ESTIMATION OF NONISOTROPIC SCATTERING IN WESTERN JAPAN USING CODA WAVE ENVELOPES - APPLICATION OF A MULTIPLE NONISOTROPIC SCATTERING MODEL

Scattering by heterogeneities is an important factor controling shapes of seismograms. Analyzing horizontal component seismograms in the ban d 1.0-8 Hz of local earthquakes in western Japan, we found that coda e nergy concentration just after the S wave arrival is more than expecte d for any multiple isotropic scattering model in uniform infinite medi um. This behavior was seen even for deep earthquakes and became strong er with increase of frequency. To try to interpret this observation, n onisotropy is introduced into the multiple scattering model. The obser ved envelopes are compared with the model in order to estimate the amo unt of nonisotropy in scattering. Coda wave envelopes are synthesized using the Monte Carlo method for the nonisotropic scattering media. Tw o types of angular dependent scattering are considered as models of th e nonisotropy with relation to the perturbation of seismic wave veloci ty in the lithosphere: perturbed media with exponential and Gaussian a utocorrelation functions. It is found that forward scattering is domin ant and the strength of sideways scattering is at least 10 times large r than that of backward scattering. The amount of the nonisotropic sca ttering means that correlation distance is estimated to be larger than several hundred meters in the case of Gaussian random media.