Theoretical Study on the heterogeneous stress field in the 3D crustal structure

地震滑动引起的库仑应力变化（ΔCFF）的计算是理解地震触发机制的有力工具。然而，在计算中通常假设均匀的半空间，尽管在震源区已经估计了高度不均匀的地壳结构。本研究的目的是了解非均匀性对静态应力场计算的影响。为了计算非均质半空间中断层滑动引起的应力场，采用三维有限差分法求解运动方程，其中考虑了拉梅常数随位置的变化和介质性质的空间分异。由于断层滑动是用双力偶系统来表示的，所以我们在有限差分网格中采用等效的体力（Graves，1996 a）。首先，我们计算了几个简单的二维模型的应力和ΔCFF场，以便于理解非均匀介质对ΔCFF场的影响。对于两层介质， ...更多信息 具有较高弹性常数的区域。然而，应力放大率小于两层之间的弹性常数的比值。这是因为两层之间的相互作用。对于简化的盆地结构和其下的浅逆冲断层，ΔCFF在盆地附近增大而在盆地内减小，并讨论了俯冲板块附近的非均匀性对ΔCFF的影响。我们发现，俯冲板块结构扩大了外隆地震发生区的正ΔCFF区域。其次，我们适用于2004年中越（新泻县中部）地震（Mw6。6)4次大余震（MJMA> 6）。Hikima和Koketsu（2005年）和Miyazawa et al.（2005）计算了均匀地壳结构下的ΔCFF分布，并得出结论：在大余震震源处，每次余震前的ΔCFF值为正值。然而，由于源极区域中的结构复杂（Kato等人，2006），重要的是要考虑异质性的影响。在计算ΔCFF时，我们使用了Hikima和Koketsu（2005）估计的断层模型和Kato等人（2006）的三维结构模型。结果表明，主震余震发生在ΔCFF为正值的区域。少

Computation of Coulomb stress change(ΔCFF) due to earthquake slip is a powerful tool in order to understand earthquake triggering. 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 understand 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 Lame' constants vary with position and spatial differentiation of these medium properties is taken into account. Since fault slip is represented by double couple force system, we apply equivalent body forces in our finite difference grids(Graves, 1996a). First, we calculated stress andΔCFF field for several simple 2D models to understand easily the effect of heterogeneous medium toΔCFF field. For two-layered medium, stress is amplified on … More the region with higher elastic constants. However, ratio of the stress amplification is less than the ratio of elastic constants between two layers. This is because of interaction between two layers. For simplified basin structure and shallow thrust fault below,ΔCFF increases near the basin but decrease in it. We also discuss the effect of heterogeneity near subducting plate onΔCFF. We found that the subducting plate structure expands the area of positiveΔCFF in the region where outer rise earthquakes occur. Second, we apply to the 2004 Chuetsu(mid-Niigata prefecture) earthquake(Mw6. 6) and four large aftershocks(MJMA> 6). Hikima and Koketsu(2005) and Miyazawa et al.(2005) calculated distribution ofΔCFF for the homogeneous crustal structure and concluded thatΔCFF values just before each aftershocks were positive at the hypocenters of the major aftershocks. However, since the structure is complex in the source region(Kato et al., 2006), it is important to consider the effect of heterogeneity. We used the fault models estimated by Hikima and Koketsu(2005) and 3D structure model by Kato et al(2006) in computation ofΔCFF. The results show that main aftershocks occurred in the area with positive values ofΔCFF. Less