Seismic Activity of Subcrustal Earthquakes and Associated Tectonic Properties in the Southeastern Part of the Kinki District, Southwestern Japan

Abstract

Seismic activity of the subcrustal earthquakes was studied to determine the tectonic properties of the southeastern part of the Kinki district. Depth contours deduced from the hypocentral distribution distinguished the seismic zone situated to the south of Lake Biwa from the seismic zone in the northern part of Kii Peninsula. One of the seismic zones corresponding to the marked activity in the Kii Peninsula was found to exhibit the inclination dipping toward N45°W with a dip angle of about 40° and to reach a depth of about 90 km beneath the east coast of Osaka Bay, whereas the zone to the south of Lake Biwa was found to be descending almost westward with a dip angle of 30°.
Focal mechanism solutions of 43 subcrustal earthquakes also indicated that the two regions were distinguished by the tectonic features of P and T axes and the fault types. The tectonic stress system of the parallel extension along the Nankai trough was verified in the north of Kii Peninsula. But the reverse faults caused by the E-W compression were found to be conspicuous in the region to the south of Lake Biwa.
A tectonic line, called "hinge line, " was supposed at the boundary of the two seismic zones. The Yoshino earthquake (1952, M=7.0, h=70 km) occurred around this area, and one of its focal planes, the right-lateral component of fault slip, lay almost in the E-W direction, parallel to this line. These facts suggest that the areas along the supposed tectonic line are capable of generating large earthquakes, though the present seismicity is quiescent thereabouts.