Origin of the concentrated deformation zone in the Japanese Islands and stress accumulation process of intraplate earthquakes

The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata-Kobe Tectonic Zone) were clarified by analyzing various observations. We made a qualitative modeling for the stress state and deformation style in and around the NKTZ. In this model a weak zone with low viscosity exists in the lower crust beneath the NKTZ. In the surrounding region, however, the viscosity in the lower crust is very high and can be regarded as elastic for the periods of a recurrence interval of intraplate earthquakes. The concentrated deformation is basically attributed to the low viscosity in the weak zone. In more details, the concentrated deformation is thought to be generated by a postseismic deformation of the weak zone to the previous large intraplate earthquake in the interseismic period (the brittle-ductile interaction model) and/or anelastic deformation in both the upper and lower crusts in the NKTZ, under a finite constant force (the anelastic deformation model).