HR: 0800h
AN: S11B-0559    [Abstracts]
TI: Detection of Diffraction Patterns at the edge of the African Superplume
AU: * Sun, D
EM: sdy@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 252-21, Caltech, Pasadena, CA 91125,
AU: Bower, D J
EM: dan@caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 252-21, Caltech, Pasadena, CA 91125,
AU: Helmberger, D
EM: helm@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 252-21, Caltech, Pasadena, CA 91125,
AB: A metastable thermal-chemical convection model has been developed to explain the African low velocity structure with its dome-like morphology. The model that best fits the seismic waveform data is referred to as the High Bulk Modulus Structure (HBMS), which has sharp walls. A recently developed WKBJ-type code (DWKM) can be used to generate approximate 3D synthetics for such structures. This method approximates 3D effects by adding out-of- plane contributions from virtual receivers at neighboring azimuths with two related to the inner Fresnel zone and two longer-period contributors sampling the outer Fresnel zone. The four responses are weighted by diffraction operators that are defined by the source duration and travel time from the edges of the structure. Here we use the HBMS model to generate 3D synthetics displaying waveform complexity patterns predicted for possible geometries. We show that these synthetics can be simulated from PREM synthetics where the shifts (δ t_i) between the four operators are determined by a grid-search. Contours of these (δ t_i) values obtained, from a grid of stations, produces a diffraction pattern footprint that suggests that array data can be processed directly to reveal 3D structure. We demonstrate the usefulness of this technique by processing both P and S wave data from the Kaapvaal array in Southern Africa. The difference between the patterns produced by P and S wave data for the same events (same ray paths) further validates the nature of these chemically sharp boundaries.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
SC: Seismology [S]
MN: 2007 Fall Meeting