Paleoseismology of Utah, Volume 9

Paleoseismic investigation of the Clarkston, Junction Hills, and Wellsville faults, West Cache fault zone, Cache County, Utah

Special Study 98, 2000, 23 pages, 3 plates, $10.50

ABSTRACT

Three large, active fault zones in and adjacent to Cache Valley in northern Utah pose a significant earthquake risk: they are the Wasatch, East Cache, and West Cache fault zones. All of these fault zones show evidence of large surface-faulting earthquakes in late Quaternary time. Trenching to determine the size and timing of prehistoric surface faulting earthquakes has been done for the Wasatch and East Cache fault zones, but prior to this study no trenching studies had been done for the West Cache fault zone.

The West Cache fault zone is a series of three related east-dipping normal faults that extend 80 kilometers (50 mi) along the west side of Cache Valley from northern Utah into southern Idaho. Faults in the West Cache fault zone are, from north to south, the Clarkston, Junction Hills, and Wellsville faults. The purpose of this study is to determine the seismic-source potential of the West Cache fault zone and evaluate the earthquake hazard presented by the fault zone to Cache Valley and northern Utah.

We investigated three sites on the West Cache fault zone for this study. The investigations included interpreting aerial photographs and previous surficial-geologic mapping along the fault zone, profiling scarps and logging two trench exposures across the Clarkston and Wellsville faults, logging a natural stream-cut exposure of the Junction Hills fault, and radiocarbon dating.

Our data show the most recent surface-faulting 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. The penultimate surface-faulting earthquake (PE) on the Wellsville fault occurred between 15,000 and 25,000 years ago, whereas the PE on the Junction Hills fault occurred some time prior to 22,500 years ago. We found no evidence of the PE on the Clarkston fault in our trenching, but a difference in elevation of the highest shoreline of Lake Bonneville along the Clarkston and Junction Hills faults suggests two or three surface-faulting earthquakes occurred on the Clarkston fault in the past 16,800 years.

Three post-lake characteristic earthquakes of average vertical displacement similar to the MRE seem to best account for the elevation difference. Our paleoseismic data show a slip rate for the Wellsville fault of 0. I I - 0.21 millimeters/year (0.004 0.008 in/yr), and geologic evidence suggests a maximum long-term slip rate for this fault of 0. 1 3 millimeters/year (0.005 in/yr) in the past 100,000 years. We could not determine slip rates for the Clarkston and Junction Hills faults, but stratigraphic and geomorphic evidence suggest that the faults have long-term slip rates less than 0.68 and 0.21 millimeters/year (0.027 and 0.008 in/yr), respectively, since the late Pleistocene.

Differences in MRE timing and slip rates between the faults, as well as structural and geologic evidence, indicate that the Wellsville, Junction Hills, and Clarkston faults are separate fault segments of the West Cache fault zone, and may be earthquake segments. Estimated paleoearthquake magnitudes are Mw 6.9 - 7.4 for the Clarkston fault, Mw 6.8 - 7.3 for the Junction Hills fault, and M,, 6.6 - 7.2 for the Wellsville fault.