Polar vortex reportedly causes ‘frost quakes’ in Chicago: What is the geological phenomenon?

Freezing temperatures in Chicago — which at one point Thursday were said to be lower than those in Alaska and Antarctica — have reportedly caused a loud, ground-shaking phenomenon to occur: so-called “frost quakes.”

Chicago residents reported hearing “loud booms” Wednesday night when temperatures in the area reached well-below zero, WGN-9 reported.

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The occurrence was likely caused by frost quakes — or "cryoseisms,” as they’re formally called — which occur “when water trapped underground freezes suddenly as the temperature drops, causing it to expand,” Live Science explained. As the water expands, pressure builds under the surface and ultimately causes the surrounding rock and soil to crack, producing a “boom” sound.

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"It's more of a noise phenomenon like a balloon popping than a physical danger," Dave Call, a meteorologist at Ball State University, told the Muncie Star Press of frost quakes.

In some cases, people have reportedly “observed actual rivets in the ground after hearing the loud boom of a frost quake,” Popular Science reported of the phenomenon.

While they may sound similar, frost quakes are not the same as earthquakes — which occur when the two blocks of the earth "suddenly slip past each other," the U.S. Geological Survey explains.