MULTIFACETED STUDY OF THE RED ROCK FAULT, SOUTHWEST MONTANA - RUPTURE AGE, SEISMIC HAZARD, AND RISK ASSESSMENT

The Red Rock Fault (RRF) in Beaverhead County, MT, is one of a system of NW-trending, range-bounding normal faults that relates to the Basin and Range-style extension as well as Yellowstone-Snake River Plane hydrothermal-magmatic extension. The RRF comprises northern (2.17 km), middle (2.00 km), and southern (4.37 km) segments which collectively separate the Tendoy Range fault-block from the Red Rock Valley graben. As part of the 2023 University of Houston summer field course, we conducted a multipronged study of the RRF at Chute Canyon to estimate its age of rupture, assess its seismic hazard and assess the shake risk to southwestern Montana. Our methods included 1) a literature review to understand context, basic geology, fault physics, and paleoseismology; 2) UAV imagery analysis and field mapping, focused on bedrock exposures, fault scarps, triangular facets, colluvial wedges, steep- and gentle-gradient alluvial fans, and stream terraces; 3) field characterization of soils in offset hanging wall fans and terraces; 4) dGPS fault scarp profiling and diffusion analysis; 5) cGPS velocity determinations from 6 GPS stations (Nevada Geodetic Laboratory GPS Network) spanning the fault; and 6) ShakeMap (USGS) to generate earthquake scenarios. The sharply-developed facets, fresh scarplets, and recent seismicity (M4.7 07/14/23) demonstrate that the fault is presently active. Soils correlated well across the fault with relatively older surfaces (T2) showing greater soil development than younger surfaces (T1). The dGPS and cGPS analyses indicate age of major rupture at 10.9-12.1 ka and slip rate average of 1.0 mm/y. Earthquake moment magnitude estimates range from M5.43 (segmented) to M6.18 (combined) and ShakeMap earthquake scenario predicts shaking intensities of VIII-III posing ground shaking, landslide and liquefaction hazards to the regional infrastructure (highways, railroads, reservoirs, and gas lines). This study provided hands-on instruction to a real-life situation that is geologically significant and societally relevant.