“It shook the whole house,” said Martha Hyvonen of Charlo about the 3.7 magnitude tremor that shook many Mission Valley residents last Wednesday.
Theodora Lambson, who lives southeast of St. Ignatius, just 4km from the epicenter, said it felt like a corner of her house had collapsed.
“A waker-upper,” Joanne Bigcrane said on Facebook.
“I got up and my house started shaking, my window rattled and the Rez girl in me was like, ‘YE! Good morning to you too, Creator, let’s start with a shake!” wrote Vina LittleOwl.
“The biggest earthquake I’ve ever felt in Montana,” wrote Michelle Miner.
Even the Polson Chamber chimed in, hinting that “even our veteran” was looking forward to the next night’s SPLASH event.
Seismograph stations, part of the Montana Regional Seismic Network, located the earthquake’s center 2.5 miles southwest of St. Ignatius and 7.4 miles below sea level. It was preceded by a magnitude 3.2 earthquake on Oct. 15 about 2 miles southwest of last week’s epicenter, followed by about a dozen tiny aftershocks.
More than 350 people in western Montana responded to the US Geological Survey’s website, Did You Feel It (earthquake.usgs.gov/data/dyfi/) about their experience.
“They don’t come out of the blue, they come out of the darkness beneath our feet,” said Mike Stickney, director of the office of earthquake studies at the Montana Bureau of Mines and Geology in Butte. “It’s not a natural feeling to feel the ground shaking.”
Stickney said several secondhand reports from people at the southern end of Mission Valley “described booms or explosions” rather than the tremors normally associated with earthquakes.
“That’s pretty typical for observers who are close to the epicenter, where there’s still a lot of high-frequency energy rising out of the ground,” he said. “You hear the seismic waves basically vibrating the atmosphere at frequencies your ears can hear.”
Observers reported hearing a rumble, an explosion, or a sound like a freight train. “Someone even thought their pickup truck had exploded in the driveway,” says Stickney. All of this “is fairly typical of a small to moderate earthquake near the epicenter.”
Although a Facebook user blamed Californians moving to Montana for bringing in that state’s earthquakes, Stickney says western Montana is an active seismic zone where small tremors are common and moderate earthquakes can occur. Mission Valley and parts of northwest Montana are in a belt of seismic activity about 100 miles wide, running roughly from Yellowstone National Park to Kalispell.
He called last Wednesday’s wake-up shake “small as an earthquake, but certainly still big enough for many, many people throughout Flathead Valley and beyond” to experience its reverberations. People from Helena to Trout Creek and Hamilton to Columbia Falls have reported feeling it, with reports pouring in from places as far away as Great Falls and Spokane.
According to Stickney, the Montana Regional Seismic Network (MRSN) operates 45 seismic monitoring stations in western Montana, including six on the Flathead Reservation. They also share the data they collect with other institutions in the area, including those in Washington, Canada and Yellowstone Park.
MRSN has worked with the Confederate Salish and Kootenai tribes to track tremors on the reservation for several decades, particularly given the many earthen dams that are located here.
“That’s primarily why the tribes got interested in monitoring seismicity,” he says. “They realized that some dams could be damaged by a very large earthquake.”
Over the past two decades, the Dam Protection Program has rebuilt the impoundments at McDonald Lake and Pablo Reservoir in Mission Valley, and at Lower Dry Fork Reservoir near Lone Pine, to modern standards.
“There are other old dams that haven’t been rebuilt, but we run active monitoring systems to track what type of activity is happening in the area and what that can tell us about the seismic hazards,” Stickney says.
In places like California, where major seismic events are more common, people tend to be better informed about precautionary measures. However, in Montana, where the last major Hebgen Lake earthquake happened in 1959, people are not particularly prepared for earthquake resilience.
According to Stickney, most injuries occur when heavy objects are thrown off shelves or bricks fall from chimneys. He says the FEMA website provides a list of recommendations for safer living in a seismic zone.
“This latest earthquake is another reminder that we live in an earthquake country and a big earthquake could come at any time,” he says. “We can’t predict them and luckily they’re not very common, but it’s better to expect them to happen and be prepared for them.”