Residents in parts of Wisconsin reported a loud boom, rattling windows, and some major concern late last week. What residents of Ozaukee County, the Green Bay area and southwestern Wisconsin experienced is something called a “frost quake.” Characterized by sudden, loud booms or rumbling, these rare natural events can occur when the temperatures start to drop.
The technical term for this phenomenon is cryoseism. Basically, it happens after the ground becomes saturated and wet and then dropping temperatures cause that water to freeze and therefore expand. When it happens in a concentrated area, the pressure becomes too much and has to be released - that's the boom that folks heard.
Frost quakes happen a good amount in the midwest thanks to our wildly varying winter weather. The Great Lakes can wreak havoc with different weather fronts and the collision of warm and cold air often leaves the Midwest with all kinds of odd weather.
Officials say there were likely a handful of quakes that occurred in Ozaukee County a week ago. After it snowed a decent amount early in the week, temperatures soared up near 50, melting all that precipitation and soaking the already-wet ground.
Though we're used to the cold here, it's rare that there would be a sudden freeze of underground water, causing quick expansion and disruption. Usually the ground freezes gradually as we fade from fall to winter, and then a big layer of snow actually acts as a ground insulator. But the fluctuating temps have everything a bit mixed up. Those two days of warmer temperatures and sunshine were likely enough to make a difference in the ground temperature and when the temp rose and the dropped dozens of degrees in a short period of time, the result was those frost quake booms.
Frost quakes don't contain near the release of energy that a regular earthquake would, so their impact is usually quite small and localized. People in neighboring towns could hear and experience completely separate frost quakes. According to the Maine Geological Survey, "Expansion that results during the process of freezing can lead to the buildup of explosive stress, which may result in fractures within the earth. Small cracks may be visible on the surface near where a cryoseism has occurred, and in some cases, shaking vibrations may also be felt within the vicinity of the frost quake, along with loud booms that sound similar to gunfire."
There is another type of cryoseism that can occur due to sudden glacial movement and obviously that's not what we're experiencing here in Wisconsin. Those types of cryoseism tend to be called "ice quakes" to differentiate.
