Synthetic waveform modelling of SS precursors from anisotropic upper-mantle discontinuities

SS precursors are a powerful tool for interrogating upper-mantle discontinuity structure. Some of these discontinuities may be defined fully or partially by a variation in anisotropy with depth. Therefore, a careful evaluation of SS precursor waveform predictions from anisotropic discontinuities is required. Here, we perform synthetic waveform modelling to evaluate the potential for using SS precursors to constrain anisotropic discontinuities. We investigate SS precursor amplitudes from models with azimuthally anisotropic discontinuities with assumed hexagonal symmetry. We demonstrate that SS precursor polarity variations are robust across a wide range of earthquake source polarizations for our anisotropic models. While polarity variations are not unique among all potential two-layer models with anisotropic discontinuities, other observables, such as the relative arrival time of the precursor and tectonic settings, may be used to constrain anisotropic structure. We discuss implications for previous imaging of upper-mantle discontinuities that may be anisotropic, such as the Lehmann discontinuity, and discontinuities in depth range of the lithosphere-asthenosphere boundary beneath the Pacific.