Numerical Approach to scattering and attenuation of high-frequency seismic waves

利用边界积分方法，研究了地壳中小尺度非均质的空间分布。如果非均质性局限在一个较小的区域，观测到的尾波振幅在经过场地效应校正后，系统地变得比其他区域大。我们首先分析了1995年兵库县南部地震余震的尾波。如果事件位于野岛断裂区，深度约10公里，频率为1~4赫兹，则尾波水平将系统性地变大。这意味着那里集中了小规模的异质性。当我们使用NIED提供的K-Net数据时，我们还检测到了北海道东南部一小块区域的异质性集中。从检测到的频率范围(1~2赫兹)，估计它们的典型尺寸约为1公里。从理论上看，我们成功地将边界积分法从均匀介质推广到垂直速度变化的介质。也就是说，我们建立了一种新的方案来计算水平分层介质中的散射波形。我们对二维SH情况进行了测试，证实了在不显著增加计算时间的情况下，可以得到准确的波形。我们模拟了非均匀介质和各层界面上散射波之间的相互作用，产生了与均匀介质中截然不同的波形。

Using the boundary integral method, we investigated spatial distribution of small-scale heterogeneities in the crust. If heterogeneities are localized in a small region, observed amplitude of coda, after corrected by site effects, becomes systematically larger than in other regions. We first analyzed coda for aftershocks of the 1995 Hyogo-ken Nambu earthquake. Coda level becomes systematically large if an event is located in the Nojima fault region at depth of about 10 km in frequency of 1〜4Hz. This implies the concentration of small-scale heterogeneities there. When we used the K-net data provided by NIED, we also detected the concentration of heterogeneities in a small region of Southeastern Hokkaido. From the detected range of frequency (1〜2Hz) , their representative size is estimated to be around 1 km.From a theoretical point of view, we were successful in extending our boundary integral method from homogeneous media into media of vertically varying velocities. That is, we established a new scheme to compute waveforms for scattering in horizontally layered media. We tested our scheme for 2-D SH case, confirming that accurate waveforms can be obtained without significant increase of computational time. We simulated the interaction between scattered waves by heterogeneities and interfaces of layers, which produces waveforms quite different from those in homogeneous media.