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SURFACE OF THE EARTH: NORTH AMERICA 2006 IRIS 5-YEAR PROPOSAL Lithospheric Structure of the Rio Grande Rift The RISTRA Group The seismic structure of the crust and upper mantle of the southwestern United States has been examined using teleseismic arrivals recorded by IRIS PASSCAL Instruments in LA RISTRA (Colorado PLAteau Rio Grande Rift-Great Plains Seismic TRAnsect). Receiver function estimation and filtering methods developed by Wilson and Aster (2005) produce receiver functions with decreased sensitivity to noise. Crustal thickness and V P /V S are estimated using both direct and reverberated P-to-S receiver function modes. Regularized receiver function migration methods produce a multiple- suppressed image of the velocity discontinuity structure of the subsurface. Crustal thickness averages 44.1 +/- 2.3 km beneath the Great Plains (GP) and 45.6 +/- 1.1 km beneath the Colorado plateau (CP). Crustal thinning beneath the RGR is broadly symmetric about the beneath the RGR (figure 2). We observe a prominent northwest-dipping discontinuity, ranging from 65-85 km deep beneath the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are consistent with preserved ancient lithospheric structures such as relict suture zones associated with Proterozoic subduction. An upper mantle discontinuity at 220-300 km depth may correlate with similar structure observed beneath eastern North America. Flat discontinuities at 410 and 660 km depth suggest that there is not a large-scale thermal anomaly beneath the RGR at these depths (Wilson et al., 2005b). Research supported by the NSF Geophysics program and Los Alamos IGPP; instruments and field support provided by IRIS PASSCAL. Figure 2. Moho modeling for specific lower-crust:upper-crust extension ratios, with the best fit being LC:UC=2.0:1. Note extensive widening of the rift structure at depth. Background colors depict S-wave variations from joint body/surface wave inversion (3.6-4.7 km/s; West et al., 2004). Both after Wilson et al. (2005a). Figure 1. The 960-km RISTRA transect across the Rio Grande rift (RGR) region. Areas of Cenozoic extension are hatched. West, M., J. Ni, W.S. Baldridge, D. Wilson, R. Aster, W. Gao, and S. Grand, crust and upper mantle shear wave structure of the southwest of the southwest United States: Implications for rifting and support for high elevation. J. Geophys. Res. 109, doi: 101029/2003JB002575, 2004. Wilson, D., Aster, R.,lter - ing, and multimode Kirchhoff migration, J. Geophys. Res., in press, 2005. Wilson, D., Aster, R., West, M., Ni, J., Grand, S., Gao, W., Baldridge, W.S., Semken, S.,Lithospheric Structure of the Rio Grande Rift, Nature, 433, doi: 10.1038/nature03297, 2005a. Wilson, D., Aster, R., Ni, J., Grand, S., West, M., Gao, W., Baldridge, W.S., Semken, S., Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau, J. Geophys. Res., in press, 2005b. 80
2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: NORTH AMERICA A Simple Approach for the Joint Tomographic Inversion of Seismic Body and Surface Waves. Michael West • University of Alaska, Fairbanks Wei Gao, Stephen Grand • University of Texas, Austin Body and surface wave tomography have complementary strengths when applied to regional-scale studies of the upper mantle. We have derived a straight-forward technique for their joint inversion which hinges on treating surface waves as horizontally-propagating rays with deep sensitivity kernels. This formulation allows surface wave measurements to be integrated directly into existing body wave tomography inversions with modest effort. To demonstrate the method, we use data from an IRIS/PASSCAL seismic array crossing 950 km of the Southwest U.S. The dense station spacing and linear array design of the RISTRA project provide a uniquely high-quality dataset suitable for both types of tomography, as well as receiver function migration. For large arrays, this method offers an improvement over the standard approach of augmenting body wave tomography with a one-dimensional model derived from surface waves. The joint inversion combines the absolute velocity of a surface wave model with the High-Resolution afforded by body waves—both qualities that are required to understand regional-scale mantle phenomena. Velocity models using body and surface wave tomographic techniques. (Top) Map of the IRIS/PASSCALsupported RISTRA deployment. Lower figures are cross sections of the Earth beneath this array. (Middle left) Image derived from traditional teleseismic body wave tomography. Color shades represent velocity perturbations relative to 1-D. This approach yields High-Resolution, but is unable to retrieve actual seismic velocities which are necessary for advanced interpretation. (Middle right) Image derived from surface wave tomography. This approach has the advantage of measuring true seismic velocities but has significantly poorer lateral resolution. True velocity contours are marked in gray. (Bottom) This image was derived using the new joint inversion technique presented in this study. It combines the benefits of both methods illustrated above. The bottom of the crust is marked in bold black. West, M., W. Gao, and S. Grand, A simple approach to the joint inversion of seismic body and surface waves applied to the Southwest U.S., Geophys. Res. Lett., 31, L15615, doi:10.1029/2004GL020373, 2004. This work was supported by National Science Foundation grants EAR 9614616, 9706094 and 9707188. 81
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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