On the Time Variation of the Velocity of Seismic Wave (Part 1)

Abstract

Recently, M. Hayakawa discussed on the time variation of the velocity of seismic wave in connection with the internal stress in the earth's crust. (not yet published.) In the present paper, we examined this problem with a somewhat different method which is simpler than that of Hayakawa's. The travel time of the first impulse to a station from two foci of different shocks, which are nearly the same point, are compared. The difference of the two seems to be closely connected with the mechanism of another larger shock occurred during the period of the two shocks under consideration; if the most part of the path of the first impulse lies across the compression (dilation) area of the intermediate shock, the travel time of the first impulse of the earlier shock is usually less (greater) than that of the later one, concording with Hayakawa's conclusion. In part 1, we show the examples of Tajima-earthquake (Fukushima Prefecture) in Au ust 1943, and Tottori-earthquake in March-October 1943.