Focal Mechanisms of Shallow Earthquakes which Occurred in San'in, Kinki, and Hokuriku Districts

Abstract

Focal mechanisms of small-size shallow earthquakes which occurred from 1976 through 1986 in San'in, Kinki, and Hokuriku districts, South West Japan, were determined. These mechanisms were obtained from the first motion of P waves recorded at the microearthquake observation stations operated by Kyoto and Nagoya Universities and as well as at the stations of Japan Meteorological Agency (JMA).
Most of the mechanisms in this region are strike-slip fault type. However, in the vicinity of Lake Biwa, there are also some earthquakes of reverse fault type.
The P axes trend mostly between ENE-WSW and ESE-WNW directions. They agree with those determined by earlier workers for small and large earthquakes in this region. In the region along the Japan Sea coast, the P axes of most of the earthquakes are directed ESE-WNW. On the other hand, in the inland region adjacent to the coast, the P axes are directed E-W to ENE-WSW. However no distinct boundary between these two regions could be deciphered. It appears that from the inland region towards the cost P axis alignment gradually makes a clockwise rotation.