A rare magnitude 5.8 earthquake has struck western Montana, briefly plunging a town into darkness and powerful enough to knock down shelves and break glass, the U.S. Geological Survey said.
It was the largest earthquake in western Montana in nearly 60 years, according to the USGS.
"It's not impossible, but it is a very rare event," USGS geophysicist Robert Sanders said.
The earthquake was felt as far away as Spokane, Wash.; Boise, Idaho; and Calgary in Canada, Sanders said. More than 10,000 people reported feeling the earthquake, with people closest to the epicenter reporting shaking as strong as intensity level 8 — capable of causing significant damage.
There were no immediate reports of severe damage, however.
An initial USGS analysis said that the earthquake struck in the area of a well-known fault system known as the Lewis and Clark line, a prominent zone that stretches from northern Idaho to east of Montana's capital, Helena. The Lewis and Clark line stretches about 250 miles and is as wide as 50 miles.
The largest earthquake in the historical record in western Montana struck in 1959, when a magnitude 7.2 earthquake struck a few miles northwest of Yellowstone National Park. The so-called Hebgen Lake earthquake hit about 175 miles away from Thursday's temblor, the USGS said, and was jammed with late summer campers when the earthquake hit before midnight.
One particularly large landslide that hit during the 1959 earthquake was so big it dumped 37 million cubic yards of broken rock into the canyon of the Madison River and blocked its flow for three weeks, forming a lake six miles long later dubbed Earthquake Lake. At least 28 people were believed to have died in that landslide, including a father and three of his children who had been camping during a trip between their home in Idaho and Yellowstone National Park.
Thursday's earthquake struck at 12:30 a.m. MDT, and was followed by aftershocks in the magnitude 3 and 4 range over the next hour.
A 76-year-old resident of Helena, which is about 34 miles away from the quake's epicenter, told the Associated Press that the earthquake was the strongest seismic activity that he had ever felt.
Ray Anderson said his wife told him the temblor woke up the dogs.
The AP reported that electricity had been restored to the town of Lincoln, population 800, after a power outage. Lincoln is about three miles away from the epicenter.
There have been two other earthquakes in Montana greater than magnitude 5 in the last two decades. There was a 5.6 in 2005 and a 5.1 in 1999, Sanders said.
Thursday's quake was unrelated to an ongoing earthquake swarm northwest of Yellowstone Lake, Sanders said. The swarm has been going on for about three weeks, with the largest a 4.5. But most have been very small, with magnitudes of 0.5 or 0.6, which are typical in places such as Yellowstone every few years, Sanders said.
The Yellowstone seismic swarm started about June 12; recently, the pace of earthquakes has dropped off.
Yellowstone is home to an active volcano; it last produced a lava flow about 70,000 years ago.
12:20 p.m.: This article was updated with more detail about when the swarm northwest of Yellowstone Lake began, and how the current pace of those earthquakes has lessened.
10 a.m.: The article was updated to reflect that Thursday's earthquake was the most powerful in the region in nearly 60 years. Also added was information on the largest earthquake in the historical record in this area.
2:30 a.m.: This article was updated to reflect past earthquakes greater than magnitude 5 in the last two decades.
2:15 a.m.: This article was updated to reflect the restoration of power in the town of Lincoln.
1:50 a.m.: This article was updated throughout with additional details and background.
12:35 a.m.: This article was updated with reports of a second quake.