On the Mechanism of Earthquake Swarm at Hamasaka

Abstract

In June 1965 a microearthquake swarm occurred at Hamasaka, Hyogo Pref.. It was composed of about 70 shocks with two main shocks of M=3.6 and 3.1, and continued for 20 days. For the purpose of investigating the characteristics of microearthquake swarm, an analysis of this swarm was made, particularly of its fine mechannism. The dimensions of focal domain were estimated at about several hundred meters. The magnitude distribution was very well fitted to the formula, log N=α-β M, with β=0.8. The strain energy which can be stored in this focal domain does not contradict that calculated by the formula, log E=11.8+1.5M. This swarm is separated into three sequences, bounded by two main shocks. Focal mechanisms of these earthquakes are well explained by the model of Type II. The result was derived by two methods, one being the analysis of the amplitude of initial P motion, and the other the analysis of S/P maximum amplitude ratio. Focal mechanisms in each sequence change with time. In this swarm the focal mechanism of component earthquake is considered to deviate somewhat from that of earthquakes in a stationary state in this region, but it seems to approach the stationary state at the end of each sequence. This variation of focal mechanism may have some relation to the mode of strain release.