Seismic waves traversing the inner core in a direction parallel to Earth’s rotation axis arrive faster than waves travelling in the equatorial plane. These observations have been explained in terms of a transversely isotropic inner-core model with a fast symmetry axis parallel to the rotation axis. In recent years, more complex models of the inner core have been developed containing strong regional variations such as hemispheres, isotropic layers and an innermost inner core, most of which assume spatially variable transverse isotropy with a fixed symmetry axis. Here we instead explain the travel times of inner-core-sensitive seismic waves in terms of tilted transverse isotropy, in which the magnitude of transverse isotropy is fixed, but the orientation of the symmetry axis is allowed to vary spatially. This model, derived from seismic tomography, fits travel time data and spatially variable fixed-axis models, yet requires fewer parameters. It features a central inner core with a strong alignment of the fast symmetry axis in the direction of Earth’s spin axis and two shallow caps beneath the Mid-Atlantic and the Indian Ocean/Indonesia regions with symmetry axes tilted towards the equatorial plane. This model indicates the potential for varying crystal orientations within the inner core, which would constrain inner-core dynamics.

沿着与地球自转轴平行的方向穿过内核的地震波比在赤道平面上传播的地震波到达得更快。这些观察结果已经用具有平行于旋转轴的快速对称轴的横向各向同性内核模型进行了解释。近年来，已经开发出更复杂的内核模型，其中包含强烈的区域变化，例如半球、各向同性层和最里面的内核，其中大多数假设具有固定对称轴的空间可变横向各向同性。这里我们用倾斜横观各向同性来解释内核敏感地震波的传播时间，其中横观各向同性的大小是固定的，但允许对称轴的方向在空间上变化。该模型源自地震层析成像，适合走时数据和空间可变固定轴模型，但需要的参数较少。它具有一个中心内核，其快对称轴与地球自转轴方向紧密对齐，并且在大西洋中部和印度洋/印度尼西亚区域下方有两个浅盖，其对称轴向赤道面倾斜。该模型表明内核内晶体取向变化的可能性，这将限制内核动力学。