HR: 10:20h
AN: S32B-01    [Abstracts]
TI: Attenuation Models (Qp and Qs) in Three-Dimensions of the Southern California Crust: Inferred Evidence for Dry Crust and Wet Faults
AU: * Hauksson, E
EM: hauksson@gps.caltech.edu
AF: Caltech Seismo Lab, 252-21, Pasadena, CA 91125 United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: Inst of Geophysics & Plan Physics, University of California, San Diego, La Jolla, CA 92093 United States
AB: We analyze high-fidelity waveform spectra to determine t* values for both P- and S-waves from earthquakes in southern California. We invert the t* values for three-dimensional (3D) frequency-independent Qp and Qs regional models of the crust. The models have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 30 km depth, and extend from the US-Mexico border in the south to the Coast Ranges and Sierra Nevada in the north. These models have generally low Qp and Qs (approximately 100) from 0 km down to 4 or 5 km depth, whereas the deeper layers have higher values of Qp from 500 to 900 and Qs from 600 to 1000, with a mean Qs/Qp = 1.3. In general, Qp and Qs increase rapidly with depth, consistent with crustal densities and velocities. The 3D Qp and Qs models image prominently the major tectonic structures and to a much lesser extent the thermal structure of the southern California crust. The near-surface low Qp and Qs zones coincide with major sedimentary basins such as the San Bernardino, Chino, San Gabriel Valley, Los Angeles, Ventura, Santa Maria basins, and the Salton Trough. In contrast, at shallow depths beneath the Peninsular Ranges, southern Mojave Desert and southern Sierras, we image high Qp and Qs zones, which correspond to the dense and high velocity rocks of the mountain ranges. Several clear transition zones of rapidly varying Qp and Qs coincide with major late Quaternary faults and connect regions of high and low Qp and Qs. At mid-crustal depths the Qp and Qs form imbricate stacks of slightly higher and lower Qp or Qs zones, which is consistent with reported crustal reflectivity. There is no obvious correlation with heat flow except for a small area within the Salton Trough, thus suggesting that other factors affect Q more strongly than heat flow. There are no sharp variations in Qp or Qs near the brittle-ductile transition, suggesting that the brittle-ductile transition may be a broad zone where the changes in shear or bulk rigidity are only gradual. In general, regions where Qs/Qp is larger than 1.0 suggest dry crust for most of southern California. A few limited regions of Qs/Qp less than 1.0 correspond to areas around some of the major strike-slip faults and the Salton Trough and suggest a larger reduction in the shear modulus compared to the bulk modulus or fluid saturation. The station corrections that correspond to the 3D models are related to unknown station calibrations and minor features in the site geology that are not included in the 3D model. The large positive station corrections correspond to stations located on the edges of sedimentary basins, possibly caused by multi-pathing or wave reverberations near the basin edge that are difficult to include in the smoothed 3D models. The negative corrections correspond to regions of very high Q in the near-surface, which due to the 15 km grid spacing and uneven ray coverage, may not be included in the final 3D model.
DE: 7200 SEISMOLOGY
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005