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Holocene Vertical Displacement on the Central Segments of the Wasatch Fault Zone, Utah

Utah Geological Survey, 1594 West North Temple, P.O. Box 146100, Salt Lake City, Utah 84114 christopherduross{at}utah.gov

Compiled per-event vertical-displacement observations from 17 paleoseismic sites along the six central segments of the Wasatch fault zone (WFZ) highlight possible biases and trends in displacement along the fault. The displacement data are consistent with a model of characteristic-type slip, but anomalous and variable displacements indicate that significant natural variability in displacement occurs. When combined into a composite distribution of displacement, 79% of the data fit within a displacement envelope that shows displacement decreasing in a half-ellipse shape from 1.4–3.5 m near the segment centers to 0.6–2.5 m near the ends. Additionally, displacements normalized by the distance from the segment centers to ends decrease from means of 2.0–3.0 m near the segment centers to 1.3–1.9 m near the ends, consistent with characteristic-type slip termination. Although several paleoseismic sites exhibit repeated, similar displacements, the data are sparse and both low-valued (0.5–0.8 m) and high-valued (4.2–4.7 m) outliers suggest complex strain release, possibly resulting from segment interaction and/or noncharacteristic events. Although a global, normal-fault-type surface-rupture-length (SRL) average-displacement regression underpredicts observed WFZ displacements, the largest displacements per segment correspond well with a SRL maximum-displacement regression. This correlation, as well as moderate variability in SRL- and displacement-based moment magnitude, suggests that the anomalous displacements represent the intrinsic variability in characteristic displacement per segment. Thus, minor variations to the characteristic slip model to account for exceptional upper- and lower-bound displacements, e.g., a hybrid characteristic-variable slip model, may be appropriate for the WFZ. Additional paleoseismic data are necessary to address data gaps and biases, to facilitate more robust tests of earthquake-slip models, and to reduce uncertainty in SRL, displacement, and magnitude.