HR: 0800h
AN: S41C-1030    [Abstracts]
TI: Upper mantle discontinuities around circum-Pacific subduction zones
AU: * Zheng, Y
EM: yzheng@es.ucsc.edu
AF: IGPP, University of California,Santa Cruz, Santa Cruz, CA 95060 United States
AU: Lay, T
EM: tlay@es.ucsc.edu
AF: IGPP, University of California,Santa Cruz, Santa Cruz, CA 95060 United States
AU: Flanagan, M
EM: flanagan5@llnl.gov
AF: Geophysics and Global Security Division, Lawrence Livermore National Laboratory, Livermore, CA 94551 United States
AB: Characterization of mantle discontinuities near subducted slabs is one approach to understanding thermal structure and mantle dynamics around subduction zones. The ubiquitous presence of the "410-km" and ``660-km" discontinuities has previously been established and effects of downwellings on these major transition zone discontinuities has been studied. Shallower upper mantle discontinuities near subduction zones have been reported in the literature but their nature is still elusive, so we focus on these structures. Precise determination of the depth and impedance contrasts of these near-subduction zone discontinuities is important to understanding their origins. We compiled a large three-component teleseismic body wave dataset for moderate size (mb~5.5-7.0) deep (>400km) earthquakes for circum-Pacific subduction zones. The data consist of digitized WWSSN long-period records (1960s-1980s) and IRIS FARM broadband data (1988-2005) and a long-period digital dataset (1976-2005). We seek and model underside reflections from upper mantle discontinuities at depth, x, (e.g., pxP, sxP and sxSH), applying linear stacking along predicted travel time curves relative to surface reflected phases (pP, sP, sS) calculated for a reference Earth velocity model to enhance any weak reflected signals. In order to assess spatial coherence of the upper mantle discontinuities, we developed a Kirchhoff migration algorithm to project the scattered energy back to its 'correct' position for an assumed migration structure. Application of these algorithms to data from South American deep events indicates that there is substantial energy reflected from a "300-km" discontinuity, as well as a "200-km" discontinuity for both P and SH -waves. Strong underside reflections from the Moho are also apparent as precursors to the surface reflections. Analyses for other subduction zones will be presented as well.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Seismology [S]
MN: Fall Meeting 2005