South Sandwich to Alaska anomalous inner core phase observations explained by upper mantle slab effects beneath Alaska

Observations of travel time anomalies of inner core-sensitive PKPdf seismic body waves, as a function of path orientation with respect to the Earth's rotation axis, have been interpreted as evidence of anisotropy in the inner core. Paths from earthquakes in the South Sandwich Islands to stations in Alaska show strongly anomalous travel times, with a large spread that stand out from data on adjacent paths. Moreover, these data show deviations in incidence angle, back-azimuth and amplitude, which vary systematically across the Alaska. Incorporating these data into models of the inner core skews the pattern of anisotropy, making it difficult to construct realistic inner core anisotropy models.

Here we assess the impact of strong velocity heterogeneity under Alaska on the travel times, directions of arrival and amplitudes of PKPdf. We use 3D ray tracing and 2.5D waveform modelling through a new, high-resolution tomography model of the upper mantle beneath Alaska. We find that the structure beneath Alaska, notably the subducting slab, has a strong effect on the patterns of PKPdf observables, which match the trends observed in the data. We conclude that at least 2 s of the travel time anomaly commonly attributed to inner core anisotropy is due to slab effects in the upper mantle beneath Alaska. We show how improvements in the 3D slab model may lead to explaining an even larger part of the observed travel time and slowness anomalies. Our observations demonstrate the importance of understanding strong heterogeneity in the upper mantle when investigating the deep Earth.