BOISE, Idaho -- About half an hour before the ground began shaking last week, Glenn Thackray had fired off an email to a colleague about launching more research on the Sawtooth Fault, a tectonic plate boundary in Central Idaho that Thackray discovered a decade ago.
So when the earth began rumbling later that night, Thackray's first thought was that the Sawtooth Fault was moving.
"The first thing I heard was there was a (magnitude) 6.5 earthquake in the Stanley area, and my first assumption was it was the Sawtooth Fault," said Thackray, who has been teaching geosciences at Idaho State University for more than 25 years.
But in the days since the March 31 earthquake, Thackray has puzzled over whether his assumption was right. The earthquake, believed to be the second-strongest on record in Idaho, originated about 21 miles northwest of Stanley, according to data from the U.S. Geological Survey.
"It's in an odd location for the Sawtooth Fault itself, being up in that corner where Highway 21 makes the big loop," Thackray said.
When he learned the earthquake was caused by a strike-slip fault -- one in which tectonic plates move past one another horizontally -- rather than a normal fault that causes vertical movement, he had even more questions.
"When the first information came out about how (the earthquake) moved, it didn't make sense for the Sawtooth Fault," Thackray said. "That's not to say the Sawtooth Fault wasn't involved in some way, but that's a detail that's of great interest to geologists." Where was epicenter of Idaho earthquake? Thackray said the geologic feature where the March 31 earthquake likely originated "appears to be a fault that cuts across the Sawtooth Fault west of Stanley." The U.S. Geological Survey said it doesn't appear there has been seismic activity in the immediate vicinity for more than 50 years.
"It's a fault that has been thought to possibly be still active," Thackray said. "We don't know a lot about that area." Thackray said two fairly large earthquakes likely occurred in a similar area in the 1940s, according to 1980s reanalysis of data from the initial events.
"Of course, now everybody is looking at those earlier earthquakes there and saying, 'Oh, maybe this is in the same fault system,' " Thackray said.
Lee Liberty, a seismologist at Boise State University, said he sees potential ties to the Sawtooth Fault.