DES MOINES, Iowa — "What's that sound?"
"Is the ground shaking?"
If you've ever found yourself asking these questions on a snowy, bitter cold winter day in Iowa, don't worry: you're not imagining anything.
You may have experienced a frost quake, also known as a cryoseism.
Frost quakes are fairly rare, but do occur most often when frigid outdoor temperatures in the winter cause groundwater or water underneath the surface to freeze and form ice.
This sometimes occurs after a heavy rainfall event or a significant snow melt, as there is excess water underneath the ground's surface.
As the ice forms under the surface, it continues to expand, putting substantial pressure on nearby soil and bedrock.
While the pressure continues to build, the ice pushes on the soil and bedrock until the pressure becomes too great, and it forces the soil and rock to crack.
Cracks in the soil and rock can make rumbling noises and, if big enough, can even cause the ground to shake a bit, much like what you might expect from a minor tremor or earthquake.
Although frost quakes may seem similar to earthquakes, they are quite different.
Earthquakes occur as a result of movement between Earth's tectonic plates, which are roughly 80 to 100 miles beneath the surface of the ground.
On the other hand, the soil and bedrock cracking to form frost quakes occurs much closer to the surface.
As a result, frost quakes, while a bit startling, are unlikely to affect you or your home in any substantial way.
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