Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 2-8
Presentation Time: 10:55 AM


MCDONALD, Greg N., HISCOCK, Adam I. and HYLLAND, Michael D., Utah Geological Survey, Geologic Hazards Program, 1594 W North Temple, Suite 3110, Salt Lake City, UT 84116

The Utah Geological Survey with assistance from the U.S. Geological Survey conducted a paleoseismic investigation for the two southernmost segments of the Wasatch fault zone—the Levan segment (LS) and the Fayette segment (FS)—in 2017. The two segments consist of mostly west-dipping faults along the western base of the San Pitch Mountains in central Utah. Prior to our investigation, limited paleoseismic data were available for the LS and no data existed for the FS.

We excavated two trenches across fault scarps cutting late Holocene deposits: one on the LS across a 3.5-m-high scarp and the other on the FS across a 1.5-m-high scarp. We logged both walls in each trench, collected soil samples for numerical dating, developed probabilistic OxCal models, and calculated earthquake timing and recurrence intervals.

Our investigation indicates the trenched scarps were each formed by single, Holocene-age surface-faulting earthquakes: 1500 to 1700 years ago on the LS and 5300 to 5500 years ago on the FS. Our timing data show no statistical overlap in these earthquake times, ruling out the possibility that a spill-over rupture between the LS and FS was responsible for the most recent surface-faulting earthquake on each segment.

Dating of the oldest deposit in the LS exposure indicates the penultimate earthquake occurred prior to 13,900 years ago. We calculated a broadly constrained minimum recurrence interval for the LS of 12,200 to 17,200 years based on stratigraphic ages and contacts for the oldest unit exposed in the trench, and calculated a maximum slip rate of 0.20–0.28 mm/yr using vertical offsets measured from correlatable stratigraphic units in the trench.

At the FS trench site, we documented secondary stratigraphic evidence of a penultimate surface-faulting earthquake that occurred between 9800 and 12,600 years ago. We calculated a broadly constrained earthquake recurrence for the southwestern strand of the FS of 4300 to 7300 years and a slip rate of 0.17–0.33 mm/yr.