S Ni adn Don V. Helmberger
Seismological Laboratory, Caltech 252-21, Pasadena,CA 91125,USA
Recent studies of the lower mantle structure beneath Africa revealed a large ridge-like structure, roughly 1200 km across and extending upward from the CMB to about 1500 km. The boundaries of this great structure appear sharp enough to produce multipathing of SKS along some edges where a 3% jump of S-velocity occurs in less than 50 km. The phase SKS jumps in travel times up to 6 secs when ray paths cross the near vertical boundaries at the CMB. We use a combination of SKS and SKKS travel time jumps to map out the position of the walls, starting at mid-Africa (15S, 5E) where it strikes roughly northwest to beyond the tip of South Africa (45S, 55E) where it bends towards the east with an abrupt turn. Several other core-phases involving a combination of S, ScS, SKS, and SKKS waveform sections are modeled along two corridors, one along strike and one at right angles. These results suggest a relatively uniform structure although some small-scale features in SKS variations, roughly a sec or two are easily recognized. These features appear to be associated with roof structure. This 7000 km ridge-segment appears to match the shape of the geoid-high contours closely suggesting a dynamic origin, probably caused by excess heating of the upper mantle.