Study of Attenuation Mechanism of Seismic Waves in the Crust

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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)