M. Hoshiba, ESTIMATION OF NONISOTROPIC SCATTERING IN WESTERN JAPAN USING CODA WAVE ENVELOPES - APPLICATION OF A MULTIPLE NONISOTROPIC SCATTERING MODEL, J GEO R-SOL, 100(B1), 1995, pp. 645-657
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.