out
I have had opportunities of twice making with my levels. After all
perceptible motion of the ground subsequent upon a moderately severe
shock had died away, I have distinctly seen the bubble in one of these
levels slowly pulsating with an irregular period of from one to five
seconds.

Although we must draw a distinction between earth waves and water
waves, we yet see that in these points they present a striking
likeness. Let us take, for example, any of the large earthquake waves
which have originated off the coast of South America, and then radiated
outwards, until they spread across the Pacific, to be recorded in Japan
and other countries perhaps twenty-five hours afterwards, at a distance
of nearly 9,000 miles from their origin. Near this origin they appeared
as walls of water which were seen rapidly advancing towards the coast.
These have been from twenty to two hundred feet in height, and they
succeeded each other at rapid intervals, until finally they died out as
a series of gentle waves. By the time these walls of water traversed
the Pacific, to, let us say, Japan, they broadened out to a swell so
flat that it could not be detected on the smoothest water excepting
along shore lines where the water rose and fell like the tide. Instead
of a wall of water sixty feet in height, we had long flat undulations
perhaps eight feet in height, but with a distance from crest to crest
of from one to two hundred miles.

If we turn to the effects of large earthquakes as exhibited on the
land, I think that we shall find records of phenomena which are only
to be explained on the assumption of an action having taken place
analogous to that which takes place so often in the ocean, or an action
similar to that exhibited by small earthquakes, and artificially
produced disturbances, if greatly exaggerated.

The only explanation for the phenomena accompanying the Lisbon
earthquake appears to be that the short quick vibrations which had
ruined so many cities in Portugal had, by the time that they had
radiated to distant countries, gradually become changed into long flat
waves having a period of perhaps several minutes. In countries like
England these pulse-like movements were too gentle to be perceived,
except in the effects produced by tipping up the beds of lakes and
ponds.

The phenomenon was not unlike that of a swell produced by a distant
storm. It would seem possible that in some cases pulsations producing
phenomena like the ‘seiches’ of Switzerland might have their origin
beneath the ocean, or deep down beneath the earth’s crust. Perhaps,
instead of commencing with the ‘snap and jar’ of an earthquake, they
may commence as a heaving or sinking of a considerable area, which
may be regarded as an uncompleted effort in the establishment of an
earthquake or a volcano.

From what has now been said it would seem that earth pulsations
are phenomena with a real existence, and that some of these are
attributable to earthquakes. On the other hand, certain earthquakes
are attributable to earth pulsations. Some of the phenomena which
have been brought forward have only a possible connection with these
movements, and they yet require investigation. Elastic tides in the
earth’s crust have for long been realities in the minds of physicists.
These, however, are due to lunar and solar influences, and are regular
in their action. The tidal-like movements called pulsations are of
greater magnitude, and their goings and comings are irregular.




                             CHAPTER XXI.

                          EARTH OSCILLATIONS.

  Evidences of oscillation—Examples of oscillation—Temple of Jupiter
    Serapis—Observations of Darwin—Causes of oscillation.


_Evidences of oscillation._—By earth oscillations are meant those slow
and quiet changes in the relative level of the sea and land which
geologists speak of as elevations or subsidences. These movements are
especially characteristic of volcanic and earthquake-shaken countries.

As evidences of elevations we appeal to phenomena like raised beaches,
sea-worn caves, raised coral reefs, and the remains of other dead
organisms like barnacles, and the borings of lithodomous shells in and
on the rocks of many coasts high above the level of the highest tides.
As a proof that subsidence has taken place, there is the evidence
afforded by submerged forests, the prolongation of certain valleys
beneath the bed of the ocean, the formation of coral islands, the
peculiar distribution of the plants and animals which we find in many
countries, and the submergence of works of human construction. Inasmuch
as these phenomena are discussed so fully in many treatises on physical
geology, the references to them here will be made as brief as possible.
Elevations and depressions which have taken place at the time of large
earthquakes in a paroxysmal manner have already been mentioned. The
movements referred to in this chapter, although generally taking place
with extreme slowness, in certain instances, by an increase in their
rapidity, have approached in character to earth pulsations. In most
instances it would appear that the upward movement of the ground, which
may be likened to a process of tumefaction, goes on so gently that it
only becomes appreciable after the lapse of many generations.

_Examples of movements._—Lyell estimated that the average rate of rise
in Scandinavia has been about two and a half feet per century. At the
North Cape the rise may have been as much as five or six feet per
century. Observations made at the temple of Jupiter Serapis, between
October 1822 and July 1838, showed that the ground was sinking at the
rate of about one inch in four years. Since the Roman period, when this
temple was built, the ground has sunk twenty feet below the waves. Now
the floor of the temple is on the level of the sea. Lyell remarks that
if we reflect on the dates of the principal oscillations at this place
there appears to be connection between the movements of upheaval and a
local development of volcanic heat, whilst periods of depression are
concurrent with periods of volcanic quiescence.[154]

As examples of movements even more rapid than those at the Temple of
Jupiter Serapis we refer to an account of the earthquakes in Vallais
(November 1755), when the ground about a mountain at a small distance
from Brigue sank about a thumb’s-breadth every twenty-four hours. This
took place between December 9 and February 26.[155]

Another remarkable example of earth movement is given in the account
of the earthquake at Scarborough, on December 29, 1737, when the head
of the spa water well was forced up in the air about ten yards high. At
this time the sands on the shore are said to have risen so slowly that
people came out to watch them.[156]

Two other examples of rapid earth movement are taken from Professor
Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to
Professor Rossi, says that a few lustres ago (one lustre = twenty
years) Mount S. Giovanni hid the towns Jenne and Subiaco from each
other. From Subiaco the church at Jenne is now visible, which a few
years ago was invisible. The people at Jenne also can see more than
formerly. The supposition is that the side of Mount S. Giovanni is
lowered. This fact corresponds to a fact stated by Professor Carina,
who says that forty or fifty years ago from Granaiola you could not see
either the church of S. Maria Assunta di Citrone or the church of S.
Pietro di Corsena. Now you can see both.[157]

For a remarkable example illustrating the connection between seismic
activity and elevation we are indebted to the patient labours of
Darwin, who carefully investigated the evidences of elevation which
are visible upon the western coasts of South America. These evidences,
consisting of marks of erosion, caves, ancient beaches, sand dunes,
terraces of gravel, &c., were traced between latitudes 45° 35′ to 12°
5′, a distance north and south of 2,075 geographical miles, and there
is but little doubt that they extend much farther. As deduced from
observations upon upraised shells alone, a summary of Mr. Darwin’s
observations are contained in the following table:—

                                                    Feet
  At Chiloe the recent elevation has been            350
  „  Concepcion            „         „      625 to 1,000
  „  Valparaiso            „         „             1,300
  „  Coquimbo              „         „               252
  „  Lima                  „         „                85

Shells, similar to those clinging to uplifted rocks, which are
evidences of these elevations, still exist in the neighbouring seas,
and in the same proportionate numbers as they are found in the upraised
beds. In addition to this, Mr. Darwin shows us that at Lima, during the
Indo-human period, the elevation has been at least eighty-five feet.
At Valparaiso, during the last 220 years, the rise was about nineteen
feet, and in the seventeen years subsequent to 1817 the rise has been
ten or eleven feet, a portion only of which can be attributed to
earthquakes. In 1834 the rise there was apparently still in progress.

At Chiloe there has been a gradual elevation of about four feet in four
years. These, together with numerous other examples, testify to the
gradual but, as compared with other parts of the globe, exceedingly
rapid rise of the ground upon the western shores of South America.[158]
The most important point to be noticed is that this district of rapid
elevation is one of the most earthquake-shaken regions of the world.
And further, judging from Darwin’s remarks, in those portions of it
where the movements have been the most extensive, and at the same time
probably the most rapid, the seismic disturbances appear to have been
the most noticeable.

