CLINTONVILLE, Wis. (WCCO) — A series of loud booms has left people in a small Wisconsin town scratching their heads.

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The U.S. Geological Survey confirmed a 1.5 magnitude earthquake just west of Green Bay in Clintonville, Wis., last week. Late Tuesday night, another loud “boom” has people fearing round two.

“It’s not unheard of that you would get some minor quakes in the Midwest, even Wisconsin,” said Val Chandler, a geophysicist with the Minnesota Geological Survey.

The Midwest certainly is not an earthquake epicenter like California, but we every 10 or 20 years there’s an earthquake strong enough to notice. In 1993, a 4.1 magnitude quake hit the small west-central Minnesota town of Graceville.

“They had a minor amount of cracking of plaster, a few people reported knickknacks knocked over on their shelves,” said Chandler.

The Wisconsin earthquake was only a 1.5 on the Richter scale, making movement nearly imperceptible. However, like the Clintonville earthquake, the 1993 quake also had an audible ‘boom.’

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“Turns out to be fairly typical of Midwestern earthquake because of their shallow focus,” said Chandler.

Mid-continent quakes are just noisier. The seismic waves generated by the quake react with the atmosphere in such a way to cause a loud noise.

As far as why these quakes are happening now, it could be any number of things. A relatively dry winter and less ground water could play a role, as could frac sand quarrying in the area. Just as likely, though, is a seemingly ancient cause. The earth might just be rebounding from the movement of glaciers thousands of years ago.

And because these quakes are so rare, it could be weeks or longer, before their cause is known.

Or as Chandler said, we may never know.

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“Sometimes these things are bump in the night kind of things that are never explained to everyone’s satisfaction,” he said.