Seismic evidence of flow at the base of the upper mantle

A combined analysis of SKS, SKKS and P660s seismic phases detected in the records of BCAO/BNG seismic station in central Africa reveals that the waveform and amplitude of the T component of P660s deviate significantly from those predicted from the observations of shear wave splitting in SKS and SKKS. The inverse problem is investigated by using the anisotropic Thomson‐Haskell code and a genetic algorithm. The discrepancy between SKS/SKKS and P660S can be explained by P‐SH conversion at the boundaries of a weakly anisotropic layer 30–40 km thick lying atop 660 km discontinuity beneath BCAO/BNG. This anisotropy is probably induced by horizontal flow at the bottom of the upper mantle, in the transition zone.