A swarm of about 25 small earthquakes in central Idaho during the last three weeks has prompted scientists to bring in additional seismographs in hopes of finding a cause, an official said.
"We have our theories, but we need more data," Idaho Geological Survey research geologist Bill Phillips told the Los Angeles Times.
A University of Utah engineer drove to Challis late Tuesday with portable seismometers that will be deployed around the epicenter of the recent earthquakes, three of which have surpassed magnitude 4. The largest, a 4.9, was recorded Sunday.
Scientists want a better sense of how deep the earthquakes are occurring and whether they might be connected to a fault in a nearby valley.
Phillips said there was no oil drilling or geothermal activity in the sparsely populated mountainous region, leaving an unmapped fault or underground fluid movement among the possible causes.
University of Utah geophysicist Kris Pankow had already been scheduled to give a talk on earthquakes in Boise on Monday, but her visit captured extra attention because of the recent temblors. Intrigued by the local interest in the seismic activity, she agreed, along with the U.S. Geological Survey, to send up the measuring equipment.
"It was fortuitous that I was there," Pankow, associate director of the university's seismograph stations, told The Times. "We try to cooperate and collaborate as we can."
The shaking hasn't led to serious damage in the region, which is filled with a "resilient crowd of ranchers that has seen it all," Phillips said.
Scientists said they didn't think the quakes were precursors to something bigger but that it's hard to know without knowing more about what's causing them.
The data gathered on the quakes so far come from distant seismographs – the closest is more than 50 miles away – and they might be missing some tiny shakes.
"Because we are a rural state, we do not have a dense seismic monitoring net," Phillips said. "We are now locating earthquakes with Wyoming, Montana, Oregon and a few devices in Idaho. There's a magnitude limit."
Julie Dutton, a geophysicist at the U.S. Geological Survey, said she understood that people in the region might be nervous. But the heart of Idaho is a seismically active area and such phenomena happen from time to time, she said.
"There's stresses and strains in the region, and when it becomes too much it gets released," Dutton told The Times. "Challis is more unusual than some places, but it's not something we're completely caught off guard by."
Idaho's largest recorded earthquake -- a 6.9 in 1983 -- was slightly east of the current swarm. That quake killed two people in Challis and seriously damaged 50 homes and offices, including the local high school. Altogether the earthquake caused more than $12 million in damage, according to the USGS. In today's dollars, that would exceed $28 million.