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.

Gifu.Nagoya.Osaka.Tokio (Imp. Univ.).
Maximum horizontal motion> 18 mm.> 26 mm.30 mm.> 35 mm.
Period of ditto2.0 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 ditto0.9 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.