Upper mantle velocity structure can contaminate differential travel-time measurements and bias estimates of inner core anisotropy causing the “Alaska anomaly”
Using a regional tomographic model of the Alaskan upper mantle to predict upper mantle effects on PKP travel times, we show that the signature of the Alaska slab is present in trends of observed absolute PKPbc, ab, and df travel times, both as a function of distance and azimuth. Moreover, we demonstrate that the effect of the slab is not fully cancelled by differential measurements. This implies that past models of the inner core are biased towards too strong average 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 mantle effects on PKP travel times, we show that the signature of the Alaska slab is present in trends of observed absolute PKPbc, ab, and df travel times, both as a function of distance and azimuth. Moreover, we demonstrate that the effect of the slab is not fully cancelled by differential measurements. This implies that past models of the inner core are biased towards too strong average anisotropy. In order to better constrain inner core anisotropy in future, differential measurements of core-phase travel times need to be more accurately corrected for upper mantle three-dimensional structure, which in turns requires the construction of higher resolution tomographic models.
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