Seismic Anisotropy in the Lowermost Mantle beneath North America from SKS-SKKS Splitting Discrepancies

Seismic anisotropy has been identified in the lowermost mantle of the Earth, the D" layer, in numerous studies, although the mechanisms that generate this anisotropy in the D" layer remain imperfectly known. Anisotropy at the base of the mantle can cause discrepancies in shear wave splitting between pairs of SKS and SKKS phases; therefore, careful documentation of SKS-SKKS splitting discrepancies can shed light on D" anisotropy. We examined SKS and SKKS data from 25 long-running seismic stations in eastern North America, along with approximately 250 stations of the USArray Transportable Array. We identified 279 pairs of high-quality splitting intensity measurements for SKS and SKKS phases; of these, a small number (15) exhibited pronounced discrepancies. Maps of the core-mantle boundary pierce points for the discrepant phases reveal that they preferentially sample the lowermost mantle beneath Texas and northern Mexico, near an isotropic low-velocity anomaly. We carried out simple forward modeling of candidate scenarios to understand the mechanism for anisotropy, including geometries that have previously been suggested for lowermost mantle anisotropy beneath North America based on other types of measurements.