Similar remarks may be applied to Japan, it being in those districts
where evidences of recent elevation are abundant that earthquakes
are numerous. Thus, in the bay of Yedo, where we have borings of
lithodomi in the tufaceous cliffs ten feet above high-water mark,
which, inasmuch as the rock in which they are found is soft and easily
weathered, indicate an exceedingly rapid elevation, earthquakes are of
common occurrence.

From the evidences of elevation which we have upon the South American
coast, Japan, and in other countries, it appears that these movements
are intermittent, there being periods of rest, when sea cliffs are
denuded, and perhaps even periods of subsidence. There is also evidence
to show that, although these movements have been gradual from time to
time, they have been aided by starts occasioned by earthquakes.

As to whether earthquakes are more numerous during periods of
elevation, or of subsidence, or during the intermediate periods of
rest, we have no evidence.

Sudden displacements which occasionally accompany earthquakes might, it
was said, sometimes be regarded as the _cause_ of an earthquake, and
sometimes as the _effect_.

The slow elevations here referred to may be looked upon as being one of
the more important factors in the production of earthquakes. By various
causes the rocky coast is bent until, having reached the limit of its
elasticity, it snaps, and, in flying back like a broken spring, causes
the jars and tremors of an earthquake.

If this is the case, then the number of earthquakes felt in a district
which is being elevated may possibly be a function of the rate of
elevation.




                               APPENDIX.

      LIST OF THE PRINCIPAL BOOKS, PAPERS, PERIODICALS, WHICH ARE
                  REFERRED TO IN THE PRECEDING PAGES.

                   •       •       •       •       •

  _For a more complete bibliography of earthquakes refer to Mallet’s
      catalogue of works given in his report to the British Association
      in 1858._

                   •       •       •       •       •

  A True and Particular Relation of the Dreadful Earthquake which
      happened at Lima, &c. (1746). 1768.
  Abbot, Gen. H. L. On the Velocity of Transmission of Earth Waves.
      _Am. Jour. Sci._ XV., March 1878.
  — Shock of the Explosion at Hallet’s Point, Nov. 14, 1876.
      _Battalion Press._
  Alexander, Prof. T. See _Trans. Seis. Soc. of Japan_.
  American Journal of Science.
  Annali del reale osservatorio meteorologico Vesuviano.
  Annual Register, The.
  Anonymous, A Chronological and Historical Account of the most
      Memorable Earthquakes in the World, &c. 1750.
  — A Vindication of the Bishop of London’s Letter occasioned by the
      Late Earthquake. 1750.
  — Phenomena of the Great Earthquake of Nov. 1, 1755.
  — Serious Thoughts occasioned by the Late Earthquake at Lisbon.
      1755.
  Asiatic Society of Japan, Transactions of.
  Ayrton, Prof. W. E. _See_ Perry, J.

  Bárceno, M. Estudio del Terremoto (May 17, 1879) Mexico. 1879.
  Beke, Dr. C. T. Mount Sinai a Volcano.
  Bissett, Rev. J. A Sermon (on account of the Earthquake at Lisbon,
      Nov. 1, 1755). 1757.
  Bittner, A. Beiträge zur Kenntniss des Erdbebens von Belluno vom
      29. Juni 1873.
  — Sitzungsb. der K. Akad. d. Wissensch., lxix. II. Abth., 1874.
  Bollettino del Vulcanismo Italiano.
  Boué, Dr. A. Ueber das Erdbeben welches Mittel-Albanien im
      October d. J. so schrecklich getroffen hat. _Die K. Akad. d.
      Wissenschaften_, Nov. 1851.
  — Parallele der Erdbeben, des Nordlichtes und des Erdmagnetismus.
  — Ueber die Nothwendigkeit die Erdbeben und vulkanischen
      Erscheinungen genauer als bis jetzt beobachten zu lassen. _Die
      K. Akad. d. Wissenschaften_, 1851 and 1857.
  Bouguer, M. Of the Volcanoes and Earthquakes in Peru.
  British Association, Reports of.
  Brunton, R. H. Constructive Art in Japan. _Trans. Asiatic Soc. of
      Japan_, II. and III., Pt. 2.
  Bryce, J. Report to British Association, 1841.
  Buffour, M. The Natural History of Earthquakes and Volcanoes.

  C. H. A Physical Discussion of Earthquakes, &c. 1693.
  Canterbury, Thomas, Lord Archbishop of, The Theory and History of
      Earthquakes.
  Casariego, E. A. See _Trans. Seis. Soc. of Japan_.
  Cawley, G. Some Remarks on Construction in Brick and Wood, &c.
      _Trans. Asiatic Soc. of Japan_, VI. Plate ii.
  Chaplin, Prof. W. S. An Examination of the Earthquakes recorded
      at the Meteorological Observatory, Tokio. _Trans. Asiatic Soc.
      of Japan_, VI. Part ii.
  Comptes Rendus.
  Credner, H. Das Dippoldiswalder Erdbeben vom Oktober 1877.
  — Zeitschr. f. d. Naturwiss. f. Sachsen u. Thüringen.
  — Das Vogtländisch-erzgebirgische Erdbeben, 23. Nov. 1875.
  — Zeitschr. f. d. gesammt. Naturwissenschaften, xlviii., Oktober.

  Dan, T. See _Trans. Seis. Soc. of Japan_.
  Darwin, Charles. Researches on Geology and Natural History.
  — Geological observations.
  Darwin, G. H. Reports on Lunar Disturbance of Gravity to British
      Association, 1881. 1882.
  Diffenbach, F. Plutonismus und Vulkanismus in der Periode von
      1868–1872, und ihre Beziehungen zu den Erdbeben im Rheingebiet.
  Doelter, C. von. Ueber die Eruptivgebilde von Fleims, nebst einigen
      Bemerkungen über den Bau älterer Vulcane.
  — lxxiv. Band d. Sitzungsb. d. K. Akad. d. Wissensch., I. Abth.,
      Dec. Heft, Jahrg. 1876.
  Doolittle, Rev. T. Earthquakes Explained and Practically Improved,
      &c. 1693.
  Doyle, P. See _Trans. Seis. Soc. of Japan_.

  Emerson, Prof., B.A. Review of Von Seebachs’ Earthquake of March 6,
      1872. _Am. Jour. Sci._, Series III.
  Ewing, Prof. J. A. Earthquake Measurement. A memoir published by
      the Tokio University. 1883.
  — See _Trans. Seis. Soc. of Japan_.

  Falb, R. Gedanken und Studien über den Vulkanismus, &c. 1875.
  — Grundzüge zu einer Theorie der Erdbeben und Vulkanausbrüche.
  — Das Erdbeben von Belluno. ‘Sirius,’ Bd. VI., Heft ii.
  Flamstead, J. A Letter concerning Earthquakes. 1693.
  Forel, F. A. Les Tremblements de Terre (Suisse). _Arch. des
      Sciences Physiques et Naturelles_, VI. p. 461.
  — Tremblement de Terre du 30 Décembre 1879.
  Fuchs, Karl. Vulkane und Erdbeben.
  — _Die Vulkanischen Erscheinungen der Erde._

  Garcia, J. C. See _Trans. Seis. Soc. of Japan_.
  Geinitz, Dr. E. Das Erdbeben von Iquique am 9. Mai 1877, &c. _Die
      K. Leop.-Carol.-Deutschen Akademie der Naturforscher_, Band
      xl., Nr. 9.
  Gentleman’s Magazine, The.
  Geographical Society, Proceedings of.
  Geological Society, Proceedings of.
  Girard, Dr. H. Ueber Erdbeben und Vulkane. 1845.
  Gray, T. See _Trans. Seis. Soc. of Japan_.
  — On Instruments for Measuring and Recording Earthquake Motions.
      Phil. Mag. Sept. 1881.
  — On Recent Earthquake Investigation. _The Chrysanthemum_, 1881.
      Guiscardi, Prof. G. Notizie del Vesuvio. 1857.
  — Il terremoto di Casamicciola del 4 Marzo. 1881.

