小地震およびその前震・余震群の発生過程とクラスター構造

抄録

Investigation of the occurrence characteristic of earthquake swarms is one of effective ways to elucidate the generating mechanism of earthquakes. We analyzed Fukusaki earth. quake sequence, which occurred at the east end of the Yamasaki fault and consists of the main shock (M 4.3), 19 foreshocks and 99 aftershocks. From precise determination of hypocenters and P waveform analysis it was made clear that foreshocks and aftershocks have a clustering structure. Earthquakes in each cluster, which has a size of tens to two hundred meters, have very similar waveforms.
The most important characteristic of the cluster is that the largest event occurred at the final stage of each cluster's activity. From this we assume that one cluster corresponds to one weak area which has patches of weaker fracture strength, and that these patches break preceding the complete fracture of the whole area. This earthquake sequence occurred in two crushed zones (vertical and oblique) which are related to fractured zones of the Yamasaki fault. The main shock is considered to be a fracture of an asperity which existed in the oblique crushed zone.