A two-dimensional hybrid method for modeling seismic wave propagation in anisotropic media

A hybrid method is developed for calculating synthetic seismograms for seismic waves propagating in two-dimensional localized heterogeneous anisotropic media. The hybrid method is a combination of analytic and numerical methods, with the numerical method (finite difference, or FD) applied in the heterogeneous anisotropic region only and the analytic method (generalized ray theory, or GRT) outside. The GRT solutions from a seismic source are interfaced with the FD calculation at the boundaries of the anisotropic region; and seismic responses at the Earth's surface are obtained from the output of FD calculation. Synthetic experiments on both isotropic and anisotropic models demonstrate the validity of the new method in simulating seismic wave propagation in complex media. We applied the hybrid method to study the SKS wave propagation in the upper mantle. We calculated the synthetics for a series of laterally varying anisotropic models, including a layer with varying anisotropy, two-layer anisotropy, separated anisotropic body and those may be related to subduction zones. For these laterally varying models, SKS waveforms exhibit higher sensitivity to the spatial variation of anisotropy than the shear wave splitting parameters do. The results of this study demonstrate the potential of the proposed hybrid method for high-resolution imaging the laterally varying anisotropic structures from seismological observations, when the tectonic structures can be approximated as two-dimensional.