Seismologists: Idaho earthquake swarm tapering off

BOISE, Idaho — A sequence of central Idaho earthquakes that rattled Challis residents for more than a month appears to be subsiding.

“All indications are that the swarm has run its course and is dying down to normal activity levels,” said Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology based in Butte, Montana.

But Stickney and other scientists said five portable seismographs have provided new information about the area that saw a sequence of quakes up to 4.9 in magnitude, peaking in mid-April.

“We have learned that there is a northwest trending zone of earthquake epicenters that’s 5 kilometers (3.1 miles) in length on the west side of the Salmon River,” Stickney told The Associated Press on Tuesday.

He said the quakes, being called the “Challis Swarm,” are at the northern tip of the Lost River Fault but it’s not clear if there’s a connection. That fault caused Idaho’s largest recorded quake, a 6.9-magnitude rumbling in 1983 near 12,667-foot Borah Peak, Idaho’s tallest peak.

The fading earthquake activity means workers will remove the portable seismographs in the middle of June, said Stickney. But the lack of activity is welcomed by Challis residents.

“A lot of people were getting worried that there were going to be a bunch of smaller ones and then the big one,” said Melanie Corrigan, a dispatcher with the Custer County Sheriff’s Office based in Challis.

The 4.9-magnitude quake occurred on April 13, and had a 4.1-magnitude quake two days before and a 4.4-magnitude a day after. Corrigan said there have been no injuries or damage reported during the earthquakes.

“We have no indication that this is leading to anything larger or more energetic,” Stickney said.

Kris Pankow, associate director of the University of Utah Seismograph Stations, said the portable seismograph stations went in between April 16 and April 19 and missed the peak.

Harley Benz, scientist in charge of the U.S. Geological Survey’s National Earthquake Information Center in Golden, Colorado, said that was still in time to capture earthquakes up to 3.7 in magnitude and narrow down the location of the swarm.

“At least we reduced the uncertainties,” he said, noting that locations of the earthquakes dropped from a range of about 10 kilometers (6.2 miles) to as little as 1 kilometer (.62 miles).”

Stickney said two types of earthquakes have occurred during the swarm. Some are from strike-slip faults that involve horizontal movement, and others are from sections of earth slipping below other sections.

“The crust surrounding these faults is being stretched in a northeast-southwest direction,” he said. “This is a common theme in southeast Idaho and southwest Montana.”

The scientists said the earthquakes near Challis aren’t the result of magma below the surface as is the case with many quakes in nearby Yellowstone National Park.

The scientists said they plan to analyze information from the portable seismographs over the next several months as time becomes available. But they said the summer field season that has them out repairing equipment and doing other work is just getting started and will keep them busy.

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