HR: 08:45h
AN: T21A-04 INVITED     [PDF]
TI: Extreme Short Scale Variations in D" Topography Beneath the Pacific Ocean Just West of Central America
AU: * Garnero, E J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences 871404, Tempe, AZ 85287-1404 United States
AU: Thomas, C
EM: tine@liv.ac.uk
AF: University of Liverpool, Department of Earth Sciences 4, Brownlow Street, Liverpool,, L69 3GP United Kingdom
AU: Lay, T
EM: thorne@es.ucsc.edu
AF: University of California, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AB: In this study we use a wavefield migration technique to infer D" reflectance and topography in a densely sampled region just west of Central America beneath the Cocos plate. High quality broadband waveforms from seismic networks in California of 13 deep focus South American earthquakes are instrument and source wavelet deconvolved to displacement, aligned on ScS as a reference phase, then studied for coherency of energy between ScS and S. A search for potential lowermost mantle reflector locations is achieved by migrating the wavefield for each earthquake to each node of a 3D grid of potential reflector locations, with spacing every 1 deg laterally and 10 km vertically (with ranges: -2 to 18 deg N; -100 to -80 deg E, 2200 to 2888 km depth). Grouping our data into densely sampled latitudinal bins resulted in 41 clusters of bounce points between 0 and 15 deg N. The migrated images for all bounce point clusters show an abrupt increase in velocity that is thickest to the north in our study area (up to 300 km and greater) and dramatically reduces to as thin as 100 km thick in the south. We also see evidence for the main positive velocity increase being underlain by a negative velocity discontinuity in the northern half of our study region, though this feature is not visible in all migrations. These results are compatible with the general picture from simpler 1D studies (which indicate a thicker high velocity D" layer to the north beneath the neighboring Central American and Caribbean than that to the south) but demonstrate increased complexity at shorter scale lengths. The thickening of the D" layer to the north coincides with inference for higher velocities there implied by ScS-S and S-SKS differential travel time residuals. Evidence for out of plane reflections is also visible in some migrated images. Such strong topographical variations (~200 km change over several hundred km laterally) are likely intimately coupled to overlying mantle currents related to subduction, such as localized flow complexities or being near the edge of a larger scale downwelling. These results, including the less well-constrained mid-D" velocity decrease, will be compared to larger scale velocity heterogeneity and gradients as inferred from tomography, as well as recent ultra-low velocity zone results.
UR: http://garnero.asu.edu/research/earthslowermantle.html
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
SC: Tectonophysics [T]
MN: 2003 Fall Meeting