Heterogeneity in the lower mantle beneath Africa, as revealed from S and ScS phases

Abstract

S and ScS phases propagated beneath Africa and its adjacent regions were analyzed using their differential travel times Δ t and the differences of anelastic parameter δ t∗ between the two phases. The residuals of the ScS — S differential travel times had strongly positive correlation with those of the ScS travel times. The largest residual of +7.9 s, was in central Africa (10.5°N, 22.6°N). The residuals decreased with increasing distance northwards from central Africa. The ScS bounce points having large positive values greater than 4 s were concentrated in a region with a horizontal length of about 500 km at the core-mantle boundary beneath central Africa. This region was characterized by an S wave velocity around 3–5% slower than prem in the D″ region. The anelastic parameter, t∗ of the S phases propagated beneath Africa was larger than that of the ScS phases, suggesting that the apparent Q structure in the mid-mantle (1000–2000 km deep) was slightly lower than that at the base of the mantle. These results probably imply a mantle plume beneath Africa.