The ground rumbled and it sounded like a big charge of dynamite exploded, farmer Ed Windom recounted to the local newspaper.
It was mid-morning in July of 1975, and an earthquake had hit Morris, Minnesota.
It was no quake like the 7 magnitude one that collapsed roads and knocked down walls in Alaska last week, but the 4.6 to 4.8 magnitude Morris earth-shaker was big enough to crack plaster, knock pictures off walls and objects off of shelves.
That’s not the only earthquake to ever strike the North Star state. Here’s a look at Minnesota’s seismic situation.
How earthquakes work
A quick middle school earth science refresher: Earth’s continents are situated on plates, pieces of the earth’s crust and top mantle that move around. Sometimes as they shift, their movement causes earthquakes.
You tend to get the biggest earthquake action where plates are grinding against one another or where one dives below another, as in the case of the Alaska quake, said Val Chandler, a senior scientist Minnesota Geological Survey. (You can read a report he wrote about Minnesota earthquakes here.)
You also get them when the plates are moving along one another, which is the problem along the San Andreas fault in California, along where Pacific plate and the North American plate meet. Those plates create a lot of tectonic friction as they move horizontally by one another.
By now you might be thinking: Minnesota’s pretty far from any of those plate boundaries. How does it have any earthquakes at all?
Minnesota is pretty squarely in the middle of the North American plate. Being so far away from plate boundaries is the main reason we don’t have a lot of earthquakes here. In fact, Minnesota is one of the least seismically active states in the U.S.
But Minnesota still does have some earthquakes. Why that is isn’t entirely clear, Chandler said.
“The prevailing idea is that the movement of the North American plate away from the mid-Atlantic ridge toward the subduction zones on the Pacific Coast set up a subtle, but pervasive stress in the interior of the continent,” Chandler said. “That stress might be sufficient to jostle old faults that formed many millions or billions of years ago in Minnesota’s case, that remain, even though the tectonic forces that built those faults are long since quiet.”
Look at a map of where the ground’s quaked in Minnesota at a magnitude of 2.5 or greater in the last several decades and most of what you find are mining or quarry explosions which tend to have magnitudes around 3 (explosions are shown as diamonds in the map below — the circles represent actual earthquakes).
The supposed biggest quake recorded in Minnesota history happened in Long Prairie in the 1860s. Its magnitude was an estimated 5. Estimated, because seismographs weren’t used to measure a significant Minnesota earthquake until later.
Several other earthquakes in the state’s history have been big enough to get people’s attention.
The Little Falls Herald reported that a 1917 Staples quake was felt in nearby Darling, Minnesota: “Darling people felt an earthquake on Monday afternoon. No damage resulted. Some people thought the war had already hit Darling” (this was months after the U.S. entered WWI).
They noticed the Staples quake in Lincoln, Minnesota, too. The Little Falls Daily Transcript reported boaters on a lake felt a jolt from the earthquake, which lasted about 20 seconds,
Chandler remembers hearing an account from someone driving into town in a horse-drawn buggy that day. The horses went wild, but the jostle of the buggy made the earthquake imperceptible to the riders. When they got to town, though, they saw some destruction.
The Morris quake was the most destructive quake in recent memory, Chandler said, but any injuries were minor — likely from excitement in the quake’s aftermath, Chandler said, such as reports of a person falling down a flight of stairs.
A local A&W drive-in manager got some press after the seismograph he built out of a coffee can, an erector set and a cheap pen was the only one to capture the Morris quake. The University of Minnesota’s wasn’t operating at the time it happened, and a Rapid City one had been disconnected due to construction, according to the Morris Sun.
In all, there have been 20 small to moderate recorded quakes in Minnesota. None of them have registered above a 5 in recorded or estimated magnitude.
Because the scale is logarithmic, an earthquake 1 point larger in magnitude than another is actually 10 times bigger in terms of ground movement and much stronger. (You can calculate the relative size and strength of earthquakes yourself here.)
So, while an earthquake with a magnitude of 5 might sound like nothing to scoff at next to a mag 7 like Alaska’s, it’s actually pretty scoff-worthy: a magnitude 7 quake is 100 times bigger and 1,000 times stronger than a magnitude 5.
The depth of quakes matters when it comes to strength, too. Typically, quakes happen 4 or 5 miles below the surface of the earth, Chandler said. When the shake-up is shallower, it may be more destructive than a deeper quake.
People like to ask Chandler whether there could be a huge quake in Minnesota. Is it possible? Sure.
“I always tell them (worrying about it’s) for people who like to worry a lot,” he said. The chance isn’t zero, but it’s also not big. It’s definitely not Minnesota’s most pressing natural disaster threat.
“Given the risk of that versus things that have a well-proven record of damage, like violent spring storms and tornadoes and blizzards, there’s enough to worry about,” he said.