Effects of upper mantle structure beneath Alaska on core-sensitive seismic wave absolute and differential measurements: Implications for estimates of inner core anisotropy

Abstract

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…