  Hales, S., D.D., F.R.S. Some Considerations on the Causes of
      Earthquakes. 1750.
  Hamilton, Sir W. Observations on Mount Vesuvius, Mount Etna, &c.
      1774.
  Hattori, I. Destructive Earthquakes in Japan. _Trans. Asiatic Soc.
      of Japan_, V. Plate i.
  Heim, Prof. A. Les Tremblements de Terre et leur Etude
      Scientifique. 1880.
  — Prof. A. Die Schweizerischen Erdbeben in 1881–1882.
  Hoeffer, Prof. H. Die Erdbeben Kärntens und deren Stosslinien.
      _Die Kais. Akademie d. Wissenschaften_, Band xlii.
  Höfer, Prof. H. Das Erdbeben von Belluno, am 29. Juni 1873.
      _Sitzungsb. der K. Akad. d. Wissensch._, I. Abth., Band lxxiv.
  Hoff, K. E. A. von. Geschichte der durch Ueberlieferung
      nachgewiesenen natürlichen Veränderungen der Erdoberfläche.
      1822.
  Hooke, R., M.D., F.R.S. Discourses concerning Earthquakes.
  Hopkins, William. Report to the British Association on the
      Geological Theories of Elevation and Earthquakes. 1847.
  Horton, Rev. Mr. An Account of the Earthquake which happened at
      Leghorn in Italy (Jan. 1742). 1750.
  Humboldt, Alexander von. Cosmos.
  — Travels.

  Jeitteles, L. A. Bericht über das Erdbeben am 15. Januar 1858.
  — Sitzungsberichte der mathem.-naturw. Classe d. K. Akad. d.
      Wissensch., xxxv. S. 511.
  Judd, J. W., Prof. Volcanoes, What they Are and What they Teach.

  Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio,
      &c. _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde
      Ostasiens_, Heft 14.
  — See _Trans. Seis. Soc. of Japan_.

  Lasaulx, A. von. Das Erdbeben von Herzogenrath am 22. October 1873.
  Lemery, M. A Physico-Chemical Explanation of Subterranean Fires,
      Earthquakes, &c.
  Lescasse, M. J. Etude sur les Constructions Japonaises, &c.
      _Mémoires de la Société des Ingénieurs Civils_.
  Lister, M., M.D., F.R.S. Of the Nature of Earthquakes.
  Little, Rev. J. Conjectures on the Physical Causes of Earthquakes
      and Volcanoes. 1820.

  Mallet, R. The Neapolitan Earthquake, Vol. II. _Reports to the
      British Association_, 1850, 1851, 1852, 1854, 1858, 1861.
  — Secondary Effects of the Earthquake of Cachar. _Proc. Geolog.
      Soc._, 1872.
  — Dynamics of Earthquakes. _Trans. Royal Irish Acad._ 1846.
  Milne, David. Reports to British Association, 1841, 1843, 1844.
  Milne, John. See _Trans. Seis. Soc. of Japan_.
  — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Trans.
      Royal Soc._ 1882.
  — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Proc.
      Royal Soc._ No. 217, 1881.
  — Earthquake Observations and Experiments in Japan (with T. Gray,
      B.Sc., F.R.S.E.) _Phil. Mag._, Nov. 1881.
  — On the Elasticity and Strength Constants of certain Rocks (with
      T. Gray, B.Sc., F.R.S.E.) _Jour. Geolog. Soc._, 1882.
  — A Visit to the Volcano of Oshima. _Geolog. Mag._, Dec. 2, Vol.
      IV., pp. 193–197, 255.
  — On the Form of Volcanoes. _Geolog. Mag._, Dec. 2, Vol. V., and
      Dec. 2, Vol. VI.
  — Note upon the Cooling of the Earth, &c. _Geolog. Mag._, Dec. 2.,
      Vol. VII., p. 99.
  — Investigation of the Earthquake Phenomena of Japan. _Rep. Brit.
      Assoc._, 1881 and 1882.
  — A Large Crater. _Popular Science Review._
  — The Volcanoes of Japan (a series of Articles). _Japan Gazette._
  — Earthquake Literature of Japan (a series of Articles). _Japan
      Gazette._
  — The Earthquake of Dec. 23, 1880. _The Chrysanthemum_, 1881.
  — Earthquake Motion. _The Chrysanthemum_, 1882.
  — Seismology in Japan. _Nature_, Oct. 1882.
  — Earth Movements. _The Times_, Oct. 12, 1882.
  Mitchell, Rev. J. Conjectures Concerning the Cause and Observations
      upon the Phenomena of Earthquakes. 1760.
  Mohr, Dr. F. Geschichte der Erde. 1875.

  Naturkundig Tijdschrift voor Nederlandsch Indie. 1875–1880.
  Naumann, Dr. E. Ueber Erdbeben und Vulkanausbrüche in Japan.
      _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde
      Ostasiens._ Heft 15.
  Noggerath, Dr. J. Die Erdbeben vom 29. Juli 1846 im Rheingebiet,
      &c.
  — Die Erdbeben im Vispthale (1855).
  — Die Erdbeben im Rheingebiet in den Jahren 1868, 1869, 1870.
  — Jahrgänge d. Verbandlungen d. Natur. Vereins für 1870. _Rheinland
      u. Westphalen_, xxvii.

  Oldham, Dr. Secondary Effects of the Earthquake of Cachas. _Proc.
      Geolog. Soc._ 1872.
  — Thermal Springs of India. _Memoirs of Geolog. Survey of India_,
      XIX. Plate 2.
  — A Catalogue of Indian Earthquakes. _Memoirs of Geolog. Survey of
      India_, XIX. Plate 3.
  — The Cachas Earthquake. _Memoirs of Geolog. Survey of India_, XIX.
      Plate 1.

  Palmer, Col. H. S. See _Trans. Seis. Soc. of Japan_.
  Palmieri, Prof. L., e Scacchi, A. Della Regione Volcanica del Monte
      Vulture, e del Tremuoto ivi avvenuto nel dì 14 Agosto 1851, 1852.
  — Annali del reale Osservatorio Meteorologico Vesuviano.
  — Il Vesuvio, il Terremoto d’ Isernia e l’eruzione sottomarina di
      Santorino. _R. Accad. d. Sci. Fis. e Mat. di Napoli_, iv. 1866.
  — Sul recente Terremoto di Corleone. _R. Accad. d. Sci. Fis. e
      Mat._, v. 1876.
  — Il Terremoto di Scio del dì 4 Aprile. _R. Accad. d. Sci. Fis. e
      Mat. di Napoli_, v. 1881.
  — Sul Terremoto di Casamicciola del 4 Marzo 1881. _R. Accad. d.
      Sci. Fis. e. Mat. di Napoli_. 1881.
  Paul, Prof. H. M. See _Trans. Seis. Soc. of Japan_.
  Perrey, Prof. A. Earthquake Catalogue and Memoirs. (For list see
      Mallet’s Report to British Association. 1858.)
  — See _Trans. Seis. Soc. of Japan_.
  Perry, J., and W. E. Ayrton. On a Neglected Principle that may be
      Employed in Earthquake Measurement.
  — See _Trans. Seis. Soc. of Japan_.
  Philosophical Magazine.
  Pickering, Rev. R. An Address to those who have either retired
      or intend to leave Town under the Imaginary Apprehension of
      the Approaching Shock of another Earthquake. 1750.

