The PKP travel time anomaly of the South Sandwich Island earthquakes in the context of inner core anisotropy

Abstract:

Using both PKP differential travel times and normal modes, the inner core (IC) has long been modeled by simple cylindrical anisotropy with fast axis parallel or quasi-parallel to the Earth's rotation axis. Such a model is popular because it can explain, to some extent, both of these data sets. However in recent years it has become clear that this simplistic model cannot explain all observed PKP differential travel times; in particular, the cluster of earthquakes from the South Sandwich Islands (SSI) recorded in Alaska, which show consistent deviation of 2-4 seconds above predicated arrival times in PKPbc-df data. Due to the sparsity of paths sampling the IC in a polar direction, these residuals form the basis of models of IC anisotropy, but their origins are not well understood.

We have created a new hand picked South Sandwich Island dataset of 500 PKPbc-df and 216 PKPab-df measurements from 36 events using a cross correlation method. In addition, we adopt two complimentary measurement techniques that have not so far been utilised on PKP waveforms, to add a measurement of uncertainty and statistical significance to existing measurements and to make sure these residual values are not a result of systematic error in picking of the core phases.

The new data set and methods suggest that these anomalous residuals are likely not due to the miss picking of core phases. Given that almost all high residual values are recorded on paths with azimuths towards Alaska and, given their scatter, a velocity anomaly due to subduction geometry at the source or receiver side is a possible cause when we consider a non-inner core origin. Here we explore whether the western hemisphere SSI residuals can be modeled using subduction geometry and velocity contrasts at the SSI subduction zone. Such a model could have significant implications for inner core anisotropy.