If you hear the ground booming and rattling during a cold snap, don't worry. It's probably just a frost quake.
The name may sound like what happens when an ice dragon lands on the ground during an episode of "Game of Thrones," but it's nothing of the sort. It's actually the combination of the cold temperatures, water and physics at work.
A frost quake, or cryoseism to use its proper name, is not an actual earthquake. An earthquake is a tectonic event that doesn't rely on a combination of geology and meteorology to occur; it's the result of tectonic plates colliding and slipping as the crust and upper mantle of the Earth moves.
A frost quake requires certain weather conditions to be met before it can even occur. First, the ground needs to be saturated with water or ice. This can easily happen as snow melts and the ground takes on more water, whether through absorption or the water trickling in through cracks. A good rainfall before a mass of cold air arrives would work, too.
Which leads to the second condition: There needs to be a sudden drop in temperature. The drop in temperature causes the water that has saturated the ground to freeze. When that water freezes, it expands. As the water, now ice, expands, it breaks up the ground surrounding it. This can split rocks and put pressure on the soil, and that in turn creates loud booming noises.
Too much snow will insulate the ground and prevent the ground from becoming cold enough for the water to freeze and then expand. If there isn't any insulation, and the conditions are just right, the booms can be quite loud. Residents of Chicago reported hearing frost quakes during a recent polar vortex blast, and likened them to someone bodyslamming the house or to the blast of a shotgun or a tree falling.
Despite knowing how they happened, we don't know a great deal about frost quakes. According to Maine's Department of Agriculture, Conservation and Forestry, "there is not much scientific data about cryoseisms" since they normally only have "minor effects" and occur only under certain conditions that are difficult to predict. They're rare enough that when Live Science reached out to the National Weather Service office in North Dakota to ask about them, meteorologists there hadn't heard of frost quakes, though one person acknowledged having heard of cryoseisms, but would have to look up what they were.
Still, some research has been done on the winter weather phenomenon. A 2016 paper posted to ResearchGate, a social media site for academics, suggests that since more reports of frost quakes are occurring, at least in southeastern Canada, they may be indicators of "large-scale atmospheric pattern changes." What the researchers suggest is that because there are more thaw days — days in which the ground becomes more saturated with water — the sudden cold snaps that cause the frost quakes will become more prevalent.
So just stay alert the next time you hear a bang during a snowy night.