  Ray, J., F.R.S. A Summary of the Causes of the Alterations which
      have happened to the Face of the Earth.
  Rockstroh, E. Informe de la Comision Científica del Instituto
      Nacional de Guatemala, nombrada por el Sr. Ministro de
      Instruccion Pública para el Estudio de los Fenómenos
      Volcánicos en el Lago de Ilopango. 1880.
  Rockwood, Prof. C. G. Notes on Earthquakes. Annually in the _Am.
      Jour. Sci._
  — Japanese Seismology. _Am. Jour. Sci._, XXII. Dec. 1881.
  Romaine, W. A Discourse occasioned by the Late Earthquake. 1755.
  Rossi, Prof. M. S. di. Intorno all’ odierna fase dei Terremoti in
      Italia, e segnatamente sul Terremoto in Casamicciola del 4
      Marzo 1881. _Società Geografica Italiana._ 1881.
  — La Meteorologia Endogena, 2 vols.
  Royal Society, Transactions of.

  Scacchi, A. _See_ Palmieri.
  Schmidt, Dr. J. F. Untersuchungen über das Erdbeben am 15. Januar
      1858.
  — Studien über Erdbeben. 1879.
  — Die Eruption des Vesuv (1855). 1856.
  Scrope, G. P. Volcanoes.
  Seebach. Das mittle Deutsche Erdbeben (1872). _Mitt. der K.K.
      geograph. Gesellsch._, II. Jahrg., 2. Heft, 1873.
  Serpieri, Prof. A. C. S. Nuove Osservazioni sul Terremoto avvenuto
      in Italia il 12 Marzo 1873. _Istituto Lombardo._ 1873.
  — Il Terremoto di Rimini della notte 17–18 Marzo 1875.
  — Documenti nuove e Riflessioni sul Terremoto della notte 17–18
      Marzo 1875. _Meteorologia Italiana_, iv. 1875.
  — Determinazione delle fasi e delle leggi del grande Terremoto
      avvenuto in Italia nella notte 17–18 Marzo 1875. _Istituto
      Lombardo._ 1875.
  — Dell’ influenza del Lume Solare sui Terremoti. _Istituto
      Lombardo._ 1882.
  Sherlock, T., D.D. (Lord Bishop of London). A Letter on the
      occasion of the late Earthquakes. 1750.
  Shower, Rev. J., D.D. Practical Reflections on the Earthquakes that
      have happened in Europe and America, &c. 1750.
  Stübel, A. (see Reiss, W.)
  Stukeley, Rev. W., M.D., F.R.S. The Philosophy of Earthquakes,
      Natural and Religious, &c. Plates 1, 2, and 3. 1756.
  Sturmius, J. C. A Methodical Account of Earthquakes.
  Suess, E. Die Erdbeben Niederösterreiches. _Die Kais. Akademie der
      Wissenschaften_, Bd. xxxiii.
  — Die Erdbeben des südlichen Italiens. _Die Kais. Akademie der
      Wissenschaften_, Bd. xxxiv.

  Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben.
      1857.

  Wagener, Dr. G. Bemerkungen über Erdbebenmesser und Vorschläge zu
      einem neuen Instrumente dieser Art. _Mitt. d. Deutsch.
      Gesellsch. für Natur- und Völkerkunde Ostasiens_, Heft 15.
  — See _Trans. Seis. Soc. of Japan_.
  Winchilsea, The Earl of. A True and Exact Relation of the late
      Prodigious Earthquake and Eruption of Mount Etna. 1669.
  Woodward, J., M.D., F.R.S. Earthquake caused by some Accidental
      Obstruction of a Continual Subterranean Heat.


                    SEISMOLOGICAL SOCIETY OF JAPAN.

  The following are a list of the papers published by this Society:—


                                VOL. I.

  Milne, J. Seismic Science in Japan. 35 pages.
  Ewing, J. A. New Form of Pendulum Seismograph. 6 pages, 3 plates.
  Gray, T. Seismometer and Torsion Pendulum Seismograph. 8 pages, 2
      plates.
  Mendenhall, T. C. Acceleration of Gravity at Tokio (abstract). 2
      pages.
  Wagener, G., and E. Knipping. New Seismometer and Observations with
      same. 18 pages, 1 plate.
  Milne, J. Earthquake in Japan of Feb. 22, 1880. 116 pages, 5
      plates, 8 woodcuts.


                               VOL. II.

  Milne, J. Recent Earthquakes of Yeddo, Effects on Buildings, &c. 38
      pages, 2 plates, and many tables.
  Mendenhall, T. C. Gravity on Summit of Fujiyama (abstract). 2 pages.
  Paul, H. M. Earth Vibrations from Railroad Trains (abstract). 4
      pages.
  Ewing, J. A. Astatic Horizontal Lever Seismograph (abstract). 5
      pages, 1 plate.
  Milne, J. Peruvian Earthquake of May, 9, 1877. 47 pages, 2 plates,
      tables. Constitution, Rules, Officers and Members of the
      Society, Dec., 1881.


                               VOL. III.

  Gray, T. Steady Points for Earthquake Measurements. 11 pages, 3
      plates.
  Milne, J. Experiments in Observational Seismology. 53 pages, 1
      plate, tables.
  — The Great Earthquakes of Japan. 38 pages, 1 plate, many tables.
  Perry, J. Theory of a Rocking Column. 4 pages.
  Knipping, E. Earthquake of July 25, 1880, with Dr. Wagener’s
      Seismometer. 4 pages.
  Ewing, J. A. Earthquake Observation at three or more Stations, &c.
      4 pages.
  — Records of three recent Earthquakes. 6 pages, 3 plates.
  — Earthquake of March 8, 1881. 8 pages, 1 plate.
  Milne, J. Horizontal and Vertical Motion in Earthquake of March 8,
      1881. 8 pages, 3 plates.
  Gray, T. Seismograph for Registering Vertical Motion. 3 pages, 1
      plate.
  Ewing, J. A. Seismometer for Vertical Motion. 3 pages, 1 plate.
  Gray, T. Seismograph for Large Motions. 2 pages.
  — Compensating a Pendulum to make it Astatic. 3 pages.
  Palmer, H. S. Note on Earth Vibrations. 3 pages.
  Kuwabara, M. The Hot Springs of Atami. 2 pages.


                               VOL. IV.

  Milne, J. Distribution of Seismic Activity in Japan. 30 pages, 1
      plate.
  Wada, T. Notes on Fujiyama. 7 pages.
  Casariego, E. Abella y. Earthquakes of Nueva Vizcaya in 1881. 23
      pages, 2 maps.
  Milne, J. Utilisation of Earth’s Internal Heat. 12 pages.
  Ewing, J. A. Earthquake of March 11, 1882. 5 pages.
  Doyle, P. Note on an Indian Earthquake. 6 pages.
  Milne, J. Systematic Observation of Earthquakes. 31 pages, 5 plates.


                                VOL. V.

  Naumann, Dr. E. Notes on Secular Changes of Magnetic Declination in
      Japan, p. 1–18.
  Casariego, Don E. Abella y. Monografía Geológica del Volcan de
      Albay ó El Máyon. p. 19–43.
  Garcia, Don J. Centeno y. Abstract of a Memoir on the Earthquakes
      on the Island of Luzon in 1880. p. 43–89.
  Ewing, Prof. J. A. Seismological Notes.
  — A Duplex Pendulum Seismometer.
  — The Suspension of a Horizontal Pendulum.
  — A Speed Governor for Seismograph Clocks, p. 89–95.
  Dan, T., S.B. Notes on the Earthquake at Atami, in the Province of
      Idzu, on September 29, 1882. p. 95–105.


                               VOL. VI.

