Mechanisms of Microearthquakes and Their Relation to Geological Structures

Abstract

For the purpose of examining relations between mechanism of microearthquakes and geological structure and also of investigating tectonic significance of microseismicity, 130 microearthquakes were analysed in an area surrounding the Yamasaki fault, which is a typical strike-slip fault in the northwestern Kinki District, Southwest Japan. Mechanisms of microearthquakes were analysed by the method of fault plane solution for 69 earthquakes out of 130, and by the smoothed radiation pattern using all earthquakes. It is remarkable that along such a clear strike-slip fault as the Yamasaki fault almost all earthquakes have strike-slip type and their maximum pressure axes lie in a nearly E-W direction. This result regarding microearthquakes is coincident with those for large and moderate earthquakes. Moreover, the strike of one nodal plane is nearly coincident with the fault strike, and it changes consistently when the fault strike changes its direction. On the other hand, in a microseismic belt along IZ-HM line (Fig. 1) where any clear strike-slip fault has not yet been detected, mechanisms of microearthquakes were very complicated, including strkie-slip, thrust and normal fault types. These results suggest that the mechanism of the microearthquake, as well as its spacial distribution, is closely related to the geological structures.