CONCLUSION.
In this concluding chapter, I propose to give a summary of the results
at which we have arrived from the study of recent earthquakes, and
this can, I think, be done best by describing what may be regarded as
an average or typical earthquake, though it may be convenient
occasionally to depart slightly from such a course. Few shocks have
contributed more to our knowledge than the majority of those described
in this volume; but, on certain points, we gain additional information
from the investigation of other earthquakes, and these are referred to
when necessary for the purpose in view.
FORE-SHOCKS.
At the outset, we are met by a question of some interest and great
practical importance--namely, whether there are any constant signs of
the coming of great earthquakes by means of which their occurrence
might be predicted and their disastrous effects mitigated.
Excluding the Ischian earthquakes, which belong to a special class, it
is evident that there is generally some slight preparation for a great
earthquake. For a few hours or days beforehand, weak shocks and
tremors are felt or rumbling noises heard within the future
meizoseismal area. But, unfortunately, it has not yet been found
possible to distinguish these disturbances from others of apparently
the same character which occur alone, so that for the present they
fail to serve as warnings.
In Japan, where the organisation of earthquake-studies is more
complete than elsewhere, it is possible that a vague forecast might be
made, if the distribution of the fore-shocks of the earthquake of 1891
should prove to be a general feature of all great earthquakes. It was
at first supposed that this earthquake occurred without preparation of
any kind; but a closer analysis of the records shows that during the
previous two years there was a very decided increase in the seismic
activity of the district, and also that the distribution of the
epicentres marked out the future fault-scarp, and at the same time
exhibited a tendency to comparative uniformity over the whole
fault-region.
For the present, then, the only warning available is that given by the
preliminary sound, which may precede the strongest vibrations by as
much as five or ten or even more seconds. Though two or three seconds
may elapse before its character is recognised, the fore-sound thus
allows time for many persons to escape from their falling houses. Some
races, however, are less capable of hearing the sound than others, and
this may be one reason why Japanese earthquakes are so destructive of
human life.
DISTURBED AREA.
It is usual with some investigators to measure the intensity of an
earthquake roughly by the extent of its disturbed area. The depth of
the seismic focus must of course have some influence on the size of
this area, and this condition is only neglected because we have no
precise knowledge of the depth in any case. Thus, Mr. Oldham regards
the Indian earthquake of 1897 as rivalling the Lisbon earthquake of
1755, which is generally considered to hold the first place, because
its disturbed area was not certainly exceeded by that of the latter.
That disturbed area is, however, an untrustworthy measure of intensity
will be evident from the following table, in which the earthquakes
described in this volume (omitting those of Ischia) are arranged as
nearly as may be in order of intensity, beginning with the
strongest:--
Earthquake. Disturbed Area
in Sq. Miles.
Indian 1,750,000
Japanese 330,000
Neapolitan 39,200
Charleston 2,800,000
Riviera 219,000
Andalusian 174,000
Hereford 98,000
Inverness 33,000
Here we see that the Charleston earthquake was perceptible over a
greater area than the Indian earthquake, while the Neapolitan
earthquake was inferior to that of Hereford in this respect. The
explanation of course is that the boundaries of the disturbed areas
are isoseismal lines corresponding to different degrees of intensity,
the inhabitants of Great Britain and the United States being evidently
more sensitive to weak tremors, or more observant, than those of
Italy, Spain, or Central Asia. The only disturbed areas that are
bounded by isoseismals of the same intensity are the two last. Very
roughly, then, we may say that the intensity of the Hereford
earthquake was three times as great as that of the Inverness
earthquake.
POSITION OF THE EPICENTRE.
One of the first objects in the investigation of an earthquake is to
determine the position and form of the epicentre. In a few rare cases,
as in the Japanese and Indian earthquakes, when the fault-scarp is
left protruding at the surface, only careful mapping is required to
ascertain both data. But, in the great majority of earthquakes, the
fault-slip dies out before reaching the surface and the position of
the epicentre is then inferred by methods depending chiefly on the
time of occurrence or on the direction or intensity of the shock.
At first sight, methods that involve the time of occurrence at
different places seem to be of considerable promise. No scientific
instruments are so widely diffused as clocks and watches; but, on the
other hand, few are so carelessly adjusted. It is the exception,
rather than the rule, to find a time-record accurate to the nearest
minute; and, as small errors in the time may be of consequence,
methods depending on this element of the earthquake are seldom
employed. If, however, the number of observations is large for the
size of the disturbed area, the construction of coseismal lines may
define approximately the position of the epicentre. In the Hereford
earthquake of 1896, the centre of the innermost coseismal line (Fig.
62) is close to the region lying between the two epicentres.
The method of locating the epicentre by means of the intersection of
two or more lines of direction of the shock was first suggested by
Michell in 1760,[79] and has been employed by Mallet in investigating
the Neapolitan earthquake, by Professors Taramelli and Mercalli in
their studies of the Andalusian and Riviera earthquakes, as well as by
other seismologists. The diversity of apparent directions at one and
the same place caused its temporary neglect, until Professor Omori
showed in 1894 that the mean of a large number of measurements gives a
trustworthy result (p. 19). His interesting observations should
reinstate the method to its former place among the more valuable
instruments at the disposal of the seismologist.
No observations, however, are at present so valuable for the purpose
in view as those made on the intensity of the shock. For many years,
it has been the custom to regard the epicentre as coincident with the
area of greatest damage to buildings; and, when the area is small, the
assumption cannot be much in error. It is of course merely a rough way
of obtaining a result that is generally given more accurately by means
of isoseismal lines; but there are exceptional cases, such as the
Neapolitan and Ischian earthquakes, when the destruction wrought by
the earthquake furnishes evidence of the greater value.
A single isoseismal accurately drawn not only gives the position of
the epicentre with some approach to exactness, but also by the
direction of its longer axis determines that of the originating fault.
When two or three such lines can be traced, the relative position
supplies in addition the hade of the fault (p. 219). The successful
application of the method requires, it is true, a large number of
observations, and these cannot as a rule be obtained except in
districts that are somewhat thickly and uniformly populated, such as
those surrounding the cities of Hereford and Inverness. In the
Charleston earthquake, also, the position and form of the epicentres
were deduced from the trend of isoseismal lines based on the damage to
railway-lines and various structures within a sparsely inhabited
meizoseismal area.
In a few cases, of which the Indian earthquake may be regarded as
typical, a fourth method has recently been found of service. The
numerous after-shocks which follow a great earthquake originate for
the most part within the seismic focus of the latter; and, as they
usually disturb a very small area, it is not difficult to ascertain
approximately the positions of their epicentres. Some, as in the
Inverness after-shocks of 1901, result from slips in the very margin
of the principal focus; but, as a rule, the seat of their activity
tends to contract towards a central region of the focus. Bearing in
mind, then, that some of the succeeding shocks originate at and beyond
the confines of the focus, and that others may be sympathetic shocks
precipitated by the sudden change of stress, it follows that the
shifting epicentres of the true after-shocks map out, in part at any
rate, the epicentral area of the principal earthquake.
DEPTH OF THE SEISMIC FOCUS.
It is much to be regretted that we have no satisfactory method of
determining so interesting an element as the depth of the seismic
focus. That it amounts to but a few miles at the most is certain from
the limited areas within which slight shocks are felt or disastrous
ones exhibit their maximum effects. Nor can we suppose that the rocks
at very great depths are capable of offering the prolonged resistance
and sudden collapse under stress that are necessary for the production
of an earthquake.
The problem is evidently beyond our present powers of solution, and
its interest is therefore mainly historical. All the known methods are
vitiated by our ignorance of the refractive powers of the rocks
traversed by the earth-waves. But, even if this ignorance could be
replaced by knowledge, most of the methods suggested are open to
objection. Falb's method, depending on the time-interval between the
initial epochs of the sound and shock, is of more than doubtful value.
Dutton's, based on the rate of change of surface-intensity, is
difficult to apply, and in any case gives only an inferior limit to
the depth. Time-observations have been employed, especially in New
Zealand; but the uncertainty in selecting throughout the same phase of
the movement, and the large errors in the estimated depth resulting
from small errors in the time-records, are at present most serious
objections. There remains the method devised by Mallet, and, though he
claimed for it an exaggerated accuracy, it still, in my opinion, holds
the field against all its successors. When carefully applied, as it
has been by Mallet himself, by Johnston-Lavis and Mercalli, we
probably obtain at least some conception of the depth of the seismic
focus.
Professor Omori and Mr. K. Hirata have recently[80] lessened the chief
difficulty in the application of Mallet's method. They have deduced
the angle of emergence from the vertical and horizontal components of
the motion as registered by seismographs, instead of from the
inclination of fissures in damaged walls. In two recent earthquakes
recorded at Miyako in Japan, they find the angle of emergence to be
7.2° and 9° respectively, the corresponding depths of the foci being
5.6 and 9.3 miles. These are probably the most accurate estimates that
we possess, and it will be noted that they differ little from the mean
values obtained for the Neapolitan, Andalusian, and Riviera
earthquakes--namely, 6.6, 7.6, and 10.8 miles.
NATURE OF THE SHOCK
In one respect, the earthquakes described above fail to represent the
progress of modern seismology. They furnish no diagrams made by
accurately constructed seismographs within their disturbed areas. The
curve reproduced in Fig. 36, as already pointed out, is no exception
to this statement. For another reason, the records that were obtained
in Japan of the earthquake of 1891 are trustworthy for little more
than the short-period initial vibrations; for, owing to the passage of
the surface-waves, visible in and near the meizoseismal area, the
Japanese seismographs registered the tilting of the ground rather than
the elastic vibrations that traversed the earth's crust.
Notwithstanding this defect, personal impressions of an
earthquake-shock give a fairly accurate, if incomplete, idea of its
nature. Nearly all observers placed under favourable conditions agree
that an earthquake begins with a deep rumbling sound, accompanied,
after the first second or two, by a faint tremor which gradually, and
sometimes rapidly, increases in strength until it merges into the
shock proper, which consists of several or many vibrations of larger
amplitude and longer period, and during which the attendant sound is
generally at its loudest; the earthquake dying away, as it began, with
tremors and a low rumbling sound.
[Illustration: FIG. 79.--Seismographic Record of Tokio Earthquake
of 1894. (_Omori._)]
The vibrations that produce the sensible shock are by no means all
that are present during an earthquake. The Indian earthquake, for
instance, seemed to last about three or four minutes at Midnapur; but
the movements of the bubble of a level showed that the ground
continued to oscillate for at least five minutes longer (p. 280). Many
of these unfelt waves are rendered manifest by seismographs, although
there are still others that elude registration either from the extreme
shortness or the great length of their periods.
In Fig. 79 is shown the principal part of a diagram obtained at Tokio
during the Japanese earthquake of June 20th, 1894 (p. 18), the curve
representing the N.E.-S.W. component of the horizontal motion during
the first 25 seconds of the record. The instrument employed is one
specially designed for registering strong earthquakes, and is
unaffected by very minute tremors. Those which formed the commencement
of this earthquake lasted for about 10 seconds, as shown by ordinary
seismographs, and the vibrations had attained a range of a few
millimetres before they affected the instrument in question. For the
first 2-1/2 seconds, they occurred at the rate of four or five a
second. The motion then suddenly became violent, and the ground was
displaced 37 mm. in one direction, followed by a return movement of 73
mm., and this again by one of 42 mm., the complete period of the
oscillation being 1.8 seconds. The succeeding vibrations were of
smaller amplitude and generally of shorter period for a minute and a
half, then dying out during the last three minutes as almost
imperceptible waves with a period of two or more seconds.[81]
Though incomplete in some respects, this diagram illustrates clearly
the division of the earthquake-motion into three stages--namely, the
preliminary tremors, the principal portion or most active part of an
earthquake, and the end-portion or gradually evanescent slow
undulations. In all three stages, however, both tremors and slow
undulations may be present; and, as the latter, owing to their long
period, are more or less insensible to human beings, the ripples of
the final stage give the impression of a tremulous termination as
described above. The duration of each stage varies considerably in
different earthquakes. Thus, in a valuable study of 27 earthquakes
recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori
and Hirata show[82] that the duration of the preliminary stage varies
from 0 to 26 seconds, with an average of about 10 seconds; that of the
principal portion from 0.7 to 26 seconds, also with an average of
about 10 seconds; and that of the end portion from 28 and 105 seconds,
with an average of about one minute. The total apparent duration,
however, depends on the instrument employed; one of the earthquakes,
that of April 23rd, 1898, disturbing the seismograph at Miyako for two
minutes; while, at Tokio, a horizontal pendulum designed by Professor
Omori oscillated for at least two hours. The periods of both ripples
and slow undulations, again, vary from one earthquake to another; but
it is worthy of notice that the average period of the undulations is
almost constant in all three stages of the motion, being 1.1, 1.3, and
1.3 seconds, respectively, for the east-west component of the
horizontal motion, and 1.0 second throughout for the north-south
component. For the ripples, the average period is .08 second in the
preliminary stage, .10 second in the principal portion, and .08 second
again in the end portion; those of the principal portion being
slightly larger in amplitude, as well as longer in period, than the
ripples of the first and third stages.
SOUND-PHENOMENA.
Besides the ripples already mentioned, there are others of still
smaller amplitude and shorter period that are sensible, but as a rule
only just sensible, to us as sounds. All the known evidence points to
the extraordinary lowness of the earthquake-sound. According to some
observers, it seems as if close to their lower limit of audibility;
while others, however intently they may listen, are unable to hear the
slightest noise. In other words, the most rapid vibrations present in
an earthquake do not recur at a rate of much more than about 30 to 50
per second; or, if they do, they are not strong enough to impress the
human ear.
To most observers, the sound seems to increase and decrease in
intensity with the shock, and so gradually and smoothly does this
change take place that the sound is frequently mistaken for that of an
underground train approaching the observer's house, passing beneath
it, and receding in the opposite direction. Some persons, especially
if situated within the meizoseismal area, hear also loud crashes in
the midst of the rumbling sound and simultaneously with the strongest
vibrations. At a moderate distance, say from 30 to 40 miles, the sound
becomes more harsh and grating while the shock is felt; and, at a
greater distance, even this change disappears, and nothing is heard
but an almost monotonous sound like the low roll of distant thunder.
The explanation of this is that the sound-vibrations are of different
periods and varying amplitude, and the limiting vibrations tend to
become inaudible with increasing distance, the lower on account of
their long period, the higher owing to their small amplitude.
The magnitude of the sound-area depends, even more than that of the
disturbed area, on the personal equation of the observers. The lower
limit of audibility varies not only in different individuals, but also
in different races. In Great Britain, it is doubtful whether an
earthquake ever occurs unaccompanied by sound; and in the meizoseismal
area the noise is heard by nearly all observers. With Italians, the
average lower limit of audibility is higher than with the Anglo-Saxon
race; slight shocks frequently occur without noticeable sound, but
with strong ones, the larger number of observers is sure to include
one or more capable of hearing the rumbling noise. The Japanese are,
however, seldom affected by the most rapid earthquake-vibrations, and
the strongest shocks may be unattended by any recorded sound. The
result is manifest in the size of the sound-area in different
countries. In the Hereford earthquake, the sound-area contained 70,000
square miles; in the Neapolitan earthquake, about 3,300 square miles;
while, in Japanese earthquakes, the sound is rarely heard more than a
few miles from the epicentre.
Another effect of this personal equation of the observers is that the
sound-vibrations apparently outrace those of longer period. The
Italians, for instance, generally hear the sound that precedes the
shock, and more rarely the weaker sound that follows it. In Japan,
only the earlier sound-vibrations, if any, seem to be audible. In
Great Britain, on the contrary, the fore-sound is perceptible to four,
and the after-sound to three, out of every five observers; and these
proportions are maintained roughly to considerable distances from the
epicentre. It follows, therefore, that the sound-vibrations and those
which constitute the shock must travel with nearly, if not quite, the
same velocity; and that the greater duration of the sound is due
either to the prolongation of the initial movement or to the
overlapping of the principal focus by the sound-focus. Neither
alternative can be regarded as improbable, but observations made on
British earthquakes point to the latter explanation as the true one.
It will be sufficient to refer to two phenomena in support of this
statement. In the first place, the percentage of observers who hear
the fore-sound varies with the direction from the epicentre. Thus,
during the Inverness earthquake of 1901, the majority of observers in
Aberdeenshire regarded the sound as beginning and ending with the
shock; while, in counties lying more nearly along the course of the
great fault, the sound was generally heard both before and after the
shock (p. 253). In this case, then, the initial and concluding sound
vibrations must have come chiefly from the margins of the seismic
focus; and those from the margin nearest to an observer would be more
sensible than those from the farther margin. Again, in slight
earthquakes, such as the Cornwall earthquake of April 1, 1898,[83] the
curves of equal sound intensity, while their axes are parallel to
those of the isoseismal lines, are displaced laterally with respect to
these curves, owing to the arrival of the strongest sound-vibrations
from the upper margin of an inclined seismic focus.
When a fault-slip occurs, the displacement is obviously greatest in
the central region, and dies out gradually towards the margins of the
focus. The phenomena described above show that the evanescent
displacement within these margins generate sound-vibrations only; and
that the greater slip within the central region produces also the more
important vibrations that compose the shock. As the former are
perceptible over a limited district, while the latter may be felt
through half a continent, it is clear that the sound-area should bear
no fixed relation in point of size to the disturbed area, but should
be comparatively greater for a slight shock than for a strong one.
VELOCITY OF THE EARTH-WAVES.
If we consider only the earthquakes here described, we see at once how
great is the diversity in the estimated velocity of the earth-waves.
On the one hand, we have a value as high as 5.2 kms. per sec. for the
Charleston earthquake, and, at the other end of the scale, a value of
0.9 km. per sec. for the Hereford earthquake. Between them, and
equally trustworthy, lie the estimates of 3.0 km. per sec. for the
Indian earthquake, and 2.1 kms. per sec. for the Japanese earthquake
and its immediate successors.
It is difficult to account entirely for such discordance. Errors of
observation may be responsible for a small part of the differences.
The initial strength of the disturbance appears to have some effect,
and the nature of the rocks traversed must be a factor of consequence
when the distances in question are not very great. In the Japanese and
Hereford earthquakes, all three may have combined to produce the
divergent results, the distance in these cases being only 275 and 142
kms. respectively.
In the Indian and Charleston earthquakes, the distances are much
greater (1944 and 1487 kms.), and the variety of rocks traversed must
tend to give a truer average. In the former, the result obtained (3.0
kms. per sec.) agrees so closely with the velocity of the long-period
undulations of distant earthquakes as to suggest that it was these
waves that were timed at the stations west of Calcutta and disturbed
the magnetographs at Bombay.[84]
Omitting, then, the Indian estimate, we find that, for the Japanese
and Charleston earthquakes, the velocity increases with the distance
as measured along the surface. To a certain extent, such a result
might have been expected, had we assumed the earthquake-waves to
travel along the chords joining the focus to very distant places of
observation.
The wave-paths that penetrate the earth are straight lines, however,
only when the conditions that determine the velocity are uniform
throughout, and such uniformity we have no reason to expect. From what
we know of the earth's interior, there can, indeed, be little doubt
that the velocity of earthquake-waves increases with the depth below
the surface, and that the wave-paths in consequence are curved lines
with their convexity downwards. It would be out of place to state more
than the principal result of the recent investigations by Dr. A.
Schmidt[85] and Prof. P. Rudzki[86] on this subject. These are based on
the assumptions that the velocity increases with the depth below the
surface, and that it is always the same at the same depth. From the
focus of the earthquake, wave-paths diverge in all directions. Those
which start horizontally curve upwards, and intersect the surface of
the earth in a circle dividing the whole surface into two areas of very
unequal size. Within the small area, the surface-velocity is infinite
at the epicentre, and decreases outwards until it is least on the
boundary-circle. In the larger region beyond, the surface-velocity
increases with the distance from the epicentre, until, at the antipodes
of that point, it is again infinite. But, as the depth of the focus is
always slight compared with the radius of the earth, the small circular
area surrounding the epicentre is practically negligible, and we may
regard the surface-velocity of the waves that traverse the body of the
earth as a quantity that continually increases with the distance from
the epicentre.
