HR: 17:00h
AN: T24B-05    [Abstracts]
TI: Mantle Deformation Beneath the Southern Cascadia Subduction Zone
AU: * Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, Dept. of Geological Sciences, Tempe, AZ 85287 United States
AB: The objective of this study is to determine the location and extent of seismic anisotropy within the southern Cascadia subduction zone to constrain mantle deformation in the region. Of particular importance is to provide an improved understanding of the role of water in strain development and seismic anisotropy in hydrated mantle. Previous seismic, geodynamic, and petrological studies of the region suggest that the mantle wedge in this region is significantly hydrated and serpentinized. Southern Cascadia is therefore an integral component in the evaluation of the relationship between observations of seismic anisotropy and mantle deformation in subduction zone settings. Data for this study comes from the 1993-1994 IRIS/PASSCAL CASC broadband seismic array, deployed by Oregon State University researchers across a $\sim$300 km swath beginning at the Oregon coast and across the Cascades Mountains. Station spacing ranges from 4 to 8 km, providing an excellent dataset for a detailed investigation of possible small-scale variations in seismic anisotropy. To this end, I apply several shear wave splitting analysis techniques and error estimation methodologies to core shear phases such as SKS to provide constraints on the location, extent, and orientation of mantle deformation across the region. The range of analysis methods, including particle motion linearization, transverse energy minimization, and error surface stacking, is designed to provide confirmation that calculated shear wave splitting parameters from each methodology are robust. Preliminary results indicate that fast polarization directions are oriented ~E-W over the western part of the array, rotating to $\sim$N50E near the eastern end of the array. Resolvable station-to-station variations in fast directions are also evident for some regions, providing a clue of small-scale variations in seismic anisotropy across the array. Splitting times range from 0.8 s to 1.6 s, but currently these variations are not well resolved between regions. While these results are preliminary, a full analysis of this dataset will provide new constraints on the lateral and depth distribution of seismic anisotropy beneath the southern Cascadia region.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting