Abstract

Recently completed surficial-geologic mapping and analyses of morphometric fault-scarp data from 52 field-measured profiles provide insight into the spatial and temporal patterns of surface faulting on the Levan and Fayette segments of the Wasatch fault zone. With the exception of the most recent surface-faulting event (MRE) on the Levan segment (~1 ka), the timing of paleoearthquakes on these segments is not well constrained. Nonetheless, different parts of the segments have apparently ruptured at different times. Levan-segment scarps provide evidence for at least one, and possibly two, partial-segment ruptures in the Holocene—latest Pleistocene. Fayette-segment scarps, or the lack thereof, provide evidence that MRE timing is different for the northern (early or middle Pleistocene?), southeastern (latest Pleistocene), and southwestern (Holocene) strands of the segment. The presence of two-event scarps on the southern part of the Levan segment and single-event scarps on deposits of similar age on the northern part of the segment could be the result of either a relatively short partial-segment rupture or a longer rupture through the Levan-Fayette segment boundary involving adjacent parts of both segments (rupture spillover). The rupture-spillover scenario is supported by the spatial pattern of vertical slip along the Levan and Fayette segments and empirical relations between surface rupture length and vertical displacement. Finally, long-term geologic (average) vertical slip rates determined for different parts of the segments locally vary by an order of magnitude; this variability is also consistent with rupture spillover, but could be related to a component of aseismic deformation. Collectively, the data and results of this study suggest a weak rupture boundary between the Levan and Fayette segments.