1987 Volume 35 Issue 6 Pages 425-448
In order to elucidate the generating mechanism of microearthquakes, we analyzed three earthquake groups around the Yamasaki fault system in southwest Japan. It has been revealed that the distribution of hypocenters in an earthquake group consists of several "clusters." Events in each cluster have the common fault plane with linear dimension of about 100 to 200 m. The fault plane is considered to be a small unit of heterogeneous structure in the source region, and it imposes restrictions on the fracture process of microearthquake sequences. Events with M≤2.5 are fractures within one fault plane. On the other hand, events with M≥2.5-3.0 are found to be multiple shocks composed of two or three subevents. It was made clear that one of the multiple shocks fractured two fault planes of clusters successively. In a series of fractures on the fault plane of a cluster, the migration of hypocenters and the increase of stress drops are observed before the final complete fracture. This can be explained by small strong patches on the fault plane of the cluster.