Surface waves in layered anisotropic structures

Abstract

We report a method to calculate surface waves in layered anisotropic media using a reflectivity algorithm. The method posits a finite stack of constant anisotropic layers over an isotropic half-space. The parametrization within each layer allows for anisotropy with an arbitrarily oriented axis of symmetry ŵ, with magnitude scaled by three constants related to wave-speed variations of P and S waves with respect to the azimuth relative to ŵ. Within each layer, eigensolutions to the equations of motion can be expressed as upgoing and downgoing oscillatory and/or evanescent plane waves, except possibly within narrow intervals of horizontal slowness p where the governing matrix becomes near-defective and an alternative set of eigensolutions is used to avoid numerical instability. Synthetic seimograms for simple anisotropic crustal models demonstrate that significant Love-Rayleigh coupling occurs between adjacent overtone branches, leading to significant 'scattering' in the absence of 3-D velocity structure. The dependence of this scattering on azimuth relative to ŵ can cause the Love-Rayleigh scattering to appear to be ralated to a spurious component of the source mechanism.