Propagation of Seismic Wave in Japan. (Eighth Report)

[摘要] The present paper is the report of the studies on the abnormal propagation of seismic waves especially in K wantô district. The study was made by the precise examination of the seismometrical data observed at 25 observatories in K wantô district, upon the 48 local earthquakes occurred there in 1931. That the apparent velocity of the seismic wave differs with the direction of propagation, was pointed out by the present author.(2)Recently, Dr. H. Honda(3) treated this phenomenon quantitatively by means of measuring the deviation of the travel time observed at a certain station from the hodograph. But the numerical value of the deviation estimated by H. Honda is only a relative one. If we wish to have an absolute value of the deviation of travel time, we must measure this quantity as the deviation from the standard hodograph correspond to the given depth of seismic focus.Fortunately, H. Honda and K. Sagisaka calculated the velocities of P-wave at the successive depth of the earth crust from the observation of a few destructive ea. thquakes occurred recently in this country. Then K. Wadati and K. Sagisaka calculated the travel time of P-wave using the above-mentioned materials. The author, from the ratio of the velocities of P and S waves observed by a few authors, calculated the travel time of S wave and made the table showing the relation between the duration of preliminary tremor γ, depth of seismic focus h and the epicentral distance Δ(Tab. II). These three tables are considered to be standard at least in the present time when studying the propagation of seismic waves in our country.Next, the depth of focus of above mentioned 48 shocks were determined by means of Tab. II and the duration of preliminary tremor τo at the epicentre, which were measured from the Δ-τ diagram. Then the deviation of the travel time of P wave ΔP and that of the duration of preliminary tremor Δτ are measured by the deviation from the standard hodograph and the Δ-τ curve corresponding to the observed depth of the focus.Fig. 8 to Fig. 15 show the abnormal propagation in the Kwantô district thus observed. The figures written besides the position of the observatory denote the deviation ΔP and Δτ in seconds and the positive sign is attached to the travel time and the duration of preliminary which exceeds the standard.A simple method of determining the depth of focus by means of the radius of iso-PS line was manifested by the author a few years ago. At that time, however, the relation between the radius of iso-PS line and the depth of focus was found under the assumption of rectilinear propagation of seismic waves. Therefore, this relation was re-calculated by means of the standard travel time of P and S waves. Fig. 16 shows this relation between the mean radius of iso-PS line and the depth of focus. This relation was tested by the results of observation of 48 shocks stated above and fair agreement was proved.Also that the area of iso-PS line is closely related with the depth of seismic focus, was proved and this relation is also shown in Fig. 16. This relation is also proved to have a fair agreement with the observed results. Thus, a new method of determining the depth of seismic focus by means of the radius and the area of iso-PS line was proposed by the author as a simple and accurate one.

[学科分类] 大气科学