  Alexander, Prof. T. The Development of the Record given by a
      Bracket Machine.
  Milne, J. Earth Pulsations.
  Ewing, J. A. Note on a Duplex Pendulum with a Single Bob.
  Gergens, F. Note on an Iron Casting, Supposed to have been
      Disturbed whilst Cooling by an Earthquake.
  West, C. D. On a Parallel Motion Seismograph.
  Ewing, J. A. Certain Methods of Astatic Suspension.
  Alexander, T. Ball and Cup Seismometer.
  Knipping, Messrs. Paul and. Report on a System for Earthquake
      Observation.
  Catalogues of Earthquakes.




                                INDEX.


  Abbadie, M. d’, on earth tremors, 309
  Abbot, General H. L., on the transmission of vibrations, 62
  Abella, M., on the earthquake in the Philippines in 1881, 77
  Activity, on seismic, 6
  Aristotle, on the classification of earthquakes, 41
  Artificial earthquakes, experiments on, 57
  — — intensity of, 61
  Aurora, on the occurrence of, with earthquakes, 264
  Ayrton and Perry, on the effect of soft foundations, 130
  — — on the period of vibration of buildings, 115
  — — on the principle of, 31

  Barometer, effect of changes of, on earthquakes, 266
  Bertelli, on aurora and earthquakes, 265
  — on earth tremors, 316, 320
  — on the normal tromometer of, 317
  Bittner, A., on the buildings of Belluno, 100
  Bridges, on earthquake, 140
  Brunton, R. H., on buildings in earthquake countries, 123
  Buckle, on the history of civilisation, 1
  Builders, interest of the study of earthquakes to, 3
  Buildings, on cracks in, 98, 108
  — the effect of earthquakes on, 96
  — on the irregular destruction of, 96
  — — effect on the end house in a row, 112
  — — church of St. Augustin at Manilla, 113
  — — relation of destruction of, to earthquake motion, 103
  — — protection of, 143
  — — pitch of the roof of, 110
  — — position of openings in the walls of, 111
  — — swing of, 115
  — — period of vibration of, 115
  — — principle of relative periods in, 116
  — — types of, for earthquake countries, 121
  — — effect of underlying rocks on, 130
  — general conclusions regarding, 144

  Cacciatore, definition of the, 18
  Caldcleugh, A., on earthquake frequency, 245
  — on barometric height and earthquakes, 266
  Carruthers, J., on earthquakes and tides, 291
  Centrum, definition of, 9
  — on the depth of, 213
  — on the maximum depth of, 218
  Centrum, determination of position of, _see_ origins
  Chaplin, W. S., on the bracket seismometer of, 27
  — on earthquakes and the position of the moon, 252
  Coast line, on the movement of, 160
  Cocks, R., on earthquakes and tides, 290
  Coseismic lines, definition of, 10
  Curves, on microseismic, 321

  Darwin, Charles, on the movement of coast lines, 160
  — George H., on earth tides, 285
  — on tidal loads, 291
  — on earth pulsation, 330
  — on the effect of fluctuations of barometric pressure, 336
  — experiments at Cambridge, 310
  Delauney, M. J., on the influence of the planets on earthquakes, 261
  Diagonic, definition of, 11
  Diastrophic, definition of, 11
  Direction of motion, from instrumental records, 198
  Distribution, on earthquake, 226
  — examples of, 231
  Disturbance, on the propagation of, 50
  Douglas, J., on South American houses, 126

  Earth particle, on the velocity and acceleration of, 79
  Earthquake motion, nature of, as deduced from the feelings, 67
  — direction of, derived from instrumental records, 69
  — duration of, 71
  — period of vibration in, 74
  — examples of extent of, 75–77
  — absolute intensity of force in, 83
  — radiation of, 85
  — velocity of propagation of, 87
  Earthquake at Lisbon, velocity of propagation of, 88
  Earthquakes, general examples of effects of, 142
  — geological changes produced by, 161
  — hunting, 187
  — distribution of, 226
  — maps, 189
  — secondary, 248
  — table of, for nineteenth century, 259
  — on the course of, 277–281
  — and tides, 290
  — prediction of, 297–304
  Elastic waves, nature of, 44
  Emergence, angle of, 9
  Energy, dissipation of, in earthquakes, 52
  — seismic, in relation to geological time, 234
  — — table of, 240
  Epicentrum, definition of, 9
  Euthutropic, definition of, 11
  Ewing, J. A., pendulum seismograph of, 25
  — astatic pendulum of, 26
  — bracket seismograph of, 26

  Falb, R., on the influence of the sun and moon on earthquakes, 286
  Fissures, on the material discharged from, 148
  — on the explanation of, 151
  Focal cavity, definition of, 9
  — on the form of, 221
  Forbes, D., on an earthquake in Mendoza, 151
  Frequency of earthquakes, 243
  Frere, Sir H. Bartle, on geological changes produced by earthquakes, 161
  Fuchs, on sea waves, 176
  — on the movement of the seismic centre, 233
  — on earthquakes and volcanic outbursts, 271
  — on hot springs, 157
  Fumaroles, the effect of earthquakes on, 156

  Geinitz, Dr., on sea waves, 182
  Geologists, on the interest of seismology to, 2
  Gray, T., astatic pendulum of, 26
  — bracket seismometer of, 27
  — conical pendulum of, 29
  — dead heat pendulum of, 22
  — on the rotation of bodies, 196
  — rolling spheres and cylinders of, 29
  — torsion pendulum seismometer of, 25
  — vertical motion seismometers of, 32, 33
  — and Milne, seismograph of, 38

  Hattori, I., on the large earthquakes of Japan, 244
  Haughton, Prof., list of active volcanoes of, 227
  — method of finding earthquake origins of, 209
  Hills, on the want of support on the face of, 136
  Höfer, on an earthquake at Belluno, 225
  Hoffmann, F., on the barometer and earthquakes, 267
  Hooke, on earthquake motion, 42
  Hopkins, on the thickness of the earth’s crust, 284
  Humboldt, on meteors and earthquakes, 261
  — on the barometer and earthquakes, 267
  — on volcanoes and earthquakes, 279

  Imagination, effect of earthquakes on the, 2
  Instruments, direction of motion derived from, 198
  Intensity, on earthquake, 51, 71
  — seismic curve of, for Kioto, 242
  Isoseismic circles, definition of, 10
  — areas, definition of, 10

  Kluge, on sea waves, 175
  — on earthquake frequency, 246
  — on simultaneous earthquakes, 248
  — on earthquakes and sun spots, 263
  — on earth pulsations, 339
  Kreil, pendulum seismometer of, 25

  Lakes, on disturbances in, 154
  Land, effect of earthquakes on, 146–162
  — on the reason of movements of, 162
  — on cracks and fissures formed in, 146
  Level, on the use of for earth pulsations, 328
  Literature, on seismic, 6
  — on Japanese earthquake, 7

  Mallet, R., on area of disturbance as a test of seismic energy, 78
  — on clock stopping, 36
  — list of works on earthquakes of, 5
  — curve of seismic energy of, 238
  — definition of earthquake of, 43
  — on earthquake frequency, 243
  — on the influence of the heavenly bodies on earthquakes, 253
  — on maximum depth of origin, 218
  — on pendulum seismometers, 20
  — projection seismometer of, 17
  — on the Neapolitan earthquake, 69, 77, 83, 97, 103, 132, 142, 218, 280
  — on sea waves, 170
  — on the swing of mountains, 135
  — on propagation from a fissure, 217
  Mallet on the temperature of focal cavity, 84
  Malvasia, M. le Conte, on earth tremors, 316
  Martin, D. S., on the New England earthquake of 1874, 142
  Meizoseismic area, definition of, 10
  Melzi, on curves of microseismic motion, 322
  Meteors, on earthquakes and, 260
  Microseismic movements, on cause of, 324
  Milne, D., on the Lisbon earthquake, 87
  — on earthquake synchronism, 247
  Mitchell, on earthquake motion, 42
  Moon, effect of, on earthquakes, 251, 285
  Mountains, on the swing of, 135

