THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891.
Although several years have elapsed since the occurrence of the
greatest of Japanese earthquakes, the final report that will embody
the labours of all its investigators is yet to be written. Several
important contributions to it, however, have already been made.
Professor Koto, in an admirable memoir, has traced the course of the
great fault-scarp and discussed the origin of the earthquake;
Professor Omori, with equal care and thoroughness, has investigated
the unrivalled series of after-shocks; Mr. Conder studied the damaged
buildings from an architect's point of view; Professor Tanakadate and
Dr. Nagaoka devoted themselves to a re-determination of the magnetic
elements of the central district,[54] while, by the compilation of his
great catalogue of Japanese earthquakes during the years 1885-92,
Professor Milne has provided the materials for a further analysis of
the minor shocks that preceded and followed the principal earthquake.
The part of Japan over which the earthquake was sensibly felt is
shown in Fig. 41. The small black area in the centre is that in which
the shock was most severe and the principal damage to life and
property occurred. The other bands, more or less darkly shaded
according to the greater or less intensity of the shock, will be
referred to afterwards. Fig. 45 represents the meizoseismal area on a
larger scale; and, as the greater part of it lies within the two
provinces of Mino and Owari, the earthquake is generally known among
the Japanese themselves as the Mino-Owari earthquake of 1891.
[Illustration: FIG. 41.--Sketch-Map of Disturbed Area and
Isoseismal Lines. (_Masato._)]
THE MEIZOSEISMAL AREA.
More than half of the meizoseismal area occupies a low flat plain of
not less than 400 square miles in extent. On all sides but the south,
the plain, which is a continuation of the depression forming the Sea
of Isé, is surrounded by mountain ranges, those to the west, north,
and north-east being built up mainly of Palæozoic rocks, and those on
the east side of granite. A network of rivers and canals converts what
might otherwise have been unproductive ground into one of the most
fertile districts in Japan. A great garden, as it has been aptly
termed, the whole plain is covered with rice-fields, and supports a
population of about 787 to the square mile--a density which is
exceeded in only six counties of England. As a rule, the soil is a
loose, incoherent, fine sand, with but little clayey matter; and it
is, no doubt, to its sandy nature that the disastrous effects of the
earthquake were largely due. In the northern half of the district, the
meizoseismal area is much narrower, and here it crosses a great
mountain-range running from south-west to north-east and separating
the river-systems of the Japan sea from those of the Pacific. To the
north, the meizoseismal area terminates in another plain, in the
centre of which lies the city of Fukui, where the destructiveness of
the earthquake was only inferior to that experienced in the provinces
of Mino and Owari. There is also a detached portion of the area lying
to the east of Lake Biwa, but it is uncertain whether the exceptional
intensity there was due to the nature of the ground or to the
occurrence of a secondary or sympathetic earthquake in its immediate
neighbourhood.
[Illustration: FIG. 42.--General Plan of Geological Structure of
Meizoseismal Area. (_Koto._)]
The general plan of the geological structure of the central district
is represented in Fig. 42. The thick line, partly continuous and
partly broken, shows the course of the great fault, to the growth of
which the earthquake chiefly owed its origin; while the thin
continuous lines represent the changing direction of strike of the
Palæozoic rocks which surround the Mino-Owari plain, and the
arrowheads the direction of the dip. It will be seen that the
direction of the strike forms an S-shaped curve, and it is clear that
the present torsion-structure of the district could not have been
produced without the formation of many fractures at right angles and
parallel to the lines of strike. Professor Koto points out that the
regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and
Itatori, indicated by broken lines in Fig. 42, have probably been
excavated along a series of transverse fractures running from
north-west to south-east; while fractures which are parallel to the
line of strike may be responsible for the zigzag course of the
valleys.
DAMAGE CAUSED BY THE EARTHQUAKE.
The great earthquake occurred at 6.37 A.M., practically without
warning, and in a few seconds thousands of houses were levelled with
the ground. Within the whole meizoseismal area there was hardly a
building left undamaged. The road from Nagoya to Gifu, more than
twenty miles in length, and formerly bordered by an almost continuous
succession of villages, was converted into a narrow lane between two
long drawn-out banks of _débris_. "In some streets," says Professor
Milne, "it appeared as if the houses had been pushed down from the
end, and they had fallen like a row of cards." Or, again, a mass of
heaped-up rubbish might be passed, "where sticks and earth and tiles
were so thoroughly mixed that traces of streets or indications of
building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and other
towns, fires broke out after the earthquake. In Kasamatsu the
destruction was absolutely complete; nothing was left but a heap of
plaster, mud, tiles, and charred timbers. At Ogaki, not more than
thirty out of 8000 houses remained standing, and these were all much
damaged. Within the whole district, according to the official returns,
197,530 buildings were entirely destroyed, 78,296 half destroyed, and
5,934 shattered and burnt; while 7,279 persons were killed, and 17,393
were wounded.
