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 ${\mathrm{D}}^{″}$ anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that ${\mathrm{D}}^{″}$ 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.

中文翻译：

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北美和东北太平洋地区广泛的 D“ 各向异性及其对上地幔各向异性测量的影响


对穿过地球最下层地幔的地震波的观测有助于深入了解深部地幔结构和动力学，通常是在相对较小的空间尺度上。在这里，我们使用在包括美国、墨西哥和中美洲在内的大片地区记录的 SKS、S2KS、S3KS 和 PKS 信号来研究北美大片地区和东北太平洋下最深的地幔。这些相通过波束形成进行增强，然后用于研究偏振和传播方向相关的横波速度（地震各向异性）。差分分裂方法使我们能够稳健地识别来自 ${\mathrm{D}}^{“}$ 各向异性的贡献。我们的结果表明，在我们研究区域的大约一半地区存在强烈的地震各向异性，这表明 ${\mathrm{D}}^{“}$ 各向异性可能比通常认为的更普遍。在某些地区，各向异性可能由下沉的冷板驱动的流动引起，而在另一些更紧凑的地区，则由上升流引起。由于最下地幔各向异性而测量的分裂足够强，以至于在某些地区由于上地幔各向异性而导致的 SKS 分裂时不可忽略，这可能会促使未来重新评估北美和中美洲以下的上地幔各向异性。