HR: 0800h
AN: DI41A-1254    [Abstracts]
TI: Detection of Subducted Lithosphere in the Midmantle From Asymmetric PP Reflections
AU: * Rost, S
EM: srost@asu.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Garnero, E J
EM: garnero@asu.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: McNamara, A K
EM: allen.mcnamara@asu.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Williams, Q
EM: qwilliams@es.ucsc.edu
AF: Department of Earth Sciences University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064 United States
AU: Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Geodynamics Research Center Ehime University, 2-5, Bunkyo-cho, Matsuyama, 790-8577 Japan
AB: The precursor wavefield to PP, a P-wave once reflected at the free surface between source and receiver, contains information about the structure of the upper mantle. Most often studied are underside reflections from the upper mantle discontinuities beneath the PP reflection point to infer temperature and composition of the mantle. However, additional energetic precursory arrivals are present, most notably in the 90-110 deg distance range. Using array data and a frequency-wavenumber analysis, we constrain the complete slowness vector (or horizontal and vertical incidence angle) for PP and all precursors. Earthquakes from the SW Pacific and Indonesia recorded by the short-period Canadian Yellowknife Array are studied, and 105 precursors not associated with upper mantle discontinuities are detected. A ray-theoretical backtracing algorithm using PP precursor slowness, backazimuth and traveltime maps distinct reflector locations throughout the central Pacific above 200 km, suggesting scattering within and below the oceanic lithosphere. However, nearly one half of all precursors are mapped to depths below 400, extending to 1000 km depth, beneath W/SW Pacific subduction zones. The deep reflectors are highly correlated with seismically fast zones in tomographic models, and are imaged from asymmetric (and off great-circle path) PP precursors, reflecting ~20 deg away from the source. These results strongly constrain the existence of subducted slab material below the 660-km discontinuity beneath the Philippine Sea plate. On the other hand, detections in the Tonga region do not indicate slab material beneath the 660 km discontinuity in agreement with previous models showing subducted material only above this depth. Forward modeling of synthetic seismograms together with geodynamical and mineral physical models permits improved constraints on the state of the subducted material in these locations.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005