P‐velocity structure of the core‐mantle boundary region inferred from PKP(AB)‐PKP(BC) differential travel times

Abstract

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 that 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 inversion. The main features are negative anomalies beneath Eurasia and North America and a positive anomaly beneath the Southwest Pacific. The inferred map is not consistent with other maps resulting from previous P‐ and S‐velocity studies, which are correlated at long wavelengths with each other. The preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the CMB.