- Associated Press - Friday, December 11, 2015

BOISE, Idaho (AP) - More than 40 small earthquakes have been recorded in east-central Idaho this week in what experts say is another earthquake swarm in the region.

Officials in the Challis area on Friday reported no damage from the micro-quakes that started Tuesday and have mostly gone unnoticed or unreported in an area with residents accustomed to more vigorous shaking.

“I didn’t know we had any,” said Custer County Sheriff Stu Lumpkin of the most recent swarm. “The tiny ones, sometimes you feel them, sometimes you don’t.”

But the temblors ranging up to 2.9 magnitude have perked up scientists trying to understand the fault system in the area where a 5.0 magnitude quake struck in January.

“It’s unclear what’s going on,” said William Phillips, a research geologist with the Idaho Geological Survey at the University of Idaho. The earthquake swarms “form a line on the map. They really line up. They occur in one tight spot and then several months go by and the earthquakes will move to the northwest or southeast.”

Idaho’s largest recorded earthquake rocked the same area in 1983 when a 6.9-magnitude quake occurred near 12,667-foot Borah Peak, Idaho’s tallest peak.

The sheriff said his agency is aware the area is prone to damage-causing earthquakes.

“We always keep it in mind,” he said. “As far as a plan, you never know when they’re going to hit.”

Phillips said the current earthquake swarm is close to the fault associated with that large earthquake but not on it.

Scientists brought in temporary seismometers last year during a previous earthquake swarm and then earlier this year put in a permanent seismometer near Challis.

“The earthquakes are being located relatively well now,” Phillips said.

The swarms are occurring within the Centennial Tectonic Belt extending west from Yellowstone National Park. The Intermountain Seismic Belt extends north and south from Yellowstone through multiple states.

The park is well known as a geologic hotspot, sitting atop magma. But scientists said that doesn’t explain the earthquake swarms in east-central Idaho, where the U.S. Geological Survey reported a 2.6-magnitude quake late Friday morning as the swarm continued.

“We have other earthquake swarms in Idaho,” Phillips said. “Nothing with this drama and intensity, though.”

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