Introduction
Utah is not the first state that comes to mind when Americans think about earthquake risk. But running along the western base of the Wasatch Range — the dramatic mountain wall that defines the eastern edge of the Salt Lake Valley — is one of the longest and most active normal faults in the world. The Wasatch Fault extends approximately 390 kilometers from southern Idaho through central Utah, and it is the dominant seismic hazard for the majority of Utah's population.
The numbers are stark. Roughly 80% of Utah's residents live along the Wasatch Front, a narrow urban corridor stretching from Ogden in the north through Salt Lake City to Provo in the south. These cities sit on the hanging wall of the Wasatch Fault — the block of crust that drops downward during an earthquake. The ancient sediments of Lake Bonneville, the predecessor of the Great Salt Lake, underlie much of the urban area and are highly susceptible to liquefaction. Older buildings, particularly unreinforced masonry structures, are common. And the fault has not produced a major earthquake in the Salt Lake City area for more than a thousand years.
The Wasatch Fault is not a single continuous fracture. It is segmented — divided into 10 distinct sections, each capable of rupturing independently to produce a large earthquake. Paleoseismic research, much of it conducted by the Utah Geological Survey and USGS, has revealed a long and active history of major earthquakes, with each segment rupturing on its own multi-century schedule. The question is not whether the Wasatch Fault will produce a major earthquake, but which segment will rupture first — and whether Utah is ready.
Geology and Tectonics
Basin and Range Extension
The Wasatch Fault is a product of the Basin and Range Province, a vast region of the western United States characterized by crustal extension — the stretching and thinning of the Earth's crust. The Basin and Range extends from the Wasatch Front in Utah westward across Nevada to the Sierra Nevada in California, and it is one of the most tectonically active continental regions on Earth.
As the crust stretches, it breaks along normal faults — faults where one block drops down relative to the other. The Wasatch Fault is a west-dipping normal fault, meaning the block to the west (the valley side) drops relative to the block to the east (the mountain side). Over millions of years, this repeated downward displacement has created the dramatic topographic contrast between the Wasatch Range, which rises more than 2,000 meters above the Salt Lake Valley floor, and the valley itself, which has filled with thousands of meters of sediment eroded from the mountains and deposited by the rivers, glaciers, and lakes that have occupied the basin.
The total extension rate across the Wasatch Front is approximately 2–3 mm/year, according to GPS measurements. This is modest compared to the slip rates of California's strike-slip faults (the San Andreas Fault moves at 20–28 mm/year), but it is substantial for a normal fault system and sufficient to produce large, damaging earthquakes.
Normal Faulting: A Different Kind of Earthquake
Most Americans are familiar with the concept of strike-slip faults — faults where two blocks of crust slide horizontally past each other, like the San Andreas. The Wasatch Fault operates differently. As a normal fault, it produces vertical displacement: during an earthquake, the valley side drops downward relative to the mountain side.
This has several important implications:
The shaking from normal-fault earthquakes tends to produce stronger vertical accelerations than strike-slip earthquakes of the same magnitude. Buildings and infrastructure are generally less well-designed to resist vertical forces than horizontal ones.
Surface rupture on a normal fault creates a scarp — a cliff-like step in the ground surface — that can be several meters high. During a large earthquake on the Wasatch Fault, structures built across the fault trace could experience vertical offsets of 1 to 3 meters, effectively destroying anything that spans the rupture.
The hanging wall (valley side) of a normal fault typically experiences stronger shaking than the footwall (mountain side) due to the geometry of seismic wave propagation. Since virtually all of the Wasatch Front population lives on the hanging wall, the shaking hazard is amplified for the areas where people actually live.
What Causes Earthquakes? The Science Behind Seismic Activity
The 10 Segments of the Wasatch Fault
The Wasatch Fault is divided into 10 segments based on geological mapping and paleoseismic evidence. Each segment is bounded by structural or geometric complexities — step-overs, bends, or changes in fault orientation — that tend to stop earthquake ruptures from propagating from one segment to the next. However, multi-segment ruptures are possible and have likely occurred in the geological past.
