A Recent Japanese Earthquake

Abstract

PROFESSOR SEKIYA, of the Imperial University, Tokio, has lately sent to this country a remarkably interesting and complete record of earthquake motion obtained by him during a severe shock which occurred at 6.52 p.m. on January 15 of this year. The most important portion of the record is shown in Fig. 1, reduced to a little more than one-third of the original size. The motion is recorded (by means of the writer's horizontal pendulum and vertical motion Seismographs) in three rectangular components—two horizontal and one vertical—on a plate of smoked glass which is caused to revolve uniformly by clockwork. The plate is started by an electric seismo-scope at the beginning of the disturbance, and for one or two seconds its motion is consequently slower than the uniform rale it afterwards attains. On this occasion the plate made one revolution in 126 seconds, and the horizontal motion continued during several revolutions. To avoid confusion only the first of these is reproduced in the figure: the motions which occurred subsequently were smaller, and, as usual, the disturbance subsided very gradually. The circles in which the three components are recorded have been arranged so that simultaneous motions are on the same radius. Radial straight *******occur, during the first part of the disturbance, with a period of about one-sixth of a second, or with about twelve times the frequency of the principal motions. The greatest amplitude of horizontal motion is found when these small oscillations have nearly died out, at the place marked A. By that time the vertical motion has become comparatively small. A few seconds later two lines, where they are drawn, mark seconds of times. The disturbance begins at a, b, and c irt Fig. 1. In its early portion it is marked very conspicuously by a feature which has been noticed (also at the beginning) in previous records—the presence of short-period oscillations superposed on larger and slower motions. These are particularly well defined in the horizontal motion, where they considerable vertical oscillations appear on the record; but the vertical component is, by a long way, the first to vanish. In the original record the horizontal components are each magnified five times, and the vertical component eight times: the same ratio between horizontal and vertical multiplication is of course maintained in the figure given here. At three places, A, B, and C, the horizontal motion has been compounded during intervals of 4, 6½, and 6½ seconds respectively: the results are shown to a magnified scale in Fig. 2, and illustrate well the complex character of earthquake motion. The greatest extent of horizontal motion is from one to the other extremity of the figure-of-eight in the first of these diagrams: its actual amount (on the ground) was 7.5 millimetres. The greatest vertical motion was 1.5 millimetres. Other records obtained by Prof. Sekiya lead him to conclude that the greatest vertical motion in Tokio earthquakes is about one-sixth of the greatest horizontal motion. In former examples published by the writer the record was in all cases taken on the soft alluvial soil on which the greater part of the city of Tokio is built. In this instance the record was taken (at the site of the new University buildings, Kaga Yashiki, Kongo) on the much harder ground which here and there rises above the alluvial plain. From a comparison of records taken at the old and the new sites of the Seismological Observatory, Prof. Sekiya concludes that the motion of the alluvial plain is generally greater than that of the; higher and stiffer soil in the ratio of two or three to one.