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

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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.