HR: 17:30h
AN: S34A-07    [Abstracts]
TI: Imaging the Las Vegas Basin: Results From Recent Seismic Refractions Experiments
AU: * Snelson, C M
EM: csnelson@unlv.nevada.edu
AF: Geoscience Department, University of Nevada Las Vegas, 4505 Maryland Parkway, MS 4010, Las Vegas, NV 89154-4010 United States
AU: McEwan, D J
EM: mcewand@unlv.nevada.edu
AF: Geoscience Department, University of Nevada Las Vegas, 4505 Maryland Parkway, MS 4010, Las Vegas, NV 89154-4010 United States
AU: Hirsch, A C
EM: hirscha2@unlv.nevada.edu
AF: Geoscience Department, University of Nevada Las Vegas, 4505 Maryland Parkway, MS 4010, Las Vegas, NV 89154-4010 United States
AU: Zaragoza, S A
EM: szargo@physics.unlv.edu
AF: Geoscience Department, University of Nevada Las Vegas, 4505 Maryland Parkway, MS 4010, Las Vegas, NV 89154-4010 United States
AB: The Las Vegas Valley sits atop a deep basin that has been shown to amplify energy from strong ground motions. As a result, a series of seismic refraction experiments have been conducted in order to better characterize the Las Vegas basin for seismic hazards and test site readiness. The basin is located within the central Basin and Range, and is characterized by local strike-slip fault zones (inactive) and a series of normal faults (active). Several of these normal faults within the Valley have been identified as potential sources of future seismic activity with the potential are capable of producing M 6 to 7 earthquakes within the highly populated Valley. In addition, within a 150-km radius of the Valley are several regional strike-slip fault zones, including the Furnace Creek fault zone, that have the potential for generating large magnitude earthquakes that could pose a significant seismic threat to the Valley. Three seismic refraction experiments have taken place over the last two years to image the geometry of the basin to better understand potential focusing effects as well as determine the depth and lithology of the basin. These projects are part of a larger collaborative study called the Las Vegas Valley Seismic Response Project (LVVSRP), which is presented in more detail by Rodgers et al. and Louie et al. (this meeting). In May 2002, the Quarry blast experiment used 434 vertical component seismic instruments to record three quarry blasts. These data were of limited use, because of the amount of cultural noise within the city. In September 2002, the Watusi experiment used 400 vertical component seismic instruments to record a chemical blast at the Nevada Test Site along the corridor of the Las Vegas Valley Shear zone (LVVSZ). The LVVSZ is a local structure that has been suspected to focus energy into the basin (see Zaragoza et al., this meeting). These data have illuminated more detail of the deeper crustal structure than has been imaged in the past. In August 2003, the SILVVER (Seismic Investigations of the Las Vegas Valley: Evaluating Risks) experiment commenced using 800 vertical and 25 three-component seismic instruments to record 9 chemical blasts within the Las Vegas Valley. This project was designed to obtain a 3D image of the basin as well as obtain the depth of the basin. Station spacing was nominally 100 m and shot point spacing was nominally 10 km. Shots ranged in size from 50 to 1000 lb. The 3D velocity shows a larger sub-basin within the main basin, indicating a change from the unconsolidated sediments to more consolidated materials. The velocities range from 2.5 to 4.5 km/s within the basin. The 4.5 km/s contour indicates the base of the basin where velocities increase to 6 km/s to the base of the model (9 km depth). Several zones of high velocity correlate to faults that have been mapped at the surface. The model shows that the deepest portion of the Valley is located to the northeast as previously estimated. Integration with the geologic and geotechnical results indicate that not only does the basin thickness effect amplification, but also the shallow sub-surface where there is a significant amount of clay deposits (see Taylor et al., this meeting). These results will be integrated with a 3D community model developed by the LVVSRP to be used for simulating ground motions in the Valley for both test site readiness as well as earthquakes.
UR: http://geoscience.unlv.edu/pub/snelson/LVSRP/
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting