Waveform Effects on Shear Wave Splitting Near Fault Zones

Abstract Shear wave splitting of teleseismic core phases such as SKS is commonly used to constrain mantle seismic anisotropy, a proxy for convective deformation. In plate boundaries, sharp lateral variations of splitting measurements near transform faults are often linked to deformation within a lithospheric shear zone below, but potential seismic waveform effects from heterogeneous structure on small scales may influence the interpretation. Here, we explore possible finite frequency effects on shear wave splitting near fault zones in a fully three‐dimensional anisotropic setting. We find that shear zones wider than 80 km, a scale set by the Fresnel zone, can be clearly detected, but narrower zones are less distinguishable. Near the edge of the shear zone, the combined effect of anisotropy and scattering generates false splitting measurements with large delay times and fast axis orientation approaching the back‐azimuth, a bias which can only be identified when records from different back‐azimuths are analyzed together. This substantiates that back‐azimuthal variations of splitting can arise not just from vertical layering but also lateral changes of anisotropic media. We also test the effects of shear zone edge geometry, epicentral distance, filtering frequency, crustal thickness, and sediment cover. Our study delineates the ability of shear wave splitting to resolve and investigate fault zones, and emphasizes the importance of good azimuthal coverage to correctly interpret observed anisotropy. Based on revisiting previous shear wave splitting and lithospheric deformation studies, we infer that many crustal fault zones are underlain by lithospheric shear zones at least 20 km wide.