3D Ground‐Motion Simulations of M w 7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault Zone: Variability of Long‐Period ( T ≥1 s) Ground Motions and Sensitivity to Kinematic Rupture Parameters

How structural segment boundaries modulate earthquake behavior is an important scientific and societal question, especially for the Wasatch fault zone (WFZ) where urban areas lie along multiple fault segments. The extent to which segment boundaries arrest ruptures, host moderate magnitude earthquakes, or transmit ruptures to adjacent fault segments is critical for understanding seismic hazard. To help address this outstanding issue, we conducted a paleoseismic investigation at the Traverse Ridge paleoseismic site (TR site) along the ∼7-km-long Fort Canyon segment boundary, which links the Provo (59 km) and Salt Lake City (40 km) segments of the WFZ. At the TR site, we logged two trenches which were cut across sub-parallel traces of the fault, separated by ∼175 m. Evidence from these exposures leads us to infer that at least 3 to 4 earthquakes have ruptured across the segment boundary in the Holocene. Radiocarbon dating of soil material developed below and above fault scarp colluvial packages and within a filled fissure constrains the age of the events. The most recent event ruptured the southern fault trace between 0.2 and 0.4 ka, the penultimate event ruptured the northern fault trace between 0.6 and 3.4 ka, and two prior events occurred between 1.4 and 6.2 ka (on the southern fault trace) and 7.2 and 8.1 ka (northern fault trace). Colluvial wedge heights of these events ranged from 0.7 to 1.2 m, indicating the segment boundary experiences surface ruptures with more than 1 m of vertical displacement. Given these estimates, we infer that these events were greater than Mw 6.7, with rupture extending across the entire segment boundary and portions of one or both adjacent fault segments. The Holocene recurrence of events at the TR site is lower than the closest paleoseismic sites at the adjacent fault segment endpoints. The contrasts in recurrence rates observed within 15 km of the Fort Canyon fault segment boundary may be explained conceptually by a leaky segment boundary model which permits spillover events, ruptures centered on the segment boundary, and segmented ruptures. The TR site demonstrates the utility of paleoseismology within segment boundaries which, through corroboration of displacement data, can demonstrate rupture connectivity between fault segments and test the validity of rupture models.

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Surficial geologic map of the Salt Lake City segment and parts of adjacent segments of the Wasatch fault zone, Davis, Salt Lake, and Utah Counties, Utah

10.34191/m-243dm ◽

2009 ◽

Keyword(s):

Fault Zone ◽

Salt Lake ◽

Salt Lake City ◽

Geologic Map ◽

Lake City ◽

Adjacent Segments ◽

Wasatch Fault

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Wasatch fault zone-Salt Lake City aqueduct system, City Creek Canyon to Provo River, Salt Lake and Utah Counties, Utah

10.34191/m-27 ◽

1969 ◽

Keyword(s):

Fault Zone ◽

Salt Lake ◽

Salt Lake City ◽

Provo River ◽

Lake City ◽

Wasatch Fault

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The 18 March 2020 M 5.7 Magna, Utah, Earthquake: Strong-Motion Data and Implications for Seismic Hazard in the Salt Lake Valley

Seismological Research Letters ◽

10.1785/0220200323 ◽

2021 ◽

Author(s):

Ivan Wong ◽

Qimin Wu ◽

James C. Pechmann

Keyword(s):

Seismic Hazard ◽

Fault Zone ◽

Salt Lake ◽

Salt Lake City ◽

Strong Motion ◽

Ground Shaking ◽

Fault Segment ◽

Salt Lake Valley ◽

Lake City ◽

Wasatch Fault

Abstract The 2020 oblique normal-faulting M 5.7 Magna mainshock has provided the best dataset of recorded strong ground motions for an earthquake within the Wasatch Front region, Utah, and the larger Basin and Range Province. We performed a preliminary evaluation of the strong motion and broadband data from this earthquake and compared the data with the Next Generation Attenuation - West2 Project (NGA-West2) ground-motion models (GMMs). The highest horizontal peak ground acceleration (PGA) recorded was 0.43g (geometric mean of the two horizontal components) at a station located above the rupture plane at a rupture distance of 8 km. Eleven stations recorded PGAs >0.20g. Most of these stations are located on the deep sedimentary deposits within the Salt Lake Valley, and all are at rupture distances <20 km. The data compare favorably with the NGA-West2 GMMs, although the expected variability was observed. PGAs exceed the GMM predictions at the closest distances for the source model that we used. The area of the strongest ground shaking encompassed the town of Magna, where some of the heaviest damage occurred. A significant implication of the 2020 Magna earthquake for seismic hazards in the Salt Lake Valley arises from the possibility that this earthquake occurred on the Salt Lake City segment of the Wasatch fault. If so, then the dip of this fault segment must decrease with depth to ≤30°–35°, as proposed by Pang et al. (2020)—at least along the northern part of the segment where the earthquake occurred. Because of the lack of information about the subsurface geometry of the Wasatch fault zone, modeling of this fault zone in seismic hazard analyses has assumed a moderate dip of 50°±15°. Assuming a more shallowly dipping fault results in higher estimates of ground shaking in future large earthquakes on this fault. Alternative interpretations of the Magna earthquake are that it occurred (1) on an auxiliary fault within the Wasatch fault zone or (2) on a listric section of the northern Salt Lake City segment that is not representative of the geometry of the whole fault segment.

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

The Seismic Record ◽

10.1785/0320210009 ◽

2021 ◽

Vol 1 (1) ◽

pp. 35-45

Author(s):

Lee M. Liberty ◽

James St. Clair ◽

Adam P. McKean

Keyword(s):

Fault Zone ◽

Seismic Data ◽

Active Fault ◽

Salt Lake ◽

Salt Lake City ◽

Ground Surface ◽

Elevated Risk ◽

Lake City ◽

Wasatch Fault ◽

Transfer Zone

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.

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