Mapping Central Utah’s Earthquake Risk

As the 2020 M5.8 Magna earthquake reminded us, Utah is earthquake country. While previous fault mapping efforts focused most heavily on the Wasatch Front, this new project turned its attention to central Utah, a geologically-critical transition zone between the actively extending Basin and Range Province and the stable Colorado Plateau. Not just an area of transitioning geology, this region is undergoing a transformation as populations and new development both accelerate.

The study covers parts of six counties: Garfield, Juab, Millard, Piute, Sanpete, and Sevier. This region is home to growing communities, vital infrastructure, and major transportation routes.The area has a history of powerful seismic events, including two of Utah’s largest historical earthquakes: the 1901 M6.6 Tushar Mountains and 1921 M6.0 Elsinore earthquakes, both of which caused widespread damage in the Sevier Valley. The UGS embarked on this project, leveraging critical federal funding from the U.S. Geological Survey’s Earthquake Hazards Program, to remap active faults, those that have ruptured the Earth’s surface in the past 2.6 million years and are capable of producing strong earthquakes.

Previous mapping of these faults relied primarily on aerial photographs and fieldwork, which often missed subtle or eroded fault scarps (a step-like feature formed on the landscape by large earthquakes), especially when hidden by dense vegetation. UGS geologists used new lidar (light detection and ranging) data to significantly update the mapping and understanding of these faults. This powerful tool allows UGS geologists to digitally “see through” vegetation, exposing the bare earth beneath, revealing small or complex fault scarps that were previously difficult to detect.

“Lidar data is a game-changer in the world of geomorphology and fault mapping due to our ability to map features on the surface of the earth, such as active faults, in much greater detail than ever before.”

Adam Hiscock, UGS Project Geologist

As part of this new mapping, UGS geologists created recommended surface-fault-rupture special-study zones around each mapped fault trace. These zones are advisory areas where a detailed, site-specific investigation by a geologist is recommended by the UGS prior to new development.
The UGS creates these maps to provide critical guidance for safe and smart community growth. UGS is tasked with helping our local governments, cities, towns, and planning commissions understand the risks below their feet when they imagine future development. Local authorities play the essential role of incorporating scientific data from the UGS into public safety and planning policy.

• Informed Planning: Planning commissions can use the special-study zones to direct new development away from high-risk areas, thereby protecting new homes, schools, hospitals, and infrastructure like water lines and highways.
• Risk Mitigation: By making development decisions based on the most accurate data available, cities and counties reduce future risks to life and property, ensuring a safer future for all Utahns.
• Guidance for Local Officials: Special-Study zones are advisory areas where the UGS recommends that a detailed, site-specific investigation by a qualified geologist take place prior to any new development.
• Ordinance Enforcement: Local officials can use these maps to create and enforce geologic-hazard ordinances, ensuring that structures are not built directly on top of or too close to a known active fault trace.

All the data from this project, including the final report, detailed fault maps, and surface-fault-rupture special-study zones, are publicly available through the Utah Geologic Hazards Portal as well as in a GIS geodatabase along with a detailed report. These resources allow homeowners, developers, and local officials to access the most up-to-date information to make informed decisions.

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