GREAT FALLS, September 22 — A magnitude 3.6 earthquake struck just north of Great Falls, Montana, on Friday, September 18, at 7:40 a.m., the U.S. Geological Survey reported, the first seismic event near the city since February 26 and the third-strongest recorded in the Great Falls area this year. The quake was centered about 3.1 miles northwest of Black Eagle, Montana, at a depth of approximately 17.2 miles. Its epicenter was nearly identical to that of a magnitude 3.7 aftershock on February 12, 2026.
No injuries, serious damage, or impacts on infrastructure were reported.
A year of aftershocks
The September 18 event follows a sequence that began earlier in 2026. A magnitude 4.2 earthquake shook the region on Thursday, January 29. A smaller magnitude 2.7 quake followed later that evening, then a magnitude 3.2 on January 31, a magnitude 3.7 on February 12, and a magnitude 2.8 on February 26.
Reporting from February 13, 2026, indicated that the four smaller quakes — and probably the one on September 18 — were aftershocks of the larger initial quake. Andres Castro, a research seismologist with the Montana Bureau of Mines & Geology, said in February that the activity near Great Falls appeared to be within the normal range of seismic behavior for the state.
“The three earthquakes that occurred about a week, a week and a half ago, are clearly part of an aftershock sequence,” Castro said. He explained that the most recent event at that time, which occurred slightly farther away, could be part of the same sequence or related to a separate nearby fault, and that determination takes time and detailed analysis. He noted that while earthquakes happen on faults, identifying exactly which fault, or how many are involved, isn’t as simple as it seems.
In many cases, earthquakes occur along faults that are not visible at the surface. “That’s actually very common,” he said.
“Sometimes we don’t see the faults mapped at the ground surface.”
Depth and historical context
Castro said the level of activity is within the normal range of seismic behavior in Montana. Although most earthquakes in Montana occur along the Intermountain Seismic Belt in the western part of the state, central Montana is not exempt. Historical records indicate an earthquake near Great Falls on January 11, 1936, that generated noticeable shaking.
The recent earthquakes occurred at depths ranging from approximately 6 to 13 miles below the surface. That depth range tells seismologists that the rocks in this part of Montana are still behaving in interesting ways at those depths, meaning they fracture and slip along faults rather than slowly deform.
The Montana Bureau of Mines & Geology website notes: “Although it has been over four decades since the last destructive earthquake in Montana, small earthquakes are common in the region, occurring at an average rate of 7-10 earthquakes per day.” The website features several interactive maps, including one example from 2018 that shows the 2,395 earthquakes recorded in Montana that year and additional earthquakes in the surrounding region. All but 23 of those quakes registered as less than 3.0, which demonstrates how small and imperceptible the vast majority are.
At the time of the February reporting, Castro said there was no indication that the recent cluster suggested something larger was coming. “We cannot predict earthquakes in space, or time, or magnitude,” he said.
“Additional aftershocks are possible.” Seismologists will continue monitoring the Great Falls area for further activity, with the Montana Bureau of Mines & Geology maintaining its interactive maps and daily earthquake records for the region.
