Residents of Ozaukee County, Wisconsin were startled by a loud boom with no visible source last Saturday. Some even reported that windows rattled and their houses shook. What felt like a miniature earthquake was actually a cryoseism, or frost quake.
Cryoseisms have been documented since the early 19th century. They are caused by an explosive release of pressure underground, when water which has drained into cracks in rocks expands as it freezes. Under most circumstances this expansion causes frost weathering, only widening cracks and gradually breaking up exposed rocks. During a cryoseism the pressure is released all at once, when a large section of rock or frozen soil fractures under the strain.
These quakes are rare because they appear to require specific conditions: saturated ground and a rapid drop in temperature. These allow the water on top to freeze first, capping the water below so that pressure can build up as it freezes.
While frost quakes affect a small area compared to earthquakes, they can be intense, causing plaster to fall and moving furniture inside buildings. However, there was no reported structural damage from the Ozaukee quake.
