HR: 10:35h
AN: U41C-02    [PDF]
TI: Determining Shear Velocity Structure of ULVZs Using Stacked ScS 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: thorne@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 J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 United States
AB: The Ultra-Low Velocity Zone (ULVZ) is an intermittent, thin (5-40 km thick) layer or mound of greatly reduced P and S velocities located just above the core-mantle boundary (CMB). ULVZs have been detected in numerous regions using stacks of high-frequency P-waves or SPdiffKS phases. Such observations indicate that ULVZ patches have P-wave velocity reductions of at least 4 to 10%. S wave velocity reductions are more difficult to determine, and are typically based on secondary sensitivity of PcP precursors to combinations of P and S velocity that control the reflection coefficient. The ULVZ is generally regarded as a region of partial melt, based on estimates of a 3:1 S- to P-wave velocity reduction, with the ratio of velocities being a critical, but poorly constrained parameter. A direct measurement of S velocity in the ULVZ, independent of the P velocity is needed to better constrain ULVZ properties. One approach is to examine wide-angle transverse component ScS reflections for evidence of precursors generated by ULVZ structure. We explore this possibility by stacking transverse component ScS wave data from sources in the Tonga-Fiji and South American regions, recorded by dense broadband seismic networks in western North America. The lower mantle regions sampled both provide evidence for low P velocity ULVZ structures. Large numbers of seismograms from events in each source region are stacked after deconvolving source wavelets in order to equalize the signals and to extend the bandwidth. ScS phases reflecting from the CMB beneath the central Pacific show opposite polarity precursors, which can be modeled well by strong reductions (8-10%) in shear velocity in the ULVZ. Spatial variations in the ULVZ shear velocity structure are examined by binning the data, with some evidence for variation in structure over intermediate scale lengths of a few hundred km. Data reflecting from the CMB beneath the Cocos Plate do not show clear evidence of ScS precursors, and we seek to bound admissible S velocity structures. We also explore the sensitivity of our ScS precursor approach to ULVZ density, thickness, and shear velocity contrasts.
DE: 7200 SEISMOLOGY
DE: 7207 Core and mantle
SC: U
MN: 2003 Fall Meeting