PKP(BC-DF) travel time residuals and short scale heterogeneity in the deepearth

Differential PKP(BC)-PKP(DF) travel time residuals for quasi-polar paths in the earth present large excursions from the trends predicted by simple mod els of inner core anisotropy. We analyze a set of such paths classified acc ording to specific source and station regions, and show that observations a re 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 impor tant contribution from the latter.