The Expression of Lithospheric Anisotropy in Ps and Sp Converted Waves

Many researchers have used the birefringence of P-to-S converted waves from the Moho discontinuity to constrain the anisotropy of Earths crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, in addition to the splitting acquired during its propagation from interface to the seismometer. We find that large amplitude variations with back-azimuth occur for P-wave anisotropy, as well as S-wave anisotropy, and that the variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations. The S-P receiver functions also constrain anisotropy at depth via the back-azimuth variation of Sp-phase amplitude on the vertical component. This Sp behavior is not typically studied, but it has the potential to test the hypothesis that the seismic lithosphere-asthenosphere boundary (LAB) is caused by a transition in anisotropic layering at the base of Earths tectonic plates. Simulations of these wave-propagation effects with the ANIMATIVITY reflectivity code, a public software package based on Matlab, suggest that the practical use of S receiver functions to investigate anisotropy will depend on accurate determination of the initial S-wave polarization. Testing for back-azimuth dependence of Sp converted-wave amplitude can focus on naturally polarized shear body waves, such as SKS phases, or can be sought in 3-D correlation in the coda of general shear waves, detected via the singular-value decomposition. Tests with synthetic seismograms verify the applicability of both strategies. We will report analyses with data from permanent stations of the US National Network and the Earthscope Transportable Array.