THE LAS VEGAS VALLEY SEISMIC RESPONSE PROJECT

We are engaged in a multidisciplinary, multi-institutional effort to characterize the seismic response of the Las Vegas Basin (LVB) to seismic ground motion. Las Vegas lies above a deep sedimentary basin (maximum depth 5-km) formed by extensional tectonics in the Basin and Range Province. The potential for large earthquakes in the region and possible future nuclear tests at the Nevada Test Site (NTS) expose Las Vegas to seismic risk. Seismologists, computer scientists, engineers at Lawrence Livermore National Laboratory (LLNL), the University of Nevada Reno (UNR) and the University of Nevada Las Vegas (UNLV) are evaluating the response of the basin to seismic ground motion and its effect on structures, with particular emphasis on large structures.

Recordings in Las Vegas of earthquakes and historical nuclear explosions at the Nevada Test Site (NTS) have provided valuable constraints on ground motion in the basin. Site response results show that ground motions in the basin can be amplified by factors of 20 or more relative to sites on Las Vegas Valley’s periphery. Site response is strongly correlated with basin depth. Additional broadband data have been recorded to augment the coverage of earlier network data. Ground motions are being synthesized using an empirical convolution (transfer function) methodology. Geotechnical investigations of specific sites using ReMi and SASW are being performed to understand the relationship between ground motions and shallow seismic structure. In particular the presence of thick deposits of unconsolidated alluvial fill and strengthening effects of carbonate cementation. Seismic refraction studies are refining the basin depth model reported by Langenheim et al. (2001) based on gravity and seismic reflection. Preliminary results provide constraints on the fill velocities and are consistent with depth-to-basement model. All these results are being integrated into a community geophysical model of the region for use in finite-difference wave propagation calculations. Observed and simulated ground motions from scenario nuclear tests and earthquakes are being used to predict the response of structures in Las Vegas.

This work performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. UCRL-JC-153010.