抄録
Subcrustal earthquakes beneath the Lake Biwa in the northeastern Kinki district, Japan have been investigated from data of microearthquake observation networks. Earthquakes, which belong to this group, are different in their occurrence mode from other subcrustal earthquakes in the Kinki and the Chubu districts. Their hypocenters, the depth of the Moho discontinuity and the Pn velocity are determined consistently. The Moho discontinuity beneath the Lake Biwa lies at a depth of 40km which is the largest in the Kinki and the Chubu districts. The Pn velocity beneath the Lake Biwa is about 7.5km/sec and is about 4% lower than the average one, say 7.8km/sec, in the Kinki district. Subcrustal earthquakes beneath the Lake Biwa are different from other subcrustal earthquakes which occur in the uppermost mantle in the neighbouring region due to the subduction of the Philippine Sea plate. Seven earthquakes occurred at the depth of the Moho discontinuity, and others occurred at 3km above and 2km below the discontinuity. The deepest one show the focal mechanism of the reverse faulting with P axis in the direction of NE-SW. Six earthquakes which occurred at the depth of and above the Moho discontinuity show the focal mechanism of the normal faulting with T axes in the direction of NW-SE, which is parallel to the direction of the slip vector of the relative motion between the Philippine Sea plate and the Asian plate and is locally perpendicular to the leading edge of the Philippine Sea plate. The bending of the crust due to the compressional stress by the Philippine Sea plate is one of the possible interpretations for the nature of the source mechanism. This idea is also supported by the focal mechanisms of crustal earthquakes around the Lake Biwa. An idea that accounts for northwest-ward migration of the Lake Biwa is also proposed based upon the occurrence characteristics of earthquakes in the region.