The five central segments are considered the most hazardous because they underlie the densely populated Wasatch Front:
| Segment | Length (km) | Most Recent Event | Recurrence Interval | Major Cities |
|---|---|---|---|---|
| Brigham City | ~40 | ~2,100 years ago | ~1,300–2,400 years | Brigham City, northern Ogden area |
| Weber | ~56 | ~590 years ago | ~1,100–1,500 years | Ogden, North Ogden, Roy |
| Salt Lake City | ~45 | ~1,200 years ago | ~1,000–1,300 years | Salt Lake City, West Valley City, Sandy, Murray |
| Provo | ~70 | ~600 years ago | ~1,000–2,500 years | Provo, Orem, American Fork, Lehi |
| Nephi | ~43 | ~1,000 years ago | ~1,000–1,500 years | Nephi, Juab County |
Source: Utah Geological Survey, USGS, Paleoseismic studies compiled by the Working Group on Utah Earthquake Probabilities
The five additional segments — Malad City, Clarkston Mountain, Collinston, Levan, and Fayette — extend the fault to the north and south but underlie less populated areas. These segments are less well-studied, though paleoseismic evidence indicates they are also seismically active.
Salt Lake City Segment
The Salt Lake City segment is the most consequential for human exposure. It extends approximately 45 kilometers from the Salt Lake Salient (a bedrock promontory near the University of Utah) southward to the Traverse Mountains near the Point of the Mountain. This segment underlies the core of the Wasatch Front's population and economic activity, including downtown Salt Lake City, the University of Utah, Salt Lake City International Airport, and the densely populated suburbs of West Valley City, Murray, and Sandy.
Paleoseismic trenching studies, including research by the Utah Geological Survey, indicate the Salt Lake City segment last ruptured approximately 1,200 years ago (around AD 800). Given a recurrence interval of roughly 1,000–1,300 years, the segment is within its expected recurrence window — though, as with all earthquake hazard assessments, the timing of the next event cannot be predicted.
Provo and Weber Segments
The Provo segment, at approximately 70 kilometers, is the longest of the five central segments and is capable of producing the largest earthquakes — potentially up to M7.5. Its most recent event occurred roughly 600 years ago, placing it earlier in its recurrence cycle than the Salt Lake City segment. The Provo segment underlies Utah County, the fastest-growing metropolitan area in the state, including the cities of Provo, Orem, American Fork, and Lehi.
The Weber segment underlies Ogden and surrounding communities. Its most recent event — approximately 590 years ago — also places it in the earlier portion of its recurrence cycle. Weber County has experienced rapid population growth and contains significant critical infrastructure, including Hill Air Force Base.
Liquefaction: The Hidden Amplifier
Lake Bonneville Sediments
One of the most significant hazards associated with a Wasatch Fault earthquake is liquefaction — the process by which saturated, loosely packed sediments lose their strength during shaking and behave like a fluid. The Wasatch Front is extraordinarily susceptible to liquefaction because of the geological legacy of Lake Bonneville.
Lake Bonneville was a massive freshwater lake that covered much of western Utah during the last ice age, reaching its maximum extent approximately 18,000 years ago. At its peak, the lake covered roughly 51,000 square kilometers and reached depths of more than 300 meters. As the lake receded over thousands of years, it deposited thick layers of fine-grained sediments — silts, clays, and sands — across the valley floors of the Wasatch Front.
These Lake Bonneville sediments are the foundation upon which Salt Lake City, Ogden, Provo, and their suburbs are built. They are exactly the type of material most susceptible to liquefaction: fine-grained, loosely consolidated, and saturated with water due to the shallow water table that exists throughout much of the Wasatch Front.
What Is Liquefaction and Why Is It So Dangerous?
What Liquefaction Means for Buildings and Infrastructure
According to the Utah Geological Survey, large areas of the Wasatch Front — particularly in the western portions of the Salt Lake Valley and Utah Valley — are classified as having high or very high liquefaction susceptibility. During a large earthquake, liquefaction in these areas could cause:
Ground settlement and tilting of buildings as the soil beneath them loses bearing capacity. Buildings may sink unevenly, causing structural damage even if the building itself is otherwise well-designed.
Lateral spreading, in which liquefied soil flows horizontally toward lower ground, carrying buildings, roads, and pipelines with it. This is particularly hazardous near the shores of the Great Salt Lake and Utah Lake, and along river channels.
Sand boils, in which liquefied sand erupts through the ground surface, flooding basements and undermining foundations.
