Credit Office of State Geologist / SD DENR
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.”
Carla Knecht felt it just before midnight.
“My husband said my name a couple times and then he said, ‘Are you OK?’ And I said, ‘yeah,’ and we both said, ‘What was that?’”
What they felt was a vibrating jolt. It lasted about 5 seconds. They felt their bed and the house shake. They heard a low rumble.
“I’ve never felt that before, thank God,” Knecht said, “and I hope I never do again.”
But they’re OK, and so is their house. It’s about 7 miles west of the small town of Bowdle in north-central South Dakota.
Geophysicist Don Blakeman said the Tuesday night earthquake’s magnitude was 3.2, well below moderate severity.