Seismic Structures in the Lower Mantle and the Core-Mantle Boundary Region beneath Southern Africa

The on-going Southern African Seismic Experiment provides an excellent dense dataset for detailed study of seismic structures in the lower mantle and the core-mantle boundary region beneath southern Africa. This network consists of nearly 60 broadband stations spanning a distance of over 2000 km. Based on global tomography, this network is situated above a complex, large-scale region of very low velocities, that extends over the entire depth range of the lower mantle, down to the core-mantle boundary. Arrival times of P, PKP, S, SKS, ScS phases are used to constrain the overall structure of this feature, and waveforms of SPdKS phases and PKP precursors are used to constrain the detailed structure in the core-mantle boundary region. Of particular geodynamic interest are the magnitude, lengthscale, sharpness of lateral transitions, and the ratio of P to S wave velocity perturbations of these structures. The complexity of seismic structures in the lowermost mantle is illustrated by the observation of strong broadband PKP precursors in one region, implying strong seismic heterogeneities at various length scales, as well as the observation of strong, long-period PKP precursors (no short-period energy) in another region suggesting the existence of a relatively large-scale, smooth seismic structure. We will perform detailed modeling of the observations and discuss possible geodynamic and mineralogical implications of these seismic structures beneath Africa