PKP(BC‐DF) Travel time residuals and short scale heterogeneity in the deep Earth

Differential PKP(BC)‐PKP(DF) travel time residuals for quasi‐polar paths in the earth present large excursions from the trends predicted by simple models of inner core anisotropy. We analyze a set of such paths classified according to specific source and station regions, and show that observations are compatible with a constant anisotropy model of at most 1.5% in the outer 400 km of the inner core. To explain local departures of up to 3 sec from the predictions of such a model, strong lateral variations on scale lengths shorter than a few hundred km need to be invoked, either in the core or in the mantle. Independent results of D' modelling argue in favor of an important contribution from the latter.