Pattern of intraplate seismicity in southwest Japan before and after great interplate earthquakes

Abstract

A fairly complete set of data on intraplate seismicity in southwest Japan during the past 170 years reveals that the seismicity before great interplate earthquakes along the Nankai trough is high over the land area adjacent to the rupture zones of the great interplate earthquakes and the seismicity after the interplate earthquakes is high in the marginal zones that border the preseismically active area. This change of seismicity distribution before and after great interplate earthquakes can be explained by the two modes of horizontal deformation in the continental-plate margin; that is, the contraction of the land area adjacent to the rupture zone of great interplate events before their occurrence and the blockwise extension of the area seaward at the time of these interplate shocks. One of the characteristic features of intraplate seismic energy release during historic times is that it is large in the narrow zones which border the land areas adjacent to the specific rupture zones of historic great interplate earthquakes. These zones must have been exposed to the shearing stress due to the blockwise extension of the areas adjacent to the specific rupture zones at the time of interplate shocks and this may provide a reason for the large seismic energy release within these marginal zones in historic times. Recent intraplate seismicity in southwestern Japan shows that intraplate earthquakes tend to cluster in the area adjacent to the expected rupture zone of a future great event off the Tokai district. A simple statistical test shows that this clustering of intraplate events in the area is significant within a 96% confidence level. The level of seismic activity in this area is 18 times larger than the normal level of activity between interplate earthquakes. This high level of activity provides another piece of evidence for a possibility of occurrence of a great interplate event off Tokai. The land area adjacent to the rupture zone off Tokai deserves high priorty for instrumentation of various types to record in the near field the destructive intraplate earthquakes which may occur over several decades before and after the future great Tokai event.