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Paleoseismic Investigation of the Levan and Fayette Segments of the Wasatch Fault Zone, Central Utah

Session: How Should Low-Probability Earthquakes be Considered in Hazard Assessments?

Type: Oral

Date: 4/23/2021

Presentation Time: 09:45 AM Pacific

Description: 

The eastern margin of the extensional Basin and Range Province is defined by the Wasatch fault zone (WFZ). The Levan segment (LS) and Fayette segment (FS) in central Utah are the WFZ’s southernmost segments and have received little paleoseismic investigation. A large discrepancy between regional geologic and geodetic slip rates could implicate evaporite-related tectonics at depth as a contributor to displacement on the LS and FS. We excavated paleoseismic trenches on the LS and FS to determine earthquake timing and to investigate if any fault displacement may be attributed to salt tectonics. A trench on the LS across a 3-m-high scarp showed evidence for a single Holocene surface-faulting earthquake. The exposure revealed ~2.6 m of vertical offset and a complex 4.4-m-wide zone of tilted, overturned, and sheared blocks of alluvial-fan strata indicating oblique displacement. A distinct soil buried by scarp colluvium on the hanging wall provides good evidence for a single surface-faulting earthquake. Numerical dating results and OxCal modeling constrain the earthquake time to 1.6 ± 0.1 ka (2σ), and stratigraphic data indicate a penultimate earthquake (PE) occurred sometime prior to 14.73 + 2.5 ka (2σ). A trench on the FS across a 1-m-high scarp revealed evidence for two Holocene surface-faulting earthquakes. Numerical dating results and OxCal modeling constrain the most recent earthquake time to 5.4 ± 0.1 ka (2σ), and stratigraphic evidence in the lower part of the trench indicated a PE time of 11.2 ± 1.4 ka (2σ). From these data and stratigraphic offsets, we determined earthquake recurrence intervals of 12.2–17.2 kyr (minimum) for the LS and 4.3–7.3 kyr for the FS, and vertical slip rates of 0.20–0.28 mm/yr (maximum) for the LS and 0.17–0.33 mm/yr for the FS. Both trenches showed faulting consistent with discrete, rapid, meter-scale displacements and long earthquake recurrence intervals that favor a seismogenic origin, rather than evaporite-related deformation.

Presenting Author: Adam I. Hiscock

Student Presenter: No

Authors