Abstract

The West Cache fault zone is a series of related east-dipping normal faults extending 80 kilometers along the west side of Cache Valley from northern Utah into southern Idaho, and includes (from north to south) the Clarkston, Junctions Hills, and Wellsville faults. The West Cache fault zone and nearby Wasatch and East Cache fault zones pose a significant seismic hazard to citizens living in Cache Valley and adjacent areas of northern Utah and southern Idaho. All of these fault zones vertically displace the surface and show evidence of large (>M 6) earthquakes in late Quaternary time. Trenching studies were performed on the Wasatch and East Cache fault zones, but no such studies existed for the West Cache fault zone prior to 1997.

To evaluate the earthquake history of the West Cache fault zone, we mapped Quaternary geology along the fault zone in 1996 and investigated three sites along the Clarkston, Junction Hills, and Wellsville faults in 1997 where displaced surficial sediments show evidence of past surface-faulting earthquakes. Our data indicate the most recent earthquake (MRE) on the faults occurred 3,600 to 4,000 years ago on the Clarkston fault, 8,250 to 8,650 years ago on the Junction Hills fault, and 4,400 to 4,800 years ago on the Wellsville fault. A penultimate surface-faulting earthquake (PE) on the Wellsville fault occurred between 15,000 and 25,000 years ago, and on the Junction Hills fault before 22,500 years ago. We found no evidence for a PE on the Clarkston fault, but a difference in Lake Bonneville shoreline elevations between the Clarkston and Junction Hills faults indicates two or three surface-faulting earthquakes on the Clarkston fault in the past 16,800 years. Timing of the MRE and PE on the Wellsville fault and average displacement from scarp profiling indicate a slip rate of 0.11-0.22 millimeters/year for this fault. Slip rates for the Clarkston and Junction Hills faults are uncertain, but stratigraphic and geomorphic relations indicate they are less than 0.68 and 0.21 millimeters/year, respectively. Differences in MRE timing and slip rates between the faults, as well as surficial-geologic evidence, indicate the Wellsville, Junction Hills, and Clarkston faults are independent segments of the West Cache fault zone. Estimated maximum paleoearthquake moment magnitudes based on surface-rupture length, average displacement, and slip rate are M_w 6.9-7.4 for the Clarkston fault, M_w 6.8-7.3 for the Junction Hills fault, and M_w 6.6-7.2 for the Wellsville fault.