High Resolution Imaging of Mantle Discontinuities across South America at 20 Degrees South

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Kelly LiuEAR-0001000This proposal will conduct a high resolution study of the major discontinuities beneath a 3000 km long profile along the 20 degree S latitude line across central South America. The study will utilize a unique data set, which is a combination of broadband data sets from three portable seismic experiments conducted during 1992-1995. Preliminary results by non-linear stacking of 752 high quality radial receiver functions suggest clear spatial variations of both the 410 and 660 discontinuities across the profile. Some interesting features include: 1) the 660 discontinuity is depressed by at least 35 km, or as large as 65 km, by the subducting slab, 2) west of the Andes, the depth to the Moho is about 40-45 km, but it increases to 65-80 km over a distance of less than 50 km and decreases to about 45-50 km beneath the eastern boundary of the Andes, 3) the transition zone thickness is about 250 km of the ocean side, and 260 km on the continent side, indicating that the former is hotter than the later, and 4) there exists a group of strong negative arrivals above the 300 km depth, but only above the slab on the continental side. The study will use a set of procedures such as phasing depth imaging of P-to-S and S-to-P conversions, conversions of waves from deep local earthquakes at discontinuities, and synthetic seismogram to improve and expand the preliminary results, and to measure the spatial variations of other properties of the discontinuities such as their sharpness and velcoity contrast. The research will develop a procedure to stack converted waves from tilted interfaces to directly map the subducting slab.