Seismic Structure of the Crust and Upper Mantle in the Southwestern United States Using Teleseismic Receiver Functions

This dissertation describes the development of receiver function processing methods to create images of the seismic structure of the crust and upper mantle from teleseismic earthquake arrivals. These methods include receiver function estimation and prestack migration techniques which reduce deconvolution instability and produce regularized, multi-phase receiver function images. The receiver function estimation technique developed here builds on frequency-wavenumber filtering methods by transforming receiver functions into the frequency-pseudo-wavenumber domain. A filter is then applied that downweights receiver function amplitudes that have a high degree of variability as a function of horizontal slowness (epicentral distance) while preserving receiver function phases that have consistent moveout characteristics. This technique is applied to synthetic receiver functions and shows excellent recovery of receiver function phases in the presence of high noise. A receiver function migration technique is developed that is based on regularized Kirchhoff migration. This technique migrates both direct and reverberated P-to-S converted phases to their true subsurface position to produce an image of the velocity discontinuity structure of the subsurface. This migration technique is applied to synthetic receiver functions to demonstrate that it is especially well suited to irregularly spaced stations and uneven data coverage. An important aspect of both the deconvolution and migration techniques is that they do not require the recording of a single earthquake at multiple stations, making them especially applicable to temporary arrays that may have irregular recording times between stations and allow the incorporation of previous teleseismic data. The receiver function processing methods described in this dissertation are applied to teleseismic arrivals recorded in the LA RISTRA (Colorado PLAteau, Rio Grande RIft, Great Plains Seismic TRAnsect) experiment. LA RISTRA was a NW-SE trending, 950.7 km linear network of broadband PASSCAL seismometers, deployed during 1999-2001 from Lake Powell, UT to Pecos, TX. Results from application of these methods to LA RISTRA data show much more topography at the base of the crust than has been observed in this region previously, with thickness changes up to 7 km over lateral distances of 50 km. Crustal thickness beneath the LA RISTRA network reaches a minimum of 35 km in the center of the Rio Grande rift (RGR), and ranges from 42 to 50 km in both the Great Plains (GP) and Colorado Plateau (CP). Crustal thinning beneath the RGR is relatively symmetric about the rift axis, with the thinnest crust located directly beneath the rift axis, suggesting a predominantly pure shear stretching of the lithosphere beneath the RGR. This is further supported by the rift centered region of low velocities observed in surface wave inversions and tomography results, as well as by regional isotopic data. Colorado Plateau crust is on average 2.7 km thicker than Great Plains crust, providing up to .5 km of Colorado Plateau uplift. This, along with added buoyancy from a deep, low-velocity channel imaged in surface wave analysis, may explain the excess elevation of the Colorado Plateau. Lithospheric receiver function images also indicate a prominent northwest-dipping discontinuity, ranging from 65 to 85 km deep beneath the CP, and possible sub-crustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, may indicate preserved ancient lithospheric structures such as relict suture zones or subducted slabs associated with Proterozoic subduction. Upper mantle receiver function images beneath the LA RISTRA network show a prominent discontinuity at 250-300 km depth that may correlate with similar discontinuities observed beneath eastern North America. This discontinuity may represent the base of an asthenospheric low velocity channel observed in surface wave velocity images. Upper Mantle discontinuities at 410 and 660 km depth are relatively flat, indicating there is not a large scale thermal anomaly beneath the RGR at these