A glacial cryoseism is a non-tectonicseismic event of the glacial cryosphere. A large variety of seismogenic glacial processes arising from internal, ocean calving, or basal processes have been identified and studied. Very large calving events in Greenland and Antarctica have been observed to generate seismic events of magnitude 5 or larger. Extremely large icebergs can also generate seismic signals that are observable at distances up to thousands of kilometers when then collide or grind across the ocean floor. Basal glacial motion be enhanced due to water accumulation underneath a glacier sourced from surface or basal ice melt. Hydraulic pressure of subglacial water can reduce the friction at the bed, allowing the glacier to suddenly shift and generate seismic waves. This type of cryoseism can be very brief, or may last for many minutes. A cryoseism, ice quake or frost quake, may also refer to a seismic event caused by a sudden cracking action in frozen soil or rock saturated with water or ice, or by stresses generated at frozen lakes. As water drains into the ground, it may eventually freeze and expand under colder temperatures, putting stress on its surroundings. This stress builds up until relieved explosively in the form of a cryoseism. The requirements for a cryoseism to occur are numerous; therefore, accurate predictions are not entirely possible and may constitute a factor in structural design and engineering when constructing in an area historically known for such events. Speculation has been made between global warming and the frequency of cryoseisms.
Effects
Cryoseisms are often mistaken for minor intraplate earthquakes. Initial indications may appear similar to those of an earthquake with tremors, vibrations, ground cracking and related noises, such as thundering or booming sounds. Cryoseisms can, however, 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 of cryoseisms, some seismic monitoring stations may not record their occurrence. Cryoseisms release less energy than most tectonic events. Since cryoseisms occur at the ground surface they can cause effects right at the site, enough to jar people awake. Some reports have indicated the presence of "distant flashing lights" before or during a cryoseism, possibly because of electrical changes when rocks are compressed. Cracks and fissures may also appear as surface areas contract and split apart from the cold. The sometime superficial to moderate occurrences may range from a few centimeters to several kilometers long, with either singular or multiple linear fracturing and vertical or lateral displacement possible.
Cryoseisms also occur in Canada, especially along the Great Lakes/St. Lawrence corridor, where winter temperatures can shift very rapidly. They have surfaced in Ontario, Quebec and the Maritime Provinces. They are also observed in Calgary.
There are four main precursors for a frost quake cryoseism event to occur: a region must be susceptible to cold air masses, the ground must undergo saturation from thaw or liquid precipitation prior to an intruding cold air mass, most frost quakes are associated with minor snow coveron the ground without a significant amount of snow to insulate the ground, and a rapid temperature drop from approximately freezing to near or below zero degrees Fahrenheit, which ordinarily occurred on a timescale of 16 to 48 hours. Cryoseisms typically occur when temperatures rapidly decrease from above freezing to subzero, and are more than likely to occur between midnight and dawn. In general, cryoseisms may 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. 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.
