The region off Fukushima Prefecture is characterized by high seismic activity. On Feb. 2, 1987, M6.4 and M6.7 earthquakes occurred there sequentially. Thereafter six earthquakes with M≥5 took place from Feb. to Apr., 1987 in this region. On June 10, we deployed five ocean bottom seismometers (OBS) to estimate focal depths accurately and to investigate the nature of the seismic activity. During the observation period (18 days) we obtained high quality seismic data from four of the OBSs, including an M5.7 earthquake and its aftershocks. We determined 289 hypocenter locations with accuracy, especially using by high quality S wave data.Almost all the earthquakes occurred in the region where the water depths were less than 2km and the region was more than 100km landward from the trench axis. The hypocenter distribution in a profile perpendicular to the trench axis shows a clear dipping plane corresponding to the boundary between the landward Eurasian plate and the subducting Pacific plate. We also observed twoearthquakes which took place at the lower plane of the double seismic zone and confirmed that the lower plane seismic activity existed at about 110km landward from the trench axis.
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Nishizawa et al. (1990) studied this question.