Low-velocity structure beneath Africa from forward modeling

Seismic waveforms observed in South Africa containing the first arrival crossover of S to SKS (70 degrees to 110 degrees) are analyzed. The data consist of analog records from the World Wide Seismographic Station Network (WWSSN) of deep events beneath South America. The S-waves arrive 2 to 3 s early relative to PREM at ranges from 70 degrees to 95 degrees and then become increasingly delayed, becoming 5 to 6 s late at 110 degrees. The SKS phase is late by 3 to 5 s over the entire range. This pushes crossover between S and SKS, normally observed at about 81 degrees, out about 2 degrees to 3 degrees, which is the most anomalous shift ever reported. To model such features, we modified Grand's tomography model [Grand et al., GSA Today 7 (1997) 1-7], and generated 2D synthetics to match the data. The overall shape and position of the lower mantle low-velocity anomaly proposed by Grand predicts good results if lower mantle anomalies are enhanced to a level of about 4%. This results in a complex tabular structure extending upward from the core-mantle boundary about 1500 km into the mantle. These features appear to be consistent with a large young plume which is erupting off the CMB. (C) 1999 Elsevier Science B.V. All rights reserved.