The Frenchman Mountain fault is an 18-km long, north- and northeast-striking, west-dipping, range-bounding normal fault on the eastern side of the Las Vegas metropolitan area. The fault is expressed along the western flank of Frenchman and Sunrise mountains as a zone of sub-parallel scarps in alluvial fan surfaces, bedrock range front scarps, and shear planes juxtaposing Quaternary sediments against bedrock. The fault is assigned a geologic vertical slip rate of 0.015 mm/yr in the 2014 USGS Hazard Map. Recent geodetic studies across Las Vegas Valley suggest potential for a 0.2 mm/yr normal slip rate. Previous studies of the fault have documented evidence for late-Quaternary paleoearthquakes, but did not include dating of deposits, recurrence estimates or slip rates.

A previously excavated exposure of the fault in Quaternary fan deposits was recently investigated in much greater detail. The excavation was originally part of an unpublished fault investigation conducted by a local consulting company for a planned housing development. The fault exposed in the excavation is buried by unfaulted, alluvial fan deposits, and is on strike with 2-8 m high Quaternary fault scarps to the north and south. Our new logging of the excavation documents evidence for three paleoearthquakes on an 85° west-dipping fault zone. Evidence for these events includes offset stratigraphy and three scarp derived colluvial wedges. Optically stimulated luminesce (OSL) samples were collected from all colluvial wedge packages, the faulted stratigraphy beneath the wedge, and the overlying unfaulted fan gravels; results of the OSL dating are pending, although preliminary data suggests all events are older than Holocene. Quaternary units beneath the colluvial wedge packages have a total vertical displacement of 1.6 m across the fault zone. The oldest recorded faulting event (E3) has a vertical displacement of 0.8 m. The two youngest events (E1 and E2) have a combined vertical displacement of 0.8 m.