Next to buildings, the embankments which border the rivers and canals
suffered the most serious damage, no less than 317 miles of such works
having to be repaired. Railway-lines were twisted or bent in many
places, the total length demolished being more than ten miles. In
cuttings, twenty feet or more in depth, both rails and sleepers were
unmoved; it was on the plains that the effects of the earthquake were
most marked. The ground appeared as if piled up into bolster-like
ridges between the sleepers, and in many places the sleepers had moved
end-ways. When the line crossed a small depression in the general
level of the plain, the whole of the track was bowed, as if the ground
were permanently compressed at such places. "Effects of compression,"
says Professor Milne, "were most marked on some of the embankments,
which gradually raise the line to the level of the bridges. On some of
these, the track was bent in and out until it resembled a serpent
wriggling up a slope.... Close to the bridges the embankments had
generally disappeared, and the rails and sleepers were hanging in the
air in huge catenaries."
ISOSEISMAL LINES AND DISTURBED AREA.
The land area disturbed by the earthquake and the different isoseismal
lines are shown in Fig. 41. The "most severely shaken" district, that
in which the destruction of buildings and engineering works was
nearly complete, contains an area of 4,286 square miles, or about
two-thirds that of Yorkshire. This is indicated on the map by the
black portion. Outside this lies the "very severely shaken" district,
17,325 square miles in area, extending from Kobe on the west to
Shizuoka on the east, in which ordinary buildings were destroyed,
walls fractured, embankments and roads damaged, and bridges broken
down. The third or "severely shaken" district contains 20,183 square
miles; and in this some walls were cracked, pendulum clocks stopped,
and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just
within this area. In the fourth region the shock was "weak," the
motion being distinctly felt, but not causing people to run
out-of-doors; and in the fifth it was "slight," or just sufficient to
be felt. These two regions together include an area of 51,976 square
miles.
Thus, the land area disturbed amounts altogether to 93,770 square
miles--_i.e._, to a little more than the area of Great Britain.
According to Professor Omori, the mean radius of propagation was about
323 miles, and the total disturbed area must therefore have been about
330,000 square miles, or nearly four times the area of Great Britain.
Considering the extraordinary intensity of the shock in the central
district, this can hardly be regarded as an over-estimate.
The isoseismal lines shown in Fig. 41 are not to be regarded as drawn
with great accuracy; for there is no marked separation between the
tests corresponding to the different degrees of the scale of
intensity. The seismographs at Gifu and Nagoya were thrown down
within the first few seconds, and failed to record the principal
motion. But a great number of well-formed stone lanterns and
tombstones were overturned, and, from the dimensions of these,
Professor Omori calculated the maximum horizontal acceleration
necessary for overturning them at fifty-nine places within the
meizoseismal area.[55] At five of these it exceeded 4000 millimetres
per second per second, an acceleration equal to about five-twelfths of
that due to gravity. Making use of these observations, Professor Omori
has drawn two isoseismal lines within the central district, which are
shown in Fig. 44. At every point of the curve marked 2, the maximum
acceleration was 2000 millimetres per second per second, and of that
marked 1, 800 millimetres per second per second. The dotted line
within the curve marked 2 represents the boundary of the meizoseismal
area, which, it will be observed, differs slightly from that given by
Professor Koto (see Fig. 45). The difference, however, is apparently
due to the standard of intensity adopted, Professor Koto's boundary
agreeing rather closely with the curve marked 2 in Fig. 44.
NATURE OF THE SHOCK.
Little has yet been made known with regard to the nature of the shock,
and the published records of the accompanying sound are so rare that
it seems as a rule to have passed unheard. The seismographs at Gifu
and Nagoya registered the first half-dozen vibrations, and were then
buried beneath the fallen buildings. In the following table, the data
from these two stations are therefore incomplete:--
PRINCIPAL MEASUREMENTS OBTAINED FROM SEISMOGRAPHIC RECORDS.
-------------------+------------+----------+----------+---------------
| | | | Tokio
| Gifu. | Nagoya. | Osaka. | (Imp. Univ.).
-------------------+------------+----------+----------+---------------
Maximum horizontal | | | |
motion | > 18 mm. | > 26 mm. | 30 mm. | > 35 mm.
| | | |
Period of ditto | 2.0 secs. | 1.3 sec. | 1.0 sec. | 2.0 secs.
| | | |
Maximum vertical | | | |
motion | > 11.3 mm. | 6.2 mm. | 8 mm. | 9.5 mm.
| | | |
Period of ditto | 0.9 sec. | 1.5 sec. | 1.0 sec. | 2.4 secs.
