Seeing the fault lines: How new technology is reshaping Utah’s earthquake maps

The land beneath central Utah tells the story of a history of earthquakes, a story that scientists are only now beginning to read clearly.

If you were to set foot in Utah’s Sevier Valley, the landscape might seem deceptively static. However, beneath this quiet lies a tumultuous history, and geologists with the Utah Geological Survey now have a way to read it.

Using lidar technology, UGS project geologist Adam Hiscock and his colleagues spent years remapping the active faults that crisscross central Utah. What they found is both instructive and sobering: The risk of earthquakes in the region is more complex and more present than ancient maps ever showed.

The tool that changed everything

Before lidar, fault mapping was difficult and inaccurate. Geologists relied on aerial photographs, paper topographic maps, and long days of fieldwork to track the minute scars that earthquakes can leave on the landscape. These scars, known as fault scarps, are graded features that form when the ground along a fault line ruptures during a large earthquake. It was easy to miss the small ones, or those hidden under dense vegetation, until lidar changed the whole equation.

By sending millions of laser pulses from aircraft and measuring how long each one takes to bounce back, lidar can digitally remove trees and brush them away to reveal the bare ground surface beneath. The result is a strikingly detailed 3D model of the Earth, one that reveals fault lines that have been quietly there for thousands of years, invisible to previous methods.

“LiDAR is an incredibly powerful tool,” Hiscock says. “Being able to create a ‘bare earth’ 3D model of the Earth’s surface and see through vegetation allows us to map very small, eroded faults that were previously impossible to detect using ancient methods.”

The new map, which covers parts of six counties: Garfield, Juab, Millard, Butte, Sunpete, and Sevier, reveals more detail and complexity than anything ever produced before. Faults that were vaguely identified on ancient maps now appear with pinpoint accuracy. New error slides have appeared that were never set at all.

An area with a seismic past

Central Utah lies at a geological crossroads. The area extends across the transition zone between the extensional basin and range province to the west, where the crust is actually being broken up, and the stable Colorado Plateau to the east. This tectonic tension has made the area a historically active seismic zone.

Two of Utah’s largest historic earthquakes struck here: a 6.6 magnitude earthquake near the Tushar Mountains in 1901 and a 6.0 magnitude earthquake near Elsinore in 1921. Both were widely felt throughout the region, with damage reported throughout the Sevier Valley. It’s a reminder that central Utah’s faults are not dormant relics of the distant past. They are active systems capable of producing strong earthquakes.

Despite the risks, over the years rough maps of the faults responsible for this seismic activity have been drawn. New work based on lidar technology changes this situation by providing what Hiscock describes as “foundational knowledge” for rapidly growing communities that are building infrastructure intended to last for generations.

Shoes on the floor

Although powerful, lidar is no substitute for fieldwork. One of Hiscock’s strongest convictions, and a lesson he will impart to any geospatial professional, is that technology works best when combined with human judgment and direct observation.

“I feel strongly that it is always important to put the ‘boot on the ground’ and check as many fault slopes and other features as you draw on the computer,” he says. “Field verification of your maps is essential.”

It’s this combination of the bird’s-eye accuracy provided by lidar and the ground-level intuition of an experienced geologist that gives the new maps their credibility. Every important feature identified in the data has been field verified, ensuring that what appears on the final maps reflects reality, and is not just a shadow in the data.

Drawing safety zones

The maps themselves are only part of the value of the project. Along with each mapped fault trace, the UGS team has created special Recommended Study Areas for Surface Fault Rupture – advisory areas where site-specific geological investigation is recommended before new development begins.

These areas are not regulations. The UGS is a non-regulatory agency, and cannot dictate what is built anywhere. But by drawing these boundaries and making them available to city and county planners, the agency gives local governments a concrete, science-based tool to make smarter growth decisions.

“We offer these areas as recommendations to help ensure safer, smarter community growth, and provide essential guidance for city and county planners to create and implement geohazard ordinances,” says Hiscock.

For communities that may not consider themselves an “earthquake country,” this guidance is important. Rural areas of Utah have seen significant population growth recently, and development is expanding into areas that historically have not been subject to the same level of scrutiny as the Wasatch Front. The new maps help fill this gap.

Make communities listen

Science that is put into a report and never reaches the people it is supposed to protect has limited value. That’s why outreach is a key part of UGS’s mission, and why Hiscock is frank about the challenges of communicating seismic risk to communities that don’t always see themselves as vulnerable.

“Geological hazards do not exist in a vacuum,” he says. “It is also as important to communicate our science as it is to actually do the research.”

UGS has developed a range of general resources, including “Putting Root in Earthquake Country,” a comprehensive guide to earthquake preparedness. The agency also offers a model geological hazards law to help municipal and county governments translate scientific findings into local policy.

The goal is to make science accessible to groups as diverse as homeowners wondering about the land under their homes, developers planning a new subdivision, or elected officials who need to balance growth with risk.

What comes next?

The Central Utah project is part of a decade-long effort by UGS to update fault maps across the state. Most of the work near Utah’s most urbanized passes is now complete, including the Wasatch Fault in northern Utah. The agency is shifting its focus to areas that currently have low population density but have great potential for future development.

This shift reflects a longer-term vision: anticipating growth rather than responding to it. The new fault maps also guide decisions about where to conduct paleoseismic research — detailed studies of ancient earthquakes recorded in the geological record — which can help scientists determine how often faults rupture and what size earthquake they can produce.

“This designation guides us in selecting sites for further paleoseismic research, allowing us to determine rates of fault activity and model earthquake probabilities,” Hiscock explains.

Data is now available

All data from the Central Utah Fault Mapping Project is publicly available. The final report, including detailed fault maps and special study areas for surface fault rupture, can be accessed through the Utah Geological Hazards Portal. A complete GIS geodatabase and detailed report are also available here.

The portal serves as a living resource, giving anyone, from a first-time homebuyer to a county engineer, access to the latest understanding of geological hazards in Utah.

Fault lines have always been there. And now, for the first time, we can see them clearly.

Adam I. Hiscock, P.J., is a project geologist in the Utah Geological Survey’s Geological Hazards Program. For more information about earthquake hazards in Utah, visit Geology.utah.gov/hazards.

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