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The sediments underlying the Salt Lake Valley are thicker in places than previously thought, indicating that current seismic hazard models likely underestimate the amount of shaking Utah's population center could experience in future earthquakes, according to new research led by University of Utah seismologists.

Five years ago, the valley trembled during the magnitude 5.7 , causing millions in damage to dozens of masonry structures in Salt Lake City and the town of Magna, a few miles to the west. Utah's urban centers, such as Ogden, Salt Lake City and Provo, lying along the Wasatch Front, remain at risk of future seismic events.

The last major earthquake exceeding magnitude 7 to hit the Wasatch Front occurred between 1,200 and 1,300 years ago. With an average recurrence interval of 900 to 1,300 years, Salt Lake City's geologic clock could be close to striking midnight once again.

In the new study now in the Journal of Geophysical Research: Solid Earth, U researchers utilized seismic data to present a refined three-dimensional seismic velocity model—an essential tool for mapping the geologic structure of the Wasatch Front and identifying seismic hazard sites.

"For this particular study, we are trying to understand the sedimentary structure within the Salt Lake area and how that might differ from previous results," said study leader Fan-Chi Lin, an associate professor of geology and geophysics. "One of the biggest questions we had was why our observations didn't agree with previous studies."

The Wasatch Front community velocity model is currently the leading reference for assessing future seismic activity. However, it has been largely informed by borehole drilling and gravity data—useful indicators, but ones that come with limitations such as private land restrictions, inconsistent documentation and limited sampling scope.

To overcome these constraints, an extensive network of seismic data probes and geophone arrays was deployed across the Salt Lake Valley—even in the backyards of private residences. Many were deployed in the month following the Magna quake in the spring of 2020 to take advantage of a steady parade of aftershocks.

"This community is incredibly supportive and happy to help. I want to emphasize that none of this would have been possible without community support, the Utah Geological Survey and the many students in our department who helped deploy hundreds of stations," Lin said.

For this study, the research team analyzed seismic waves from only distant earthquakes, using interferometry analysis—comparing measurements of the same signal from two different stations—and conversion phase analysis—comparing the incident P-wave and the S-wave converted at the base of the sediment.

This analysis gleaned insights into the subsurface structure of the Salt Lake Valley, which was once the bed of ancient Lake Bonneville that covered northern Utah as recently as 14,000 years ago.

The goal wasn't to predict strong earthquakes but to predict the severity of ground motion they could produce. The team was also pursuing academic questions.

"We are interested to understand how the tectonic forces or tectonic movements form the basin itself," Lin said. "Why there's a basin here? What controls the depth of the basin?"

The team's analysis led to a proposed revision of the Wasatch Front velocity model. While still consistent with earlier studies, the new model indicates thicker sediment bands in some places than previously estimated. Thicker sediments increase seismic hazard by intensifying ground shaking during an earthquake.

"In a basin, seismic shaking is amplified. It's very important to understand the thickness and rigidity of sediment to better assess potential shaking. The sediment is thicker than previously thought—especially in the heavily populated area south of Salt Lake City," Lin said.

"So our findings reinforce the idea that a hazard exists in the valley and that shaking could be stronger than expected. Many houses in Salt Lake are unreinforced masonry and could be vulnerable in a big quake. Buildings should be reinforced, and people should be prepared."

Numerous factors influence an earthquake's intensity and the hazard it poses to populated regions. The earthquake's source mechanism, magnitude, fault geometry and subsurface composition all play a role. Sediment, in particular, can dramatically elevate hazard levels—amplifying ground motion and weakening structural integrity, potentially causing liquefaction.

"If we know the subsurface structure very well, then we can predict how strong the ground motion will be when the big earthquake happens," Lin said. "And that will allow us to collaborate with the engineers to determine which buildings are potentially hazardous when the big earthquake hits."