ACTIVE FAULTING AND INTRAPLATE EARTHQUAKE POTENTIAL IN THE NORTHERN WIND RIVER BASIN, WYOMING

The Wind River Basin of Wyoming contains multiple active WNW-ESE faults as indicated by fault scarps in Late Quaternary alluvium. Owing to low slip rates, surface expressions such as fault scarps can have subtle landscape expressions. Along the northern boundary of the Wind River Basin, the WNW-ESE striking Stagner Creek Fault is expressed as fault scarps in numerous Late Quaternary geomorphic surfaces, both east and west of the Boysen Reservoir. East of the reservoir, near Birdseye Creek, a single, co-linear scarp has developed in different late Quaternary alluvial surfaces. A high-resolution digital surface model of the study area was constructed using low-altitude digital photogrammetry. A notable result is identification of fault-related scarp within the Holocene-age surface – a previously undocumented result that changes the estimate of timing for the last surface-rupturing event. The 30 cm height of the scarp suggests a paleoseismic event of magnitude 6.0 – 6.2. Shallow seismic reflection profiles demonstrate the deformed nature of the Wind-River formation, including north-dipping faults below the surface scarp. Shallow seismic reflection profiles were obtained using a rolling-spread technique with geophone spacing of 2 meters, and shot spacing of 6 meters. A fixed-spread seismic refraction line was also obtained for use in creation of a seismic velocity model for the area. West of Boysen Reservoir, the fault system consists of a broader zone of multiple scarps in alluvial fan surfaces. Analysis of recently available LiDAR data allows more detailed mapping and assessment of scarp morphology. The geometry of these scarps is consistent with secondary, bending moment faulting above the hanging wall of a fault or the hinge zone an anticline. Due to the high-angle nature of this reverse faulting observed in the seismic data, its proximity to the adjacent Owl Creek Mountains, and the tectonic setting of the Wind River Basin, this faulting likely involves reactivation of an inherited Laramide-age structure.