Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa

We present data which indicate that the broad, low shear velocity anomaly beneath southern Africa is stronger and more extensive than previously thought. Recordings of earthquakes in the southwestern Atlantic Ocean at an array of broadband seismic stations in eastern Africa show anomalously large propagation time delays of the shear phases S, ScS, and SKS which vary rapidly with epicentral distance. By forward modeling, we estimate that the low velocity anomaly extends from the core-mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3% lower than in standard models such as PREM. Strong velocity contrasts exist at its margins (2% over about 300 km). These seismic characteristic are consistent with recent numerical simulations of lower mantle mega-plume formation.