Abstract
In October-November, 1989, five M≥6 earthquakes, including an M7.1 earthquake, sequentially took place off Sanriku, northeastern Japan, and the earthquake activity became very high in this region. Since the seismically active zone was far from the land observation network, it was difficult to obtain accurate hypocenter locations. We carried out ocean bottom seismographic observation in order to locate the hypocenters precisely and investigate characteristics of the activity. Combining the OBS and land station data for hypocenter determination, we obtained two main conclusions. (1) Almost all epicenters of the present activity distribute landward from the 4 km isobath. Some earthquake clusters were observed including an M5.6 earthquake and its aftershocks. (2) Focal depths of the earthquakes are shallower than 20 km and the activity is interpreted to take place around the boundary between the subducting Pacific plate and the landward plate. Taking account of the mechanisms of the larger earthquakes in this active period, this activity is a thrust fault type and is different from the 1933 Sanriku earthquake which was a normal fault type and might have broken the entire oceanic lithosphere.
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