Microseismicity in the inter-seismic period in the rupture area of the 1994 Sanriku-Oki Earthquake by ocean bottom seismographic observation

The off-Sanriku region, northern part of the Japan Trench subduction zone, is characterized by high seismicity and large (M>7) earthquakes occur almost once per decade. In fall 2000, we observed microseismicity in the rupture area of the 1994 Sanriku-Oki Earthquake (Mw 7.7) using an Ocean Bottom Seismometer(OBS)-network to probe temporal change in seismicity by comparing the hypocenter distribution at present and that of the aftershock activity. Land GPS network data shows that the ultra-slow post seismic failure has ceased and interplate coupling is recovering in this area. We expect that present seismicity pattern is different from that of the aftershock activity reflecting such change. The previous OBS study on the aftershock distribution of the 1994 earthquake showed that the aftershocks were distributed over the entire rupture area forming many clusters, except a gap in aftershock activity in the central part corresponding to an asperity with the largest coseismic moment release. The result of the OBS observation in 2000 also shows almost no seismicity in the same gap, separating the aftershock area into two parts with different seismicity pattern: In the trenchward half, many earthquakes during the present observation are located within clusters identical to those in aftershock period, indicating these clusters persistently active. In contrast, microseismicity in the landward half does not show evident cluster pattern as in the aftershock distribution. In the trenchward area, the moment release in the 1994 mainshock was significantly smaller, suggesting weaker seismic coupling. The different pattern of the seismicity of the inter-seismic period revealed by the present study may be correlated to such variation in interplate coupling strength.