We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential traveltimes, as a function of the angle ζ between the Earth’s rotation axis and the ray path in the inner core. Previous research relied heavily on body waves originating in the South Sandwich Islands (SSI) and travelling to seismic stations in Alaska to sample inner core velocities with low ζ (polar paths). These SSI polar paths are problematic because they have anomalous travel time anomalies, there are no ultra-polar SSI paths with ζ < 20° and they only cover a small part of the inner core. Here we improve constraints on inner core anisotropy using recently installed seismic stations at high latitudes, especially in the Antarctic, allowing us to measure ultra-polar paths with ζ ranging from 20°–5°. Our new data show that the SSI’s polar events are fast but still within the range of velocities measured from ray paths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data are particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab, which is currently located just above the core mantle boundary. This fast velocity region results in mantle structure being misinterpreted as inner core structure and we correct for this using a P-wave tomographic model. We also analyse the effect of velocity changes on the ray paths within the inner core and find that faster velocities significantly change the ray path resulting in the ray travelling deeper into the inner core and spending more time in the inner core. To remove this effect, we propose a simple but effective method to correct each event-station pair for the velocity-dependent ray path changes in the inner core, producing a more reliable fractional traveltime measurement. Combining the new ultra-polar data with mantle and ray path corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of 1.9–2.3 per cent for the whole inner core. This is lower than previous body wave studies (3 per cent anisotropy) and in better agreement with the value of inner core anisotropy measured by normal modes (2 per cent anisotropy). We also identify regional variation of anisotropic structure in the top 500 km of the inner core, which appears to be more complex than simple hemispherical variations. These regional variations are independent of the SSI data and are still present when these data are excluded. We also find a potential innermost inner core with a radius of 690 km and stronger anisotropy.

中文翻译：

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使用新的超极性PKIKP路径测量的内核内各向异性

我们使用PKPbc-PKPdf和PKPab-PKPdf差分传播时间来测量内芯的地震各向异性，它是地球自转轴与内芯射线路径之间的角度ζ的函数。先前的研究严重依赖于起源于南桑威奇群岛（SSI）并传播到阿拉斯加地震台的体波，以采样具有低ζ（极路径）的内芯速度。这些SSI极路径是有问题的，因为它们具有异常的旅行时间异常，不存在ζ<20°的超极SSI路径，并且它们仅覆盖内核的一小部分。在这里，我们使用最近安装的高纬度地震台站，特别是在南极，改善了对内核各向异性的约束，使我们能够测量ζ范围为20°-5°的超极路径。我们的新数据表明，SSI的极地事件速度很快，但仍在从其他地方发出的射线路径测得的速度范围内。我们进一步调查了地幔结构对我们数据集的影响，发现SSI数据特别受制于俯冲南乔治亚板块（目前位于地幔核心边界上方）的SSI下的快速速度。这个快速的速度区域导致地幔结构被误解为内芯结构，我们使用 目前位于核心地幔边界上方。这个快速的速度区域导致地幔结构被误解为内芯结构，我们使用 目前位于核心地幔边界上方。这个快速的速度区域导致地幔结构被误认为是内部岩心结构，我们使用P断层扫描模型。我们还分析了速度变化对内核内部光线路径的影响，发现更快的速度会显着改变射线路径，从而导致光线更深地进入内核并在内核中花费更多时间。为了消除这种影响，我们提出了一种简单而有效的方法来校正每个事件站对，以校正内核中速度相关的射线路径的变化，从而产生更可靠的分数行程时间测量。将新的超极性数据与地幔校正和射线路径校正相结合，可以实现更可靠的内核各向异性测量，整个内核的整体测量各向异性为1.9–2.3％。这比以前的体波研究（3％的各向异性）要低，并且与通过正常模式测量的内核各向异性的值（2％的各向异性）更好地吻合。我们还确定了内芯顶部500 km内各向异性结构的区域变化，该变化似乎比简单的半球形变化更为复杂。这些区域差异与SSI数据无关，并且在排除这些数据时仍然存在。我们还发现了一个潜在的最内层核心，其半径为690 km，各向异性更强。这些区域差异与SSI数据无关，并且在排除这些数据时仍然存在。我们还发现了一个潜在的最内层核心，其半径为690 km，各向异性更强。这些区域差异与SSI数据无关，并且在排除这些数据时仍然存在。我们还发现了一个潜在的最内层核心，其半径为690 km，各向异性更强。