M. Sylvander et al., SEISMIC VELOCITIES AT THE CORE-MANTLE BOUNDARY INFERRED FROM P-WAVES DIFFRACTED AROUND THE CORE, Physics of the earth and planetary interiors, 101(3-4), 1997, pp. 189-202
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.