How often do earthquakes happen in Minnesota?

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Windows rattled. Dishes fell from shelves.

Cracks appeared in the foundations of several buildings. And across the border in Clark, S.D., the rumbling caused one woman to tumble out of her bed, according to newspaper accounts.

On July 9, 1975, Minnesota’s largest earthquake ever recorded shook the area around Morris, in the west-central part of the state. It measured 4.6 to 4.8 on the Richter scale.

No one was injured. It did shake people up a little, though.

“State quake mostly surprises people,” read the Minneapolis Tribune headline the next day.

Rhonda Broberg, who lives in Robbinsdale, recorded the event in her childhood diary. “My grandma called — long distance — panicked,” she recalled. “An earthquake had just shaken her house in Johnson.”

The short answer? Not that often, especially compared to states like California and Alaska.

“It’s very infrequent,” said Aaron Hirsch, a geophysicist at the Minnesota Geological Survey. “There have been about nine earthquakes since 1975 that have been of note.”

The reason Minnesota experiences relatively little seismic activity has to do with our location — smack dab in the middle of a tectonic plate — and the age of the rocks beneath our feet, said Hirsch.

Most earthquakes happen along the boundaries of the Earth’s tectonic plates.

“We are nowhere near a plate boundary. We are, like, the near center of the North American plate,” Hirsch said.

The 1975 quake may not have been a tectonic earthquake caused by plate movement at all, Hirsch explained.

“It may have been associated more with what’s called isostatic rebound,” he said.

This process was caused by the glaciers that came down from Canada 14,000 years ago, pushing down on the Earth’s crust, he said.

“If you imagine standing on your bed, you’re going to depress the bed, you know, with your feet, but then when you move your feet, that then rebounds back up,” he said. “The Earth can kind of respond similarly.”

But the Earth’s rebound is happening much, much more slowly. The land is still rising — and earthquakes can happen as this rebound triggers long-dormant faults.

At the time, U.S. Geological Survey geophysicist John Bellini told the Star Tribune that it measured 2.5 in magnitude and likely “felt like a truck rumbling by or thunder.”

Before 1900, earthquake data in Minnesota was not well documented, so it’s difficult to determine the magnitude of earthquakes that happened here before that, according to a 2014 Minnesota Geological Survey paper by geophysicist Val Chandler.

But the Morris earthquake and a 1917 quake in Staples — estimated to have a magnitude of 4.3 based on observations of its intensity — are considered the two largest in state history that are well documented.

“Earthquake Jars Section of Minnesota,” was the headline in the Park Rapids Enterprise after the 1917 quake. While buildings shook in Staples, in Park Rapids “the vibrations were so slight that when the reports first began to gain circulation, those who had not noticed the disturbance invariably looked for the joke,” the paper wrote. “Reports came from so many places, however, that they could not be doubted.”

In 1979, the University of Minnesota’s Harold Mooney estimated that Minnesota’s average recurrence rate for a magnitude 4 earthquake is every 10 years. He put that at 30 years for a magnitude 4.5 quake.

“Although we cannot assign a zero probability of a seriously damaging earthquake occurring in the time span of a human life, the threat is very small compared to other natural hazards that have well-proven records of destruction, such as winter storms, tornadoes, and flooding.”

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