  Naumann, E., on meteors and earthquakes, 261
  — on sun spots and earthquakes, 263

  Ocean, on disturbances in, 163–186
  Origin, definition of, 9
  — on the determination of, 187
  — position of, deduced from direction of motion, 192
  — — from destruction of buildings, 194
  — — from rotation of bodies, 195
  — — from time of occurrence, 199
  — — examples of methods of calculating, 200–212
  Oscillations, on earth, 344
  Overturning moment, on the area of greatest, 53

  Palmer, Col. H. S., on earth tremors, 307
  Palmieri, on clock stopping, 36, 62
  Paul, H. M., on earth tremors, 308
  Perrey, A., on the influence of the moon on earthquakes, 251
  Perrey on the periodicity of earthquakes, 8
  Perry, J., on position of openings in walls, 111
  Physicists, on the interest of earthquakes to, 2
  Planets, influence of, on earthquakes, 260
  Plantamour, M., on earth pulsations, 328
  Pleistoseists, definition of, 10
  Poly, M. A., on earthquakes and sun spots, 263
  — on earthquakes and revolving storms, 294
  Prost, M. le Baron, on earth tremors, 316
  Pulsation, on earth, 4, 326–343

  Records, on receivers of, 33
  Rivers, on disturbances in, 154
  Rockwood, Prof., on American earthquakes, 6
  Ronaldson, T., on San Francisco houses, 129
  Rossi, M. S. di, on an eruption of gas in the Tiber, 153
  — aurora and earthquakes, 264
  — earth tremors, 317, 320
  — earth oscillations, 346
  — earth pulsations, 327
  — microseismograph of, 318
  — microphonic observations of, 319, 323
  — normal tromometer of, 317

  Schmidt, on the influence of barometric pressure on earthquakes, 267
  Sea waves, on nature of, 165
  — on cause of, 171
  — seldom produced by earthquakes which originate inland, 175
  — on velocity of propagation of, 177
  — examples of, 179
  Seasons, frequency of earthquakes at different, 254
  Seebach, on the determination of origins, 211
  — on the focal cavity, 224
  Seismic vertical, definition of, 9
  Seismic and volcanic phenomena, relation of, 270
  — — conclusions regarding, 275, 295
  Seismology, definition of, 9
  Seismometers, on various forms of, 17–40
  Seismoscopes, on various forms of, 13–20
  Serpieri, P. A., on distribution of seismic movement, 231
  Shadows, on earthquake, 137
  Spring, on frequency of earthquakes during, 156
  Stukeley, on earthquake motion, 42
  — earthquakes and aurora, 265
  Succussatore, definition of, 10
  Sun, on the effect of, on earthquakes, 253, 285

  Temperature, effect of changes of, on earthquakes, 268, 294
  Terremoto, definition of, 10
  Thomson, Sir W., on the rigidity of the earth, 285
  Time, on recording apparatus for, 35
  Travagini, F., on earthquake motion, 42
  Trembelores, definition of, 10
  Tremors, on earth, 3, 306–325

  Understanding, effects of earthquakes on, 2

  Verbeck, on the ball and plate seismometer of, 31
  Vibration, on the nature of earthquake, 12
  Vorticose motion, on, 70
  — definition of, 10

  Wagener, on the pendulum seismometer of, 25
  — vertical motion seismometer of, 33
  — list of earthquakes, 76
  Wave paths, definition of, 9
  Waves, on the nature of earthquake, 55
  — on the interference of, 138
  Wells, on the effect of earthquakes on, 156
  Wenthrop, on the New Zealand earthquake of 1855, 79
  West, on the parallel motion seismometer of, 28
  Winslow, on pulsations of the ocean, 334
  Wolf, R., on earthquakes and sun spots, 263
  Woodward, on earthquake motion, 42

  Young, Dr. T., on earthquake motion, 43

  Zantedeschi, M. F., on the influence of the sun and moon on earthquakes, 285
  Zöllner, on the bracket seismometer of, 27
  — on earth tremors, 309




