Quake rocks Idaho

6.5-magnitude temblor between Challis and Cascade rattles communities from Coeur d’Alene to Boise

Bill Richards didn’t feel the earth roll Tuesday at 4:52 p.m. in Coeur d’Alene but he got a notification on his cellphone.

“I didn’t feel much,” Richards, a North Idaho College geology professor said. “Just a little.”

His cellphone app though, which sends him notifications of earthquakes of over 3.5 magnitude, said Tuesday’s 6.5-magnitude quake in central Idaho near Stanley sent shock waves across the Gem State.

Its epicenter was directly between Challis and Cascade, Richards said.

“If you draw a line between them, it was right at the center of the line,” he said.

It is an area of faults that has produced large earthquakes in the past.

The 1983 Borah Peak earthquake, which had a magnitude of around 7.2 started in the same area, killing two people and causing millions in damages in the cities of Challis and Mackay.

Tuesday’s quake started 6 miles underground, Richards said.

University of Idaho geology professor Kenneth F. Sprenke, who lives in Moscow, said his house shook and the chandeliers swung from the ceiling.

“The house was swaying,” Sprenke said. “My seismographs were going crazy.”

Even a half hour later, Sprenke said, Moscow felt smaller aftershocks.

“This was a big earthquake,” Sprenke said. “This was a very significant earthquake and unusual too.”

A 5.7-magnitude earthquake near Salt lake City earlier this month, which was widely felt by more than 35,000 people, was 30 times less noticeable than Tuesday’s quake, he said.

Tuesday’s quake was inside the Challis-Stanley National Forest a remote and rugged landscape in the Sawtooth Mountain area almost directly north of Stanley.

“Probably no one lives near there,” he said.

The quake was on a strike-slip fault meaning two opposite sides slid in different directions alongside each other, Sprenke said.

Usually in Idaho, earthquake activity is along a normal fault where one block moves up or down.

“This is very unusual for an earthquake in the Intermountain West,” Sprenke said. “This kind of thing is what happens in California.”

Sprenke said Tuesday’s quake was the largest in Idaho since the Borah Peak quake, although quakes along the Sawtooth fault are not unusual, he said.

Tuesday’s quake was felt throughout the state, he said.

“I know it was felt in Coeur d’Alene and Spokane,” he said.

Claudio Berti of the Idaho Geological Survey at the University of Idaho, said Tuesday’s quake was not associated with the fault zone to the east near Yellowstone, where small magnitude quakes are not unusual.

“Those are very likely unrelated,” Berti said. “That is a different geological structure.”

Berti said Tuesday’s quake, despite having originated in an area known for its faults, was unusual for its magnitude.

“We know those faults have been moving recently,” he said.

According to the USGS, Tuesday’s quake struck during evening rush hour in Boise where ground movement was stronger than in North Idaho.

“Perceived shaking for the quake was very strong. The event was widely felt,” according to the USGS blog, which documents “Did You Feel It?” — reports of people calling in to ask if others had felt the quake or to make sure they weren’t going nuts. Minutes after the quake, more than 16,000 people had filed reports with the USGS.

Because its epicenter was in a remote Idaho backcountry, Berti said the quake is likely to have low impact.

Richards and Sprenke speculated the quake probably left a mark, like a tear, on the landscape.

It’s in the middle of a national forest in central Idaho, Sprenke said. “It will be interesting to see if it left a fault scarp.”

Tuesday’s quake was felt as far away as Calgary, Alberta, according to the USGS.

Two areas of faults affect North Idaho including the Spokane Fault that was named after small earthquakes were recorded there about 20 years ago. A large fault system runs through the Silver Valley that regularly produces low magnitude quakes, Richards said.