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

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″ ${\mathrm{D}}^{{\prime\prime}}$ anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that D″ ${\mathrm{D}}^{{\prime\prime}}$ 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. Plain Language Summary Earthquakes emit seismic waves that travel through Earth's deep interior. In some parts of Earth, the speed of these waves depends on their vibrational directions. In these cases, the material they travel through can be described as seismically anisotropic. Such seismic anisotropy is often caused by material deformation due to convective flow in Earth's interior. We show in this work that Earth's mantle beneath North America and the northeastern Pacific Ocean is seismically anisotropic in many places just above its boundary with the core at a depth of ∼2900km ${\sim} 2900\,\text{km}$. In some of these places, the deformation may be caused by sinking subducted slabs, upwelling flow, or changes in horizontal flow at the mantle's lower boundary layer. We additionally show that potential contributions from the lowermost mantle anisotropy cannot always be neglected when measuring seismic anisotropy in other parts of the mantle. Key Points We infer D″ ${\mathrm{D}}^{{\prime\prime}}$ seismic anisotropy beneath North America and parts of the Pacific Ocean from beamformed SKS, S2KS, S3KS, and PKS data We find widespread seismic anisotropy