Inner core anisotropy is often studied using differential travel times between the inner core phase, PKPdf, and one of two outer core phases, either PKPbc or PKPab, to eliminate contamination by crust and upper mantle structure. In particular, given the similarity of the two ray paths in the shallow Earth, the PKPbc-df differential travel time is assumed to robustly reflect the structure of the inner core, and not be influenced by mantle structure. Measurements of PKPab-df and PKPbc-df differential times reveal that the inner core is anisotropic: PKPdf rays travel through the inner core ~3% faster along polar paths than along equatorial paths. Even so, measurements of both PKPbc-df and PKPab-df differential travel times on quasi-polar paths between sources in the South Sandwich Islands and stations in Alaska present greater scatter (6 s spread) than other equivalent quasi-polar paths from other parts of the globe (2 s spread). While the South Sandwich Islands to Alaska data help increase spatial sampling of the inner core, including these data in inner core models significantly increases estimates of average global inner core anisotropy strength, by more than 1%. Whether this reflects real spatial variability in the strength of inner core anisotropy or else results from complexity outside of the inner core is uncertain but is crucial for constraining the inner core composition and growth history. Using a regional tomographic model of the Alaskan upper mantle to predict upper mantle effects on PKP travel times, we show that the signature of the Alaska slab is present in trends of observed absolute PKPbc, ab, and df travel times, both as a function of distance and azimuth. Moreover, we demonstrate that the effect of the slab is not fully cancelled by differential measurements. This implies that past models of the inner core are biased towards too strong average anisotropy. In order to better constrain inner core anisotropy in future, differential measurements of core-phase travel times need to be more accurately corrected for upper mantle three-dimensional structure, which in turns requires the construction of higher resolution tomographic models.

通常使用内芯相PKPdf和两个外芯相之一PKPbc或PKPab之间的差异传播时间来研究内芯各向异性，以消除地壳和上地幔结构的污染。特别是，考虑到浅层地球中两个射线路径的相似性，假定PKPbc-df的差分传播时间能稳健地反映内核的结构，而不受地幔结构的影响。对PKPab-df和PKPbc-df时差的测量表明，内芯是各向异性的：PKPdf射线沿内芯的传播速度比沿赤道的传播速度快约3％。尽管如此，在南桑威奇群岛的源与阿拉斯加的台站之间的准极路径上对PKPbc-df和PKPab-df差分传播时间的测量，比来自全球其他地方的其他等效准极路径具有更大的分散性（扩展了6 s）。 （传播2秒）。尽管南桑威奇群岛至阿拉斯加的数据有助于增加内核的空间采样，但在内核模型中包含这些数据将使全球平均内核各向异性强度的估计值明显增加1％以上。这是否反映出内核各向异性强度的真实空间变化性还是内核外部复杂性的结果尚不确定，但对于限制内核的组成和生长历史至关重要。使用阿拉斯加上地幔的区域层析成像模型来预测上地幔对PKP传播时间的影响，我们表明，观察到的绝对PKPbc，ab和df传播时间的趋势中都存在阿拉斯加平板的特征，这两者都是距离和方位角。此外，我们证明了差分测量不能完全抵消平板的影响。这意味着过去的内核模型偏向于太强的平均各向异性。为了将来更好地约束内芯各向异性，需要对上地幔三维结构更准确地校正芯相移动时间的差分测量，这反过来又需要构建更高分辨率的层析成像模型。我们表明，阿拉斯加平板的特征存在于观测到的绝对PKPbc，ab和df传播时间的趋势中，该趋势既是距离又是方位角的函数。此外，我们证明了差分测量不能完全抵消平板的影响。这意味着过去的内核模型偏向于太强的平均各向异性。为了将来更好地约束内芯各向异性，需要对上地幔三维结构更准确地校正芯相移动时间的差分测量，这反过来又需要构建更高分辨率的层析成像模型。我们表明，阿拉斯加平板的特征存在于观测到的绝对PKPbc，ab和df传播时间的趋势中，该趋势既是距离又是方位角的函数。此外，我们证明了差分测量不能完全抵消平板的影响。这意味着过去的内核模型偏向于太强的平均各向异性。为了将来更好地约束内芯各向异性，需要对上地幔三维结构更准确地校正芯相移动时间的差分测量，这反过来又需要构建更高分辨率的层析成像模型。这意味着过去的内核模型偏向于太强的平均各向异性。为了将来更好地约束内芯各向异性，需要对上地幔三维结构更准确地校正芯相移动时间的差分测量，这反过来又需要构建更高分辨率的层析成像模型。这意味着过去的内核模型偏向于太强的平均各向异性。为了将来更好地约束内芯各向异性，需要对上地幔三维结构更准确地校正芯相移动时间的差分测量，这反过来又需要构建更高分辨率的层析成像模型。