Study of Attenuation Mechanism of Seismic Waves in the Crust

地震地面运动的定量预测有三个主要贡献：震源、路径和场地。这里我们重点研究第二和第三个主题：作为路径效应的散射和衰减；作为场地效应的硬岩石的场地放大研究。本文根据赤木县Ashio的野外观测资料，阐明了P波和S波在花岗岩类坚硬岩石上的频率相关放大因子(Yoshimoto等，1993b)。发展了传统的尾波归一化方法，我们首先测量了关东地区P波和S波衰减的频率依赖性(Yoshimoto等，1993a)。基于能量输运理论(Sato，1994a，1994b，1995b)，我们发展了一个新的能量在散射和衰减介质中传播的数学公式。该公式适用于任何类型的非各向同性散射，对于高频地震记录包络的研究是有用的。基于该公式的数值合成很好地解释了能量密度的空间均匀分布(1995b)。这种方法被应用于宫城县女川获得的地震图。估计的S波衰减因子与频率成正比，散射损失对总衰减的贡献比地震反照率小，约为0.25。如果我们增加前向散射，地震反照率将增加一倍。根据对地震记录包络的分析，我们发现日本关东地区火山前缘西侧的岩石圈随机性比东侧的要大得多(Obara和Sato，1995A)。

There are three major contributions for the quantitative prediction of seismic ground motion : source ; path, and site. Here we focused on the second and the third subjects : scattering and attenuation as a path effect ; site amplification study of hard rocks as a site effect. Based on the filed observation at Ashio, Tochigi prefecture, we have clarified the frequency dependent amplification factors of both P and S waves at hard rocks as granite (Yoshimoto et al., 1993b). Developing the conventional coda normalization method, we first measured the frequency dependence of both P and S wave attenuation in the Kanto area (Yoshimoto et al., 1993a). We have newly developed a mathematical formulation of the energy propagation through scattering and attenuation media based on the energy transport theory (Sato, 1994a, 1994b, 1995b). This formalism adopt any type of no-isotropic scattering, and is useful for the study of high frequency seismogram envelopes. The numerical synthesis based on this formulation well explains the spatially uniform distribution of energy density (1995b). This method is applied to seismograms obtained at Onagawa, Miyagi prefecture. The estimated S wave attenuation factor is proportional to frequency, and the contribution ratio of scattering loss to the total attenuation, the seismic albedo, is small as about 0.25. If we increase the forward scattering, the seismic albedo becomes twice. Based on the analysis of seismogram envelopes, we found that the randomness of the lithosphere is much larger under the west of the volcanic front compared with that under the east in Kanto-Tokai area, Japan (Obara and Sato, 1995a)