-------------------+------------+----------+----------+---------------
If the period of the principal vibrations were known, the observations
of Professor Omori on the overturning of bodies would enable us to
determine the range of motion at different places. For instance, the
maximum acceleration at Nagoya was found by these observations to be
2,600 millimetres per second per second, and if we take the period of
the greatest horizontal motion to be the same as that of the initial
vibrations--namely, 1.3 second, the total range (or double amplitude)
would be 223 millimetres, or 8.8 inches. With the same period, and the
maximum acceleration observed (at Iwakura and Konaki) of more than
4,300 millimetres per second per second, the total range would be
greater than 14.5 inches.[56]
In the meizoseismal area, many persons saw waves crossing the surface
of the ground. At Akasaka, according to one witness, the waves came
down the streets in lines, their height being perhaps one foot, and
their length between ten and thirty feet. To the north of the same
area, we are told that "the shoreline rose and fell, and with this
rising and falling the waters receded and advanced." Even at Tokio,
which is about 175 miles from the epicentre, the tilting of the ground
was very noticeable. After watching his seismographs for about two
minutes, Professor Milne next observed the water in an adjoining tank,
80 feet long and 28 feet wide, with nearly vertical sides. "At the
time it was holding about 17 feet of water, which was running across
its breadth, rising first on one side and then on the other to a
height of about two feet." Still clearer is the evidence of the
seismographs in the same city. Instead of a number of irregular waves,
all the records show a series of clean-cut curves. The heavy masses in
the horizontal pendulums were tilted instead of remaining as steady
points. They were not simply swinging, for the period of the
undulations differed from that of the seismograph when set swinging,
and also varied in successive undulations. It was ascertained
afterwards, by measurement with a level, that to produce these
deflections, the seismograph must have been tilted through an angle of
about one-third of a degree.
_Direction of the Shock._--Shortly after the earthquake, Professor
Omori travelled over the meizoseismal area and made a large number of
observations on the directions in which bodies were overturned, taking
care to include only those in which the direction of falling would not
be influenced by the form of the base, such as the cylindrical stone
lanterns so frequently found in Japanese gardens. At some places
these bodies fell in various directions, at others with considerable
uniformity in one direction. For instance, at Nagoya, out of 200 stone
lanterns with cylindrical stems, 119 fell between west and south, and
36 between east and north; the numbers falling within successive
angles of 15° being represented in Fig. 43. The mean direction of fall
is W. 30° S., coinciding with that in which the majority of the
lanterns were overturned. Similar observations were made at forty-two
other places within and near the meizoseismal area, and the resulting
mean direction for each such place in the Mino-Owari district is shown
by short lines in Fig. 44, the arrow indicating the direction towards
which the majority of bodies at a given place were overturned. It will
be seen from this map that the direction of the earthquake motion was
generally at right angles, or nearly so, to that of the neighbouring
part of the meizoseismal zone, and that on both sides of it, the
majority of overturned bodies at each place fell towards this zone.
[Illustration: FIG. 43.--Plan of Directions of Fall of Overturned
Bodies at Nagoya.]
[Illustration: FIG. 44.--Map of Mean Directions of Shock and
Isoseismal Lines in Central District. (_Omori._)]
VELOCITY OF THE EARTH-WAVES.
The times of the great earthquake and of sixteen minor shocks on
October 28th and 29th and November 6th were determined at the Central
Meteorological Observatory at Tokio, and at either two or three of the
observatories of Gifu, Nagoya, and Osaka, each of which is provided
with a seismograph and chronometer. The after-shocks referred to
originated near a point about 6 miles west of Gifu, and the difference
between the distances of Tokio and Osaka from this point is 89-1/2
miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles.
The mean time-intervals between these three pairs of places were 67,
111, and 128 seconds respectively; and these give for the mean
velocity for each interval 2.1 kilometres (or 1.3 mile) per second.
Thus there appears in these cases to be no sensible variation in the
velocity with the distance from the origin.
As might be expected, an earthquake of such severity was recorded by
magnetometers at several distant observatories. Disturbances on the
registers of Zikawei (China), Mauritius, Utrecht, and Greenwich have
been attributed to the Japanese earthquake, but the times at which
they commenced are too indefinite to allow of any determination of the
surface-velocity of the earth-waves to great distances from the
origin.
THE GREAT FAULT-SCARP.
As in all disastrous earthquakes, the surface of the ground was
scarred and rent by the shock. From the hillsides great landslips
descended, filling the valleys with _débris_; and slopes which were
formerly green with forest, after the earthquake looked as if they had
been painted yellowish-white. Innumerable fissures cut up the plains,
the general appearance of the ground, according to Professor Milne,
being "as if gigantic ploughs, each cutting a trench from 3 to 12 feet
deep, had been dragged up and down the river-banks." But by far the
most remarkable feature of the earthquake was a great rent or fault,
which, unlike the fissures just referred to, pursued its course
regardless of valley, plain, or mountain. Although at first sight
quite insignificant in many places, and some time hardly visible to
the untrained eye, Professor Koto has succeeded in tracing this fault
along the surface for a distance of forty miles, and he gives good
reasons for believing that its total length must be not less than
seventy miles.
[Illustration: FIG. 45.--Map of Meizoseismal Area. (_Koto._)]
[Illustration: FIG. 46.--Ploughshare Appearance of the Fault near
Fujitani. (_Koto._)]
[Illustration: FIG. 47.--The Fault-scarp at Midori. (_Koto._)]
The general character of the fault-scarp changes with the surface
features. On flat ground, where the throw is small, it cuts up the
soft earth into enormous clods, or makes a rounded ridge from one to
two feet high, so that it resembles, more than anything else, the
pathway of a gigantic mole (Fig. 46). When the throw is
considerable--and in one place it reaches from 18 to 20 feet--the
fault-scarp forms a terrace, which from a distance has the appearance
of a railway embankment (Fig. 47). Or, again, where the rent traverses
a mountain ridge or a spur of hills, "it caused extensive landslips,
one side of it descending considerably in level, carrying the forest
with it, but with the trees complicatedly interlocked or prostrate on
the ground."
[Illustration: FIG. 48.--Displacement of Field Divisions by the
Fault near Nishi-Katabira. (_Koto._)]
At its southern end, the fault was seen for the first time crossing a
field near the village of Katabira. The field was broken into clods of
earth, and swollen up to a height of 5-1/2 yards, while a great
landslip had descended into it from an adjoining hill. A little
farther to the north-west, the ground was sharply cut by the fault,
the north-east side having slightly subsided and at the same time been
shifted horizontally through a distance of 3-1/4 to 4 feet to the
north-west Adjoining fields were formerly separated by straight mounds
or ridges running north and south and east and west, and these mounds
were cut through by the fault and displaced, as shown in Fig. 48. From
this point the fault runs in a general north-westerly direction, the
north-east side being always slightly lowered with respect to the
other and shifted to the north-west. Near Seki it takes a more
westerly direction, and continues so to a short distance east of
Takatomi, where the north side is lowered by five feet, and moved
about 1-1/4 feet to the west. At the north end of Takatomi, a village
in which every house was levelled with the ground, the fault is
double, and the continuous lowering towards the north has converted a
once level field into sloping ground. At this point, the small river
Toba, flowing south, is partially blocked by the fault-scarp, and an
area of about three-quarters of a square mile, on which two villages
stand, was converted into a deep swamp (Fig. 49), so that, as the
earthquake occurred at the time of the rice-harvest, the farmers were
obliged to cut the grain from boats. After passing Takatomi, the fault
again turns to the west-north-west, but, the throw being small, it
resembles here the track of an enormous mole. At Uméhara it crosses a
garden between two persimmon trees, appearing on the hard face of the
ground as a mere line; but the trees, which were before in an
east-and-west line, now stand in one running north and south, without
being in the least affected by the movement (Fig. 50). From here to
Kimbara, where the fault enters the Neo valley, the north side is
always depressed and shifted westwards by about 6-1/2 feet.
[Illustration: FIG. 49.--Map of Swamp formed by stoppage of River
Toba by Fault-scarp. (_Koto._)]
[Illustration: FIG. 50.--Shifting of Trees by fault at Uméhara.
(_Koto._)]
It was in the Neo valley that the supreme efforts of the earthquake
were manifested. Landslips were so numerous that the greater part of
the mountain slopes had descended into the valley, the whole
appearance of which had changed. "Unfamiliar obstacles," remarks
Professor Koto, "made themselves apparent, and small hills covered
with forest had come into sight which had not been seen before." But
the ground was not only lowered and shifted by the fault; it was
permanently compressed, plots originally 48 feet in length afterwards
measuring only 30 feet. In fact, "it appears," in the words of
Professor Milne, "as if the whole Neo valley had become narrower."
A few miles after entering the Neo valley, the throw of the fault
reaches its maximum at Midori. But instead of the relative depression
of the east side, which prevails throughout the rest of the line, that
side is here about 20 feet higher than the other. It is, however,
shifted as usual towards the north, by about 13 feet; and this
displacement is rendered especially evident by the abrupt break in the
line of a new road to Gifu (Fig. 47). That the east side has really
risen is clear, for, a little higher up, the river has changed from a
shallow rapid stream 30 yards wide into a small lake of more than
twice the width, and so deep that a boatman's pole could not reach the
bottom. At Itasho, about a mile north of Midori, both sides are nearly
on the same level, the fault appearing like a mole's track; and seven
miles farther, at Nagoshima, the east side is relatively depressed by
more than a yard, and at the same time shifted about 6-1/2 feet to the
north.
[Illustration: FIG. 51.--Daily frequency of after-shocks at Gifu
and Nagoya.]
At Nogo, the main Neo valley turns off at right angles to the east,
and the fault continues its course up a side valley, the east side,
with respect to the other, being continually depressed and shifted
towards the north. It was traced by Professor Koto through Fujitani
(Fig. 46), where there were many unmistakable evidences of the
violence of the shock, as far as the eastern shoulder of Haku-san; and
here, after following the fault for 40 miles, the lateness of the
season compelled him to return. There can be no doubt, however, that
it runs as far as Minomata; and it is probable, from the linear
extension of the meizoseismal area, that it does not entirely die out
before reaching the city of Fukui, 70 miles from its starting-point at
Katabira.
MINOR SHOCKS.
