Small, medium quakes common in Southern Utah; but what about the ‘big one?’

ST. GEORGE — It’s happened before, and it’ll happen again — a “rolling shake” underfoot lasting seconds, maybe longer. Lower magnitude earthquakes are common in seismically active Southern Utah, but what about the “big one?”

A crack from a doorway at a St. George area business caused by the earthquake that took place 50 miles south of St. George, Utah, Sept. 19, 2022 | Photo reader submitted, St. George News

On Sept. 19, St. George News reported a 4.4 magnitude earthquake with a focus point on Shivwits Plateau in Arizona, 31 miles south of St. George. The “shallow shaker” occurred at a depth of 6.2 miles.

The quake reportedly was felt throughout the St. George metro area, Mesquite, Kanab, Cedar City and as far as Brian Head, according to one report. However, no injuries or property damages were reported.

About 1,500 earthquakes occur annually in Utah, including aftershocks, with an average of approximately 13 measured at a magnitude of 3.0 or more, according to the University of Utah. About 2% of these are felt.

1992 earthquake

Damage From Springdale Landslide, Springdale, Utah, Sept. 2, 1992 | Photo courtesy of the Utah Geological and Mineral Survey, St. George News

On Sept. 2, 1992, Washington County was rocked by a 5.8 magnitude earthquake, which was the largest in recent history.

Brett Barrett, an advertising coordinator with Canyon Media, said he was woken up by the “most awful noise,” followed by a bright light. When he looked at his bedroom wall, it appeared to be waving.

“My first thought was, ‘They nuked us — we’re all going to die,'” he said.

Barrett said he left his two-story apartment with his infant daughter in his arms and found himself on the lawn outside with no recollection of how he got there. The air was full of what he first thought was smoke but soon realized was dust kicked up by the shaking.

Unsure of whether water lines were disrupted, Barrett went across the street to purchase supplies from Smiths. In one of the aisles, broken pickle jars littered the floor, he said.

Elsewhere, Barrett’s mom Sue Jolley was in her water bed when she heard what sounded like a train running through her backyard. Because she woke from a sound sleep to the noise and movement, she was disoriented.

Damage From Springdale Landslide, Springdale, Utah, Sept. 2, 1992 | Photo courtesy of the Utah Geological and Mineral Survey, St. George News

“It was really hard to get out (of bed) because the mattress was just rolling,” she said.

She had an expensive porcelain Lladró figurine from Spain on her dresser and could hear it falling and bouncing. One piece broke and she kept the rest of it wrapped in a towel for the next few days in anticipation of aftershocks, Jolley said.

Nothing else in the house broke and nothing was lost, Jolley said.

“I was trying to get out to save my Lladró figures but my concern was to make sure nearby family members were OK,” she said.

While some houses saw minor damage in St. George, the most destruction was reported near Springdale, according to the University of Utah.

Highway Buckle — Toe Of Balanced Rock Hills Landslide, Springdale, Utah, Sept. 2, 1992 | Photo courtesy of the Utah Geological and Mineral Survey, St. George News

State Route 9 was closed near the entrance of Zion National Park because of a landslide measured at one point to be two-thirds of a mile long and close to 1,000 feet wide.

Three houses on the hillside were destroyed in the slide, the university states. The residents of one of the homes were reportedly forced to abandon their car because their way was blocked by rocks and cracks in the road, and they either walked or slid down the hillside.

People from a large area surrounding the epicenter reported feeling the quake, including those in Las Vegas, Nevada, Flagstaff, Arizona, Escalante and Richfield.

There were no fatalities or serious injuries reported in connection to the earthquake, the university states.

Before 1992, the largest earthquake in Washington County was a magnitude 6.34 in 1902, according to the Five County Association of Governments Multi-Jurisdictional Natural Hazard Mitigation Plan for 2022-2027.

The Hurricane Fault

A fault is a fracture in the Earth’s crust that has shifted, said Casey Webb, a geology professor at Southern Utah University. The crust in the West’s Basin and Range Province is being pulled apart and the tensional stress of this event has caused fractures — the Hurricane Fault and its subsidiary faults.

The Hurricane Fault runs through the Shurtz Canyon area, where earthquakes have formed scarps, Cedar City, Utah, Sept. 27, 2022 | Photo by Alysha Lundgren, St. George News

Visible along parts of Interstate 15  are steep slopes or scarps, which were formed by the fault’s displacement of the ground.

The subsidiary faults run parallel to the main fault and also might be seismically active. However, these don’t typically cause as much displacement and are less likely to generate large earthquakes, Webb said.

As is typical for long, “normal faults,” where one side has moved down relative to the other, it is composed of “discrete segments that tend to rupture independently,” according to a Utah Geological Association 2019 publication. For instance, the Cedar City segment can rupture independently of the Shivwits segment.

For comparison, the San Andreas Fault is a “strike-slip” fault, which moves horizontally and can generate more “energetic and damaging” earthquakes, according to the University of Utah.

The natural wonder is about 155 miles long and stretches from Cedar City to south of the Grand Canyon, the document states. It can potentially generate a magnitude 7.0 earthquake, which can cause significant damage.

However, earthquakes of this magnitude are uncommon, averaging approximately 18 per year globally, the mitigation plan states.

The Hurricane Cliffs, a scarp formed by earthquakes generated by the Hurricane Fault as seen from Leeds, Utah, Sept. 27, 2022 | Photo by Alysha Lundgren, St. George News

The Hurricane Fault hasn’t been studied as thoroughly as other faults, like the San Andreas, which makes it more difficult for researchers to understand the timing of previous earthquakes and to predict future events, Webb said.

There is no way to predict an earthquake with 100% certainty, but higher-risk areas can be identified, Webb said. One way to do so is by comparing the last time the fault event was and the average time span between events.

Foreshocks also could indicate a forthcoming earthquake as evidence of stress being applied to the fault. These do not always lead to a large earthquake.

It is normal for a tectonically active area to experience many small to medium earthquakes. But because of their lower magnitude, this activity might not relieve the stress within the fault that can sometimes lead to larger quakes, Webb said.

Because Southern Utah is in a seismically active area, even though a large earthquake is not guaranteed to occur any time soon, it is important to be prepared, Webb said.

Southern Utah earthquake risk

Earthquakes are considered a moderate risk in Iron and Washington counties and St. George but a high risk for Cedar City, according to the Multi-Jurisdictional Natural Hazard Mitigation Plan.

Rockfall Off West Black Ridge In St. George, Utah, Sept. 2, 1992 | Photo courtesy of the Utah Geological and Mineral Survey, St. George News

In Cedar City, two medical facilities, an assisted living facility, two schools and some residential properties sit within one-quarter mile of a fault, the plan states. I-15 and several city roads cross fault lines, particularly in the Cross Hollow Hill area.

A magnitude 4.8 earthquake was recorded in Cedar City in 1942, but no earthquake higher than 4.2 has been reported since then, the plan states.

Another factor considered in the mitigation plan was the age of buildings and their construction types.

Historical structures, typically not reinforced and made of brick, stone or concrete blocks are at higher risk of earthquake damage, according to the plan. Such buildings can be found throughout Southern Utah and were constructed before Utah began enforcing seismic building codes in 1975.

“This construction type can topple during shaking and damage property and people in and outside of the structure,” the plan states.

Higher magnitude earthquakes release greater amounts of energy and people might experience increased shaking intensity, Webb said. Other factors, however, will determine the amount of shaking an individual feels; for instance, how close they are to the earthquake and the materials the earth beneath them is made of.

Liquefaction Along Virgin River Near St. George And Washington, Utah, 1992 | Photo courtesy of the Utah Geological and Mineral Survey, St. George News

Houses built on loose sediment typically are more at risk than those built on solid bedrock. In St. George, many homes are built on bedrock made of weaker stone like mudstone and gypsum, which de-form more easily than harder rock, like granite, Webb said.

Liquefication occurs when shaking causes the ground to become fluid, according to the plan.

“It’s basically the result of closing the pore spaces (in sediment) as the shaking is occurring,” Webb said. “It’s collapsing those pore spaces. The fluid has to go somewhere — they’re going to go up and saturate the surface.”

It is primarily hazardous near areas that have been developed as the phenomenon can damage buildings and infrastructure, according to the mitigation plan. Fires, flooding, rockslides and landslides are also hazards associated with earthquakes.

Quakes generally are not triggered by other natural events; they can be caused by volcanic eruptions and drilling or mining, the plan states.

The Utah Geological Survey provides an online geological hazards map, which Southern Utahns can use to assess their individual risk and prepare.

A ground shaking hazard map of Southern Utah | Image courtesy of the Utah Geological Survey, St. George News

People should be prepared for quake damage to buildings and utilities and for the possibility of going for an extended period without water or access to roads, Webb said. Additionally, he suggests anchoring furniture at risk of tipping over, like bookshelves, to walls.

In case of an earthquake, suggests individuals create a disaster supplies kit and keep it where it is easily accessible in an emergency. The Great Utah ShakeOut also provides safety tips and resources for earthquake safety.

To watch Professor Casey Webb’s YouTube video about the Hurricane fault, click here.

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