DETAILED MAPPING OF THE WASHINGTON, HURRICANE, AND SEVIER/TOROWEAP FAULT ZONES, UTAH AND ARIZONA—USING NEW HIGH-RESOLUTION LIDAR DATA TO REDUCE EARTHQUAKE RISK  

Abstract / Description

The Utah Geological Survey and the Arizona Geological Survey mapped Quaternary-active faults in southwestern Utah and northwestern Arizona using recently collected airborne high-resolution topographic lidar data. The late Cenozoic, west-dipping Washington, Hurricane, and Sevier/Toroweap fault zones define the seismically active transition zone between the Colorado Plateau and Basin and Range physiographic provinces in southwestern Utah and northwestern Arizona. These three fault zones pose a significant earthquake hazard to the St. George metropolitan area, which is the largest metropolitan area in Utah outside of the Wasatch Front and one of the fastest growing metropolitan areas in the U.S. We also took advantage of available lidar data and re-mapped additional faults adjacent to the Hurricane fault, including the Enoch graben, Parowan Valley, and Paragonah faults. Previously, the surface location and extent of fault traces associated with these fault zones were not well understood in many areas, owing to limited aerial photography coverage, heavy vegetation near range fronts, and the difficulty in recognizing moderate (<1 m) displacements in the field or on aerial photographs. Previous geologic mapping, paleoseismic investigations, historical aerial photography, and field investigations were also used to identify and map surface fault traces and infer fault locations. For faults that are mapped in Utah, special-study areas were delineated around faults to facilitate understanding of the surface-rupturing hazard and associated risk. Defining these special-study zones encourages the creation and implementation of municipal and county geologic-hazard ordinances dealing with hazardous faults in Utah. We identified 72 potential paleoseismic investigation sites where fault scarps appear relatively pristine, are located in geologically favorable settings, and where additional earthquake timing data would be beneficial to earthquake research of the faults mapped in this study. More accurate mapping and characterization of these faults help to mitigate earthquake risk in southwestern Utah and northwestern Arizona by creating surface-fault-rupture hazard maps (in Utah), refining fault segmentation models, and developing the paleoseismic fault parameters necessary for regional earthquake-hazard assessments.