HR: 0800h
AN: T41B-1190    [Abstracts]
TI: Upper Mantle P and S Velocity Structure Beneath Eastern Mexico Derived Through Waveform Inversion
AU: * Gao, W
EM: gao@maestro.geo.utexas.edu
AF: Dept. of Geological sciences, Jackson School of Geosciences, University of Texas at Austin, Dept. of Geological sciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Matzel, E
EM: matzel@mantle.llnl.gov
AF: Lawrence Livermore National Laboratory, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AU: Grand, S P
EM: steveg@maestro.geo.utexas.edu
AF: Dept. of Geological sciences, Jackson School of Geosciences, University of Texas at Austin, Dept. of Geological sciences, University of Texas at Austin, Austin, TX 78712 United States
AB: The majority of detailed models of seismic velocity in the transition zone have been derived from observations of triplicated P and S waves that have turning depths within the transition zone. The waveforms of these waves have been modeled largely through trial and error and thus resolution is difficult to assess. Furthermore, it is very rare that P and S studies are done with similar source-receiver geometries making P to S ratio studies uncertain. We present P and S models for the upper mantle beneath eastern Mexico derived through waveform inversion of triplicated P and S waves produced from the same earthquake. The models were derived from seismic waveform data produced by an event located at the border of Mexico and Guatemala recorded by the La Ristra passive seismic array. The La Ristra array consisted of 54 broadband seismometers arranged linearly from west Texas to southeastern Utah spanning about 950 kms. The data span from 18.5 to 26.5 degrees in distance. The orientation of the array lies approximately along a great circle from the Guatemala source making the data set ideal for investigating the upper mantle. We shifted the data in time to account for shallow crustal and upper mantle variations beneath the array using the results of a local tomography and receiver function study. The waveforms were inverted for mantle velocity from 40 to 800 km depth using a conjugate gradient algorithm. In the inversion we tried a suite of starting models with different depths of the 410 and 660 discontinuities and different gradients. The best fitting models have discontinuities of 8% for P and 8% for S at 410 km depth and 3% for P and 6% for S at 670 km depth. A common feature of the models is a low velocity zone above the 410 km discontinuity that is much more prominent in the S model than the P model. This feature may be due to partial melt induced by water release from the transition zone as proposed by Bercovici and Karato. The overall jump in velocity at 410 km is also larger than in previously published models with a lower gradient below. A second feature is an unusually high gradient beginning at about 550 km depth extending to the 660 km discontinuity. This may be a thermal anomaly due to a flat lying slab or perhaps represents a phase change in the transition zone. The data do not require a discontinuity near 520 km depth.
DE: 8180 Tomography
DE: 7260 Theory and modeling
DE: 7299 General or miscellaneous
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting