不均質構造中で発生する地震が作るΔCFF

抄録

To understand earthquake triggering, computation of Coulomb stress change (ΔCFF) associated with earthquake slip is a powerful tool. However, uniform half space is usually assumed in the computation, though highly heterogeneous crustal structures have been estimated in source regions. The purpose of this study is to evaluate the effect of heterogeneity on the calculation of static stress field. In order to calculate stress field from fault slip in a heterogeneous half space, we solve the equation of motion using 3D finite difference method in which the Lamé's constants vary with position. Since fault slip is represented by double couple force system, we apply equivalent body forces in our finite difference grids. First, we calculated stress and ΔCFF field for several simple 2D models to easily understand the effect of heterogeneous medium on ΔCFF field. For two-layered medium, stress is amplified on the region with larger elastic constants. The ratio of the stress amplification is less than the ratio of elastic constants between two layers, which is explained by interaction between two layers. For simplified basin structure and shallow thrust fault below, ΔCFF is larger near the basin and smaller in it than that of uniform structure. We also discuss the effect of heterogeneity near subducting plate on ΔCFF. We found that the subducting plate structure expands the area of positive ΔCFF in the region where outer rise earthquakes occur. Second, we apply the 3D calculation to the 2004 Chuetsu (mid-Niigata prefecture) earthquake (M_w 6.6) and four large aftershocks (M_JMA >6). Hikima and Koketsu (2005) and Miyazawa et al. (2005) calculated distribution of ΔCFF for the homogeneous crustal structure and concluded that ΔCFF values just before each aftershock was positive at the hypocenters of the major aftershocks. However, since the structure is complex in the source region, it is important to consider the effect of heterogeneity. We used the fault models estimated by Hikima and Koketsu (2005) and the 3D structure model by Kato et al. (2006) in computation of ΔCFF. The results show that the large aftershocks occurred in the area with positive values of ΔCFF.