Scientists puzzled by flurry of quakes in central Idaho

Scientists puzzled by flurry of quakes in central Idaho
The series of quakes are clustered in a sparsely populated area of central Idaho. (U.S. Geological Survey map)
BOISE, Idaho (AP) - Three portable seismographs will be installed in the Challis area in central Idaho to help experts better understand a recent flurry of earthquakes.

Harley Benz, scientist in charge of the U.S. Geological Survey's National Earthquake Information Center, said scientists decided Tuesday to put in the devices. One of them should be in place by Wednesday and two more within a week to record future earthquakes.

"It certainly has gotten the attention of the state and our regional partners," Benz said. "So what we're trying to do is put in an array to get a better feel for the location of the events and the depths and the rate of activity."

The U.S. Geological Survey has recorded a sequence of quakes rumbling the area, the largest of them being a 4.1-magnitude quake on Thursday, a 4.9 quake on Sunday and a 4.4 on Monday.

Smaller quakes have also been recorded, including five on Monday ranging from 2.5 to 3.3 in magnitude. Three of the quakes took place within a 40-minute span starting about 9:12 p.m. Monday.

The quakes have ranged from about 6 to 15 miles northwest of Challis in lightly populated Custer County.

"People are asking: 'Is this going to lead to a bigger earthquake?' " said Benz, based in Golden, Colo. "And the answer is we simply don't know."

He noted the earthquakes are in the same region as Idaho's largest recorded quake, a 6.9-magnitude in 1983 near Borah Peak, Idaho's tallest peak at 12,667 feet.

Linda Lumpkin, a dispatcher for the Custer County Sheriff's Office in Challis, said she personally is expecting another big one, but residents in general have become accustomed to the recent temblors.

"At this point, everybody is not getting real shook up about anything because we're getting them almost every day," she said.

Benz said the three portable seismographs will provide real-time information and, using triangulation, allow scientists to pinpoint the locations of the earthquakes, including depth, should they continue. They'll also be able to record smaller quakes, down to about a 1.0 magnitude.

He said the nearest seismograph in place now is about 71 miles away in Montana, meaning earthquake locations in the Challis area could be off by 4 miles. Depths are also difficult to plot accurately. By putting three stations within 6 to 12 miles, the error could be reduced to about half a mile, he said.

"We can find out where they are actually occurring and at what depth," Benz said. "One station will significantly help, and three will do better."

The plan is to put them in an L shape, he said.

"Ideally, you would like them to be in as quiet a spot as possible," Benz said. "The other problem with this part of the world is there is lots of snow. Finding sites is not going to be straightforward."

The seismographs will be installed by a field engineer from the University of Utah, he said. The engineer has two of the portable seismographs, and the U.S. Geological Survey is sending a third.

Idaho's quakes, Benz said, are caused by broad-scale deformation of the Western United States as a result of plate tectonics. The three portable seismographs could help scientists identify which fault is currently active, if the quakes continue.