A Study of Recent Earthquakes Part 14

[48] Professor Uzielli has also published a map of the isoseismal lines for the Italian part of the disturbed area.

[49] It seems doubtful whether this movement was connected with the earthquake. M. Offret does not include Nice in his list of observatories at which magnetographs were disturbed.

[50] This is the time given by M. Offret. According to M. Mascart, it should be 6h. 25m. 40s.

[51] In order to test the truth of this explanation, M. Moureaux suspended a bar of copper at the Parc Saint-Maur observatory by two threads in the same way as the horizontal force-magnet. The direction of this bar was also registered photographically, and it remained unmoved during the Verny earthquake of July 12th, 1889, and the Dardanelles earthquake of October 25th, 1889, while one or more of the magnets were disturbed. The experiment, however, was ineffective; for, in order that the magnet may rest in a horizontal position, its centre of gravity must be at unequal distances from the two points of support.

[52] The hour-marks in Fig. 38 refer to Paris mean time, and those in Fig. 39 to Genoa mean time.

[53] In the seventeenth century, the maximum seismic activity was manifested in the neighbourhood of Nice, and in the eighteenth century in Piedmont.

CHAPTER VII.

THE j.a.pANESE EARTHQUAKE OF OCTOBER 28TH, 1891.

Although several years have elapsed since the occurrence of the greatest of j.a.panese 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 j.a.panese earthquakes during the years 1885-92, Professor Milne has provided the materials for a further a.n.a.lysis of the minor shocks that preceded and followed the princ.i.p.al earthquake.

The part of j.a.pan 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 princ.i.p.al 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 j.a.panese themselves as the Mino-Owari earthquake of 1891.

[Ill.u.s.tration: 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 Ise, is surrounded by mountain ranges, those to the west, north, and north-east being built up mainly of Palaeozoic rocks, and those on the east side of granite. A network of rivers and ca.n.a.ls converts what might otherwise have been unproductive ground into one of the most fertile districts in j.a.pan. 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 j.a.pan 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 f.u.kui, 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.

[Ill.u.s.tration: 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 Palaeozoic 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 _debris_. "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 ma.s.s of heaped-up rubbish might be pa.s.sed, "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 ca.n.a.ls 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 Yorks.h.i.+re. 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 s.h.i.+zuoka 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 princ.i.p.al 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 pa.s.sed 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:--

PRINc.i.p.aL 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 princ.i.p.al 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 sh.o.r.eline 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 ma.s.ses 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 j.a.panese 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.

[Ill.u.s.tration: FIG. 43.--Plan of Directions of Fall of Overturned Bodies at Nagoya.]

[Ill.u.s.tration: 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 j.a.panese 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 _debris_; 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.

[Ill.u.s.tration: FIG. 45.--Map of Meizoseismal Area. (_Koto._)]

[Ill.u.s.tration: FIG. 46.--Ploughshare Appearance of the Fault near Fujitani. (_Koto._)]

[Ill.u.s.tration: FIG. 47.--The Fault-scarp at Midori. (_Koto._)]