SPATIAL-DISTRIBUTION OF EARTHQUAKES ASSOCIATED WITH THE PACIFIC PLATE SUBDUCTION OFF NORTHEASTERN JAPAN REVEALED BY OCEAN BOTTOM AND LAND OBSERVATION

Precise earthquake locations in the vicinity of a trench axis elucidate the characteristics of the shallower part of a subducting plate. We split the Japan Trench into two regions, off-Sanriku in the north and off-Fukushima in the south, on the basis of the seismic properties of the regions and the fine microearthquake structure investigated by ocean bottom seismographic surveys, as well as recent observation of major seismic activity. From ocean bottom seismograph data, the hypocentral distributions along a profile perpendicular to the trench axis, indicate that almost all earthquakes occur along the boundary separating the subducting Pacific plate and the landward plate. A difference of the hypocentral distribution between off-Fukushima and off-Sanriku is found at the shallow plate boundary level. The seismic thrust zone begins at more than 100 km landwards from the trench axis off-Fukushima, whereas it begins at 50 km off-Sanriku and is nearly flat between 50 and 100 km from the trench axis. This reflects regional variations of the interplate coupling. In spite of these variations, a common characteristic in recent observations indicates that in both regions almost all the hypocenters are deeper than 10 km. This suggests the plate boundary to be decoupled at least in the part shallower than 10 km.