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An earthquake occurred Tuesday evening about 4 miles north of state Highway 21 at the point where the highway crosses Cape Horn Creek and makes a loop around the northern edge of the Sawtooth Mountains. Dozens of aftershocks occurred since then, mostly to the southwest of the initial quake.

Already on edge due to a deadly viral pandemic, Wood River Valley residents were further rattled Tuesday night by the second largest earthquake ever recorded in Idaho.

The U.S. Geological Survey pinpointed the 6.5 magnitude quake to a spot deep underground about 20 miles northwest of Stanley. It occurred at 5:52 p.m.

“Perceived shaking for the quake was very strong,” the USGS reported Wednesday. “The event was widely felt, with close to 16,000 ‘Did You Feel It?’ reports thus far submitted, but [is] likely to have low impact.”

According to a USGS response map, the quake was felt as far north as southern British Columbia and Alberta, and as far west as western Washington and Oregon.

Idaho’s strongest recorded earthquake was the 6.9 magnitude Borah quake—four times as strong as the recent one—which caused about $12.5 million in damage and killed two children in Challis on Oct. 28, 1983.

According to the Custer County Sheriff’s Office, no damage was reported in Stanley or Challis from the recent quake. The Blaine County Sheriff’s Office reported receiving only a handful of reports of minor damage such as muddy well water, cracked stucco and cracked drywall.

Stanley Mayor Steve Botti said he felt “severe shaking” at his home in the southwestern part of town as did other residents with whom he talked. He estimated that the shaking lasted for about 20 seconds, with items falling off shelves.

“It started here with a really loud rumbling noise,” he said. “At first I thought it was a big piece of machinery. It stopped all of a sudden, like somebody had flipped a switch.”

The USGS has reported dozens of aftershocks at various epicenters near the initial quake, the largest being magnitude 4.6 at 6:27 p.m. Tuesday about 8 miles to the south. Botti said that at one point Tuesday night the USGS had reported nine aftershocks, “and I think I felt all of them.”

The agency stated Thursday that aftershocks are likely to continue during the next week, with a less than a 1 percent chance of any surpassing the initial quake in size.

Ketchum resident Todd Dean was snowshoeing across Redfish Lake when the quake struck. He had been camping across the lake from the lodge for a few days and was returning Tuesday when he stopped about three-quarters of a mile from shore to build a snowman.

“I was trying to be funny and artistic,” he said. “As I’m making the first snowball for the snowman this super loud roaring from under the ice occurred. I thought the ice was cracking, so I grabbed my backpack and ran to shore as fast as I could.”

It took about five minutes for Dean to make it to the shore.

“I looked and I noticed the entire lake shifted,” he said. “I’m still not thinking earthquake, but I’m thinking what could have that big of an impact? Maybe a snowslide from the other side of lake? Then as I assessed more, I’m like, ‘Holy smokes. That was an earthquake!’”

The Idaho Transportation Department reported that the quake caused “extensive” landslides in the Canyon Creek section of state Highway 21 west of Banner Summit. There was no traffic on that section of the highway at the time because ITD had closed it at 2 p.m. Tuesday due to a threat of avalanches from a 27-inch snowfall.

ITD reported that its crews will remain out of the canyon until the USGS determines that the risk of aftershocks is reduced.

“At this time, the number and extent of landslides on this section are unknown,” ITD stated in a press release. “There is no timeframe when crews can re-enter the canyon and begin clearing the road. The department is advising it may be many days before this section of Highway 21 reopens.”

Seismic instruments indicated the earthquake originated at a depth of 6.2 miles, the USGS reported, and was the result of “strike slip faulting”—mostly horizontal movement along vertical fractures—unlike the Borah quake, which moved vertically.

Hailey resident Ginna Lagergren, who experienced both quakes, said she felt a distinct difference, with the movement this time “vibrating, not bumping up and down like the 1983 quake.”

College of Southern Idaho geology professor Shawn Willsey, author of “Geology Underfoot in Southern Idaho,” said the recent earthquake occurred in the western part of the Central Idaho Seismic Zone, an area centered around Challis that is being stretched in a southwest-northeast direction.

Willsey said the quake’s energy dissipated quickly as it traveled south through the deep volcanic rocks of the Snake River Plain, and was not felt strongly where he lives in Twin Falls. In contrast, he said, the harder granite that runs up the spine of Idaho transferred the energy more efficiently toward the north.

He said the immediate vicinity of the epicenter has some history of strong earthquakes, with a 6.0 magnitude quake occurring in 1944 and a 6.1 quake occurring in 1945.

Willsey said that if there’s going to be an earthquake, this was a safe place to have it.

“This was a good-size earthquake way up in the mountains,” he said. “The same-size quake, if you stuck it under Boise, you’d have widespread structure damage and injuries, maybe casualties.”

Willsey said the recent 6.5-magnitude quake is the second strongest recorded in the state since accurate measurement of seismic activity began about in the 1940s, and, based on early accounts, probably the second strongest since the beginning of white settlement here. However, he said geological evidence of prehistoric times indicates that “we definitely have an environment where 7-7.5 isn’t out of the question.”

Email the writer: gmoore@mtexpress.com

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