日本海中部地震発生による三陸沖地震活動の低下と東北日本のブロック構造

抄録

Seismicity in the east-off Sanriku region decreased conspicuously after 26 May 1983, when the central Japan Sea earthquake with a magnitude of 7.7 occurred on the west coast of Tohoku, northeastern Japan. The decrease was restricted to shallow seismicity in the depth range of 0-40 km. For deeper earthquakes in the depth range of 41-70 km rate of earthquake occurrence rather increased. Seismicity in the land area of northern Tohoku also decreased at the same time. However, seismic activity in the Oga-Oshika tectonic zone increased remarkably after the central Japan Sea earthquake. Increase of seismicity was also observed in the sea region corresponding to the extension of the Oga-Oshika tectonic zone. We consider that the coincidence of the notable decrease of shallow seismicity in the coupling region between the Pacific plate and the land plate with the central Japan Sea earthquake occurring on the west coast of Tohoku indicates the following tectonic regime. The northern Tohoku district constitutes one rigid-like block and stress fields in the both sides of the block, e. g., stress at the east-off Sanriku subduction zone and that at the active tectonic zone along the coast of the Japan Sea, influence each other. On the other hand the increase of seismic activity in the Oga-Oshika tectonic zone and in its extension after the central Japan Sea earthquake is considered to show that the zone corresponds to the southern boundary of the northern Tohoku block and change of stress is likely to be transmitted along the zone. Seismicity in the Oga-Oshika tectonic zone has a tendency to increase always when large earthquake occurs in the zone or in its extension. It should be noted that decrease of stress in the coupling region between the Pacific and the land plates due to the occurrence of the central Japan Sea earthquake was confined to the north of the Oga-Oshika tectonic zone. Seismicity in the subduction region to the south of the Oga-Oshika tectonic zone did not show any change when the central Japan Sea earthquake occurred. It is interesting to note as well that seismicity in the Pacific coast and sea regions did not change at all by the 1964 Niigata earthquake with a magnitude of 7.5 whose focal region was located on the east coast of the Japan Sea, but to the south of the Oga-Oshika tectonic zone. These facts suggest that the eastern Japan, eastern side to the Itoigawa-Shizuoka tectonic line, does not constitute a single microplate, but is separated mechanically at the Oga-Oshika tectonic zone.