Estimation of nonisotropic scattering in western Japan using coda wave envelopes: Application of a multiple nonisotropic scattering model

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Abstract
Scattering by heterogeneities is an important factor controling shapes of seismograms. Analyzing horizontal component seismograms in the band 1.0–8 Hz of local earthquakes in western Japan, we found that coda energy concentration just after the S wave arrival is more than expected for any multiple isotropic scattering model in uniform infinite medium. This behavior was seen even for deep earthquakes and became stronger with increase of frequency. To try to interpret this observation, nonisotropy is introduced into the multiple scattering model. The observed envelopes are compared with the model in order to estimate the amount of nonisotropy in scattering. Coda wave envelopes are synthesized using the Monte Carlo method for the nonisotropic scattering media. Two types of angular dependent scattering are considered as models of the nonisotropy with relation to the perturbation of seismic wave velocity in the lithosphere: perturbed media with exponential and Gaussian autocorrelation functions. It is found that forward scattering is dominant and the strength of sideways scattering is at least 10 times larger than that of backward scattering. The amount of the nonisotropic scattering means that correlation distance is estimated to be larger than several hundred meters in the case of Gaussian random media.