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

计算地震滑动引起的库仑应力变化（ΔCFF）是了解地震触发的有力工具。然而，在计算中通常假设均匀的半空间，尽管在震源区域估计了高度不均匀的地壳结构。本研究的目的是了解非均质性对静应力场计算的影响。为了在非均质半空间中计算断层滑动的应力场，我们采用三维有限差分法求解运动方程，该方法考虑了拉梅常数随位置的变化，并考虑了介质性质的空间分异。由于断层滑动是由双耦合力系统表示的，我们在有限差分网格中应用等效的体力（Graves, 1996a）。首先，我们计算了几种简单二维模型的应力场andΔCFF，以便于理解非均质介质toΔCFF场的影响。对于两层介质，应力在弹性常数较高的区域被放大。但应力放大比小于两层间弹性常数比。这是因为两层之间的相互作用。对于简化的盆地构造和下方的浅层逆冲断层，ΔCFF在盆地附近增大，在盆地内减小。我们还讨论了俯冲板块附近非均质性的影响onΔCFF。我们发现，俯冲板块构造扩大了外隆起地震发生区域positiveΔCFF的面积。其次，我们适用于2004年中越（新泻县中部）地震（Mw6）。6)和4次大余震（MJMA bb0.6）。Hikima and Koketsu（2005）和Miyazawa et al.（2005）计算了均匀地壳结构的分布ofΔCFF，并得出每次余震前的thatΔCFF值在主要余震震源处为正。然而，由于震源区域结构复杂（Kato et al., 2006），考虑异质性的影响是很重要的。我们使用Hikima和Koketsu（2005）估计的断层模型和Kato等人（2006）的三维结构模型进行计算ofΔCFF。结果表明，主要余震发生在正值ofΔCFF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