INVESTIGATING THE LAS VEGAS VALLEY: ANALYSES FROM THE LAS VEGAS VALLEY BROADBAND ARRAY USING DIFFERENTIAL TRAVEL TIME RESIDUALS AND INTER-STATION PHASE VELOCITIES

Las Vegas Valley, Nevada is a northwest trending valley situated in the southern portion of the Basin and Range province of western North America. It is recognized as being seismically active, containing at least eight tectonically active faults capable of producing mid-scale earthquakes within a highly populated region. In the mid 1950’s through the early 1990’s, Blume & Associates deployed an array of seismometers in Las Vegas Valley to monitor ground-motions created from nuclear testing at the Nevada Test Site. The coverage area was constrained by the city limits during this era. However, with rapid growth of the city of Las Vegas within the last decade, there is a lack of data within the city’s recently expanded northeastern region; the area estimated to be the deepest portion of the basin. To address this issue, Lawrence Livermore National Laboratory (LLNL) and the University of Nevada Las Vegas (UNLV) deployed an array of twelve broadband seismometers in the northern region of the valley in September 2002. These stations recorded data continuously through late January 2003.

To better model the depth to basement and basin geometry, differential travel time residuals were calculated through the cross-correlation of P-waves from global teleseismic events. The calculated delay times in the northern regions show variations up to 0.5 seconds over distances of 15 km or less. The residual pattern is consistent across the region and is attributed to zones of thicker basin fill. This supports earlier models of the Las Vegas basin. Additionally, a collection of regional events in close proximity to the town of Lavic, in southern California, is inline with the broadband array. The regional Rayleigh and Love waves will be used to calculate the inter-station phase velocity along a portion of the ray path and a phase velocity dispersion curve. This analysis will provide insight to the shear velocity structure of the basin. These studies will contribute to improved models of Las Vegas basin by assessing basin shape, volume, and shear velocity structure of the basin fill. Results of this study will be pertinent to assessing and modeling ground-motions in Las Vegas Valley in the event of an earthquake or future nuclear testing at the Nevada Test Site.