Polarization effects of seismic waves on the basis of radiative transport theory

Radiative transfer theory provides a good framework for the study of multip le scattering in the randomly inhomogeneous lithosphere. Envelopes of high- frequency seismograms (mainly S coda waves) of local earthquakes have been synthesized on the basis of this theory, and inversions for some Earth para meters such as intrinsic attenuation, scattering attenuation and degree of non-isotropic scattering have been carried out. However, a scalar model has often been assumed because of its mathematical relative simplicity. The si mplification amounts to neglecting the polarized nature of the underlying m otion. This approach is only valid for long lapse times when S waves become unpolarized because of high-order scattering, and cannot be justified by o nly assuming that the source is unpolarized. We show that incoming unpolari zed S waves can be up to 80 per cent polarized after single scattering. Dep olarization of S waves after multiple scattering is studied by a Monte Carl o method. We show that the scattering of S waves off different kinds of inh omogeneities gives rise to different polarization and depolarization patter ns. Consequently, polarization should provide valuable information for the understanding of the physics of wave motion and the properties of the Earth 's lithosphere.