Digging Up Evidence of Past Earthquakes in Salt Lake Valley

by Adam I. Hiscock, Emily J. Kleber, Michael D. Hylland, and Greg N. McDonald

The West Valley fault zone and the Salt Lake City segment of the Wasatch fault zone. Yellow dots indicate previous research trenches; pink dots indicate previous consultant trenches that yielded earthquake timing data. Indiana Avenue trench site shown as yellow triangle. Red, orange, and black lines show mapped faults; bar-and-ball symbols along lines indicate downthrown side of fault.

In the late 1800s, geologist Grove Karl Gilbert recognized the large elevation difference between the spectacular and rugged Wasatch Range and the Salt Lake Valley as being created by episodic fault movement associated with large earthquakes, each time raising the mountains and dropping the valley floor. Gilbert was the first geologist to identify the Wasatch fault zone and the seismic hazard it presents to residents along the Wasatch Front. What he did not recognize at the time was the presence of a second fault system running through the middle of Salt Lake Valley—the West Valley fault zone.
The West Valley fault zone is antithetic to the Salt Lake City segment of the Wasatch fault zone; i.e., the fault plane of the Salt Lake City segment dips westward underneath the Salt Lake Valley, whereas the fault plane of the West Valley fault zone dips eastward and towards the Salt Lake City segment. The West Valley fault zone consists of two separate faults: the Taylorsville (eastern strand) and Granger (western strand) faults. These faults connect with the Salt Lake City segment several miles below the valley surface to form a V-shaped, downthrown crustal block known as a graben. Fault scarps (i.e., slopes formed from ground movement during past surface-rupturing earthquakes) that define the Taylorsville and Granger faults are generally much smaller than scarps along the Wasatch fault zone. Many of these fault scarps have been disturbed or destroyed by human activity in the Salt Lake Valley. Unlike the Wasatch fault zone, which has been the subject of dozens of detailed scientific studies, the West Valley fault zone has had comparatively little scientific study despite the fact that it is still capable of generating large, damaging earthquakes. 

The inferred relationship between the Salt Lake City segment of the Wasatch fault zone and the West Valley fault zone (specifically, the Taylorsville and Granger faults). Vertical motion on the west-dipping Wasatch fault zone has uplifted the Wasatch Range and down dropped Salt Lake Valley. The West Valley fault zone is antithetic to the Wasatch fault zone to the east, forming a V-shaped graben. Modified from UGS Special Study 149 (Hylland and others, 2014).

To study past earthquakes, geologists conduct paleoseismic (ancient earthquake) studies by excavating trenches across fault scarps and studying and dating the geologic deposits in the excavation to determine past earthquake timing. Ideally, sites for paleoseismic studies have fault scarps that have not been disturbed by human activity. Since the West Valley fault zone runs through the heavily urbanized Salt Lake Valley, sites suitable for paleoseismic trenching studies are very sparse, and the few sites that do exist are disappearing rapidly due to urban growth and development. 

Previous paleoseismic investigations have identified evidence that earthquakes have occurred at roughly the same time on both the West Valley and Wasatch fault zones (see Survey Notes, v. 44 no. 2, May 2012, and Survey Notes, v. 48, no. 2, May 2016). Data from these studies suggest that fault movement on the Wasatch fault zone can trigger earthquakes on the antithetic West Valley fault zone. Seismological data from the March 2020 Magna earthquake sequence indicate the magnitude 5.7 mainshock occurred on the Salt Lake City segment several miles beneath the Salt Lake Valley, and the aftershocks included a sequence of small earthquakes in the vicinity of the West Valley fault zone. Data from the Magna earthquake sequence as well as from previous paleoseismic studies have improved our understanding of how these major faults interact beneath Salt Lake Valley; however, significant uncertainties remain. Detailed paleoseismic studies have only been conducted on a few strands of the West Valley fault zone; thus, the current earthquake record is far from complete. Additional paleoseismic research is needed to help fill gaps in the earthquake record for the West Valley fault zone.

In the summer of 2022, the Utah Geological Survey (UGS) set out to collect additional data from the Taylorsville fault of the West Valley fault zone to help fill these gaps in the earthquake record. We excavated two paleoseismic fault trenches at the Indiana Avenue trench site across a 3- to 5-foot-high scarp on one of the last undeveloped parcels of land along the West Valley fault zone. We estimate the oldest sediments exposed in the trenches were deposited about 15,000 years ago, when Salt Lake Valley was submerged under Lake Bonneville. Younger sediments exposed in the trench excavations were likely deposited after Lake Bonneville had receded, when the modern-day Jordan River and Great Salt Lake were depositing sediments across parts of Salt Lake Valley. Evidence for potentially two surface-rupturing earthquakes was observed in the trenches. Unfortunately, due to a shallow groundwater table at the site, we were unable to trench deeper to search for evidence of additional, older earthquakes. 

Samples were collected to determine ages of exposed geologic units and to determine the timing of fault movement at the site. Two dating methods are typically used in paleoseismic studies: radiocarbon and optically stimulated luminescence (OSL) dating. Radiocarbon dating measures the concentrations of the radioactive isotope carbon-14 and OSL dating measures trapped electrons that accumulate over time in quartz-bearing minerals in buried sediments. The samples collected are currently being processed, but when the results are received, they will be used to develop a model for the timing of past earthquakes at the Indiana Avenue site. This model will be compared to models from other trench sites on the West Valley fault zone and the Salt Lake City segment of the Wasatch fault zone to fill gaps in the earthquake record, improve earthquake probability estimates, and further our understanding of the relationship between these two active fault systems.