Earthquake Risk in the Salt Lake Valley

01 August 2013
Published in Science and Society
Written by  Kim Schuske
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Earthquake Risk in the Salt Lake Valley Michael Hylland, Utah Geological Survey
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The Wasatch Front is one of the most seismically at risk areas in Utah and in the Intermountain West. Scientists are looking at thousands of years of earthquake history to learn more about the hazard we face.

In Christchurch, New Zealand in February, 2011, a magnitude 6.3 earthquake struck six miles from the city center. The sandy type of soil present in the area caused the ground to basically liquefy during shaking. Close to 200 hundred people died and recent estimates put the cost of rebuilding at $40 Billion.

“Liquifaction occurs during earthquake shaking,” explains Chris DuRoss, a senior geologist at the Utah Geological Survey. “Basically a soil that is strong before the earthquake, during the shaking loses its strength, and essentially behaves like a liquid. More or less like quicksand.” Buildings and other structures can be partially sucked into the earth. Once the shaking stops, the loose, sandy, saturated soil solidifies, and returns to its original state. This is one reason why the Christchurch earthquake caused so much damage.

Much of the infrastructure along the Wasatch Front is built on a similar type of soil, says DuRoss. “Just like in Christchurch we could have strong ground shaking, and we have soils that are susceptible to liquefaction.”However, he says, the type of fault along the Wasatch Front is different than in Christchurch. Ongoing studies to characterize the potential for liquefaction in Salt Lake Valley should help better define the risk.

What is not in question is that we are due for a large, magnitude 6.5 to 7.5 earthquake. “We could have an earthquake at anytime. Basically enough seismic energy has accumulated on the fault,” says DuRoss. “The fault has been sitting here without any movement for long enough, that we could have a large earthquake.”

Why are geologists convinced that we are due for a big one? DuRoss’ group looks for signs of prehistoric earthquakes that have occurred over thousands of years. They dig trenches across fault scarps, or scars in the earth, to get a better look at the disruptions caused by earthquakes. Vertical displacement of the ground surface allows them to date geologic deposits, giving them a detailed historical record of the quakes.

Unlike in California, there have not been any large earthquakes over the past 150 years since the Wasatch Front was settled. In fact, DuRoss’ group and others have found that there have only been about four earthquakes in the last 5,000 years along the Salt Lake City segment of the Wasatch Fault.

“In Salt Lake City we have earthquakes that occur about every 1,300 years over the last 5,000 years of history of the fault. We call that a return period, or recurrence time,” says DuRoss. “So, about 1,300 years between magnitude seven earthquakes and it’s been about 1,400 years since the last large earthquake.”

DuRoss says one of the concerns in Utah is that since we haven’t experienced earthquakes, building codes over the last century have not taken into account the seismic nature of the area. Most houses or buildings built before the 1970’s are at risk from shaking and liquefaction, depending on the epicenter and strength of the next big one.

“We have building sock, we call these unreinforced masonry buildings. These were built in the ‘20s, and ‘30s, and ‘40s, up to about the ‘60s. These are typically brick homes that you see all throughout, say, Sugarhouse and other areas. The bricks are just stacked up,” says DuRoss. “While they’re historic buildings and they look great, they’re not designed to withstand horizontal motions that occur due to ground shaking from a large earthquake. So they tend to have considerable damage and even failure.”

Data from the Federal Emergency Management Agency suggests damage in Salt Lake City from a large earthquake will run over $42 Billion and 9,000 people will suffer life threatening injuries or death. DuRoss says the best thing to do to prepare is to strap down your water heater, have an earthquake kit available with food and water, and have an earthquake plan ready with family, schools and place of work.

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