HR: 14:10h
AN: V53F-03 INVITED    [Abstracts]
TI: Regional Tectonics of the Coso Geothermal Area: Three-dimensional Vp and Vp/Vs Models, Spatial-Temporal Seismicity Patterns, and Seismogenic Deformation Along the Intracontinental Plate Boundary in Central Eastern California
AU: * Hauksson, E
EM: hauksson@gps.caltech.edu
AF: California Institute of Technology, Seismological Laboratory, Pasadena, CA 91125, United States
AU: Unruh, J
EM: unruh@lettis.com
AF: William Lettis & Associates, 1777 Botelho Dr., Suite 262, Walnut Creek, CA 94596, United States
AB: We use regional earthquake data to synthesize the tectonics of the southern Walker Lane belt and Coso Range in central eastern California. We calculated three-dimensional models of the Vp and Vp/Vs structure of the upper and middle crust. Using these models, we also determine three-dimensional Vs and Poisson's ratio models. The changes in seismic velocities across the region are small, except for low velocities in sedimentary basins and a ~2-km positive elevation of the basement velocities (Vp > 6 km/s) beneath the southern Sierra Nevada. Localized low-Vp and low-Vs zones beneath the central Coso Range image a geothermal reservoir at 0- to 3-km depth, as well as distinct low-velocity anomalies in the depth range of ~8 to ~12 km. Because the Vp/Vs has average crustal values within this broader zone, we interpret the anomaly to indicate a zone of few percent geothermal brines extending from 8 to 12 km depth. In addition, an embedded highly localized poorly resolved zone (possibly as small as 1 km³) of slightly above average Vp/Vs and higher Poisson's ratio is a tentative suggestion of a small volume percent of magma present at depth of ~10 km. We also relocated the seismicity in the Coso region using absolute traveltimes and differential traveltimes determined from waveform cross correlation. The relocated seismicity forms several spatially clustered lineaments along the southeast side of the Sierra Nevada and in the Indian Wells Valley and vicinity of the Coso geothermal field, which coincide with mapped late Quaternary faults in the region. The base of seismicity shallows from a regional depth of about ~11 to ~5 km beneath the central Coso Range, which we interpret as evidence for shallowing of the brittle-ductile transition zone beneath the geothermal field. In addition to abundant background seismicity, two large earthquake swarms, located 5 to 8 km to the west of Coso, occurred in April to May 1992 and May to June 2001. Two dual main shock-aftershock sequences also occurred as follows: the 1994 sequence near Ridgecrest and the later Coso earthquake sequence from late 1996 to early 1998, with the pairs of main shocks spaced 47 days and 16 months apart, respectively. Kinematic analysis of the focal mechanisms indicates that the crustal stress loading process varies across the region. The low-Vp anomaly, abundant seismicity, and crustal thinning provide quantitative evidence for the Coso region being an extensional releasing step over between two northwest-striking dextral faults: The Airport Lake fault zone to the south, and the Owens Valley fault to the north.
DE: 7203 Body waves
DE: 7215 Earthquake source observations (1240)
DE: 7270 Tomography (6982, 8180)
DE: 8419 Volcano monitoring (7280)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting