The effect of source-side subduction on PKP differential times and implications for inner core anisotropy

Abstract

Directional variation in PKP differential times provides compelling evidence that Earth's inner core is anisotropic. These phases, however, are also sensitive to Earth's heterogeneous mantle. Disentangling the causal structures of PKP differential time anomalies is difficult but nonetheless important if we are to fully understand the structure and evolution of Earth's inner core. A large proportion of earthquakes used to study the inner core originate from subduction zones, which are associated with strongly positive upper mantle seismic velocity anomalies, but the effect of these on the measurements has not yet been investigated.

In this study, we use AxiSEM3D to simulate the effect of source-side subduction on PKP differential time measurements. We find that some combinations of slab parameters result in artefacts up to several seconds in magnitude, while for others the effect is negligible. We subsequently examine existing data sets of measurements to assess if source-side subduction has a detectable influence on the data, and also to assess if the Scotia slab is the cause of observed anomalous measurements originating from the South Sandwich Islands. We find that the signal of source-side subduction in the data is possibly present but weak, and that the magnitude of anisotropy required by the data is not affected by source-side subduction. Furthermore, the Scotia slab is unlikely to be the cause of the anomalous measurements from the South Sandwich Islands. Nevertheless, we advise caution for future studies as the artefacts caused by source-side subduction may, in some cases, be significant.