GROUND-RUPTURING EARTHQUAKES ON THE MEAD SLOPE FAULT, ARIZONA

The Mead Slope fault (MSF) is one of a number of fault zones in the Las Vegas area that show evidence of late Quaternary activity. The close proximity of the MSF to Hoover Dam is concerning as it impounds the largest reservoir by volume in the United States and supplies water to millions of people in Arizona, Nevada, and Southern California. The MSF has long been recognized as an active fault system, but because of its inaccessibility, a detailed study had not been completed. During our study of the MSF we generated high-resolution DEMs of the fault zone using drone imagery we collected from multiple low-altitude drone surveys. These DEMs provided a base for detailed mapping of the MSF, as well as a base for more precise offset measurements of offset channels, debris lobes, and alluvial fans. We mapped the MSF as two main strands with multiple orthogonal connecting faults. The most recently active strand is a high-angle, predominantly left-lateral, SW-striking fault that eventually steps westward, where its location is covered by Lake Mead. The connecting faults are the result of wrenching between this fault and a parallel fault that cuts cut Quaternary landforms but has a more subtle geomorphic expression. The Mead Slope fault displaces late Quaternary alluvial fans and terraces, perhaps younger than 20,000 yrs old based on surface characteristics and soil age estimates. Due to the multiple incursions of the Lake Mead shoreline, these relatively younger faulted sediments appear to be the only location in the area where faulting of younger deposits has been preserved. Mapping of landforms and offsets measurements include estimates of left-lateral offset of over 100 m, and vertical offset of over 24 m since 0.5-1.0 Ma. OSL samples were collected from the younger faulted sediments, and Be10 cosmogenic samples were collected from older landforms in hopes of getting better age constraints on the amount of offset over time, with results pending. Miocene-Pliocene Bullhead Alluvium, Bouse Formation, and pre-Bouse alluvium thought to be equivalent to the Muddy Creek Formation were discovered in the mapping area and are progressively deformed along MSF down section, suggestive of long-lived MSF activity, from Miocene to present.