SEISMIC VELOCITIES AT THE CORE-MANTLE BOUNDARY INFERRED FROM P-WAVES DIFFRACTED AROUND THE CORE

The very base of the mantle is investigated with core-diffracted P wav e (P-diff) travel times published by the International Seismological C entre (ISC) for the period 1964-1987. Apparent slownesses are computed for two-station profiles using a difference method. As the short-peri od P-diff mostly sample a very thin layer above the core-mantle bounda ry (CMB), a good approximation of the true velocity structure at the C MB can be derived from the apparent slownesses. More than 27 000 profi les are built, and this provides an unprecedented P-diff sampling of t he CMB. The overall slowness distribution has an average value of 4.62 s/deg, which corresponds to a velocity more than 4% lower than that o f most mean radial models. An analysis of the residuals of absolute IS C P and P-diff travel times is independently carried out and confirms this result. It also shows that the degree of heterogeneities is signi ficantly higher at the CMB than in the lower mantle. A search for late ral velocity variations is then undertaken; a first large-scale invest igation reveals the presence of coherent slowness anomalies of very la rge dimensions of the order of 3000 km at the CMB. A tomographic inver sion is then performed, which confirms the existence of pronounced (+/ -8-10%) lateral velocity variations and provides a reliable map of the heterogeneities in the northern hemisphere. The influence of heteroge neity in the overlying mantle, of noise in the data and of CMB topogra phy is evaluated; it seemingly proves minor compared with the contribu tion of heterogeneities at the CMB. Our results support the rising ide a of a thin, low-velocity laterally varying boundary layer at the base of the D '' layer. The two principal candidate interpretations are th e occurrence of partial melting, or the presence of a chemically disti nct layer, featuring infiltrated core material. (C) 1997 Elsevier Scie nce B.V.