Western US Anisotropy: Role of Slabs, Lateral Viscosity Variations and Small‐Scale Heterogeneity

Abstract The observed SKS splitting in the western United States (WUS) defines a distinctive pattern, with a swirl in the SKS fast‐axis directions. This pattern circles around the southern end of the Juan de Fuca (JdF) but is as wide as the entire Great Basin. In this study, we address the origin of this SKS pattern using density‐driven models of mantle convection to predict the anisotropy directions. We find that a well‐defined JdF slab is crucial for generating this anisotropy pattern. The toroidal flow around the southern edge of the JdF is widend by the presence of the East Pacific Rise (EPR) near the WUS. The E‐W orientation of the northern arm of the observed anisotropy swirl is caused by the westward corner flow associated with the JdF. The eastward push from the EPR, and the westward flow of the mantle from the Yellowstone plume down the Snake River Plain region constitutes other components of the observed anisotropy pattern. The small‐scale features seen in seismic tomography models in the WUS asthenosphere do not affect the flow field nor the resultant anisotropy pattern.