Abstract We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.

中文翻译：

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来自 SKS-SKKS 分裂强度差异的北美最下地幔地震各向异性

摘要 我们研究了对北美最下地幔进行采样的相位的 SKS-SKKS 分裂强度差异，此前已使用其他分析技术显示该地幔表现出地震各向异性。我们检查了来自 25 个长期运行的地震台站以及位于美国东部、东南部和西部的 244 个临时 USArray 可移动阵列台站的数据 我们确定了 279 个高质量 SKS-SKKS 波对，这些波对产生了良好约束的分裂强度测量对于两个阶段。在 279 对中，相对较少的数（15）表现出 0.4 秒或更大的分裂强度差异，表明对最下地幔各向异性的一相或两相分裂有贡献。由于本研究中检查的 SK(K)S 相中只有一小部分显示出受最下地幔各向异性影响的证据，因此认为这些相的分裂主要反映台站正下方的上地幔各向异性的传统解释是合适的。差异对表现出显着的地理趋势，对美国南部和墨西哥北部下方的最低地幔进行采样，而其他地区则以非差异对为主。我们对简单的各向异性情景进行了射线理论建模，这些情景之前曾被建议用于北美最下层地幔，调用后钙钛矿的排列是由于残余法拉隆板撞击核-地幔边界引起的流动。