HR: 10:35h
AN: U42A-02    [Abstracts]
TI: The Lowermost Mantle Beneath Central America Imaged by Kirchhoff Migration of Scatterers and Reflectors
AU: * Hutko, A
EM: ahutko@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AU: Lay, T
EM: thorne@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota Twin Cities, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, United States
AB: We use tens of thousands of seismograms from South and Central American earthquakes recorded by western North American seismic networks to image the lowermost mantle beneath Central America using a 3D Kirchhoff migration method. P wave studies of the deep mantle often rely on some form of stacking of many records in order to enhance the signal-to-noise ratio of weak phases generated by deep structure, such as reflections off of the D" discontinuity. These methods, however, often assume one-dimensional structure, which is at odds with the evidence for significant heterogeneity. Kirchhoff migration is a three-dimensional stacking method that allows interactions with structure outside of the source-receiver plane, thus illuminating a much larger volume. The D" discontinuity beneath Central America has been readily observed in S wave studies and may be the result of the shear wave velocity increase associated with the recently discovered perovskite to post-perovskite phase transition. This phase transition is expected to have weaker effects on P wave velocities than on S wave velocities and the sharpness of this transition is unknown. Using data at post-critical distances, we observe structures consistent with a P velocity discontinuity about 200 km above the core-mantle boundary (CMB). Observing this using short period data suggests that the boundary must be less than a few 10s of km thick, while observation with lower frequency broadband data exclude the possibility of it being a thin layer. Whether this discontinuity is co-located for both P and S waves is difficult to resolve. Both the broadband and the short period P wave data sets also reveal a sharp out-of-plane scatterer, which may be located close to the CMB. The short period data also indicate reflectivity about 400 km above the CMB, well above the D" discontinuity, and similar reflectivity is observed under the Central Pacific. This feature appears to be more consistent with a discontinuity than a scatterer, is hinted at in the broadband data set and is not observed with S waves. We also present results using P wave data from pre-critical distances. While reflection and scattering coefficients are much lower at these distances, the imaging volume is much larger. Results from these data hint at a complicated lower mantle with multiple features whose origins may lie anywhere between the core-mantle boundary and many hundred km above it.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
DE: 7208 Mantle (1212, 1213, 8124)
SC: Union [U]
MN: 2007 Joint Assembly