The combination of strong shaking from a normal fault earthquake, thick deposits of liquefiable Lake Bonneville sediments, and a high water table creates a liquefaction hazard that is among the most severe in the western United States.
[CHART: Line Chart — Wasatch Fault Segment Recurrence Timelines] Data: X-axis shows years (AD 0 to present); Y-axis shows the five central segments (Brigham City, Weber, SLC, Provo, Nephi). Plot the most recent events for each segment with error bars representing dating uncertainty: Brigham City (~2,100 years ago), Weber (~590 years ago), Salt Lake City (~1,200 years ago), Provo (~600 years ago), Nephi (~1,000 years ago). Overlay average recurrence intervals as shaded ranges. Source: Utah Geological Survey paleoseismic database.
Earthquake Scenario: M7.0 on the Salt Lake City Segment
Projected Impact
The most consequential earthquake scenario for Utah is a M7.0 event on the Salt Lake City segment of the Wasatch Fault. This scenario has been modeled by the USGS, the Utah Geological Survey, and FEMA, with consistent results painting a picture of severe regional disruption.
According to scenario modeling, a M7.0 earthquake on the Salt Lake City segment would produce:
An estimated 2,000 to 2,500 fatalities, with the majority caused by building collapses — particularly unreinforced masonry structures.
Approximately $33 billion in direct economic losses, including structural damage, infrastructure repair, business interruption, and emergency response costs.
Widespread liquefaction across the western Salt Lake Valley, causing building settlement, pipeline breaks, and lateral spreading.
Surface rupture along the fault trace with vertical offsets of 1 to 3 meters, destroying any structures spanning the rupture.
Disruption to I-15 (the primary north-south highway serving the Wasatch Front), TRAX light rail, the Union Pacific rail corridor, and Salt Lake City International Airport.
Damage to the State Capitol building, the University of Utah campus, and numerous hospitals and emergency facilities.
Unreinforced Masonry: The Primary Killer
The single greatest contributor to casualties in a Wasatch Fault earthquake scenario is unreinforced masonry (URM) buildings — structures built with brick, stone, or concrete block walls without steel reinforcement. URM buildings are extremely vulnerable to earthquake shaking; their walls can collapse outward, burying occupants and pedestrians.
According to the Utah Seismic Safety Commission, thousands of URM buildings remain standing along the Wasatch Front, including schools, churches, commercial buildings, and historic structures. Many of Salt Lake City's most iconic buildings — particularly in the downtown and Capitol Hill neighborhoods — are URM construction.
Utah has made progress in addressing this vulnerability. The state adopted the International Building Code with seismic provisions, and Salt Lake City has undertaken seismic evaluations of critical facilities. However, mandatory retrofit ordinances — like those adopted in Los Angeles and San Francisco for URM buildings — do not exist along the Wasatch Front. The cost of retrofitting URM buildings ranges from roughly $15 to $60 per square foot, and most building owners have not voluntarily undertaken the work.
[MAP: Wasatch Fault Segments and Hazard Zones] Data source: Utah Geological Survey, USGS. Features: All 10 Wasatch Fault segments with labels, five central hazard segments highlighted, major cities (Ogden, Salt Lake City, Provo, Nephi, Brigham City), Lake Bonneville sediment extent (showing high-liquefaction zones in western valleys), Great Salt Lake and Utah Lake shorelines, I-15 corridor, University of Utah, Salt Lake City International Airport. Inset: Wasatch Fault location within the Basin and Range Province.
Historical Seismicity
No Major Earthquake in Recorded History
One of the most challenging aspects of earthquake hazard communication in Utah is that the Wasatch Fault has not produced a major earthquake in the approximately 175 years of European-American settlement along the Wasatch Front. The largest instrumentally recorded earthquake in the Wasatch Front area was the M5.7 Magna earthquake of March 18, 2020, centered near the western edge of the Salt Lake Valley. This event, which caused moderate damage to buildings and infrastructure (including cracking of the spire on the Salt Lake Temple), was a reminder of the region's seismic hazard but was far smaller than what the Wasatch Fault is capable of producing.
The absence of a major historical earthquake creates a dangerous complacency. Unlike California, where the 1906, 1971, 1989, and 1994 earthquakes have built institutional memory and public awareness, Utah lacks a modern benchmark event. Residents who have lived in the state for decades may never have experienced significant earthquake shaking.
