Northwest Montana rattled by two more earthquakes felt in McGregor Lake area

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McGregor Lake

Just three weeks after a pair of rare earthquakes rattled windows and bookshelves, two more seismic events were reported in the Flathead Valley and scientists say this could be an indication that a seismic "swarm" has begun in the area.

On Dec. 1 at 5:40 a.m., a 3.0 magnitude earthquake shook the McGregor Lake area, almost the exact same area impacted by the 4.0 quake on Nov. 11. The epicenter of the earthquake was 3.6 miles beneath the surface. Less than three hours later at 8:13 a.m., a 3.5 magnitude earthquake 5.3 miles below the surface shook the same exact area.

Mike Stickney, director of the Montana Bureau of Mines and Geology's Earthquake Studies Office, said there were a number of reports from local residents who said they felt the second quake.

Earthquakes are rare but not unheard of in Northwest Montana and Stickney said the series of shakes could indicate that a seismic "swarm" is beginning.

An earthquake swarm is a series of minor to moderate earthquakes in a single area that can last for months. They are common in places with lots of volcanic activity, such as Yellowstone National Park, but also happen in places with little volcanic connections, such as Northwest Montana.

"Swarms of earthquakes can last anywhere from a few hours to a few months," Stickney said. "They're not well understood."

Stickney said the 3.9 earthquake on Nov. 14 that shook the Whitefish area was most likely not related to the McGregor Lake swarm.

"It can be frightening for people to feel repeated earthquakes and you may start wondering what's going on with the earth but as of right now we have no indication that this activity is leading up to a major seismic event," Stickney said.

Stickney said Northwest Montana has a history of earthquake swarms, most notably in the Polson area from 1969 until 1971. In April 1969, a 4.7 magnitude quake caused damage in Big Arm, Dayton, Lake Mary Ronan and Polson. There were at least 21 aftershocks felt during the following month and 325 minor aftershocks were reportedly felt from May 1969 until December 1971. In 1995, another swarm of earthquakes impacted the Kila area for months.

The largest earthquake to ever hit Montana in recorded history was near Hebgen Lake, near Yellowstone National Park, in 1959. The 7.3 magnitude quake resulted in 28 fatalities and $11 million in damage.

Stickney said that earthquakes are hard to predict but that this recent swarm of quakes in Northwest Montana are a subtle reminder that the state is "earthquake country" and prone to seismic activity.