SURFACE RUPTURE, PALEOSEISMOLOGY, AND SEISMIC HAZARD ASSESSMENT OF THE HOLOCENE CALIFORNIA WASH FAULT, SOUTHERN NEVADA: IMPLICATIONS FOR RISK TO GREATER LAS VEGAS AREA

The California Wash fault (CWF) is a Quaternary fault that lies in the Lake Mead Tectonic Domain (LMTD) of southern Nevada. The young faults in this domain are relatively unstudied, but have the capability of generating M 6-7 earthquakes. Thus, they pose a significant hazard and risk to the rapidly expanding Las Vegas area population and structures. To provide data and analyses to be used to better define the required mitigation types and the Quaternary faulting history of the region, we addressed the following questions about the CWF. What are the geometry, kinematics and history of the CWF? What are the hazards along the fault? What role does the CWF play in the LMTD? We mapped two regions of the fault at 1:12,000 scale, defined Quaternary units, collected seven topographic profiles across the fault, and performed a paleoseismic study along a recent wash cut through the CWF. The number of fault strands mapped across strike ranges from 1 to 11. Most strands strike NNE and dip steeply W, but a few dip E. A comparison with previous work, leads us to suggest that multiple strands are typical of Quaternary LMTD faults. A significant geometric bend crops out near a transition from a few strands to multiple strands. Thus, this bend may be a geometric segment boundary that appears to influence surface ruptures. Topographic profiles across the strands suggest at least three relative ages of rupture on the strands. A diffusion model done on the profile from the paleoseismic study site, yielded an age of 3.056 ± 0.286 ka for the last surface rupture. Three separate paleoevents are shown by colluvial wedges in that cut. Displacements of documented single events range between ~ 0.3 m and ~ 0.6 m. These values are comparable to earthquakes of M 6.5 - 6.7. Because the CWF bounds the eastern side of a basin, such a quake would produce strong ground motions due to amplification in the basin fill. Thus, a significant seismic risk is posed by the CWF to a nearby power plant, a chemical factory, small towns, I-15, and Las Vegas. In summary, the CWF is a geometrically complex, Holocene fault that had multiple earthquakes, and that has the multiple strands, which we suggest are typical of LMTD faults.