Widespread D ″ ${\mathbf{D}}^{\mathbf{{\prime\prime}}}$ Anisotropy Beneath North America and the Northeastern Pacific and Implications for Upper Mantle Anisotropy Measurements

Abstract

Observations of seismic waves that have passed through the Earth's lowermost mantle provide insight into deep mantle structure and dynamics, often on relatively small spatial scales. Here we use SKS, S2KS, S3KS, and PKS signals recorded across a large region including the United States, Mexico, and Central America to study the deepest mantle beneath large swaths of North America and the northeastern Pacific Ocean. These phases are enhanced via beamforming and then used to investigate polarization- and propagation direction-dependent shear wave speeds (seismic anisotropy). A differential splitting approach enables us to robustly identify contributions from $D^{″}$ anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that $D^{″}$ anisotropy may be more prevalent than commonly thought. In some regions, the anisotropy may be induced by flow driven by sinking cold slabs, and in other, more compact regions, by upwelling flow. Measured splitting due to lowermost mantle anisotropy is sufficiently strong to be non-negligible in interpretations of SKS splitting due to upper mantle anisotropy in certain regions, which may prompt future re-evaluations of upper mantle anisotropy beneath North and Central America.