Study on the regionality of earthquake Source spectra

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For assessing the regionality of earthquake source spectra, we analized waveforms of micro- to small-sized earthquakes ocurring in different regions in Japan. One of the most important results is the finding of systematic difference in stress drop for shallow earthquakes occurring in the aftershock region of the 1984 Naganoken-Seibu earthquake (M=6.8) and in the Asio region where stationary seismic activity is observed. Detailed analysis of waveforms revealed that the ratio of P to S wave attenuation is higher than unity in contrast to past studies. We also revealed that site amplification effect is not negligible even at hard-rock observatiion sites for high-frequency seismic waves. We further attempted to separate the effects of seismic wave dissipation due to intrinsic attenuation and scattering, based on the theoretical formulation of scattering behavior that was done in this study. The theoretical work includes formulation of analitic solutions of the two-dimensional multiple isotropic scattering, and seismogram envelope synthetics including P-S conversions.The broad-band seismometer that was newly installed by this study is in full operation, and the observation is to be continued. The high quality date from this observation will largely contribute to extend our study to larger earthquakes.