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.