‘Curved’ Wasatch fault may be even more destructive and deadly than previously thought

In the year since a magnitude 5.7 earthquake, Utah’s complicated Wasatch fault has come into sharper focus, thanks to thousands of aftershocks that scientists were able to observe and analyze, revealing the paths of cracks in the earth like an MRI machine showing views of otherwise hidden human tissues.

It turns out the fault’s Salt Lake City segment does not plunge downward at a uniform angle, but actually bends into a shallow angle, indicating that a rupture would lead to more violent shaking at the surface.

That could lead to more damage, more destruction, more death.

“As [the fault] goes down underground, it’s like a snow shovel; it kind of curves. So that bottom part is actually shallower than what we thought. If it went straight down, it would go deeper,” said Keith Koper, director of the University of Utah’s Seismograph Stations, which led various studies. “... And so when we make our predictions for how much shaking we would get if we had a magnitude 7, those predictions now are being recalculated.”

That picture was based on an initial study of 2,600 aftershocks recorded during the six weeks after the March 18, 2020, quake from the Seismograph Stations’ network of fixed instruments. This research — which is ongoing as U. seismologists analyze terabytes of data they amassed after Utah’s strongest earthquake on record — came up at a forum Thursday timed with the quake’s anniversary.

The aftershocks may have rattled plenty of nerves but they were a gift to seismologists, providing a chance to map the part of the long-running fault system through the heart of Utah’s most populous region. A 217-mile network of cracks in the the earth running south from Malad, Idaho, past Nephi, the Wasatch fault has a 57% chance of unleashing a magnitude 6 earthquake in the next 50 years, or a 40% chance in the next 30 years, according to Steve Bowman, the geologic hazards program manager for the Utah Geological Survey.

This prospect poses a major threat to life and property, because 85% of Utah’s population lives near the fault. Major roads, pipelines and communications networks also could be disrupted in a big quake.

Magnifying Utah’s problem is the large number of unreinforced masonry homes and other structures built before 1950 that are particularly vulnerable.

Bowman said the U.’s data will be used to update seismic hazard maps.

“New buildings are designed based on the information that is in these maps,” Bowman said. “A structural engineer could use that data and do an evaluation of an historic building and give an opinion of how that structure would fare in an earthquake.”

Post-quake legislation fails

Dozens of historic masonry buildings were damaged in last year’s temblor, centered near Magna, and one historic home, the Sears mansion, in Salt Lake City’s Liberty Wells neighborhood had to be torn down.

“This is the one building we lost in the earthquake,” said Utah State Historic Preservation Officer Chris Merritt. “I don’t want to lose any more historic structures.”

He was speaking from his office in the 1911 Rio Grande Depot, which sustained among the worst damage of any large building and is still undergoing repairs. A massive crack was apparent in the wall behind Merritt, a testament to the destructive power of an earthquake that would hardly hold a candle to the magnitude 6.5 to 7 that scientists forecast for the Wasatch Front.

The quake’s most visible impact unfolded on Magna’s historic Main Street, where bricks crumbled off many century-old commercial buildings, filling the sidewalks with debris.

Rep. Clare Collard, a Magna resident, was showering on the morning of the quake, and braced herself as she heard glass breaking and objects fall. That experience prompted her to champion legislation last session aimed at boosting public awareness about the dangers of unreinforced masonry and what homeowners can do to mitigate the risks. HB366 would have funded a public education campaign, while fellow Democratic Rep. Andrew Stoddard’s HB214 would have required disclosure in real estate transactions.

“There’s just a lot of education that needs to take place. The fact that we have 98,000 unreinforced masonry structures in Salt Lake County and almost 200,000 in the state is sobering,” Collard said. “Knowledge is power and when you are dealing with people’s lives, it’s important to get the information out there.”

To her disappointment, both bills failed, but she plans to resurrect these proposals in interim sessions.

“Many of my colleagues were extremely concerned with the issue, but just didn’t see the urgency to it,” Collard said. “We were told things like, ‘People can move.’ OK, so people can move, but we have a housing shortage and an affordability issue here in the state of Utah. These homes will always be occupied regardless. Typically, these residents don’t have the funding to retrofit the homes.”

Expanding Fix the Bricks

Thursday night’s virtual forum, hosted by the Utah State Historic Preservation Office, highlighted the Fix the Bricks program, in which federal grants are used to subsidize retrofits of masonry homes. Currently, the program is available only in Salt Lake City. Collard and others want to see it extended to the rest of Salt Lake County and the state, wherever there are historic brick homes at risk of collapse in a sizable quake.

“It’s a costly enterprise to retrofit your home, and Fix the Bricks has done a tremendous job, but it’s very limited in scope and has a tremendously long waiting list,” Collard said. “Retrofitting 25 or 30 homes a year is just not going to cut it.”

To further help understand the risks Utahns face, U. seismologists, such as Amir Allam, fanned out across the Salt Lake Valley after the quake, installing portable seismometers, known as nodal geophones, in the ground around the Wasatch fault and its branches. For 35 days, these instruments recorded thousands of aftershocks. Through triangulation, this trove of seismic data was used to pinpoint the precise locations of the fault lines.

The 180 geophones Allam’s teams embedded in the ground doubled the number of aftershocks the U. was able to record, up to 5,000, according to Koper.

“We have literally thousands more aftershocks in that first month that we were able to locate and detect,” Koper said, “just because of that nodal data that helps us figure out more about the twists and turns in the fault structure down there.”

The fault’s Salt Lake City segment plunges at a steep 70-degree angle, dipping toward the west for 6 to 8 miles before curving to a shallower angle. That means when the fault moves, the quake’s epicenter is closer to the surface.

“Our observations suggest that the rupture area of future large earthquakes will be closer to the surface than previously thought, which would cause increased ground-shaking in the Salt Lake City metropolitan area with its 1.2 million residents,” the scientists report in a paper published last year by the American Geophysical Union.

Previously, it was assumed the fault descended at a uniform 50-degree angle, much less steep than what scientists have now learned.

“There is this flattening shape to the Salt Lake City segment of the Wasatch fault,” Koper said. “It’s causing us to rethink our models of the area. We’re going to redo our hazard forecast because of that.”

According to the geological record, the Wasatch fault releases a magnitude 7 quake every 1,300 years or so. The last quake of that size hit about 1,400 years ago.

“There’s still a large amount of stress. It’s still loaded,” Koper said. “The 5.7 [Magna quake] did not relieve a significant amount of the stress that’s loaded right underneath us.”