P-VELOCITY STRUCTURE OF THE CORE-MANTLE BOUNDARY REGION INFERRED FROMPKP(AB)-PKP(BC) DIFFERENTIAL TRAVEL-TIMES

PKP(AB)-PKP(BC) differential travel time residuals from the bulletins of the International Seismological Centre are inverted to retrieve the P-velocity anomalies in the lowermost 300 km of the mantle. We show t hat a realistic core-mantle boundary (CMB) topography may have only a marginal contribution to the residuals. The block parametrization used allows us to discard poorly sampled areas prior to a least-squares in version. The main features are negative anomalies beneath Eurasia and North America and a positive anomaly beneath the Southwest Pacific. Th e inferred map is not consistent with other maps resulting from previo us P- and S-velocity studies, which are correlated at long wavelengths with each other. The preferred explanation for this challenging incon sistency is the presence of a thin heterogeneous layer just above the CMB.