  +--------------------------------------------------------------------+
  |                                                                    |
  |                             FOOTNOTES:                             |
  |                                                                    |
  | [1] _Mémoires de l’Académie Imp. de Dijon_, vols. xiv. and xv.,    |
  |     2nd Series, 1855–56.                                           |
  |                                                                    |
  | [2] _Trans. Seis. Soc. of Japan_, vol. iii. p. 65.                 |
  |                                                                    |
  | [3] _Gentleman’s Magazine_, 1753.                                  |
  |                                                                    |
  | [4] 1 Kings xix. 11, 12.                                           |
  |                                                                    |
  | [5] ‘Notes on the Great Earthquake of Japan.’ J. Milne, _Trans.    |
  |     Seis. Soc. of Japan_, vol. iii.                                |
  |                                                                    |
  | [6] See Mallet’s List of Works on Earthquakes, _Report of the      |
  |     British Association_, 1858, p. 107.                            |
  |                                                                    |
  | [7] _Quarterly Review_, vol. lxiii. p. 61.                         |
  |                                                                    |
  | [8] _De Mundo_, c. iv.                                             |
  |                                                                    |
  | [9] See _Phil. Trans. R. S._, Part III. 1882.                      |
  |                                                                    |
  | [10] _Report of the British Association_, 1851.                    |
  |                                                                    |
  | [11] ‘On the Velocity of Transmission of Earth Waves,’ by General  |
  |     H. L, Abbot, _American, Journal of Science and Arts_, vol. xv. |
  |     March 1878; ‘Shock of the Explosion at Hallet’s Point,’ by     |
  |     Bvt. Brig.-Gen. Henry L. Abbot, read before the Essayons Club  |
  |     of the Corps of Engineers, Nov. 1876.                          |
  |                                                                    |
  | [12] _West. Rev._, July 1849.                                      |
  |                                                                    |
  | [13] _Phil. Trans._, L., 1755.                                     |
  |                                                                    |
  | [14] The solution is taken from Mallet’s _Account of the           |
  |     Neapolitan Earthquake_, vol. i. p. 155.                        |
  |                                                                    |
  | [15] _Neapolitan Earthquake_, ii. p. 300.                          |
  |                                                                    |
  | [16] See _Edinburgh Phil. Trans._, vol. xxxi.                      |
  |                                                                    |
  | [17] See _Report of British Association_, 1858, p. 10.             |
  |                                                                    |
  | [18] _Meteorologia Endogena_, i. p. 306.                           |
  |                                                                    |
  | [19] See remarks on the Earthquake ‘Push,’ p. 162.                 |
  |                                                                    |
  | [20] See _Researches in Geology and Natural History_, p. 374.      |
  |                                                                    |
  | [21] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_,  |
  |     March, 1883.                                                   |
  |                                                                    |
  | [22] Mallet, _Dynamics of Earthquakes_.                            |
  |                                                                    |
  | [23] Stud Mill at Haywards.                                        |
  |                                                                    |
  | [24] See ‘Constructive Art in Japan,’ by R. H. Brunton, C.E.,      |
  |     F.R.G.S., F.G.S., _Transactions of Asiatic Society of Japan_,  |
  |     December 22, 1873, and January 13, 1875.                       |
  |                                                                    |
  | [25] _Journal of the American Geographical Society_, vol. x.       |
  |                                                                    |
  | [26] _Phil. Trans._, li. 1760.                                     |
  |                                                                    |
  | [27] _Ibid._, xviii.                                               |
  |                                                                    |
  | [28] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_,  |
  |     March 1883.                                                    |
  |                                                                    |
  | [29] T. Ronaldson, _A Treatise on Earthquake Dangers &c._          |
  |                                                                    |
  | [30] _Principles of Geology_, Lyell, vol. ii. p. 106.              |
  |                                                                    |
  | [31] _The Neapolitan Earthquake of 1857_, R. Mallet, vol. ii. p.   |
  |     359.                                                           |
  |                                                                    |
  | [32] _Am. J. Sci._ x. 191.                                         |
  |                                                                    |
  | [33] _Reports of British Association_, 1858, p. 106.               |
  |                                                                    |
  | [34] See chapter ‘Causes of Earthquakes’ for details of this myth. |
  |                                                                    |
  | [35] _Am. Jour. Sci._ vol. x. p. 191.                              |
  |                                                                    |
  | [36] _The Earth_, p. 599.                                          |
  |                                                                    |
  | [37] Lyell, _Principles of Geology_, vol. ii. chap. xxix.          |
  |                                                                    |
  | [38] _Gent. Mag._ vol. xx. p. 212.                                 |
  |                                                                    |
  | [39] _Trans. Seis. Soc._ vol. v. p. 67–68.                         |
  |                                                                    |
  | [40] _Am. Jour. Sci._ vol. iv.                                     |
  |                                                                    |
  | [41] _Phil. Trans._ vol. xviii.                                    |
  |                                                                    |
  | [42] Oldham and Mallet, ‘Cachar Earthquake,’ _Proc. Geolog. Soc._  |
  |     1872.                                                          |
  |                                                                    |
  | [43] _Phil. Trans._ vols. li. and xviii.; _Gent. Mag._ vol. xx.    |
  |     212.                                                           |
  |                                                                    |
  | [44] _Trans. Royal Geog. Soc._ vol. vi.                            |
  |                                                                    |
  | [45] _Phil. Trans._ vols. xxxvi. and xxxix.                        |
  |                                                                    |
  | [46] _Am. Jour. of Sci._ 1865, vol. xl. p. 365.                    |
  |                                                                    |
  | [47] _Proc. Geolog. Soc. Ap._ 1875, p. 270.                        |
  |                                                                    |
  | [48] _Gent. Mag._ vol. xxi. p. 569.                                |
  |                                                                    |
  | [49] _Jahrb. f. Min._ 1840, p. 173.                                |
  |                                                                    |
  | [50] Oldham and Mallet, ‘Cachar Earthquake,’ _Trans. Geolog. Soc.  |
  |     Ap._ 1872.                                                     |
  |                                                                    |
  | [51] O. Volger, _Unters üb. d. Phän. d. Erdb._ vol. iii. p. 414.   |
  |                                                                    |
  | [52] _Meteorologia Endogena_, vol. i. p. 166.                      |
  |                                                                    |
  | [53] _Gent. Mag._ vol. xxvi. p. 91.                                |
  |                                                                    |
  | [54] _Compte Rendu_, 1873, p. 66.                                  |
  |                                                                    |
  | [55] _An Historical Account of Earthquakes_, p. 46.                |
  |                                                                    |
  | [56] _Phil. Trans._ vol. xlix. p. 436.                             |
  |                                                                    |
  | [57] _Am. Jour. Sci._ vol. xlv. p. 129.                            |
  |                                                                    |
  | [58] _Phil. Trans._ vol. xlix. p. 547.                             |
  |                                                                    |
  | [59] _Ibid._ vols. xlii. and xxxix.                                |
  |                                                                    |
  | [60] _Phil. Trans._ vol. xlix, part i.                             |
  |                                                                    |
  | [61] _Compte Rendu_, 1873, part ii. p. 66.                         |
  |                                                                    |
  | [62] _Die Vulcan. Ers. d. Erde_, C. W. C. Fuchs.                   |
  |                                                                    |
  | [63] _Comte Rendu_, 1875, p. 693.                                  |
  |                                                                    |
  | [64] _Gent. Mag._ vol. xix. p. 190.                                |
  |                                                                    |
  | [65] _Phil. Trans._ vol. xlix. p. 115.                             |
  |                                                                    |
  | [66] _Gent. Mag._ vol. xxi. 1751.                                  |
  |                                                                    |
  | [67] _Jour. Royal Geo. Soc._ vol. vi. p. 319.                      |
  |                                                                    |
  | [68] Darwin, _Geolog. Observations_, p. 232.                       |
  |                                                                    |
  | [69] _Ibid._ p. 245.                                               |
  |                                                                    |
  | [70] Lyell, _Principles of Geology_, vol. ii. pp. 107–8.           |
  |                                                                    |
  | [71] _Gent. Mag._ 1733, vol. iii. p. 217.                          |
  |                                                                    |
  | [72] ‘Earthquakes of Cutch,’ _Jour. Royal Geo. Soc._ vol. xl.      |
  |                                                                    |
  | [73] M. Daussy, ‘Sur l’existence probable d’un volcan sousmarin    |
  |     situé ar environ 0° 20′  de lat. S., et 22° 0′  de long, ouest,’ |
  |     _Comptes Rendus_, vol. vi. p. 512.                             |
  |                                                                    |
  | [74] _Am. Jour. Sci._ vol. xlv. p. 133.                            |
  |                                                                    |
  | [75] _Am. Jour. Sci._ vol. xiv. p. 209.                            |
  |                                                                    |
  | [76] D. C. F. Winslow, ‘Tides at Tahiti,’ _Am. Jour. Sci._ 1865,   |
  |     p. 45; also Mallet’s _Catalogue of Earthquakes_.               |
  |                                                                    |
  | [77] _Am. Jour. Sci._ vol. i. p. 469.                              |
  |                                                                    |
  | [78] Darwin, _Researches in Geology, &c._, p. 378.                 |
  |                                                                    |
  | [79] Kluge, _Jahrb. f. Min._ 1861, p. 977.                         |
  |                                                                    |
  | [80] Darwin, _Voyage of a Naturalist_, p. 309.                     |
  |                                                                    |
  | [81] Prof. A. D. Bache, _United States Coast Survey Report_,       |
  |     1855, p. 342.                                                  |
  |                                                                    |
  | [82] _United States Coast Surrey Report_, or _Am. Jour. Sci._ vi.  |
