Earth Planets Space, Vol. 56 (No. 12), pp. 1339-1345, 2004
Yoshihisa Iio1, Takeshi Sagiya2, Norihito Umino3, Takuya Nishimura4, Kunihiko Takahashi5, and Takahiro Homma5
1Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho Uji 611-0011, Japan
2Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
3Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578, Japan
4Geographical Survey Institute, Kitasato-1, Tsukuba, Ibaraki 308-0811, Japan
5Fuji Research Institute Corporation, Chiyoda-ku, Tokyo 101-8443, Japan
(Received May 29, 2004; Revised August 24, 2004; Accepted September 7, 2004)
A two-dimensional finite element model was constructed along a cross section almost perpendicular to the Nagamachi-Rifu Fault Zone, in order to clarify the stress accumulation process on an intraplate earthquake fault. We explain the surface deformations observed by the dense GPS network and leveling surveys using models with heterogeneities in the crust. These heterogeneities are identified from various geophysical surveys in the region. We found that the observed surface deformations cannot be explained by a model having a weak zone in the upper crust, but can be explained by models having a weak zone in the lower crust. Models having an aseismic fault or fault zone in the lower crust can reproduce the spatial pattern of the observed deformations, but amplitudes predicted by these models are smaller than those observed. The weak zone in the lower crust probably plays an important role in the stress accumulation process on the Nagamachi-Rifu fault zone.
Key words: Intraplate earthquake, lower crust, ductile fault zone, stress accumulation, plate boundary.