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Session: Advances in Intraplate Earthquake Geology [Poster]

Type: Poster

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

[Withdrawn] Paleoseismic Investigation of the Levan and Fayette Segments of the Wasatch Fault Zone, Central Utah

The east margin of the Basin and Range extensional province is defined by the Wasatch fault zone (WFZ), whose central segments have been trenched for decades. The Levan segment (LS) and Fayette segment (FS) in central Utah are the southern segments of the WFZ and have little paleoseismic data available. Presence of evaporites at depth and a large discrepancy between regional geologic and geodetic rates could implicate salt tectonics as a contributor to displacement on the LS and FS. The Utah Geological Survey in cooperation with the U.S. Geological Survey excavated paleoseismic trenches on the LS and FS to determine Holocene earthquake timing and 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 1 – 2 Holocene surface-faulting earthquake(s). A trench on the FS across a 1-m high scarp revealed evidence for a single post-Lake Bonneville surface-faulting earthquake. The LS trench showed ~2.6-m of vertical offset and a complex 4.4-m wide zone of tilted, overturned and sheared blocks of alluvial-fan strata suggesting strike slip and horizontal extension. A soil covered by scarp colluvium on the hanging wall provides evidence for the surface-faulting earthquake that created the deformation zone. Scarp-derived colluvium overlying soil developed on older colluvial wedge suggests a second, smaller displacement event. Well-defined stratigraphic and fault relationships exposed in the trench across the FS scarp indicate a single surface-faulting earthquake with ~1 m of vertical displacement. Preliminary dating results for the FS trench site indicate it ruptured at 5.4 ± 0.1 ka (2s). A single Holocene surface-faulting earthquake on the FS is consistent with an expected lower slip rate and longer recurrence interval at the southern end of the WFZ. The deformation exposed in both trenches is consistent with discrete, rapid, meter-scale displacements and indicative of seismogenic earthquakes.

Presenting Author: Greg N. McDonald

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