SEISMIC HAZARD AND PALEOSEISMICITY IN SOUTHERN NEVADA: THE CALIFORNIA WASH FAULT

Young deformation and seismic hazards are suggested by more than 20 Quaternary fault scarps; 1000’s of earthquakes, mostly less than M4.0; and relatively small geodetic motions between the Southern Nevada Transverse Seismic Zone/Belt and the Las Vegas-Lake Mead area in the Central Basin and Range; however, reconciling these various data set is, in part, precluded by a lack of field and paleoseismic data on most of the Quaternary surface-rupturing faults. To address this data deficit, we investigated a prominent fault, the California Wash Fault (CWF). The CWF is a NNE-striking, W-dipping normal fault zone with a surface rupture length of 29 km that lies about 22 km from the Las Vegas metropolitan area. The seismic hazard posed by the CWF, and the well-exposed fault and scarp were the basis for selecting the CWF.

Combined, our geologic map data, scarp profiles, trench logs including retrodeformation, ¹⁴C dates, and geomorphic and seismic hazard analysis establish that the CWF is an active normal fault zone with significant paleoseismicity. Map data show that the CWF offsets Quaternary deposits and reveal an overstep where the fault zone comprises overlapping right-stepping en echelon faults situated 5 km from the southern fault tip. Five topographic profiles show scarp heights up to 0.6 m high in young alluvium and up to 6 m in older alluvium. The latter have topographic bevels suggesting that the scarps formed in at least 2-3 surface-rupturing events, which is consistent with fault log data. A log of a natural dissection across the CWF shows nine fault strands with normal offsets including at least two that are reactivated reverse faults and two colluvial wedges. The two colluvial wedges yielded ¹⁴C dates of 30 to 220 Cal yr CE (1920-1730 Cal yr BP) and 80 Cal yr BCE to 80 Cal yr CE (2030-1870 Cal yr BP) representing two closely spaced events. Geomorphic analysis based on the mapped distribution and relative ages of a series of alluvial units supports a tectonic control on deposition. Calculations based on surface rupture length and average displacement suggest an average earthquake of Mw 6.7, which is large enough to pose a seismic hazard to the Las Vegas metropolitan area. These new data and analyses on the CWF help to fill the data gap in southern Nevada, and suggest that Quaternary faulting is of significant magnitude and remarkably recent.