[SCG63-P06] Modeling strain concentration zones in Kyushu Island considering heterogeneous rheological structure
Keywords:strain concentration zone, Kyushu, finite element method, thermal structure, Okinawa Trough
Geothermal gradients according to data obtained from Hi-net (Matsumoto et al. 2007) and from the Geological Survey of Japan (Tanaka et al. 2004) indicate high geothermal anomalies in the Beppu–Shimabara graben zone, in particular, regions that include the Kujyu and Aso volcanoes and the Beppu region. Geothermal gradients are also high in the regions of the Kirishima and Sakurajima volcanoes. The distribution of the geothermal gradient is similar to that of the cut-off depth of the seismicity obtained by Matsumoto et al. (2016).
This study models strain concentration zones by considering the heterogeneous rheological structure using the finite element method (Shibazaki et al., 2007), with nonlinear viscoelasticity and Mohr–Coulomb plasticity. From the observed geothermal gradient distribution, we assume that temperature increases with depth. We set the boundary conditions as extensional in the N–S direction and compressional in the E–W direction. We reproduce the strain concentration zone along the Futagawa fault, where the 2016 Kumamoto earthquake occurred. However, we cannot reproduce the strain concentration zone along the Hinagu fault. Therefore, we include the low-strength region around the Hinagu fault zone considering the existence of an ancient accretional complex, and the low-strength region in the northern part of the Okinawa Trough. By considering these low-strength regions, we can reproduce the strain concentration zones along the Futagawa and Hinagu fault zones, the pattern of which is similar to the observed one.