A Broad, Distributed Active Fault Zone Lies beneath Salt Lake City, Utah

Abstract

Although the Wasatch fault is currently known to have a high-seismic hazard from motion along range-bounding faults, new seismic data reveal faulted and folded 13,000–30,000-yr-old Lake Bonneville strata beneath Salt Lake City (SLC). Coupled with previous excavation trench, borehole, and other geologic and geophysical observations, we conclude that a zone of latest Pleistocene and/or Holocene faulting and folding kinematically links the East Bench and Warm Springs faults through a 3 km wide relay structure and transfer zone. We characterize faults beneath downtown SLC as active, and these faults may displace or deform the ground surface during an earthquake. Through offset but linked faults, our observations support throughgoing ruptures across faults of the Wasatch fault zone (WFZ) and an elevated risk of earthquake-induced building damage.

Fault Zone Imaging With Distributed Acoustic Sensing: Body‐To‐Surface Wave Scattering

“…Seismic surveying produces detailed images of the subsurface, with which fault locations can be inferred (e.g. Liberty et al, 2021;Lay et al, 2021), but surveys are often expensive and logistically challenging, particularly in urban settings. Satellite imagery is also used to map faults, often by identifying topographic anomalies in images (Joyce et al, 2009).…”

“…Geologic mapping over decades has produced excellent records of Quaternary faults (e.g., USGS and CGS, 2022), but discerning faults using geologic mapping requires careful fieldwork and evidence of faulting at the surface. Seismic surveying produces detailed images of the subsurface, with which fault locations can be inferred (e.g., Lay et al., 2021; Liberty et al., 2021), but surveys are often expensive and logistically challenging, particularly in urban settings. Satellite imagery is also used to map faults, often by identifying topographic anomalies in images (Joyce et al., 2009).…”

3D Shear Wave Velocity Model of Salt Lake Valley via Rayleigh Wave Ellipticity across a Temporary Geophone Array

“…The SLCS consists of three faults: the Warm Springs fault (WSF), East Bench fault (EBF), and Cottonwood fault (CF). The WSF and CF separate the Salt Lake basin from the Wasatch range to the east with prominent topographic scarps, whereas the EBF appears as a prominent intrabasin fault scarp with little known about fault geometry at depth and its association with other faults (Liberty, Clair, et al, 2021). The WVFZ and SLCS cut through the most densely populated area in Utah, and paleoseismic records reveal evidence of recurrent large magnitude earthquakes generated by both systems (DuRoss and Hylland, 2015).…”

“…Since its establishment, the model has gone through several revisions, in which near-surface constraints, such as those derived from spatial autocorrelation (SPAC) microtremor arrays (Stephenson and Odum, 2010), geotechnical borehole logs, and other near-surface site response measurements were included in the model construction. A recent seismic study (Liberty, Clair, et al, 2021) using a land streamer has revealed shallow fluid and soil properties and identified faults and folds across Salt Lake City, though the results have not been incorporated into the CVM yet. Because of the limitations of both the seismic and borehole data, deeper CVM basin structures are mostly only constrained solely by earlier gravity measurements.…”