1968年11月8日焼岳に発生した群発地震の発震機構について

Abstract

The mechanism of earthquake swarm that occurred at Mt. Yakedake on November 8, 1968, have been studied. The swarm began at Olh3Om and terminated at 14h51m. 43 shocks wererecorded at Kamitakara by the vertical component seismograph for the observation of micro-earthquakes. The frequency distribution of the P-IS interval times of the swarm corresponds toone of four peaks in that distribution which were observed at the station during four monthsbefore the occurrence of the earthquake swarm.The foci of these shocks have been determined from the P-IS times observed at the near micro-earthquake observation stations. They are confined within a small volume whose dimensionis about 500 meters.The time variation of the S/P which means the ratio of the maximum amplitudes of P and Swaves have been investigated. The push-pull distributions of the initial P motions have beenalso studied. The results show that the focal mechanism changes in relation to the activityof the earthquake swarm and seems to approach the stationary state at the end of the swarm.It is supposed that the occurrence of the main shocks disturbed the stress field around theirfoci and gave the great influences upon the mechanism of the smaller shocks.This earthquake swarm is separated into four sequences. In the first sequence the large shockof M=~3.4 occurred in isolation. The main shock of M=3.3 and many aftershocks took placein the second sequence, the main shock of M =03.1 with some foreshocks and aftershocks occurredin the third one and the swarm of some smaller shocks occurred in the fourth one. This variationof the characteristics of the activity of each sequence seems to be related to the development ofthe fractures in the focal region.