HR: 16:15h
AN: U44A-02    [Abstracts]
TI: Absence of P-wave Reflectivity Near the D" S-wave Velocity Discontinuity in the Lowermost Mantle beneath the Cocos Plate
AU: * Hutko, A
EM: ahutko@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth Sciences, Santa Cruz, CA 95064, United States
AU: Lay, T
EM: tlay@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth Sciences, Santa Cruz, CA 95064, United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota Twin Cities, Department of Geology and Geophysics, Minneapolis, MN 55455, United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tuscon, AZ 85287, United States
AB: An abrupt 1-2.5% S-wave velocity increase has been observed in the lowermost mantle beneath the Cocos plate in several studies. This is commonly attributed to the perovskite to post-perovskite phase transition. This phase transition is expected to have much weaker effects on P-wave velocities than on S-wave velocities and the depth range of the transition is expected to depend on Al and Fe content of the perovskite. We image lowermost mantle P-wave reflectivity beneath the Cocos plate using 1D stacking and 3D Kirchhoff migration. Our carefully processed data set comprises 8000 seismograms from deep South American earthquakes recorded by broadband and short-period seismic networks in western North America. Stacked P-wave source wavelets are deconvolved from the data for each event, allowing band-pass filtered signals to be combined for many events. Events are discarded if individual event-stacks do not show an impulsive PcP arrival with significant signal-to- noise ratio. Depth shifts are applied to each event to align PcP arrivals in the combined stacks. These shifts increase systematically from south to north. We observe a widespread weak P-wave reflector about 320 km above the CMB modeled well by a P-wave velocity (Vp) change of –0.2 to –0.4%, depending on the thickness of the velocity change. The depth of this relatively flat reflector is tightly constrained and is a few tens of km shallower than the local S-wave reflector, which may have regional topography of up to 100 km. We model a clear feature in the P-wave reflectivity with a change in dVp/dZ about 180 km above the CMB, accompanied by a sharp 0.2% increase in Vp. Different narrow band filters up to 2 Hz and forward modeling of double-array stacks show that this small velocity increase must occur over less than 10 km in depth. This also does not directly correspond to any significant feature in the S-wave velocity structure. The high signal-to-noise ratio of our locally binned data stacks allows us to preclude the existence of any other abrupt Vp change stronger than ±0.1 to ±0.4%. These upper bounds are influenced by the source-receiver geometry of the data and the thickness of the velocity change. A strong, positive velocity contrast, out-of-plane scatterer is observed in the migrations, and appears to originate shallower than 200 km above the CMB. Finally, we show that the PREM values for P-wave attenuation in the lowermost mantle are too low, and need to be at least three times greater locally to match our broadband observations.
DE: 3654 Ultra-high pressure metamorphism
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
SC: Union [U]
MN: 2007 Fall Meeting