For some hours after the earthquake, shocks were so frequent in the
meizoseismal area that the ground in places hardly ever ceased from
trembling. Without instrumental aid, detailed record was of course
impossible; but fortunately the buried seismographs at Gifu and Nagoya
were uninjured, and in about seven hours both were once more in
working order. To the energy by which this result was accomplished, we
owe our most valuable registers of the after-shocks of a great
earthquake.
[Illustration: FIG. 52.--Monthly frequency of after-shocks at
Gifu. (_Omori._)]
Until the end of 1893--that is, in little more than two years--the
total number of shocks recorded at Gifu was 3,365, and at Nagoya
1,298. None of these approached the principal earthquake in severity.
Nevertheless, of the Gifu series, 10 were described as violent and 97
strong; while of the remainder, 1,808 were weak, 1,041 feeble, and 409
were sounds alone without any accompanying shock. The slight intensity
of most of the shocks is also evident from the inequality in the
numbers recorded at Gifu and Nagoya, from which it appears that nearly
two-thirds were imperceptible more than about 25 miles from the chief
origin of the shocks. Only 70 of the after-shocks during the first two
years were registered at Osaka, and not more than 30 at Tokio.
_Distribution of After-shocks in Time._--The decline in frequency of
the after-shocks was at first extremely rapid, the numbers recorded at
Gifu during the six days after the earthquake being 303, 147, 116, 99,
92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, half
of the shocks up to the end of 1893 occurred by November 23rd at Gifu,
and by November 6th at Nagoya. The daily numbers at these two places
are represented in Fig. 51, in which the crosses correspond to the
numbers at Gifu, and the dots to those at Nagoya; and the curves drawn
through or near the marks represent the average daily number of shocks
from October 29th to November 20th. It will be seen that these curves
are hyperbolic in form, the change from very rapid to very gradual
decline in frequency taking place from five to ten days after the
great earthquake. Fig. 52 illustrates the distribution in time of the
after-shocks at Gifu to the end of 1893, the ordinates in these cases
representing the number of shocks during successive months.[57]
A similar rapid and then gradual decline in frequency characterises
the strong and weak shocks recorded at Gifu. Of the ten violent
shocks, only one occurred after the beginning of January 1892; and of
the 97 strong shocks, only three after April 1892. But at the
commencement of the series, feeble shocks (_i.e._, shocks that could
just be felt) and earth-sounds without any accompanying movement were
comparatively rare, and did not become really prominent until two
months had elapsed. Of the 308 after-shocks recorded in 1893, none
could be described as strong, only 10 were weak, while 263 were feeble
shocks and 35 merely earth-sounds.
The last two diagrams show at a glance that the decline in frequency
of after-shocks is very far from being uniform. Some of the
fluctuations are due to the occurrence of exceptionally strong shocks,
each of which is followed by its own minor train of after-shocks.[58]
Others seem to be periodic, and possibly owe their origin to external
causes unconnected with the earthquake.[59]
_Method of representing the Distribution of After-shocks in
Space._--The maps in Figs. 54-57 show the distribution of the
after-shocks in space during four successive intervals of two months
each. They are founded on Professor Milne's great catalogue of
Japanese earthquakes, which give, among other data, the time of
occurrence and the position of the epicentre for every shock until the
end of 1892. For the latter purpose, the whole country is divided by
north-south and east-west lines into numbered rectangles, each
one-sixth of a degree in length and breadth; and the position of an
epicentre is denoted by the number of the rectangle in which it
occurs. The area included within the maps is bounded by the parallels
34° 40' and 36° 20' lat. N., and by the meridians 2° 10' and 3° 50'
long. W. of Tokio, so that ten rectangles adjoin each side of the map.
The number of epicentres lying within each rectangle having been
counted, curves are then drawn through the centres of all rectangles
containing the same number of epicentres, or through points which
divide the line joining the centres of two rectangles in the proper
proportion. Taking, for example, the curve marked 5, if the numbers in
two consecutive rectangles are 3 and 7, the curve bisects the line
joining their centres; if the numbers are 1 and 6, the line joining
their centres is divided into five equal parts, and the curve passes
through the first point of division reckoned from the centre of the
rectangle in which six epicentres are found. Thus the meaning of the
curve marked, say, 5 may be stated as follows:--If any point in the
curve be imagined as the centre of a rectangle whose sides are
directed north-south and east-west, and are respectively one-sixth of
a degree of latitude and longitude in length; then the number of
epicentres within this rectangle is at the rate of 5 for the time
considered.
_Preparation for the Great Earthquake._--At first sight, there appears
to have been but little direct preparation for the great earthquake.
Except for a rather strong shock on October 25th, at 9.14 P.M., it
occurred without the warning of any preliminary tremors. But a closer
examination of the evidence shows, as we should indeed expect, that
there was a distinct increase in activity for many months beforehand.
The region had become "seismically sensitive." Of the hundred
rectangles included in the maps in Figs. 53-57, there are thirteen
lying along the meizoseismal area of the earthquake of 1891, in which
nearly all the after-shocks originated. During the five years 1885-89,
53 out of 125 earthquakes (or 42 per cent.) had their epicentres lying
within the thirteen rectangles; or, in other words, the average
frequency in one of the rectangles of the meizoseismal area was five
times as great as in one of those outside it. In 1890 and 1891 (until
October 27th), the percentage in the thirteen rectangles rose to 61,
and the average frequency in one of them to ten times that of one of
the exterior rectangles.
The curves in Fig. 53 illustrate the distribution of epicentres during
the latter interval. It will be seen that they follow roughly the
course of the meizoseismal area southwards to the Sea of Isé, and that
to the south-east they continue for several miles the short branch of
the meizoseismal area which surrounds the southern end of the
fault-scarp.
[Illustration: FIG. 53.--Distribution of preliminary Shocks in
Space. (_Davison._)]
Thus, the preparation for the great earthquake is shown, first, by the
increased frequency of earthquakes originating within its meizoseismal
area; and, secondly, by the uniformity in the distribution of
epicentres throughout the same region, the marked concentration of
effort which characterises the after-shocks being hardly perceptible
during the years 1890-91.
[Illustration: FIG. 54.--Distribution of After-shocks in Space
(November-December 1891). (_Davison_).]
_Distribution of After-shocks in Space._--We have seen that the
after-shocks were subject to a fluctuating decline in frequency, rapid
at first, and more gradual afterwards. It is evident, from Figs.
54-57, that a similar law governs the area within which the
after-shocks originated. During the first two months, epicentres occur
over nearly the whole of the meizoseismal area, but afterwards they
are confined to a smaller district, which slowly, though not
continually, decreases in size.
[Illustration: FIG. 55.--Distribution of After-shocks in Space
(January-February, 1892). (_Davison._)]
[Illustration: FIG. 56.--Distribution of After-shocks in Space
(March-April). (_Davison._)]
The most important feature in the distribution of the epicentres is
the central region of extraordinary activity; but there are also
districts of minor and more short-lived activity near the three
extremities of the meizoseismal band. The seat of chief seismic action
shifts slightly from one part to another of the epicentral region,
especially about the end of 1891, as will be seen by comparing the
innermost curves of Figs. 54 and 55. Thus, with the decline in
frequency of the after-shocks and the decrease in their sphere of
action, there took place concurrently a gradual but oscillating
withdrawal of that action to a more or less central region of the
fault.
[Illustration: FIG. 57.--Distribution of After-shocks in Space
(May-June, 1892). (_Davison._)]
_Sound Phenomena of After-shocks._--While comparatively few observers
seem to have noticed any noise with the principal earthquake, many of
the after-shocks were accompanied by sounds. Professor Omori describes
them as belonging to two types. They were either rushing feeble noises
like that of wind, or loud rumbling noises like those of thunder, the
discharge of a gun, or the fall of a heavy body. In the Neo valley,
sounds of the second type were most frequent and distinct, but they
either occurred without any shock at all, or the attendant tremor was
very feeble; while, on the other hand, severe sharp shocks were
generally unaccompanied by distinctly audible sounds.
It is remarkable, also, that sounds were less frequently heard with
the early than with the later after-shocks. In November 1891, the
percentage of audible shocks was 17, and from December to the
following April always lay between 10 and 12. In May the percentage
suddenly rose to 39, and until the end of 1892 was always greater than
32, while in November 1892, it rose as high as 49. This, of course,
agrees with Professor Omori's observation that sounds attended feeble
shocks more often than strong ones.
The distribution of the audible after-shocks in space is shown in Fig.
58. These curves are drawn in the same way as those in Figs. 53-57,
but they represent the percentages, not the actual numbers, of shocks
accompanied by sound. It will be noticed that all three groups of
curves lie along the meizoseismal area, or the continuation of the
south-east branch; while the axis of the principal group of curves
lies to the west of the central regions in which most after-shocks
originated.
[Illustration: FIG. 58.--Distribution of Audible After-shocks in
Space (November 1891-December 1892). (_Davison_.)]
The explanation of these peculiarities is no doubt connected with the
comparative inability of the Japanese people to perceive the deep
sounds which in Europe are always heard with earthquake shocks. The
sounds are rarely heard by them more than a few miles from the
epicentre.[60] We may therefore conclude that slight after-shocks
originated nearer the surface than strong ones, that the mean depth
of the foci decreased with the lapse of time, and that the axes of the
systems of curves in Fig. 58 mark out approximately the lines of the
growing faults. The separation of the two westerly groups of curves
appears to show that the main branch of the meizoseismal area is
connected with a fault roughly parallel to that traced by Professor
Koto, but of which no scarp (if it existed) could be readily
distinguished among the superficial fissures produced by the great
shock.
EFFECT OF THE EARTHQUAKE ON THE SEISMIC ACTIVITY OF THE ADJOINING
DISTRICTS.
So great and sudden a displacement as occurred along the fault-scarp
could hardly take place without affecting the stability of adjoining
regions of the earth's crust, and we should naturally expect to find a
distinct change in their seismic activity shortly after October 28th.
