Intra-plate seismicity and inter-plate earthquakes: Historical activity in Southwest Japan

Abstract

Over the last four hundred years the spatial variation of intra-plate seismicity in Southwest Japan correlates well with the occurrence of great inter-plate earthquakes. For fifty years before an inter-plate earthquake the intra-plate seismic activity is highest along a belt inland. For ten years afterwards it falls off in this belt, but rises on both sides along the Philippine Sea and Japan Sea coasts. Then it becomes low and remains low throughout the whole region until fifty to thirty years before the next inter-plate event, as shown by Utsu in 1974. An intermittent underthrusting drag exerted by the Philippine Sea plate seems to control the intra-plate seismicity, which partly takes up the relative plate motion as internal deformation. When a great inter-plate earthquake occurs, tectonic stress is released and seismic activity falls off in the central belt. The breaking of the plate boundary temporarily weakens the coupling between the two plates along the shallower part of the interface, which gently dips toward the Japan Sea coast. The decoupling causes stress concentration in the deeper part and results in increased seismic activity along the Japan Sea coast. The activity along the Philippine Sea coast may be interpreted as aftershock activity.