Cryoseisms (Frost Quakes)

Residents in cold climates sometimes report a loud boom in the middle of a winter night, followed by a distinct shaking of the ground -- with no earthquake, explosion, or storm to explain it. The cause is usually a cryoseism, sometimes called a frost quake, and it has nothing to do with tectonic plates. Cryoseisms occur when a rapid, severe temperature drop follows a period of wet ground or standing water. As the water in the saturated soil freezes, it expands with tremendous force, since ice occupies roughly nine percent more volume than the liquid water it froze from. If that expansion happens quickly enough, the surrounding soil or rock has no time to accommodate the pressure gradually, and it fractures instead, releasing a sudden burst of energy that produces both the sound and the shaking. Unlike a true earthquake, a cryoseism originates at or very near the surface, which is why the boom is often sharp and localized rather than a rolling tremor felt over a wide area. Cryoseisms are most common in regions with sandy or loamy soil that retains a lot of moisture, and they tend to strike in the early hours of the morning, when temperatures are at their lowest point of the day. They cause no lasting seismic risk, unlike tectonic earthquakes, though the sudden noise has been mistaken for gunfire, a collapsing structure, or even a meteor strike by startled residents.