2001 年 54 巻 2 号 p. 267-280
In an attempt to examine the characteristic behavior of fault asperities (large slip areas), we comparatively studied two large earthquakes: the Tokachi-oki earthquake (M 7.9) of May 16, 1968 and the Sanriku-oki earthquake (M 7.5) of December 28, 1994, which have a partially common source area. Both the strong motion records at a regional network and the teleseismic body waves at global networks were analyzed to determine the detailed spatio-temporal distribution of moment release. The aftershock distribution, which may provide us with a more reliable location of asperity, was also re-examined using the same underground structure and the same algorithm for both events.
The total seismic moment, Mo, and the source duration, T are obtained as: Mo=3.5×1021Nm; T=90s for the 1968 event, and Mo=4.4×1020Nm; T=60s for the 1994 event. It is also shown that the 1968 event consists of more than two asperities, one of which took a role of asperity again for the 1994 event. The distribution of relocated aftershocks, which fringe the major asperities, strongly supports this fact. A simple calculation indicates that the seismic coupling is almost perfect (100%) in this common asperity. We thus propose that there exist characteristic sites for asperities where fault slip occurs only as a seismic event, and that the individual asperities usually manifest M 7 class earthquakes but sometimes synchronize to cause M 8 class earthquakes.