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

Radiative transfer theory provides a good framework for the study of multiple 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 parameters 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 simplification amounts to neglecting the polarized nature of the underlying motion. This approach is only valid for long lapse times when S waves become unpolarized because of high-order scattering, and cannot be justified by only assuming that the source is unpolarized. We show that incoming unpolarized S waves can be up to 80 per cent polarized after single scattering. Depolarization of S waves after multiple scattering is studied by a Monte Carlo method. We show that the scattering of S waves off different kinds of inhomogeneities gives rise to different polarization and depolarization patterns. Consequently, polarization should provide valuable information for the understanding of the physics of wave motion and the properties of the Earth's lithosphere.