MICRO-EARTHQUAKES OCCURRING IN THE VICINITY OF KYOTO (2)

Abstract

One of the conclusions drawn in the first paper that micro-earthquakes are very frequent in a belt-like region running from Wakasa Bay to Osaka Bay is strongly confirmed by use of data obtained after the publication of the first paper. According to the analysis of data, the focal depths of the micro-earthquakes are shallow in the east end of the belt-like region, and become deeper toward the west end. The deepest of focal depths, as was already pointed out, lie in the crust. For making the focal mechanism of the micro-earthquakes clear, the first motions of P waves are also examined. As a result, the distribution of the first motions of P waves is of quadrant type, and the horizontal component of the maximum pressure lie in the E-W direction, which agrees comparatively well with the direction of the maximum pressure of very shallow earthquakes occurring in Kinki District. The frequent occurrence of not only micro-earthquakes but the larger earthquakes in the belt-like region might have a close relation to the geologically distincive features found in Bouguer anomaly, horizontal dis. placements and Quaternary faults. It is also shown that the region concerned, if uniform, has the mean velocity of 5.8 to 6.0km/sec for P, and 3.4 to 3.5km/sec for S.