USGS: “Be ready for more earthquakes” following 6.5 magnitude temblor in Idaho

The “did you feel it” map held by the U.S. Geological Survey demonstrates how far the powerful 6.5 magnitude quake was felt across the West. (USGS)

Seismic stations in central Idaho have recorded dozens of aftershocks stemming from Tuesday night’s 6.5 magnitude temblor that rattled much of the Western United States, including Montana.

And the U.S. Geological Survey said to be ready for more.

The quake struck at 5:52 p.m., just as the local evening news was set to conclude and many Missoula residents were setting up for dinner.

A map of felt activity kept by the USGS recorded hits from across the West, include Seattle and Portland, and as far away as Denver. Those in Idaho and western Montana reported moderate shaking, while some near Boise and closer to Challis – the epicenter – reported strong to very strong ground activity.

Michael Stickney, director of the earthquake studies office at the Montana Bureau of Mines and Geology, said the quake occurred on an unknown fault line.

“At this point, it’s not known,” he said. “It appears to be a strike-slip earthquake. A 6.5 earthquake – there’s a lot of energy in that.”

A forecast prepared by the USGS after the event cautioned area residents to “be ready for more earthquakes.” That’s not uncommon after a significant event, Stickney said, and some aftershocks could be larger than the main shock.

“According to our forecast, over the next 1 Week there is a 4 % chance of one or more aftershocks that are larger than magnitude 6.5,” the forecast reads. “The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily.”

The forecast also suggests than over the next week, up to 710 aftershocks with a magnitude 3 or higher are likely to occur. Anything over 3 is large enough be felt by those closest to the epicenter.

The USGS said a chance of an aftershock with a magnitude 6 or higher is 10%, with as many as two possibly occurring. The chance of an earthquake of magnitude 7 or higher is 1%, though the probability remains low.

“Our closest seismic station has been in almost continuous ground motion just before six last night,” Stickney said. “That’s expected for an earthquake of this magnitude. They’re typically accompanied by thousands of aftershocks that can go on for even longer.”

While damage was minimal, the quake caused an avalanche on Idaho State Highway 21. The Idaho Transportation Department has closed the road roughly 23 miles south of Stanley.

The last time such a powerful quake hit Idaho was in 1983, when a 6.9 magnitude earthquake struck east of the Lost River fault zone. The last significant quake to hit Montana was a 5.8 that occurred in July 2017 with an epicenter roughly 80 miles east of Missoula.