Mantle Shear Wave Splitting based on 3D Seismic Wave Simulations

Abstract:

Constraining the patterns and properties of seismic anisotropy in the Earth can help reveal relationships between mineral physics, deformation, mantle convection, and seismology. Sources of anisotropy in the lithosphere as frozen-in anisotropy, transition zone, and D" complicate shear wave splitting measurements, resulting in shear wave splitting that can differ from plate motion. In addition, complications from 3D mantle and crustal heterogeneity may affect shear wave splitting as well – especially phases that graze along the core-mantle boundary. Ray theory is commonly used in modeling seismic anisotropy and is appropriate within certain limits, but not all implications have been explored. Ray theory’s validity depends on the period of waves, the scale of heterogeneities, the length of its propagation path, and the superposition of multiple arrivals, making interpreting seismic anisotropy observations more difficult. In this work, we explore different background tomography models (PREM [Dziewonski and Anderson, 1981], S40RTS [Ritsema et al., GJI, 2011], and GLAD-M25 [Lei et al., GJI, 2020]) and their effects on various phases used for shear wave splitting (S, ScS, SKS, etc.). We also calculate sensitivity kernels of various phases for travel time and anisotropic parameters. The goal of this work is to explore the effects of ray theoretical assumptions made in shear wave splitting as well as tease out discrepancies between different background mantle models and observations of anisotropy within the Earth. We conduct our numerical simulations via 3D global wave propagation solver SPECFEM3D_GLOBE (Komatitsch & Tromp, 2002). Ultimately, we aim to improve the understanding of how seismic anisotropy observations, like SKS shear wave splitting, are related to models of deformation in the Earth's mantle and the sources of anisotropy in the crust and mantle, specifically the D" layer.