HR: 15:25h
AN: S33E-08    [Abstracts]
TI: Imaging Seismic Attenuation in the Crust and Upper Mantle by Ambient Noise Correlation
AU: * Matzel, E
EM: matzel1@llnl.gov
AF: LLNL, 7000 East Ave, Livermore, Ca 94551, United States
AB: The cross correlation of ambient noise data yields an estimate of the Green's function (EGF) corresponding to a point source located at one station and recorded at the other. As the distance between the two stations goes to zero (the autocorrelation) this is equivalent to the zero-offset record of the "source". The energy in this "source" can be used to normalize the amplitude of the EGF computed between station pairs to get a measurement of seismic attenuation (Q). Here, we present a 2D tomogram of Q in the crust and upper mantle beneath the Ristra PASSCAL deployment in New Mexico. The Ristra experiment consisted of 54 STS2 instruments with an average interstation spacing of 18 km and was fully deployed for 18 months between 1999 to 2001. Arranged along a line stretching from Texas to Utah, it crossed the western Great Plains, the Rio Grande Rift and 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 studying crust and upper mantle seismic structure and for investigating the resolution of new techniques such as noise correlation. We calculated the EGFs using a year's worth of data for each pair of stations in the array. The raw data were instrument corrected and converted to displacement, then whitened and finally converted to single-bit signals. These were then crosscorrelated to produce the final EGFs. We inverted the resulting waveforms to create a 2D velocity profile. The data are particularly sensitive to structure at mid-crustal depths, and resolve heterogeneities into the upper mantle. The broad features of our velocity model match those of a traditional surface wave inversion of the same profile (West et al. [2004]), although the noise-correlation result has sharper features with higher velocity contrasts. In the course of the study, we observed that there is a distinct and predictable decrease in amplitude of the EGFs with respect to distance when normalized by the number of days in the stack. Key to our analysis is the assumption that enough data has been correlated that the equipartition assumption is valid. On the Ristra profile, the interstation amplitude, normalized by the source term, becomes stable after a few months of data have been stacked. Once the data were properly normalized, we were able to measure finer details related to the subsurface geology - for example the rapid loss of amplitude along paths crossing through the Rio Grande Rift. By combining all our measurements we are able to create a profile of Q beneath the array.
DE: 5144 Wave attenuation
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting