Part 14 (1/2)

[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.