More small temblors in wake of Tuesday’s Challis quake

More than 300 aftershocks have rumbled around Idaho since Tuesday’s 6.5 magnitude earthquake near Stanley, geologists said.

The aftershocks commonly follow a sizable quake like the one felt from Boise to Canada, according to the USGS.

“Idaho has been rockin’ and rollin’,” Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology, said.

Geologists have been picking up small temblors, many of them deep underground with magnitudes of 3 or smaller.

“There have been hundreds and hundreds of smaller earthquakes since then,” Stickney said. “That’s pretty standard.”

The epicenter of Tuesday’s quake felt at 4:52 p.m. in Coeur d’Alene, was 6 miles underground in a location between Challis and Cascade, north of Stanley in the Challis-Stanley National Forest. The area is known for its remote and rugged landscape in the Sawtooth Mountains where seismic activity is common.

A series of small faults undercut the entire area, Stickney said.

The general area produced the 1983 Borah Peak earthquake, which had a magnitude of around 7.2 that caused an estimated $12 million in damages to the cities of Challis and Mackay and resulted in the death of two people.

Aftershocks from the quake will likely be felt for weeks near the epicenter but unless they reach magnitudes above 3.5 they will not be felt outside the immediate area.

“We wouldn’t have felt a 3.5 (magnitude quake),” said Bill Richards, professor of geology at North Idaho College.

Three-magnitude quakes often occur in North Idaho’s Silver Valley, and may be mining induced, Richards said.

The largest aftershock from Tuesday’s quake — it registered 4.6 magnitude — happened within a half hour of the main quake, Stickney said.

“There were several dozen magnitude 3 quakes,” he said.

Historically central Idaho has been active, with back-to-back quakes in 1944 and 1945 “that were sizable,” Stickney said.

Seismic equipment wasn’t as sensitive then as it is now and the quakes likely registered in the four or five range and their exact location was not pinpointed.

Tuesday’s quake was unusual in that it was a strike-slip quake in which two opposing land masses slide alongside each other instead of moving up or down.

The immediate area is not tied to the Yellowstone land mass that also produces regular small quakes. A 7.2 magnitude quake in 1959 resulted in Lake Hebgen, near West Yellowstone when a chunk of the nearby mountains broke away during the earthquake and blocked a river, which produced the lake.

Stickney said geologists won’t know why the strike-slip quake occurred or what it means for the region.

“We probably won’t know for a couple of years until the research comes out,” he said. “It’s still a bit of a mystery.”

The quake occurred about 10 miles north of the major Sawtooth fault, a 40-mile fault that runs along the eastern base of the Sawtooth Range.