The 2020 Magna Earthquake
The M5.7 Magna earthquake on March 18, 2020 — which struck just days after COVID-19 lockdowns began — was not on the Wasatch Fault itself but on a secondary fault west of the Salt Lake Valley. The earthquake caused an estimated $100 million in damage, displaced residents from older apartment buildings, and produced more than 2,500 aftershocks over the following months.
The Magna earthquake served as a partial wake-up call. It demonstrated that even a moderate earthquake could cause significant disruption in the Salt Lake City metro area, and it revealed vulnerabilities in the region's building stock and emergency response systems. But a M5.7 releases roughly 1,000 times less energy than a M7.0 — the Magna earthquake was a tremor compared to what the Wasatch Fault will eventually produce.
Preparedness and Mitigation
Building Codes and Retrofitting
Utah adopted the International Building Code (IBC) with seismic provisions, and new construction along the Wasatch Front is generally required to meet modern seismic standards. However, the vast majority of the existing building stock was constructed before modern seismic codes were adopted or enforced.
The Utah Seismic Safety Commission has advocated for more aggressive retrofit policies, particularly for URM buildings, schools, and essential facilities. Some progress has been made — the Utah State Capitol underwent a major seismic retrofit in the early 2000s, and several hospitals and emergency facilities have been upgraded. But the scale of the problem dwarfs the resources allocated to address it.
The Great Utah ShakeOut
Utah participates in the annual Great ShakeOut earthquake drill, typically held in April. The drill is coordinated by the Utah Division of Emergency Management and the Utah Seismic Safety Commission, and participation has grown steadily. The ShakeOut is an important public awareness tool, but awareness alone does not retrofit buildings, strengthen bridges, or extend water system resilience.
Earthquake Insurance
Earthquake insurance take-up in Utah is low, mirroring the pattern seen in the New Madrid Seismic Zone and other areas where large earthquakes are infrequent. Standard homeowner's insurance policies do not cover earthquake damage, and separate earthquake policies typically carry high deductibles (10–20% of the dwelling value). Given the potential for severe damage along the Wasatch Front, the Utah Division of Insurance has encouraged residents to evaluate their earthquake insurance options.
What to Do During an Earthquake
Utah Earthquake History and Current Risk
The Wasatch Fault in Context
The Wasatch Fault differs from California's major faults in fundamental ways, but the underlying reality is similar: a large, active fault underlies a major population center, and the next major earthquake is a certainty — only the timing is unknown.
Compared to the Hayward Fault in California's East Bay, the Wasatch Fault has a longer recurrence interval (1,000+ years vs. ~150 years) but a larger potential for vertical displacement and ground failure due to its normal-fault mechanics and the pervasive liquefaction hazard. Compared to the New Madrid Seismic Zone, the Wasatch Fault has better-constrained paleoseismic data and a more clearly defined fault structure, but shares the challenge of low public awareness and a building stock that was not designed for major earthquakes.
The Wasatch Fault also faces a challenge unique to Utah: the state's rapid population growth. Utah has been one of the fastest-growing states in the country for several decades, with much of the growth concentrated along the Wasatch Front. Cities like Lehi, Eagle Mountain, and Saratoga Springs — all in the southern Salt Lake Valley and northern Utah Valley — have expanded rapidly on land underlain by Lake Bonneville sediments with high liquefaction potential. Growth outpacing seismic preparedness is a recurring theme along the Wasatch Front.
Sources
- USGS, Wasatch Fault Zone — USGS Earthquake Hazards Program
- Utah Geological Survey, "Paleoseismology of the Wasatch Fault Zone" — Utah Geological Survey
- DuRoss, C.B., et al. (2016), "Fault segmentation: New concepts from the Wasatch Fault Zone, Utah, USA," Journal of Geophysical Research: Solid Earth
- Working Group on Utah Earthquake Probabilities (WGUEP, 2016), "Earthquake probabilities for the Wasatch Front region in Utah, Idaho, and Wyoming," Utah Geological Survey Miscellaneous Publication 16-3
- FEMA Hazus-MH earthquake loss estimation modeling for Utah
- Utah Seismic Safety Commission — Utah Seismic Safety Commission
- Personius, S.F., et al. (2012), "Paleoseismic studies of the Wasatch fault zone," USGS Professional Paper
- Utah Division of Emergency Management, Great Utah ShakeOut