Imaging the Crust and Upper Mantle Beneath the Southwestern U.S. Using Noise Cross- Correlation.

Cross correlation of ambient seismic noise has shown great promise for resolving Earth structure independently of the presence of earthquake sources. Because the noise correlation function is equivalent to the waveform that would be obtained from an impulsive source located at one of the stations and recorded at the other, many of the techniques developed in earthquake seismology can be applied to the correlations themselves. Here, we present a high resolution shear wave seismic velocity profile for the crust and upper mantle beneath New Mexico derived from waveform inversion of the cross-correlation of ambient seismic noise between individual elements of the Ristra PASSCAL deployment. The Ristra array consisted of 54 STS2 instruments with an average interstation spacing of 18 km which was fully deployed for 18 months between 1999 to 2001. Arranged along a line stretching from western Texas to southeastern Utah, it crossed the western Great Plains, the Rio Grande Rift and reached into the Colorado Plateau. The dense, regular interstation spacing, and uniform instrumentation along a 950 km line makes the data from this array ideal for investigating localized crust and upper mantle seismic structure and for investigating the resolution of new techniques such as noise correlation. We calculated the cross-correlations of several days of data for each 2-station combination in the array, resulting in over 1200 usable waveforms. Each of these waveforms was inverted using a conjugate gradient method to find a 1D average shear velocity model between each station pair. These models were then combined to create a 2D profile beneath the array. The data are particularly sensitive to structure at mid- crustal depths, however, a substantial percentage of the data are sensitive to structures as deep as 100 km - allowing us to resolve heterogeneities well into the lithospheric mantle.