abstract

All of the active periods of intraplate earthquakes for the past 100 years in Tohoku, northeast Japan correlate with the occurrence of great interplate earthquakes along the Japan trench. The seismically active periods are preceded by a period of quiescence lasting 10 to 15 years. One-fourth of intraplate earthquakes of magnitude 5.8 and above preceded interplate earthquakes by less than 5 years; a further one-half followed them by less than 10 years. All foreshocks and aftershocks are excluded in these statistics. The duration of the high-seismic activity in Tohoku before interplate earthquakes is about one-tenth of that in southwest Japan before interplate earthquakes along the Nankai trough. If the preseismic activity is triggered by an increase in the regional tectonic stress before a great interplate earthquake, the short duration of the activity found in Tohoku suggests a slow rate of earthquake strain accumulation, conformable to geomorphological studies of active faults in Tohoku. The postseismic activity at least in part is probably due to stress concentration near the base of the interface between the oceanic and continental plates.

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