Tilted transverse isotropy in Earth's inner core

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. A seismic tomographic model shows that the directional dependence of the travel time of seismic waves through Earth's inner core can be explained by a spatially varying orientation of the transverse isotropy symmetry axis, which is simpler than other proposed structures.