Study on the regionality of earthquake Source spectra

为了评价震源谱的地域性，我们分析了日本不同地区发生的微、小地震波形。其中最重要的结果之一是发现发生在1984年长野根-西武地震（M=6.8）余震区的浅层地震的应力降与观测到静止地震活动的亚洲地区的应力降有系统差异。对波形的详细分析表明，与以往的研究相比，P波与S波衰减的比值大于1。我们还发现，即使在高频地震波的硬岩观测点，场地放大效应也不容忽视。我们进一步尝试在本研究散射行为理论公式的基础上，分离固有衰减和散射对地震波耗散的影响。理论工作包括二维多重各向同性散射的解析解的建立和包括P-S转换在内的地震记录包络合成。本研究新安装的宽带地震仪已全面投入使用，观测工作将继续进行。这次观测的高质量数据将在很大程度上有助于将我们的研究扩展到更大的地震。

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.