  |     p. 77.                                                         |
  |                                                                    |
  | [83] _Petermann’s Mittheilungen_, 1877, Heft xii. S. 454,          |
  |     and _Nova Acta der Ksl. Leop. Carol. Deutschen Acad. d         |
  |     Naturforscher_, Band xl. No. 9.                                |
  |                                                                    |
  | [84] J. Milne: ‘Peruvian Earthquake of May 9, 1877.’ See _Trans.   |
  |     Seis. Soc. of Japan_, vol. ii.                                 |
  |                                                                    |
  | [85] _Report of British Association_, 1847, p. 84.                 |
  |                                                                    |
  | [86] _Das Erdbeben von Herzogenrath, &c._, p. 134.                 |
  |                                                                    |
  | [87] _Phil. Trans._ vol. li.                                       |
  |                                                                    |
  | [88] See _Am. Jour. Sci._ 1872.                                    |
  |                                                                    |
  | [89] David Milne says that ‘out of 110 shocks recorded in          |
  |     England, thirty-one originated in Wales, thirty-one along the  |
  |     south coast of England, fourteen on the borders of Yorkshire   |
  |     and Derbyshire, and five or six in Cumberland.’                |
  |                                                                    |
  | [90] E. Suess, _Die Erdbeben Niederösterreiches_.                  |
  |                                                                    |
  | [91] H. Hoeffer, _Die Erdbeben Kärntens_.                         |
  |                                                                    |
  | [92] _Six Lectures on Physical Geography_, by Rev. S. Haughton,    |
  |     F.R.S., chap. i.                                               |
  |                                                                    |
  | [93] Ramsay, ‘Geological History of Mountain Chains,’ _Mining      |
  |     Journal_.                                                      |
  |                                                                    |
  | [94] A notable example of a rapid diminution in the number of      |
  |     earthquakes felt at a place is that of Comrie in Scotland. In  |
  |     1839–40, no less than sixty shocks were felt in eleven months. |
  |     In 1842–43, about thirty shocks were felt, and in the          |
  |     following year thirty-seven. Since this time the number of     |
  |     shocks has decreased until they are almost of as rare          |
  |     occurrence at Comrie as in other portions of the British       |
  |     Isles.                                                         |
  |                                                                    |
  | [95] _Phil. Trans._ vol. i. 1836.                                  |
  |                                                                    |
  | [96] _Am. Jour. of Sci._ vol. xxxvii. p. 1.                        |
  |                                                                    |
  | [97] Milne, ‘British Earthquakes,’ _Edin. Phil. Jour._ vol. xxxi. |
  |                                                                    |
  | [98] _Phil. Trans._ vol. xlix. pt. i.                              |
  |                                                                    |
  | [99] _Compte Rendus_, 1875, p. 690.                                |
  |                                                                    |
  | [100] _Am. Jour. Sci._ vol. xi. p. 233.                            |
  |                                                                    |
  | [101] _Transactions of the Asiatic Society of Japan_, vol. vi.     |
  |     pt. i. p. 353.                                                 |
  |                                                                    |
  | [102] Kluge, _Ueber die Ursachen_, &c., p. 74.                     |
  |                                                                    |
  | [103] _Am. Jour. Sci._ vol. xix. p. 162.                           |
  |                                                                    |
  | [104] _Mitt. d. Deutsch._ Ges., Aug. 1878.                         |
  |                                                                    |
  | [105] _Report to British Association_, 1850, p. 74.                |
  |                                                                    |
  | [106] Fuchs, _Die Vulkanischen Erscheinungen der Erde_, p. 424.    |
  |                                                                    |
  | [107] _Bern. Naturf. Gesellschaft_, 1852.                          |
  |                                                                    |
  | [108] _Comptes Rendus_, 1874, Jan. to June, p. 51.                 |
  |                                                                    |
  | [109] Boué, _Parallele der Erdbeben, Nordlichter und               |
  |     Erdmagnetismus, in Sitz. der K. A. d. Wissensch_. 1856, vol.   |
  |      iv. p. 395.                                                   |
  |                                                                    |
  | [110] _Meteorologia Endogena_, vol. i. p. 107, &c.                 |
  |                                                                    |
  | [111] _Phil. Trans._ vol. lxviii. p. 221.                          |
  |                                                                    |
  | [112] _Gent. Mag._ vol. xxvii. p. 508.                             |
  |                                                                    |
  | [113] _Die Vulkanischen Erscheinungen der Erde_, p. 419.           |
  |                                                                    |
  | [114] Petermann’s _Geogr. Mitth._ 1858, sec. 246.                  |
  |                                                                    |
  | [115] _Notes on volcanoes of the Hawaiian Islands_, W. T.          |
  |     Brigham, Mem. Boston Soc. of Nat. Hist., 1868.                 |
  |                                                                    |
  | [116] _Gent. Mag._ vol. xxiii., 1753.                              |
  |                                                                    |
  | [117] _Jour. Royal Geog. Soc._ vol. vi.                            |
  |                                                                    |
  | [118] _Ibid._ vol. vi.                                             |
  |                                                                    |
  | [119] _Phil. Trans._ vol. xlii.                                    |
  |                                                                    |
  | [120] _Am. Jour. Sci._ vol. x. p. 191.                             |
  |                                                                    |
  | [121] ‘Earthquakes of San Salvador, December 21–30, 1879.’ _Am.    |
  |     Jour. Sci._ vol. xix. p. 415.                                  |
  |                                                                    |
  | [122] _Gent. Mag._ 1757, p. 323.                                   |
  |                                                                    |
  | [123] _Phil. Trans._ vol. li., 1760.                               |
  |                                                                    |
  | [124] Mallet, _Report to Brit. Ass._, 1858, p. 67.                 |
  |                                                                    |
  | [125] Von Lasaulx, _Earthquakes of Herzogenrath_.                  |
  |                                                                    |
  | [126] Lyell, _Principles_, vol. ii. p. 51.                         |
  |                                                                    |
  | [127] Lyell, _Principles_, vol. i. p. 402.                         |
  |                                                                    |
  | [128] Fuchs, p. 464.                                               |
  |                                                                    |
  | [129] _Comptes Rendus_, August 1854.                               |
  |                                                                    |
  | [130] _Nature_, April 26, 1883.                                    |
  |                                                                    |
  | [131] _Phil. Soc._, Wellington, New Zealand, 1875.                 |
  |                                                                    |
  | [132] _Phil. Trans._, vol. xlii.                                   |
  |                                                                    |
  | [133] M. S. di Rossi, _Earthquakes of Casamicciola_.               |
  |                                                                    |
  | [134] _Phil. Trans._, vol. xviii. 1683–5.                          |
  |                                                                    |
  | [135] _Ibid._ vol. xlix.                                           |
  |                                                                    |
  | [136] H. D. Warner, ‘The City of Earthquakes,’ _Atlantic           |
  |     Monthly_, March 1833.                                          |
  |                                                                    |
  | [137] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148.      |
  |                                                                    |
  | [138] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148.      |
  |                                                                    |
  | [139] Paul, _Trans. Seis. Soc. of Japan_, vol. ii. p. 41.          |
  |                                                                    |
  | [140] G. H. and H. Darwin, _Reports of British Association_, 1881. |
  |                                                                    |
  | [141] _Reports of British Association_, 1881.                      |
  |                                                                    |
  | [142] _Comptes Rendus_, 1875, January to June, p. 685.             |
  |                                                                    |
  | [143] _Tel. Jour._, November 15, 1881.                            |
  |                                                                    |
  | [144] _Minutes and proceedings of the Institute of Civil           |
  |     Engineers_, vol. lx. p. 412, and vol. lxiv. p. 343.            |
  |                                                                    |
  | [145] _See_ ‘Earth Tremors,’ p. 309, experiments of M. d’Abbadie,  |
  |     &c.                                                            |
  |                                                                    |
  | [146] _Meteorologia Endogena._                                     |
  |                                                                    |
  | [147] _Ibid._                                                      |
  |                                                                    |
  | [148] _Phil. Trans._ vol. xlix. p. 544.                            |
  |                                                                    |
  | [149] _Annual Register_, vol. iv. 1761, p. 92.                     |
  |                                                                    |
  | [150] _Phil. Mag._, May 1876, p. 447.                              |
  |                                                                    |
  | [151] _Boston Soc. Nat. Hist._, 1868.                              |
  |                                                                    |
  | [152] ‘Notes on Tides at Tahiti,’ &c., _Am. Jour. Sci._ 1866,      |
  |     vol. xlii. p. 45.                                              |
  |                                                                    |
  | [153] _Trans. Seis. Soc. of Japan_, vol. iv. Milne, _Systematic    |
  |     Observation of Earthquakes_.                                   |
  |                                                                    |
  | [154] _Principles of Geology_, vol. ii. 177.                       |
  |                                                                    |
  | [155] _Gent. Mag._, vol. xxvii. p. 448.                            |
  |                                                                    |
  | [156] _Phil. Trans._, vol. xli. p. 805.                            |
  |                                                                    |
  | [157] _Meteorologia Endogena_, vol. i. pp. 186, 187.               |
  |                                                                    |
  | [158] Darwin, _Geological Observations_, p. 275 _et seq._          |
  |                                                                    |
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Transcriber’s Notes:
 - Text enclosed by underscores is in italics (_italics_).
 - Text enclosed by equals is in bold (=bold=).
 - Blank pages have been removed.
 - Silently corrected typographical errors.
 - Spelling and hyphenation variations made consistent.
 - Front publication list moved to the back.
 - Tables pages 77, 89: removed unneeded right braces.
 - Tables pages 240, 257: changed to use cell borders instead of right braces.
 - Table page 259: Northern Hemisphere average 15·0 corrected to 150.