Study of Attenuation Mechanism of Seismic Waves in the Crust

地震地面运动定量预测的贡献主要有三个方面：震源；路径和站点。这里我们重点讨论第二和第三个主题：作为路径效应的散射和衰减；现场放大研究作为现场效应的硬岩。通过在栃木县ashhio的野外观测，我们明确了花岗岩等坚硬岩石中P波和S波的频率相关放大因子（Yoshimoto et al., 1993b）。我们发展了传统的尾数据归一化方法，首先测量了关东地区P波和S波衰减的频率依赖性（Yoshimoto et al., 1993a）。我们在能量输运理论的基础上，建立了能量在散射和衰减介质中传播的数学公式（Sato, 1994a, 1994b, 1995b）。这种形式适用于任何类型的非各向同性散射，对高频地震记录包络的研究是有用的。基于此公式的数值综合很好地解释了能量密度的空间均匀分布（1995b）。该方法应用于宫城县女川地震记录。估计的S波衰减系数与频率成正比，散射损失对总衰减的贡献率（地震反照率）很小，约为0.25。如果增加前向散射，地震反照率将增加一倍。通过对地震记录包膜的分析，我们发现在日本关东-东海地区，火山锋西部岩石圈的随机性要比东部大得多（Obara and Sato, 1995）。

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)