Earth Planets Space, Vol. 56 (No. 8), pp. 831-842, 2004
Yoshihisa Iio1, Takeshi Sagiya2, and Yoji Kobayashi3
1Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan
2Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
3Institute for Geoscience, University of Tsukuba, Tsukuba, Ibaragi 305-0006, Japan
(Received December 18, 2003; Revised July 13, 2004; Accepted July 13, 2004)
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).
Key words: Intraplate earthquake, lower crust, anelastic deformation, weak zone, concentrated deformation, NKTZ.