HR: 0830h
AN: S51C-0054    [PDF]
TI: Waveform Modeling Constraints on the South African Superplume
AU: * Helmberger, D
EM: helm@gps.caltech.edu
AF: California Institute of Technology, 1200 East California Blvd., Pasadena, CA 91125 United States
AU: Ni, S
EM: stone@gps.caltech.edu
AF: California Institute of Technology, 1200 East California Blvd., Pasadena, CA 91125 United States
AB: Tomographic studies of the lower mantle structure beneath South Africa reveal large-scale slow velocities from the core-mantle-boundary (CMB) to about a depth of 1500 km. Predicted SKS delay patterns (up to 3 sec) for some of these models fit observations (South African Array Data) quite well except for magnitude level, explaining less than ($\frac{1}{2}$) the anomaly. Moreover, the sharpness in travel time offsets and waveform complications require nearly vertical walls separating the anomalous structure from the normal PREM mantle. We present numerous record sections along with 2D and 3D synthetics displaying multi-pathing of arrivals (S$_{diff}$, SKS, SKKS, S and S$_{c}$S) based on our previous plume model. This ridge model has a crescent shape with one apex beneath the Indian Ocean (Kerguelen) with the other beneath the mid-Atlantic (Cape Verde). The structure extends upward through the lower mantle similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM, although there is some evidence for a still slower core or upward doming near the middle of the structure. Some record sections fit the observed waveform complexity better than our standard model by injecting a thin (50 km wide) thermal boundary layer along the walls with depressed velocities. We have not found any evidences for ULVZ's beneath the main structure but ample evidence at some locations near the edges. We also analyzed P$_{diff}$ and the differentials of (P$_{c}$P-P) along the same great circle paths (same events). The P-velocity is not very anomalous, perhaps -.5%, arguing for a chemical origin.
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2003 Fall Meeting