Seismic velocities at the core-mantle boundary inferred from P waves diffracted around the core

Details Seismic velocities at the core-mantle boundary inferred from P waves diffracted around the core Seismic velocities at the core-mantle boundary inferred from P waves diffracted around the core

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997pepi..101..189s&link_type=abstract

Physics of the Earth and Planetary Interiors, v. 101, p. 189-202.

Physics

12

Scientific paper

The very base of the mantle is investigated with core-diffracted P-wave (Pdiff) travel times published by the International Seismological Centre (ISC) for the period 1964-1987. Apparent slownesses are computed for two-station profiles using a difference method. As the short-period Pdiff mostly sample a very thin layer above the core-mantle boundary (CMB), a good approximation of the true velocity structure at the CMB can be derived from the apparent slownesses. More than 27 000 profiles are built, and this provides an unprecedented Pdiff sampling of the 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 of most mean radial models. An analysis of the residuals of absolute ISC P and Pdiff travel times is independently carried out and confirms this result. It also shows that the degree of heterogeneities is significantly higher at the CMB than in the lower mantle. A search for lateral velocity variations is then undertaken; a first large-scale investigation reveals the presence of coherent slowness anomalies of very large dimensions of the order of 3000 km at the CMB. A tomographic inversion 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 heterogeneity in the overlying mantle, of noise in the data and of CMB topography is evaluated; it seemingly proves minor compared with the contribution of heterogeneities at the CMB. Our results support the rising idea of a thin, low-velocity laterally varying boundary layer at the base of the D'' layer. The two principal candidate interpretations are the occurrence of partial melting, or the presence of a chemically distinct layer, featuring infiltrated core material.

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