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

SUMMARY 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 Earth’s interior. While it is relatively straightforward to resolve seismic anisotropy in the uppermost mantle directly beneath a seismic station, measurements of deep mantle anisotropy are more challenging. This is due in large part to the fact that measurements of anisotropy in the deep mantle are typically blurred by the potential influence of upper mantle and/or crustal anisotropy beneath a seismic station. Several shear wave splitting techniques are commonly used that attempt resolve seismic anisotropy in deep mantle by considering the presence of multiple anisotropic layers along a raypath. Examples include source-side S-wave splitting, which is used to characterize anisotropy in the deep upper mantle and mantle transition zone beneath subduction zones, and differential S-ScS and differential SKS-SKKS splitting, which are used to study anisotropy in the D″ layer at the base of the mantle. Each of these methods has a series of assumptions built into them that allow for the consideration of multiple regions of anisotropy. In this work, we systematically assess the accuracy of these assumptions. To do this, we conduct global wavefield modelling using the spectral element solver AxiSEM3D. We compute synthetic seismograms for earth models that include seismic anisotropy at the periods relevant for shear wave splitting measurements (down to 5 s). We apply shear wave splitting algorithms to our synthetic seismograms and analyse 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 source-side direct S splitting and S-ScS differential splitting, are typically reliable for the fast polarization direction ϕ but not always for the time lag δt, and their accuracy depends on the details of the upper mantle elastic tensor. 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 built 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 that are often used to infer lowermost mantle anisotropy. 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 analyse the lowermost mantle beneath east Asia.