四国東部および周辺地域における地震の発震機構

Abstract

Focal mechanisms of 60 earthquakes are determined in the eastern part of Shikoku and its vicinity from P first motions. The events shallower than 23 km have maximum pressure axes of east-west direction, on the contrary events deeper than 23 km have these of north-south direction. The distribution of the earthquakes in the uppermost mantle dips from the Nankai trough to the Median Tectonic Line and maximum pressure axes show down-dip comprresion along the dip direction of distribution of them. Furthermore, maximum tension axes lie east-west direction in the distribution of the deeper earthquakes. A few earthquakes with normal faults are found in the upper mantle of central Shikoku and Kii Channel. This complex feature of focal mechanisms shows the detailed structure of the Philippine sea plate in this region.