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.