IMAGING THE LAS VEGAS BASIN: RESULTS FROM SEISMIC REFRACTIONS EXPERIMENTS

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 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 State Line and Furnace Creek fault zones, which have the potential for generating large magnitude earthquakes that could pose a significant seismic threat to the Valley. A series of seismic refraction experiments (2002 to 2003) were undertaken 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). The 3D velocity distribution 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 indicates that the portions of the shallow sub-surface where there is a significant amount of clay deposits is the area expected to experience the most severe ground shaking where the deeper Tertiary portion of the basin has less amplification potential.