How fully this interesting theoretical result has been confirmed is
well shown in Mr. Oldham's recent and very valuable investigation on
the propagation of earthquake-motion to great distances.[87] A study
of the records of the Indian earthquake revealed the existence of
three series of waves, the first two consisting in all probability of
longitudinal and transversal waves travelling through the body of the
earth, and the third of undulations spreading over its surface (pp.
282-285). Extending his inquiries to ten other earthquakes originating
in six different centres, Mr. Oldham distinguishes the same three
phases in their movements; the third phase being the most constantly
recorded, the second less so, while the first phase is the most
frequently absent. With the exception of a few very divergent records,
the initial times of these phases and the maximum epoch of the third
phase are plotted on the accompanying diagram (Fig. 80), in which
distances from the epicentre in degrees of arc are represented along
the horizontal line and the time-interval in minutes along the
perpendicular line. The dots near the two lower curves refer to the
records of the heavily weighted Italian instruments, and the crosses
to those of the light horizontal pendulums, which respond somewhat
irregularly to the motion of the first two phases (p. 282). In the
third phase, there is less divergence between the indications of the
two classes of instruments, and dots are used in each case for the
initial, and crosses for the maximum epoch.
[Illustration: FIG. 80.--Time-curves of principal epochs of
earthquake-waves of distant origin. (_Oldham._)]
Of the smoothed curves drawn between these series of points, those
marked A, B, and C represent the time-curves of the beginnings of the
first, second, and third phases respectively, while D is the
time-curve for the maximum of the third phase.
The concavity of the two lower lines towards the horizontal base-line
shows that the surface-velocity of the corresponding waves increases
rapidly with the distance, far more so than would be possible with
rectilinear motion. The rates at which these waves travel through the
earth therefore increase with the depth, and the wave-paths must in
consequence be curved lines convex towards the centre of the earth.
If the time-curves A and B were continued backwards to the origin,
their inclinations at that point to the horizontal line give the
initial velocities of the corresponding waves, which prove to be about
5 and 3 kms. per sec. respectively. Now, according to recent
experiments made by Mr. H. Nagaoka on the elastic constants of
rocks,[88] the mean velocity of seven archaean rocks is 5.1 kms. per
sec. for the longitudinal waves, and 2.8 kms. per sec. for the
transversal waves--values which agree so closely with those obtained
for the first two series of earthquake-waves as to leave little doubt
with regard to their character.
The other time-curves, C and D, corresponding to the initial and
maximum epochs of the third phase, are practically straight lines.
Some of the records are slightly discordant for the average curve,
especially for the initial epoch; but it is often difficult to define
the commencement of this phase with precision. At any rate, the
observations show no distinct sign of an increase in the
surface-velocity of these waves with the distance from the origin. It
may therefore be concluded that they travel along the surface with
velocities which are practically constant for each individual
earthquake, the largest waves at the rate of about 2.9 kms. per sec.,
and the advance waves with a velocity of about 3.3 kms. per sec.,
rising occasionally to over 4.0 kms. per sec.
STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
Changes of elevation have long been known as accompaniments of great
earthquakes, though many of the earlier observations and measurements
left much to be desired in accuracy and completeness. The Japanese
earthquake of 1891, however, placed the reality of such movements
beyond doubt, and revealed the existence of a fault-scarp, with a
height in one place of 18 or 20 feet, and a length of at least 40, if
not of 70, miles. In the Indian earthquake of 1897, the fault-scarps
were shorter, though more pronounced in character, the largest known
(the Chedrang fault) being about 12 miles long, and having a maximum
throw at the surface of 35 feet. In some other recent earthquakes,
also, remarkable fault-scarps have been developed. After the great
shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure
was traced for a distance of about 34 miles, running in an
east-south-east and west-north-west direction through the epicentral
district, and varying in width from an inch or two to more than three
yards. That it was a fault, and not an ordinary fissure, was evident
from its great length, its uniform direction, and its independence of
geological structure. The throw was generally small, in no place
exceeding five feet.[89] Again, in British Baluchistan, after the
severe earthquake of December 20th, 1892, a fresh crack was observed
in the ground running for several miles in a straight line parallel to
the axis of the Khojak range. It coincided almost exactly with a line
of springs, and was clearly produced by a fresh slip along an old line
of fault, for before the earthquake it had the appearance of an old
road, and the natives assert that the ground has always cracked along
this line with every severe shock. In 1892, the change in relative
height of the two sides of the fault was small, in one place where it
was measured being only two inches.[90]
But other changes, besides those in a vertical direction, occasionally
take place; though, owing to their recent discovery, comparatively few
examples are as yet known. While the throw of the Japanese fault
varied greatly in amount, and once even in direction, there was also a
constant shift towards the northwest of the ground on the north-east
side of the fault, the displacement at one spot being as much as 13
feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar
shift took place, though to what extent is unknown. There is,
moreover, evidence of actual compression of the earth's crust at right
angles to the fault-line. The Neo valley, traversed by the Japanese
fault, was apparently narrower after the earthquake than it was
before, and plots of ground were reduced from 48 to 30 feet in
length--_i.e._, by nearly 40 per cent. In British Baluchistan, the
formation of the fissure referred to above was accompanied both by
compression perpendicular, and by shifting parallel, to the fault. The
actual displacement in each direction is unknown, but the resultant
was not less than 27 inches.
There can be no doubt that a fault-scarp is formed in the first place
with great rapidity. So abrupt, indeed, were the structural
displacements in the epicentral area of the Indian earthquake, that
they contributed very materially to the intensity of the shock, giving
rise to the excessive velocities observed at Rambrai and elsewhere (p.
273). The growth of the scarp does not, however, always cease with the
first great earthquake, though it may take place in a contrary sense,
as in the elevation connected with the Conception earthquake of 1835.
The principal shock, according to Darwin, was followed during the few
succeeding days "by some hundred minor ones (though of no
inconsiderable violence), which seemed to come from the same quarter
from which the first had proceeded; whilst, on the other hand, the
level of the ground was certainly not raised by them; but, on the
contrary, after an interval of some weeks, it stood rather lower than
it did immediately after the great convulsion."[91]
AFTER-SHOCKS.
A series of after-shocks, more or less long, is a constant attendant
on every great tectonic earthquake, and few are the earthquakes of
any degree of strength that can be regarded as completely isolated.
Even in those which visit this country, after-shocks are seldom
absent. For instance, confining ourselves to the last few years, the
Pembroke earthquake of 1892 was followed by 8 shocks, the Inverness
earthquake of 1890 by at least 10, and possibly by 19 shocks, and that
of the same district in 1901 by 15 well-defined after-shocks in
addition to many others recorded by one observer. Of 300 Italian
earthquakes strong enough to cause some damage to buildings, Dr.
Cancani finds that every one was either preceded or followed, and
chiefly followed, by its own train of minor shocks.
For some hours, and even for days, after a great earthquake, the
shocks are so numerous that it is often impossible to keep count of
them. Many local centres spring into activity in different parts of
the epicentral area; and, though only the strongest shocks can be
identified elsewhere, it is clear that as a rule the shocks felt at
any one station are quite distinct from those observed at another.
The enormous number of after-shocks that follow some earthquakes can
only be realised when they are subjected to continuous seismographic
registration; and, even then, countless earth-sounds and the slightest
tremors must escape detection. The shocks may, indeed, succeed one
another so rapidly that one begins before another ends, and the result
is an almost incessant tremulous motion rendered manifest by the
quivering of water-surfaces or the swinging of chandeliers. Of the
total number of after-shocks, we may form some idea from recent
records in Japan. After the Mino-Owari earthquake of 1891, 3,365
shocks were recorded within little more than two years at Gifu, and
1,298 at Nagoya, but neither of these figures includes the shocks felt
within the first few hours. Of the Kumamoto earthquake of July 28th,
1889, the after-shocks recorded at Kumamoto until the end of 1893
amount to 922; and those of the Kagoshima earthquake of September 7th,
1893, recorded at Chiran until the end of January 1894, to 480. During
the first 30 days, the numbers recorded were 1,746 at Gifu, 340 at
Kumamoto, and 278 at Chiran; showing, as Professor Omori remarks, that
the after-shocks diminish in frequency with the size of the disturbed
areas,[92]--_i.e._, roughly with the initial intensity of the shocks.
Next to absolute number, the rapid decline in general frequency is the
most marked characteristic of after-shocks. Professor Omori has shown
that, excluding minor oscillations, it follows the law represented
geographically by the curves in Fig. 51, and algebraically by the
equation y = k / (h + x), where _y_ is the frequency at time _x_ and
_h_ and _k_ are constants for one and the same earthquake. By means of
this formula, it is possible to estimate roughly the interval of time
that must elapse before the seismic activity of the central district
resumes its normal value. For the Mino-Owari earthquake, this proves
to be about forty years, for the Kumamoto earthquake about seven or
eight years, and for the Kagoshima earthquake about three or four
years.
In a recent memoir on Italian after-shocks,[93] Dr. Cancani has urged
that other factors besides initial intensity determine the duration of
a seismic period, and prominently among these he places the depth of
the seismic focus. When the depth is very small, the duration of the
period is short, not much more than ten days; when the depth is
moderate, the duration may extend to three months; and, when great, it
may amount to several years.
The principal law that governs the distribution of after-shocks in
time may be regarded as well-established. It is otherwise with regard
to their distribution in space. This has been examined only in the
cases of the Japanese earthquake of 1891 and the Inverness earthquake
of 1901. So far as we can judge from the evidence which they furnish,
after-shocks appear to be most numerous within and near the central
portion of the seismic focus; though the area of maximum activity is
subject to continual oscillation. In this region, also, there is
evidence of a gradual decrease in the depths of the after-shock foci;
while, near the extremities of the epicentral area, there occur
districts of slightly greater frequency than elsewhere. With the lapse
of time, there seems therefore to be a constant extension, both
upwards and longitudinally, of the area over which the principal
fault-slip took place.
ORIGIN OF EARTHQUAKES.
In the introductory chapter, a brief sketch is given of the different
causes to which earthquakes are assigned. With those due to rock-falls
in subterranean channels, we need have little to do. The shocks are
invariably slight, and the part they play in the shaping of the
earth's crust is insignificant. Volcanic earthquakes possess a higher
degree of interest. They represent, no doubt, incipient or
unsuccessful attempts to produce an eruption. They may be the
forerunners of a great catastrophe.
Of far higher importance in the history of our globe is the third
class of earthquakes, including all those connected with the manifold
changes which the crust has undergone. In the slow annealing process,
to which it has been subjected from the earliest times, the crust has
been crumpled and fractured, elevated into the loftiest mountain
ranges or depressed below the level of the sea. Every sudden yielding
under stress is the cause of an earthquake. It is chiefly, perhaps
almost entirely, in the formation of faults that this yielding is
manifested. The initial fracturing may be the cause of one or many
shocks, but infinitely the larger number must be referred to the slow
growth of the fault, the intermittent slips, now in one part, now in
another, which, after the lapse of ages, culminate in a great
displacement. Of the length of time occupied in the formation of a
single fault, we can make no estimate in years. The anticlinal fault
of Charnwood Forest dates from a pre-carboniferous period. In 1893 it
had not ceased to grow.[94]
Still less can we conceive, however faintly, the number of elemental
slips that constitute the history of a single fault. We may think, if
we please, of the 143 tremors and earth-sounds noted at Comrie in
Perthshire during the last three months of 1839, of the 306
earthquakes felt in the Island of Zante during the year 1896, or the
1,746 shocks recorded at Gifu during thirty days in 1891; but we shall
be as far as ever from realising the vast number of steps involved in
the growth of a fault, let alone a mountain-chain.
Yet, all over the land-surface of the globe, the crust is intersected
by numberless faults, and hardly any portion is there in which some or
many of these faults are not growing. One country, indeed, such as
Great Britain, may have reached a condition of comparative stagnancy;
the fault-slips are few and slight, and earthquakes in consequence are
rare and generally inconspicuous. In another, like Eastern Japan and
the adjoining ocean-bed, the movements are frequent, occasionally
almost incessant, and few years pass without some great convulsion by
which cities are wrecked and hundreds of human lives are lost. At such
times, we magnify the rôle of earthquakes, and are in some danger of
forgetting that, in the formation of a mountain-chain or continent,
they serve no higher purpose than the creaking of a wheel in the
complex movements of a great machine.
FOOTNOTES:
[79] _Phil. Trans._, vol. li., pt. ii., 1761, pp. 625-626.
[80] _Journ. Sci. Coll. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
194-195.
[81] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. vii., pt. v., 1894,
pp. 1-4; _Ital. Sismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
[82] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
161-195.
[83] _Quart. Journ. Geol. Soc._, vol. lvi., 1900, pp. 1-7.
[84] There is no reason why the surface-undulations of the Indian
earthquake should not have produced a sensible shock even as far as
Italy. Taking their amplitude in that country at 508 mm. and their
period at 22 sec. (p. 283), the maximum acceleration would be about 40
mm. per sec., corresponding to the intensity 2 of the Rossi-Forel
scale. (_Amer. Journ. Sci._, vol. xxxv., 1888, p. 429.)
[85] _Nature_, vol. lii., 1895, pp. 631-633.
[86] Gerland's _Beiträge zur Geophysik_, vol. iii., pp. 485-518.
[87] _Phil. Trans._, 1900A, pp. 135-174.
[88] _Publ. of Earthq. Inves. Com. in For. Langs._ (Tokyo), No. 4,
1900, pp. 47-67.
[89] S.A. Papavasiliou, Paris, _Acad. Sci., Compt. Rend._, vol. cxix.,
1894, pp. 112-114, 380-381.
[90] _Geol. Mag._, vol. x., 1893, pp. 356-360.
[91] _Geol. Soc. Trans._, vol. v., 1840, pp. 618-619.
[92] The disturbed areas of these earthquakes contained, respectively,
221,000, 39,000, and 30,000 square miles.
[93] _Boll. Sismol. Soc. Ital._, vol. viii., 1902, pp. 17-48.
[94] _Roy. Soc. Proc._, vol. lvii., 1895, pp. 87-95.
INDEX.
Acceleration, maximum, of wave-motion in Japanese earthquake, 184, 185;
in Indian earthquake, 272
After-shocks, definition, 4;
frequency, 198, 256, 296, 344;
distribution in space, 200, 203, 298, 326, 345;
sound-phenomena, 207, 300;
connection with fault-scarps, 300;
outlining of epicentre by, 326;
origin of, 257;
of Neapolitan earthquake, 40;
of Ischian earthquakes, 56, 65;
of Andalusian earthquake, 97;
of Charleston earthquake, 133;
of Riviera earthquake, 167;
of Japanese earthquake, 198;
of Hereford earthquake, 240;
of Inverness earthquake, 256;
of Indian earthquake, 296;
of British earthquakes, 343;
of Italian earthquakes, 343;
of Japanese earthquakes, 344
Agamennone, G., 93, 94, 101, 319
Alluvium, displacement of, by Indian earthquake, 287
Amplitude of wave-motion, definition, 4;
in Neapolitan earthquake, 34;
in Japanese earthquake, 185;
in Indian earthquake, 270
Andalusian earthquake, preparation for, 75;
investigation of, 76;
damage caused by, 77;
isoseismal lines and disturbed area, 78;
the unfelt earthquake, 82;
position of epicentre, 84;
depth of focus, 85;
nature of shock, 87;
sound-phenomena, 91;
velocity of earth-waves, 92;
connection between geological structure and intensity of shock, 95;
fissures, 96;
landslips, 97;
effect on underground water, 97;
after-shocks, 97;
origin of, 99;
bibliography, 101
Animals, effects of earthquakes on, 143
Baluchistan earthquake of 1892, 288, 341
Baldacci, L., 70, 73
Baratta, M., 320
Barrois, C., 76
Bergeron, C., 76
Bertelli, T., 175
Bertrand, M., 76
Birds, effects of earthquakes on, 143
Bordwar, crust-fracture at, 309
Bréon, R., 76
Burton, W.K., 214
Cancani, A., 281, 282, 320, 343, 345
Castro, M.F. de, 76, 101
Charleston earthquake, investigation of, 102;
damage caused by, 103;
isoseismal lines and disturbed area, 104;
preparation for, 107;
nature of shock, 108;
double epicentre, 111;
origin of double shock, 120;
depth of foci, 122;
velocity of earth-waves, 126;
fissures, 130;
sand-craters, 130;
effects on human beings, 131;
feeling of nausea, 132;
after-shocks, 133;
origin of, 134;
bibliography, 137
Charlon, E., 175
Chedrang, fault-scarp at, 304
Clocks, untrustworthiness of time-records of stopped, 39, 94, 121, 127
Conder, J., 177, 213
Coseismal lines, 227, 324
Covelli, N., 67, 69
Damage caused by Neapolitan earthquake, 10, 24;
by Ischian earthquakes, 50, 56;
by Andalusian earthquake, 77;
by Charleston earthquake, 103;
by Riviera earthquake, 139;
by Japanese earthquake, 181;
by Hereford earthquake, 217;
by Inverness earthquake, 247
Darwin, H., 281
Daubrée, A., 73
Davison, C, 202-206, 208, 210, 213, 215-261, 295
Death-rate of Neapolitan earthquake, 24;
of Ischian earthquakes, 50, 56;
of Andalusian earthquake, 77;
of Charleston earthquake, 104;
of Riviera earthquake, 140;
of Japanese earthquake, 182
Denza, F., 155, 175
Depth of seismic focus, methods of determining, 25, 86, 122, 326;
of Neapolitan earthquake, 28;
of Ischian earthquakes, 54, 61;
of Andalusian earthquake, 86;
of Charleston earthquake, 122, 125;
of Riviera earthquake, 150;
of Japanese earthquakes, 328
Derby earthquake of 1903, 236
Direction of shock, 22, 33, 186, 225, 325
Disturbed area, definition of, 3;
of Neapolitan earthquake, 10;
of Ischian earthquakes, 51, 58;
of Andalusian earthquake, 80;
of Charleston earthquake, 107;
of Riviera earthquake, 145;
of Japanese earthquake, 183;
of Hereford earthquake, 219;
of Inverness earthquake, 249;
of Indian earthquake, 265;
connection between intensity of shock and, 323
Dolomieu, 11
Du Bois, F., 73
Dutton, C.E., 103-137
Dutton's method of determining depth of seismic focus, 122, 327
Earthquake-motion, nature of, 280, 282, 328, 330, 337;
propagation of, to great distances, 337
Earth-sound, definition of, 4
Edinburgh, record of Indian earthquake at, 281, 283, 285
Ellis, W., 83
Emergence, angle of, 13
Epicentre, definition of, 3;
methods of determining position of, 14, 52, 60, 324;
of Neapolitan earthquake, 22, 23;
of Ischian earthquakes, 53, 60, 67;
of Andalusian earthquake, 84;
of Charleston earthquake, 111;
of Riviera earthquake, 146;
of Hereford earthquake, 224;
of Inverness earthquake, 248;
of Indian earthquakes, 264, 276, 302
Epomeo, 45, 61, 71
Falb's method of determining depth of seismic focus, 86, 327
Fallen pillars, evidence of, 17, 19
Fault, originating, of Hereford earthquake, 219;
of Inverness earthquake, 249
Fault-scarp of Japanese earthquake, 189;
general appearance, 189;
length, 192;
throw, 193;
horizontal shift, 193;
course, 193;
swamp formed by it, 194
Fault-scarps of Indian earthquakes, 273, 304;
Chedrang fault, 304;
Samin fault, 308;
of Greek earthquake of 1894, 340, 341;
of Baluchistan earthquake of 1893, 341, 342;
formation and growth of, 342
Fault-slips, tectonic earthquakes due to, 5, 43, 100, 135, 174, 211,
219, 224, 241, 249, 255, 317, 346
Fishes, destruction of, by Riviera earthquake, 162
Fissures, caused by Andalusian earthquake, 96;
by Charleston earthquake, 130;
by Inverness earthquake, 247;
by Indian earthquake, 285
Focus, seismic, definition of, 3
Focus, seismic, depth of, methods of determining, 25, 86, 122, 326;
of Neapolitan earthquake, 28;
of Ischian earthquakes, 54;
of Andalusian earthquake, 86;
of Charleston earthquake, 122, 125;
of Riviera earthquake, 150;
of Japanese earthquakes, 328
Focus, dimensions of seismic, of Hereford earthquake, 224;
of Inverness earthquake, 250
Fore-shocks, 321;
of Neapolitan earthquake, 40;
of Ischian earthquake, 57;
of Andalusian earthquake, 76;
of Charleston earthquake, 107;
of Riviera earthquake, 142;
of Japanese earthquake, 201;
of Hereford earthquake, 239;
of Inverness earthquake, 246
Fouqué, F., 76, 84, 101
Fracture, crust-, at Bordwar, 309
Fractures in buildings, evidence of, 14, 15, 26
Fuchs, C.W.C., 102
Galli, I., 82
Geological structure and intensity of shock, connection between, 95,
106, 113, 115, 135, 164, 265
Gifu, records of Japanese after-shocks at, 183, 197
Gray, T., 295
Great Glen fault and Inverness earthquakes, connection between, 245
Greek earthquake of 1894, fault-scarp of, 340
Hayden, E., 103
Heath, T., 283, 320
Hereford earthquake, investigation of, 215;
preparation for, 215, 238;
isoseismal lines and disturbed area of, 216;
damage caused by, 217, 294;
position of originating fault, 219;
nature of shock, 220;
origin of double series of vibrations, 223;
position and dimensions of the two foci, 224;
direction of the shock, 225;
coseismal lines and velocity of earth-waves, 227;
sound-phenomena, 229;
isacoustic lines and sound-area, 234;
fore-shocks, 238;
after-shocks, 240;
origin of earthquake, 240;
bibliography, 261
Hills, changes in aspect of, after Indian earthquake, 311
Hirata, K., 327, 331
Human beings, effects of Charleston earthquake on, 131
Hypocentre, 3
Iberian peninsula, earthquakes of, 75
Indian earthquake, investigation of, 262;
isoseismal lines and disturbed area, 264;
nature of shock, 266;
visible earth-waves, 268;
elements of wave-motion, 270;
sound-phenomena, 274;
velocity of earth-waves, 275;
the unfelt earthquake, 280;
earth-fissures, 285;
displacements of alluvium, 287;
sand-vents, 288;
rise of river-beds, etc., 290;
landslips, 291;
rotation of pillars, 293;
after-shocks, 296;
structural changes in epicentral area, 301, 315;
structure of epicentral district, 302;
fault-scarps, 304;
crust-fractures, 309;
lakes and pools not due to faulting, 310;
changes in aspects of hills, 311;
revision of trigonometrical survey, 312;
origin of earthquake, 317;
bibliography, 319
Inverness earthquake, preparation for, 246;
damage caused by, 247;
fissure in ground, 247;
isoseismal lines and disturbed area, 247;
position of originating fault, 249;
nature of shock, 250;
sound-phenomena, 253;
origin of earthquake, 255;
after-shocks and their origin, 256;
sympathetic earthquakes, 259;
comparison with Japanese earthquake, 260;
bibliography, 261.
Investigation, Mallet's methods of, 12, 21
Isacoustic lines, 234;
of Hereford earthquake, 235;
of Derby earthquake, 236
Ischia, volcanic history of, 45, 70;
characteristics of eruptions, 49;
seismic history, 49
Ischian earthquake of 1881, investigation of, 50;
isoseismal lines and disturbed area, 51;
position of epicentre, 52;
depth of focus, 54;
nature of shock, 55;
after-shocks, 56;
origin of, 70;
bibliography, 73
Ischian earthquake of 1883, investigation of, 56;
preparation for, 57;
isoseismal lines and disturbed area, 58;
position of epicentre, 60;
depth of focus, 61;
nature of shock, 64;
landslips, 64;
after-shocks, 65;
origin of, 70;
bibliography, 73
Ischian earthquakes, characteristics of, 66; origin of, 70
Isoseismal lines, definition of, 3;
their use in determining position of epicentre, 219, 249, 325;
of Neapolitan earthquake, 9;
of Ischian earthquakes, 51, 58;
of Andalusian earthquake, 78;
of Charleston earthquake, 104;
of Riviera earthquake, 143;
of Japanese earthquake, 178, 182;
of Hereford earthquake, 216;
of Inverness earthquake, 247;
of Indian earthquake, 264
Issel, A., 139, 163, 164, 166, 175
Japanese earthquake of 1887, 18
Japanese earthquake of 1891, investigation of, 177;
structure of meizoseismal area, 179;
damage caused by, 181;
isoseismal lines and disturbed area, 182;
nature of shock, 184;
the great fault-scarp, 189;
minor shocks, 197;
distribution of after-shocks in time, 198;
preparation for, 201;
distribution of after-shocks in space, 203;
sound-phenomena of after-shocks, 207;
sympathetic earthquakes, 209;
origin, of, 211;
bibliography, 213
Japanese earthquake of 1894, 18, 329
Johnston-Lavis, H.J., 50-72, 327
Kilian, W., 76
Koto, B., 177, 180, 181, 184, 190-196, 209, 212, 213
Lakes formed by bending of river-bed during Indian earthquake, 310
Lakes formed by fault-scarp of Japanese earthquake, 194;
of Indian earthquake, 305
Landslips caused by Ischian earthquake, 64;
by Andalusian earthquake, 97;
by Indian earthquake, 291
Lévy, M., 76
Lisbon earthquake of 1755, 75, 82
McGee, W.J., 134
Macpherson, J., 101
Magnetographs, earthquakes recorded by, 82, 157, 160, 189, 277, 282
Mallet, R., 7-44, 85, 102, 124, 150, 294-296, 325
Mallet's method of determining depth of focus, 25, 327
Masato, H., 178, 213
Mascart, E., 159, 160
May Hill anticlinal and Hereford earthquake, connection between, 242
Meizoseismal area, definition of, 3;
of Andalusian earthquake, 99;
of Japanese earthquake, 179
Mercalli, G., 11, 57, 58, 60, 61, 63, 67, 70-73, 76, 80, 84, 85, 88,
90, 101, 138-175, 325, 327
Michell, J., 325
Milne, J., 35, 177, 181, 182, 186, 189, 200, 213, 281, 283
Minor shocks of Neapolitan earthquake, 40;
of Japanese earthquake, 197
Mountain ranges, effect of, on intensity of shock, 95, 106
Moureaux, T., 161
Nagaoka, H., 177, 214, 339
Nagoya, records of Japanese after-shocks at, 183, 197
Nature of shock, Neapolitan earthquake, 30;
Ischian earthquakes, 55, 64;
Andalusian earthquake, 87;
Charleston earthquake, 108;
Riviera earthquake, 150;
Japanese earthquake, 184;
Hereford earthquake, 220;
Inverness earthquake, 250;
Indian earthquake, 266
Nausea, feeling of, caused by Charleston earthquake, 132
Neapolitan earthquake, investigation of, 7, 12;
isoseismal lines and disturbed area, 9;
damage caused by, 10;
position of epicentre, 14;
depth of focus, 25;
nature of shock, 30;
sound-phenomena, 37;
velocity of earth-waves, 39;
minor shocks, 40;
origin, 41;
bibliography, 44
Ness, Loch, connection between Inverness earthquakes and formation of,
255, 257, 261
Nogués, A.F., 101
Oddone, E., 175
Offret, A., 76, 158, 159, 175
Oglialoro, A., 73
Oldham, R.D., 262-320, 337, 340
Omori, F., 19, 20, 177, 183-186, 188, 197-199, 207, 214, 262, 325, 327,
329, 331
Origin of earthquakes, 2, 5, 345;
of Neapolitan earthquake, 41;
of Ischian earthquakes, 70;
of Andalusian earthquake, 101;
of Charleston earthquake, 134;
of Riviera earthquakes, 174;
of Japanese earthquake, 211;
of Hereford earthquake, 240;
of Inverness earthquake, 255;
of Indian earthquake, 317
Overturned bodies, maximum acceleration deduced from, 184, 272
Palmieri, L., 57, 72, 73
Periodicity of Japanese after-shocks, 199
Perrey, A., 7
Potenza, evidence of damaged church at, 15, 26
Prediction of earthquakes, possible, 322
Preparation for earthquakes, 40, 57, 76, 107, 142, 201, 238, 246, 321
Rails, flexure of, by Charleston earthquake, 112;
by Japanese earthquake, 182;
by Indian earthquake, 288
Railway-tunnels, observations of Riviera earthquake in, 166
Rebeur-Paschwitz, E. von, 281
River-beds, rise of, caused by Indian earthquake, 290
Riviera earthquake, investigation, 138;
damage caused by, 139;
preparation for, 142;
isoseismal lines and disturbed area, 143;
position of epicentre, 146;
depth of principal focus, 149;
nature of shock, 150;
sound-phenomena, 156;
the unfelt earthquake, 157;
effects of earthquake at sea, 162;
destruction of fishes, 162;
seismic sea-waves, 163;
connection between geological structure and intensity of shock, 164;
observations in railway-tunnels, 166;
after-shocks, 167;
recent movements in the Riviera, 170;
seismic history of the Riviera, 171;
origin of, 171;
bibliography, 175
Rocca di Papa, record of Indian earthquake at, 281, 282, 285
Rossi, M.S. de, 57, 74, 82, 101, 175
Rossi-Forel scale of seismic intensity, 104, 216, 247
Rotation of pillars, caused by Hereford earthquake, 294;
by Indian earthquake, 293;
explanation of, 295
Rudzki, P., 336
Rumi, Prof., 169
Samin, fault-scarp at, 308
Sand-craters caused by Charleston earthquake, 130;
by Indian earthquake, 288
Schmidt, A., 336
Seismic sea-waves of Riviera earthquake, 142, 163
Seismic vertical, 12, 29, 62
Seismographic records of Riviera earthquake, 154;
of Japanese earthquake of 1894, 329
Sekiya, S., 18, 19
Serpieri, A., 74
Shillong, nature of Indian earthquake at, 266
Sloan, E., 103, 117-119, 134, 135
Sound-area, definition of, 3;
of Neapolitan earthquake, 38;
of Andalusian earthquake, 92;
of Hereford earthquake, 234;
of Inverness earthquake, 252;
of Indian earthquake, 275
Sound-phenomena, nature of sound, 38, 229, 252, 332;
inaudibility to some observers, 231, 274; its cause, 233;
isacoustic lines, 234-236;
variations in nature of sound throughout sound-area, 237;
time-relation of sound and shock, 238, 253;
origin of earthquake-sounds, 334;
sound-phenomena of Neapolitan earthquake, 37;
of Andalusian earthquake, 91;
of Charleston earthquake, 133;
of Riviera earthquake, 156;
of Japanese after-shocks, 207;
of Hereford earthquake, 229;
of Inverness earthquake, 252;
of Indian earthquake, 274
Structural changes, distribution of, in Indian earthquake, 315
Subsultory shock, 5
Sympathetic earthquakes of Japanese earthquake, 209;
of Inverness earthquake, 259
Tanakadate, A., 177, 214
Taramelli, T., 76, 84, 85, 88, 90, 101, 138, 150, 165, 175, 325
Tectonic earthquakes, 5
Thrust-plane, Indian earthquake due to movement along, 318
Time-curve of Indian earthquake, 278;
of principal epochs of earthquake-waves of distant origin, 338
Time-records, general inaccuracy of, 324
Time-relations of sound and shock in Hereford earthquake, 238;
in Inverness earthquake, 253
Trigonometrical survey, revised, of Khasi hills after Indian
earthquake, 312;
interpretation of results, 314
Twin earthquakes, origin of, 32, 89, 120, 153, 174, 223;
Neapolitan earthquake, 31;
Andalusian earthquake, 87;
Charleston earthquake, 108;
Riviera earthquake, 149, 150;
Hereford earthquake, 221
Undulatory shock, 5
Unfelt earth-waves, Andalusian earthquake, 82;
Riviera earthquake, 157;
Indian earthquake, 280
Uzielli, G., 143, 176
Velocity, maximum, of wave-motion, in Neapolitan earthquake, 35;
in Indian earthquake, 272
Velocity of earth-waves, methods of determining, 39, 93, 127, 229;
variation with depth, 336;
form of wave-paths, 336;
velocity of different phases, 339;
of Neapolitan earthquake, 39;
of Andalusian earthquake, 92;
of Charleston earthquake, 126;
of Japanese earthquakes, 188;
of Hereford earthquake, 229;
of Indian earthquake, 275, 279, 284
Visible earth-waves in Charleston earthquake, 110;
in Japanese earthquake, 186;
in Indian earthquake, 268
Volcanic earthquakes, 5, 70
Vorticose shock, 5
Water, effect of Andalusian earthquake on underground, 97
Waterfalls caused by fault-scarps of Indian earthquake, 305
Wave-path, 13
West, C.D., 272
Woolhope anticlinal and Hereford earthquake, connection between, 241
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End of Project Gutenberg's A Study of Recent Earthquakes, by Charles Davison