||The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (August 2011)|
A cryoseism, also known as a frost quake, may be caused by a sudden cracking action in frozen soil or rock saturated with water or ice. As water seeps down into the rock, it freezes and expands, putting stress on surrounding rock. This builds up until it is relieved explosively in a cryoseism.
Another form of cryoseism, known as an ice quake, is a non-tectonic seismic event caused by sudden glacial movements, sometimes due to the formation of a thin veneer of liquid water under a glacier from surface melting. Usually, a glacier would stick to the rough bedrock, but the hydraulic pressure acts as a lubricant, allowing it to slide up to many metres at once. This type of cryoseism can last for tens of minutes.
As the requirements for cryoseisms to occur are not yet defined, their prediction is still not possible and constitute a significant factor in structural design and engineering. Links have been made between global warming and the frequency of cryoseisms.
Cryoseisms are often mistaken for minor or intraplate earthquakes. Although the outward signs often appear similar to those of an earthquake, with tremors, vibrations, ground cracking and related noises such as thundering or booming sounds, cryoseisms can be distinguished from earthquakes through meteorological and geological conditions. Cryoseisms can have an intensity of up to VI on the Modified Mercalli Scale. Furthermore, cryoseisms often exhibit high intensity in a very localized area, in the immediate proximity of the epicenter, as compared to the widespread effects of an earthquake. Due to lower frequency vibrations than earthquakes, some seismic monitoring stations may not record their occurrence. Although cryoseisms release less energy than most tectonic events, they can still cause damage or significant effects at the site.
Some reports have indicated the presence of "distant flashing lights" before or during a cryoseism, possibly due to the electrical changes when the rocks are compressed. Cracks and fissures can also appear, as the ground may contract and split apart from the cold. These, usually superficial, cracks range from several centimetres to several kilometres long, with either a single linear fracture or multiple ones as well as small vertical and moderate lateral displacement.
Geocryological processes were identified as the cause of tremors as early as 1818. In the United States, such events have been reported in the states of Connecticut, Maine, Massachusetts, Michigan, Ohio upstate New York, Vermont and Wisconsin, all in the Northern and Northeastern United States. Cryoseisms have also surfaced in Ontario, Canada. Also, glacier-related phenomena have been reported in Greenland, Alaska, the Antarctic Prince Charles Mountains and Ross Island, and Iceland (Grímsvötn).
Cryoseisms typically occur when temperatures rapidly decrease from above freezing to subzero, in the first cold snap of spring, usually between midnight and dawn (during the coldest parts of night). However, due to the permanent nature of glacial ice, glacier-related cryoseisms may also occur in the warmer months of summer. In general, cryoseisms occur 3 to 4 hours after significant changes in temperature. Perennial or seasonal frost conditions involved with cryoseisms limit these events to temperate climates that experience seasonal variation with subzero winters. Additionally, the ground must be saturated with water, which can be caused by snowmelt, rain, sleet or flooding. The site of a cryoseism generally has little or no snow cover to insulate the ground. Geologically, areas of permeable materials like sand or gravel, which are susceptible to frost action, are likelier candidates for cryoseisms. Following large cryoseisms, little to no seismic activity will be detected for several hours, indicating that accumulated stress has been relieved.
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