LIDAR-BASED OBSERVATIONS OF FAULTS IN WESTERN WYOMING AND IMPLICATIONS FOR SEISMIC SOURCE CHARACTERIZATION

We compiled, processed, and interpreted 80,000 km² of publicly available airborne lidar data from Wyoming, Idaho, and Utah to evaluate evidence for and against Quaternary-active faulting and to improve the characterizations of fault sources used as input to regional seismic hazard analyses. Using the high-resolution lidar topographic data, we mapped fault scarps in Quaternary deposits and bedrock to better define the extents of previously mapped faults in the region. We produced hundreds of fault-normal topographic profiles to estimate fault vertical separations, and from these we developed fault-specific, along-strike slip distributions to identify possible rupture scenarios. Fault-parallel profiles were used to evaluate potential lateral components of slip on some faults. Our lidar-based mapping and geologic field reconnaissance observations indicate the following: (1) the Quaternary-active Rock Creek and Greys River faults are significantly longer than depicted in the U.S. Geological Survey’s Quaternary Fault and Fold Database (QFFD) and National Seismic Hazard Maps (NSHM); (2) the Rock Creek, Greys River, and Bear River faults each show evidence for clustered earthquake activity, with high rates of activity since the latest Pleistocene that were preceded by longer periods of relative quiescence, consistent with McCalpin (1993) and Hecker et al. (2021); (3) the Hogsback fault does not appear to be an active fault, despite its inclusion in the QFFD and NSHM; and (4) the enigmatic 1882 M ~6.6 “Colorado” earthquake may have ruptured faults in the Saratoga Valley of south-central Wyoming, which exhibit fresh-looking scarps in latest Holocene deposits and that show evidence for recurrent faulting.