In Fig. 59 two such regions are shown, bounded by the straight dotted
lines. The district in which the principal earthquake and its
after-shocks originated is enclosed within the undulating dotted
lines. The continuous lines inside all three districts are the curves
corresponding to 10 and 5 epicentres for the years 1885-92. Not far
from the axes of the outer groups of curves there are probably
transverse faults, approximately parallel to the great fault-scarp and
the main branch of the meizoseismal band, and distant from them about
45 and 55 miles respectively.
[Illustration: FIG. 59.--Map of Adjoining Regions in which
Seismic Activity was affected by the Great Earthquake.
(_Davison._)]
In the district represented in the north-east corner of Fig. 59, 29
earthquakes originated between January 1st, 1885, and October 27th,
1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 of
the latter number occurring in November 1891. In the south-west
district, the corresponding figures before and after the earthquake
are 20 and 36, 8 of the latter occurring in November 1891. Thus, in
the north-east district, for every shock in the interval before the
earthquake there were six in an equal time afterwards, and at the rate
of 10 during November 1891; and in the south-west district, for every
shock before the earthquake there were 10 afterwards, and at the rate
of 16 during November 1891.
Now, it is unlikely that the gradual increase of stress should be so
nearly proportioned everywhere to the prevailing conditions of
resistance as to give rise to a marked and practically simultaneous
change in seismic activity over a large area; whereas the paroxysmal
occurrence of a strong earthquake might alter the surrounding
conditions with comparative rapidity, and so induce a state of seismic
excitement in the neighbourhood. It therefore seems very probable
that the increased activity in the two districts here described was a
direct consequence of the occurrence of the great earthquake.
ORIGIN OF THE EARTHQUAKE.
The preponderance of preliminary earthquakes within the meizoseismal
area and the outlining of the fault-system by the frequency curves of
1890-91 (Fig. 53) point to the previous existence of the originating
fault or faults, and to the earthquake being due, not to the formation
of a new fracture, as has been suggested, but to the growth of an old
fault.
The last severe earthquake in the Mino-Owari plain occurred in 1859,
so that for more than thirty years there had been but little relief to
the gradually increasing stresses. Now, the distribution of stress
must have been far from uniform throughout the fault-system, and also
the resistance to displacement far from proportional to the stresses
at different places. At certain points, therefore, the effective
stress would be greater than elsewhere, and it would be at these
points that fault-slips would first occur. Such slips tend to remove
the inequalities in effective stress. Thus, the function of the slight
shocks of 1890 and 1891 was, briefly, to equalise the effective stress
over the whole fault-system, and so to clear the way for one or more
great slips throughout its entire length.
As to which side of the fault moved during the great displacement, or
whether both sides moved at once, we have no direct evidence but as
regards the neighbourhood of Midori, and there the conditions were
exceptional. Professor Koto thinks that it was probably the rock on
the north-east side that was generally depressed and always shifted to
the north-west But the disturbance in reality seems to have been more
complicated. That this was the case, that displacement occurred along
more than one fault, is probable from the branching of the
meizoseismal area, the isolation of the audibility curves of the
after-shocks (Fig. 58), and the sudden increase in seismic activity
both to the north-east and south-west of the epicentre. The detached
portion of the meizoseismal area near Lake Biwa may also point to a
separate focus. The whole region, indeed, was evidently subjected to
intense stresses, and the depression on the north-east side of the
fault-scarp can hardly fail to have been accompanied by other
movements, especially along a fault running near the western margin of
the main branch of the meizoseismal area.
The later stages of the movements are somewhat clearer. From a study
of the after-shocks, we learn that the disturbed masses began at once
to settle back towards the position of equilibrium. At first the slips
were numerous and took place over the whole fault-system, but chiefly
at a considerable depth, where no doubt the initial displacement was
greatest. After a few months, stability was nearly restored along the
extremities of the faults; slips were confined almost entirely to the
central regions, while a much larger proportion of them took place
within the superficial portions of the faults.
The official records bring down the history to the end of 1893. Since
that time more than one strong shock has been felt in the Mino-Owari
plain; but the stage of recovery from the disturbances of 1891 is
probably near its end, and we seem rather to be entering on a period
in which the forces are once more silently gathering that sooner or
later will result in another great catastrophe.
REFERENCES.
1. CONDER, J.--"An Architect's Notes on the Great Earthquake of
October 1891." _Japan Seismol. Journ._, vol. ii., 1893, pp.
1-91.
2. DAVISON, C.--"On the Distribution in Space of the Accessory
Shocks of the Great Japanese Earthquake of 1891." _Quart.
Journ. Geol. Soc._, vol. liii., 1897, pp. 1-15.
3. ---- "On the Effect of the Great Japanese Earthquake of 1891 on
the Seismic Activity of the Adjoining Districts." _Geol.
Mag._, vol. iv., 1897, pp. 23-27.
4. ---- "On the Diurnal Periodicity of Earthquakes." _Phil. Mag._,
vol. xiii., 1896, pp. 463-476, especially pp. 466-468.
5. ---- "On Earthquake-Sounds." _Phil. Mag._, vol. xlix., 1900, pp.
31-70--especially pp. 49-53, 60-61.
6. KOTO, B.--"The Cause of the Great Earthquake in Central Japan,
1891." _Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893,
pp. 295-353.
7. MASATO, H.--"Report on Earthquake Observations in Japan." _Cent.
Meteor. Obs. of Japan_ (Tokio, 1892), pp. 16-18, 41, and map
30.
8. MILNE, J.--"A Note on the Great Earthquake of October 28th,
1891." _Japan Seismol. Journ._, vol. i., 1893, pp. 127-151;
_Brit. Assoc. Rep._, 1892, pp. 114-128.
9. ---- "A Catalogue of 8,331 Earthquakes recorded in Japan between
1885 and 1892." _Japan Seismol. Journ._, vol. iv., 1895, pp.
1-367--especially pp. 134-234, 303-353.
10. ---- "On Certain Disturbances in the Records of Magnetometers
and the Occurrence of Earthquakes." _Brit. Assoc. Rep._, 1898,
pp. 226-251--especially pp. 227, 232, 234, 241, and 245.
11. MILNE, J., and W.K. BURTON.--"The Great Earthquake in Japan."
_Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893, pp.
295-352.
12. OMORI, F.--"On the After-shocks of Earthquakes." _Journ. Coll.
Sci. Imp. Univ. Japan_, vol. vii., 1894, pp. 111-200; abstract
in _Japan Seismol. Journ._, vol. iii., 1894, pp. 71-80.
13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th,
1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_,
No. 4, Tokio, 1900, pp. 13-24.
14. ---- "Sulla velocità di propagazione e sulla lunghezza delle
onde sismiche." _Ital. Soc. Sismol. Boll._, vol. i., 1895, pp.
52-60--especially pp. 52-57.
15. ---- "Sull' intensità e sull' ampiezza del movimento nel gran
terremoto giapponese del 28 ottobre 1891." _Ital. Soc. Sismol.
Boll._, vol. ii., 1896, pp. 189-200.
16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of
Oct. 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign
Languages_, No. 7, Tokio, 1902, pp. 27-32.
17. ---- "Note on the relation between Earthquake Frequency and the
Atmospheric Pressure." _Tokyo Phys.-Math. Soc. Reports_, vol.
ii., 1904, No. 8.
18. TANAKADATE, A., and H. NAGAOKA. "The Disturbance of Isomagnetics
attending the Mino-Owari Earthquake of 1891." _Journ. Coll.
Sci. Imp. Univ. Japan_, vol. v., 1893, pp. 149-192.
FOOTNOTES:
[54] I have not referred to the results of this survey, for, though
changes in all the magnetic elements (especially in horizontal
intensity) have taken place between 1887 and 1891-92, these changes
cannot be ascribed with confidence to the earthquake in the absence of
a thorough knowledge of the secular variation.
[55] From the formula a=x*g/y, where _a_ is the maximum horizontal
acceleration, _g_ the acceleration due to gravity, _y_ the height of
the centre of gravity, and _x_ its horizontal distance from the edge
about which the body was overturned.
[56] These estimates are made, on the supposition of simple harmonic
motion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is the
total range or double amplitude, _a_ the maximum acceleration, and _t_
the period of the vibration.
[57] Professor Omori finds that the mean daily number of earthquakes
_y_ during the month _x_ (reckoned from November 1891) may be
approximately represented by the equation--
y = 16.9 / (x + 0.397);
or, taking the semi-daily earthquake numbers during the five days
between October 29th and November 2nd, 1891, by the equation--
y = 440.7 / (x + 2.314),
where _y_ denotes the number of earthquakes observed during the twelve
hours denoted by _x_, the time being measured from the first half of
October 29th. It is interesting to notice that, taking account of the
mean annual frequency of earthquakes in ordinary years, the number of
shocks observed at Gifu during the two years 1898-99 should, according
to the latter formula, be 163; the actual number recorded was 160.
[58] The last violent shock before the end of 1893 occurred on
September 7th, 1892, and its effects on the frequency of after-shocks
is shown by the daily numbers recorded at Gifu during the first
fortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September
7th), 8, 8, 5, 4, 3, 2, 4, 3.
[59] The periodicity of after-shocks is discussed in the papers
numbered 4, 12, 16, and 17 at the end of this chapter. In these, the
existence of diurnal and other periods is clearly established.
Professor Omori also shows that the mean daily barometric pressure is
subject to fluctuations with maxima occurring on an average every
5-1/2 days, and that earthquakes are least frequent on the days of the
barometric maxima and minima, and more frequent in the days
immediately preceding and following them.
[60] Of the Japanese earthquakes of 1885-92 originating beneath the
land, twenty-six per cent. were accompanied by a recorded sound; but
less than one per cent. of those originating beneath the sea and not
more than ten miles from the coast.