5-15. Research on the generating mechanism of the stress field in the Japanese islands based on earthquake activities and GPS data.

 

  We listed problems concerning the generating mechanism of the stress field in the Japanese islands and examined possible answers to the problems.

  In Southwest Japan and the Hidaka, Hokkaido area, the direction of the maximum compressional stress is almost perpendicular to the direction of relative plate motion (parallel to the azimuth of the subducting oceanic plate). It seems that the above observations contradict the ordinary concept that subducting oceanic plates ‘push’ continental plates. This indicates that the coupling force between a subducting oceanic plate and continental plate is smaller than the stress component parallel to the azimuth of the subducting oceanic plate in a continental plate. It is likely that the frictional coefficient on plate boundary faults is much smaller than those predicted from laboratory experiments.

  The magnitude of the coupling force between an oceanic plate and a continental plate is an important problem not only for the forecasting of large interplate earthquakes, but also for the understanding of the stress field in intraplate regions. To solve this problem, it is important to perform the stress measurements close to plate boundaries, (for examples, near the Itoigawa-Shizuoka Tectonic Line and along the Pacific coast in the Tohoku district) and to utilize earthquake data, for example, fault plane solution data.

  The relative plate motion around the eastern edge of the Japan Sea probably has a more significant role on the deformation and stress field in the intraplate region in Japan than that on the plate boundary along the Pacific ocean. At present, however, the convergent process along the southern and eastern boundary of the Amurian plate is not clear. Further, it is also pointed out the possibility that the deformation around the eastern edge of the Japan Sea is an intraplate deformation. Further, the heterogeneity of the stress field in the intraplate region in Japan, in particular, in the South West Japan, is governed by pre-existing crustal structures.

The GPS network (GEONET) revealed that a horizontal strain rate is the largest in the Niigata-Kobe tectonic zone. Recently, denser GPS networks found a GPS velocity jump and/or change in trend of the GPS velocity field along the northern part of the Itoigawa-Sizuoka Tectonic Line and the Atotsugawa fault. There seems to be a secondary deformation zone with a width of a few kilometers and a velocity of a few cm/yr in a broader deformation zone. It is very important to clarify such a complicated deformation process there, not only for the understanding of boundary conditions of plates but also for the forecasting of intraplate earthquakes.