LAS VEGAS VALLEY SEISMIC RESPONSE PROJECT (LVVSRP): INITIAL RESULTS FROM SEISMIC REFRACTION EXPERIMENTS

Las Vegas Valley, Nevada is located in the southern Basin and Range Province. Extension along the Las Vegas Valley Shear Zone, a right-lateral strike-slip fault, has produced sub-basins as deep as 5 km that contain Cenozoic to recent sediments. Eight tectonically active faults carry the potential to produce a M6.5-7.0 in the Valley. In an attempt to assess seismic hazards, we undertook two active source refraction experiments.

In May 2002, we deployed 434 single-channel seismic recorders in two northeast and east-northeast trending lines across the Valley. Shot point locations were placed at three quarries in the valley, one to the north, one to the east and one to the southwest. The profiles cross the Las Vegas Valley Shear zone as well as a prominent NW/SE trending step in the basin floor across which the basement drops from 2 to 4 km. Station spacing was 500 m in urbanized areas and 300 m outside the city. First arrivals were clearest closest to the source, but the data displayed significant cultural noise from the Las Vegas metropolitan area. Two preliminary P-wave velocity models revealed an upper sub-basin with an average velocity of 3.5 km/s and a lower sub-basin with an average velocity of 4.5 km/s, consistent with a previous gravity model of the basin.

To examine seismic hazards from energy sources outside the basin, in September 2002 we utilized 400 seismic instruments to record the Watusi chemical blast at the Nevada Test Site. The profile extended from Ann Road and I-95 northwest to the town of Indian Springs with 150 m station spacing. The blast was expected to have the energy equivalent to 40,000 tons of TNT, but most of the energy was lost into the air, and falling debris produced diffractions in the data. We picked first arrivals and analyzed these data with forward modeling utilizing MacRay to produce a 2-D velocity model, comparing this with the quarry blast models. This third model shows mid to lower crustal arrivals with an average apparent velocity of 6 km/s.

To further study the basin and its velocity structure, we are acquiring additional seismic refraction data in August 2003. These new data will be used to produce a 3-D velocity model of the entire basin and will be integrated into a community model, which is being produced by the Las Vegas Basin Seismic Response working group to further assess the site response of the basin.