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We investigated seismic activities before large interplate earthquakes which occurred in the subduction zone along the Japan trench since 1970s and found that following features are commonly observed: (1) conspicuous quiescence appears in the focal region of the main shock several years before its occurrence, (2) in the deeper zone than the focal region seismic quiescence also appears at the same time or somewhat earlier, but the seismicity recovers preceding occurrence of the main shock, (3) migration of seismic activities from deeper parts to shallower regions-from the land side to the trench side-is observed, (4) remarkable foreshock activity appears one to several days before the occurrence of the main shock. Besides these common features before large interplate earthquakes, we found that seismic activity in the sea region off -Fukushima prefecture decreased obviously about two years before the 1978 Miyagi-ken-oki earthquake and the seismicity recovered when an M6. 7 earthquake occurred off-Miyagi prefecture on February 20, 1978. Further, it is found that a clear seismic quiescence appeared around the focus of the 72 km-depth earthquake with a magnitude of 6. 6 occurring in the Sanriku seashore in 1987. Recovery and migrational tendency of seismic activities in the deeper part of the interplate coupling zone before large earthquakes are considered to reflect a ductile feature in that region.

Quiescence and Precursory Seismic Activity before Large Interplate Earthquakes along the Japan Trench