Effects of mantle structure on models of seismic anisotropy in the inner core

Mapping inner core (IC) seismic anisotropy at high resolution provides important insight on the growth of the inner core through time, its internal dynamics, and its role in the production of the geodynamo. Since the discovery of IC anisotropy (ICA) in 1986, numerous studies have suggested the presence of significant lateral and depth variations in its character and strength. In particular, there is controversial evidence for the presence of a distinct region, spanning the central third of the IC in radius, referred to as the “innermost inner core”. Yet, obtaining robust constraints on the 3D structure of ICA is hampered by the uneven sampling by seismic waves passing through the inner core, and the possible contamination of measurements by unmodelled 3D mantle structure, to which all seismic core phases are sensitive.

Typical ICA models rely on differential travel time measurements between the inner core traversing wave PKPdf (PKIKP) and a reference phase that has a similar path in the mantle, but does not enter the IC (PKPab, PKPbc, PKPcd and related phases). This kind of measurement is thought to minimize mantle contamination, but the global dataset is limited by the distribution of earthquake sources and stations.

Recently, Pham and Tkalcič (2023) devised a clever method to augment the available dataset by including measurements from exotic pairs of core phases that reflect several times at the earth’s surface and repeatedly sample the central part of the inner core, referred to as PKPn. They proposed a new model for ICA and in particular a distinct model for the innermost inner core.

However, we found that their model does not fit the travel time data measured using conventional PKPdf. We investigated the possible cause of this discrepancy by selecting PKPdf measurements on paths sampling similar portions of the mantle as the 16 measurements by Pham and Tkalcič (2023). While some measurements agree, the discrepant data correspond to paths that repeatedly interact with subducted slabs in the mantle.  We thus proceeded to analyse the effects of mantle structure, particularly subducting slabs, on differential travel times of core-sensitive phases. We assessed observed PKPdf and PKPdfn differential times for systematic bias. We find that the higher the “n”, i.e. the greater the number of passages through the mantle, the greater the effect of mantle structure on the PKPdfn measurements, suggesting that the discrepancies between the proposed ICA model constrained by these measurements compared with traditional direct PKP observations are likely due to mantle heterogeneity.