HR: 0800h
AN: T11E-1320    [Abstracts]
TI: Imaging Lower Mantle Structure Beneath the Central Pacific by Stacking S Wave Data
AU: * Avants, M S
EM: mavants@es.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@es.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 Box 871404, Tempe, AZ 85287 United States
AB: The lowermost $\sim$250 km of Earth's mantle is being revealed as a region of surprisingly complex structure and inferred dynamics. Recent waveform stacking efforts indicate the presence of rapid P and S velocity increases about 230 km above the core-mantle boundary (CMB) beneath the central Pacific. Tangential component S-wave (SH) data from the Tonga-Fiji region recorded by broadband networks in western North America are used to map small-scale variability in the topography and magnitude of this central Pacific D" discontinuity, as well as small-scale structure within the D" layer. Our approach involves deconvolving source wavelets, binning and stacking localized subsets of data, and modeling the stacks with synthetics to provide localized spatial resolution of structure in our study region. The D" discontinuity under the central Pacific is found to vary in depth from 2577 km to 2730 km, with shear velocity increases from 0.3% to 2.1%. If the reflector is continuous, it has topography as dramatic as 100 km over scale lengths of $\sim$150 km. We also determine the shear velocity structure in an ultra-low velocity zone (ULVZ) present just above the CMB beneath the central Pacific. ULVZ's have been detected in numerous regions using high frequency P waves or SPdiffKS phases. Results of such studies indicate P wave velocity reductions of 4% to10% in the ULVZ, but the S wave velocity reduction is poorly resolved due to tradeoffs between the P and S velocities and density in the reflection coefficient at the top of the ULVZ. The ULVZ is generally regarded as a region of partial melt, based on estimates of a 3:1 S velocity to P velocity reduction, but the ratio of velocity decrements remains a critical, and poorly constrained, parameter. Direct measurement of S velocity in the ULVZ, independent of P velocity, is needed to better constrain ULVZ properties. We examine our data set for precursors generated by ULVZ structure by aligning the data on ScS and stacking, finding spatially variable precursors with opposite polarity to ScS. Modeling the ScS precursors indicates shear wave velocity reductions of 0% to 22% in the ULVZ across our study region, with the estimates trading off with any assumed density increase. Evidence for abrupt velocity reductions at shallower depth within the D" layer is also found. The overall shear velocity structure under this central Pacific region will be presented.
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting