Constraining deep mantle anisotropy with shear wave splitting measurements: Challenges and new measurement strategies

Determinations of seismic anisotropy, or the dependence of seismic wave velocities on the polarization or propagation direction of the wave, can allow for inferences on the style of deformation and the patterns of flow in the Earths interior. A major challenge when attempting to measure deep mantle anisotropy is the fact that these measurements are typically blurred by the potential influence of upper mantle and/or crustal anisotropy beneath a seismic station. Techniques commonly used to resolve seismic anisotropy in the deep mantle by considering the presence of multiple anisotropic layers along a raypath include source-side S, differential S-ScS, and differential SKS-SKKS splitting. Each of these methods has a series of assumptions built in to them, whose accuracy we assess in this work. We conduct global wavefield modelling using the spectral element solver AxiSEM3D, computing synthetic seismograms down to 5s. We apply shear wave splitting algorithms to our synthetic seismograms and analyze whether the assumptions that underpin common measurement techniques are adequate, and whether these techniques can correctly resolve the anisotropy incorporated in our models. Our simulations reveal some inaccuracies and limitations of reliability in various methods. Specifically, explicit corrections for upper mantle anisotropy, which are often used in S-ScS differential splitting, are typically reliable for the fast polarization direction but not (always) for the time lag t. We find that several of the assumptions that underpin the S-ScS differential splitting technique are inaccurate under certain conditions, and we suggest modifications to traditional S-ScS differential splitting approaches that lead to improved reliability. We investigate the reliability of differential SKS-SKKS splitting intensity measurements as an indicator for lowermost mantle anisotropy and find that the assumptions inbuilt into the splitting intensity formula can break down for strong splitting cases. We suggest some guidelines to ensure the accuracy of SKS-SKKS splitting intensity comparisons. Finally, we suggest a new strategy to detect lowermost mantle anisotropy which does not rely on explicit upper mantle corrections and use this method to analyze the lowermost mantle beneath east Asia.