Large-Scale Earthquake-Hazard Class Mapping by Parcel in Las Vegas Valley, Nevada

Clark County and the city of Henderson, Nevada, completed the United States' first effort to map earthquake site classifications with systematic, direct measurements through an entire urban area. Urban development, disaster response planning, and especially building code implementation and enforcement motivated map development. The municipalities contracted the Nevada System of Higher Education to classify similar to 1500 km(2) including urban Las Vegas Valley and exurban areas of future development. The resulting Parcel Map includes over 10,000 surface-wave array measurements accomplished within three years using Optim's SeisOpt ReMi refraction microtremor measurement and processing technology adapted for large-scale data collection. The noisy urban setting necessitated use of microtremor as the seismic source. With a nominal measurement spacing of 300 m or less, the Parcel Map classifies every parcel on the National Earthquake Hazards Reduction Program V-S30 site-class scale. The most important revelation of the Parcel Map was that 84% of the mapped region was found to be stiffer than the default site class D category previously imposed on the whole of the Las Vegas Valley. Details of the mapping also revealed the high variability of V-S30 over short distances (< 200 m) in many areas. We describe a class C+ for sites with class B V-S30 velocities but soft surface soil more than 3 m thick. Correlation between the occurrence of class C+ sites and topographic slope varied spatially from north to south along the western margin of Las Vegas Valley. Of particular significance was the delineation of an alluvial fan surface that was not topographically expressed in the southwest part of the valley. Given the density of velocity measurements required for code enforcement, ReMi proved to cost-effectively produce the desired Parcel Map within the three-year contract period.