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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 is not compatible with simple models of anisotropy. 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, is reflected in the patterns of these PKPdf observations, and this can be replicated by our model. We also find similar patterns in observed teleseismic P waves that can likewise be explained by our slab model. We conclude that at least 2 s of the travel time anomaly often attributed to inner core anisotropy is due to slab effects in the upper mantle beneath Alaska.

The Altai–Sayan region of central Asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between India and Eurasia. To add constraints to the role of the upper mantle in this type of tectonic environment, we analyzed intermediate-period, analogue records of SKS and SKKS to determine the state of mantle anisotropy in this region. We find that for all stations located in actively deforming areas, the fast direction of anisotropy correlates well with indicators of Cenozoic to the present-day deformation, and correlates with older structures only when they happen to align with younger ones. The general east-northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors. At one station (NVS), located outside of any actively deforming region, the N–S fast direction may correspond to the long-wavelength strain field of the mantle beneath much of Siberia. These correlations, along with the size of the delay times (1–2 s), suggest that the anisotropy in the upper mantle beneath the Altai–Sayan is responding to recent tectonic activity.