There are almost 100 documented earthquakes in South Dakota history, including a recent one that shook the Bowdle area.
That 3.2-magnitude quake may have been caused by massive plates shifting along faults underground. Or, said State Geologist Tim Cowman, there could be another cause.
“The thing that probably more likely caused this earthquake,” he said, “is something that we call ‘glacial rebound’ or ‘isostatic rebound.’”
It sounds complicated, but it’s simple.
“During the last Ice Age,” Cowman said, “the ice sheets that covered this part of North America were very thick and put a lot of weight on the Earth’s crust.”
That immense weight caused compression.
“And we think the crust is slowly rebounding back to its near-original form,” Cowman said, “and occasionally there might be a little bit of a jump in that rebound that causes these small earthquakes.”
It's been about 12,000 years since the last ice sheets receded, and Earth’s crust may have been rebounding ever since. Cowman thinks that may have caused the Bowdle quake and many other South Dakota earthquakes.
He gets that impression not only from geology, but also from reading reports by people who’ve experienced the quakes.
“It’s a pretty common theme,” Cowman said. “They start to feel this low rumble, and they might hear some shaking of some things on shelves or on cupboards, and it lasts about 5 seconds, and then that’s it.”
Cowman didn’t trade notes with Carla Knecht, who lives in rural Bowdle and felt Tuesday night’s earthquake. But she described it like Cowman expected: a momentary, loud phenomenon that was scary but caused no damage.
"It was probably 5 seconds, if that long,” Knecht said. “But we definitely felt the house move, or something moving, and we were wondering what the heck it was.
“It was a noise, too, like something that you had never heard before. It was just kind of a rumble, but not a thunder rumble. With the house shaking, too, it just wasn’t something we’d ever felt before, so we knew it wasn’t just regular thunder.”
Cowman experienced an earthquake himself almost 40 years ago near Yankton, when his collegiate interest in geology was budding into a lifelong passion.
“The forces of the earth, the geologic forces that are happening, they really kind of put in perspective just how small we are in terms of the larger scheme of this Earth that we live on,” Cowman said.
Cowman’s office keeps records of South Dakota earthquakes. The earliest quake documented in writing is from 1872, almost 20 years before statehood, near Pierre. Since then, earthquakes have been documented in South Dakota at a rate of about two every three years.
-Contact reporter Seth Tupper by email.
