HR: 0800h
AN: U41B-0410    [Abstracts]
TI: D" Shear Wave Velocity Structure Under the Cocos Plate: Discontinuity Sharpness and Double Crossing
AU: * Lay, T
EM: thorne@pmc.ucsc.edu
AF: Department of Earth and Planetary Sciences, University of California, Santa Cruz, Earth and Marine Sciences Building, Santa Cruz, CA 95064, United States
AB: The possible occurrence of a perovskite to post-perovskite (Pv-pPv) phase transition in the deep mantle may account for complex seismic wave velocity structure in the D" region overlying the core-mantle boundary (CMB). A rapid increase in shear wave velocity several hundred kilometers above the CMB is commonly associated with the Pv-pPv phase change, while velocity decreases within 100 km of the CMB have been attributed to the reverse transformation (double-crossing) resulting from rapid temperature increase in the CMB thermal boundary layer. Establishing the sharpness of the velocity increase is important for evaluating the phase boundary interpretation, as mineral physics experiments suggest the likelihood of a significant pressure range for the two-phase domain. Analysis of stacked broadband shear waves sampling the D" region beneath the Cocos Plate is used to establish constraints on the sharpness of the 1.1-2.2 percent shear velocity increase 232-296 km above the CMB in this region. Separate stacks aligned on direct S or ScS are modeled, with lower mantle models being constrained to match ScS-S differential times. Bounds on the inferred discontinuity sharpness will be discussed in the context of Pv-pPv properties. Evidence for and implications of a rapid velocity decrease found 47-75 km above the CMB in this region will also be presented.
DE: 3924 High-pressure behavior
DE: 5430 Interiors (8147)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Union [U]
MN: 2007 Fall Meeting