Opinion: How many more houses will slide away?
When it comes to earthquake preparedness, a lot of people (myself included) have focused on the need to shore up the approximately 140,000 aging homes and buildings along the Wasatch Front made of unreinforced masonry. When the ground shakes, these tend to crumble, injuring anyone beneath. In the 5.7-magnitude quake that hit the Salt Lake Valley in 2020, falling bricks from these buildings posed the biggest danger.
But now, it’s clear that landslides ought to be a big concern, as well. Utah’s majestic mountains have a way of shaking themselves off from time to time, like irritated sleeping giants.
Actually, that’s not a news flash. You don’t have to dig too hard in the archives to find all kinds of examples of hillsides collapsing around the state, taking houses with them. The two houses in Draper that slid into a ravine last month were just the latest examples.
Plenty of people and commissions have been willing to point out the problem. Lawmakers have even done a little to deal with it. They should do more.
How many houses would slide if the big one, a magnitude 7.0 earthquake or higher, hit?
No one knows the answer to that, but history gives us lots of clues about the dangers.
In 1992, a 5.8-magnitude quake hit St. George. Oddly, the force of that quake pushed itself away from St. George and toward Hurricane. It ended up causing a powerful landslide in Springdale, about 27 miles away, that destroyed three homes.
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A report at the time said this was the farthest recorded distance “for a coherent landslide of this type” from such an earthquake.
That hillside was a “large old mass of rocks that were just on the brink of exceeding equilibrium,” Ben Erickson, senior geologist for the Utah Geological Survey, told me. Hillsides are under various levels of stress depending on the angle of the slope and the type of soils they contain. In Utah, ancient volcanic ash has turned to clay in many areas, and water makes that clay swell.
Why should we be concerned along the Wasatch Front? Plenty of reasons.
A 2022 report by the Utah Seismic Safety Commission placed a good deal of attention on those unreinforced masonry buildings, but it also sounded warnings about four major aqueducts along the Wasatch Front that provide water to more than 2 million people. Three of these intersect the Wasatch Fault zone and the fourth is in an area of risk for landslides.
One of them, the Salt Lake Aqueduct, is a 42-mile concrete pipe that runs from the base of Deer Creek Dam through Utah County and up to the mouth of Parleys Canyon in Salt Lake County.
“The pipeline, which serves around 450,000 people, was built in the 1940s and has several segments that are subject to earthquake damage where they cross the Wasatch fault,” the report said. “A recent risk assessment identified a high risk of joint failure during an earthquake due to ground deformation and ground shaking.”
If they fail, landslides are sure to follow. The report said a risk assessment found that segments above Pleasant Grove, Cedar Hills, Draper and Cottonwood Heights were the most critical.
In the last legislative session, state lawmakers appropriated $50 million toward repairing and shoring up these pipes. Ari Bruening, chief executive officer of Envision Utah, told me that water districts said they could come up with all but $175 million of what is needed to do the job, meaning the state’s appropriation provided less than one-third of that.
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State lawmakers haven’t done enough to address this problem, nor to keep developers from wanting to inch farther and farther up hillsides. Courts may have to determine who was to blame for the landslides in Draper. But plenty of hillside homes have suffered from landslides during the years, without an earthquake, including one that crushed a house in North Salt Lake in 2014.
In the mid-1990s, developers proposed a subdivision on a level piece of land about 100 feet up and 450 feet back from the nearest road in Olympus Cove, accessible only by a tram or cable car. Thankfully, Salt Lake County said no. But a lot of other less-audacious hillside projects since then have gotten a yes from various local governments.
Despite all the surpluses in recent years, lawmakers haven’t done much to shore up those unreinforced masonry homes, although they have spent money to shore up old schools.
They have come only partway on the aqueducts. They haven’t addressed hillside developments.
That 2022 earthquake report said, “Without proactive measures, an expected m(agnitude) 7.0 earthquake on the Salt Lake City segment of the Wasatch fault (‘The Big One’) would be among the deadliest disasters in U.S. history. It would leave hundreds of thousands of Utahns without shelter and critical lifeline services for many months.”
Let’s hope we don’t end up lamenting what we could have done to avoid a lot of misery.
