HR: 1340h
AN: T33A-1350    [Abstracts]
TI: Anisotropy in the upper mantle beneath Mexico
AU: * Matzel, E
EM: matzel1@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave L-206, Livermore, CA 94550 United States
AB: We have developed a 3D model of the seismic velocity structure for the upper mantle beneath Mexico including measurements of radial anisotropy with the ultimate goal of establishing its thermal structure and composition. In particular, we want to establish the variability of structure between different tectonic provinces. We obtained high resolution of the region by using data from earthquake sources in the south to dense collections of seismometers in the north, in particular the RISTRA seismic array and the California network. Initially, we defined paths between a single source and a collection of receivers and calculated 1D models for each path using a seismic waveform inversion procedure designed to match the body and surface waves of the radial, vertical and tangential components simultaneously. These 1D models were then combined to create the 3D tomogram. The results show that the mantle beneath Mexico is characterized by a radially anisotropic seismic lid, which thickens from west to east, overlying a low velocity zone. Most of the lateral variation in the models occurs in the top 200 km, generally related to tectonic regime. S anisotropy is highest (7%) in the thin lid beneath the tectonically active western region. The anisotropy beneath central and eastern Mexico is fairly uniform (4-5%) and extends deeper into the mantle (150 km depth). P anisotropy is not well resolved, but P structure mimics the SH velocity structure, suggesting that P is also anisotropic within the lid. The VP/VSH ratio (1.86) is nearly constant as a function of depth from the Moho through the transition zone.
DE: 7260 Theory and modeling
DE: 7299 General or miscellaneous
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting