Mapping heterogeneity of the uppermost inner core using two pairs of core phases

Two pairs of core phases, PKPcd-PKPdf and PKPbc-PKPdf, are analysed to map the P-wave velocity (Vp) of the uppermost inner core. Waveform analysis of PKPcd-PKPdf shows that Vp relative to PREM (δVp) within 100 km from the inner core boundary (ICB) is laterally heterogeneous by -0.5% to +0.1% with a wavelength of nearly 1000 km. Differential travel times of PKPbc-PKPdf relative to PREM (δTbc-df) are positive (negative) for the regions of positive (negative) δVp near the ICB. This correlation indicates that the lateral Vp variation exists predominantly in the inner core side of the ICB. Absolute values of δTbc-df for both positive and negative δVp regions are too large to be explained with models in which lateral δVp variation is restricted within the upper 200 km of the inner core. The lateral variation appears to extend at least to 300 km below the ICB.