HR: 17:15h
AN: S34C-06    [Abstracts]
TI: Double-Layer ULVZ Shear Velocity Structure Imaged With Stacked ScS Data
AU: * Avants, M
EM: mavants@pmc.ucsc.edu
AF: University of California Santa Cruz, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AU: Lay, T
EM: tlay@pmc.ucsc.edu
AF: University of California Santa Cruz, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences PO Box 871404, Tempe, AZ 85287 United States
AB: The ultra-low velocity zone (ULVZ) has been imaged as a thin (5-40 km thick) layer just above the core-mantle boundary (CMB), with P and S velocities reduced by up to 10 and 30%, respectively. Accurate characterization of the ULVZ is important, as it may relate to, for example, the role of the lowermost mantle in Earth's evolution, mantle and outer core convection, the geodynamo, and heat flux into the mantle. A direct measurement of S velocity in the ULVZ, independent of the P-wave velocity, is needed to better constrain ULVZ properties. We establish tangential component ScS data as a new probe of ULVZ shear velocity properties. Lowermost mantle structure beneath the central Pacific is studied using data from 38 deep focus Tonga-Fiji earthquakes, recorded by dense broadband seismic networks in western North America. Our data set consists of 442 instrument-deconvolved displacement seismograms, which are additionally deconvolved by average source-time functions (source wavelets) constructed for each event, in order to equalize the signals and to extend the signal bandwidth to high frequencies. The resulting traces are used in a double-beam stacking approach to enhance the signal-to-noise ratio of any coherent precursory reflections of the wide-angle transverse component ScS data, which should be detectable if ULVZ structure is present beneath our central Pacific study region. Our stacks reveal two distinct ScS precursors, which indicate a double layer ULVZ structure in this region. Both layers show strong lateral variations in shear velocity reduction (dVs) and thickness. The deeper ULVZ layer is well modeled by dVs drops varying from 3.3-7.4% (relative to PREM) with a thickness range of 24-30 km. The overlying layer has dVs reductions from 0.8-2.0% (relative to PREM), and 60-86 km thickness. Thus the imaged 2-layered ULVZ has dVs reductions far milder than previous studies (10-30%), which have argued for a partial melt origin to the ULVZ. Finer subdivisions of data will be examined and the variability across the study region of these ScS precursors will be investigated. Tradeoffs between layer thicknesses, shear velocity reductions, and density contrasts across the layer boundaries will also be explored, and results will be presented in the greater context of the entire D" region in this central Pacific study area.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005