cryoseism

cryoseism, the sudden fracturing of soil or rock caused by rapid freezing of water in saturated ground. Such seismic events are sometimes mistaken for true earthquakes because they produce seismic vibrations, loud booms, jolts, and shaking at the ground surface. Cryoseisms may also occur in polar and mountainous regions where glaciers lurch forward intermittently, sliding or skidding against a rocky substrate, on their journey toward the ocean.

Cryoseisms are not triggered by tectonic events, such as shifting tectonic plates or rising magma. They occur when liquid groundwater saturates pores in the soil and cools rapidly to solid ice. Water is at its densest at 4 °C (39.2 °F), and it expands when it freezes. The freezing of water and its subsequent expansion begin at the surface and continue downward into the soil. Ice forming at or near the surface creates a barrier that hinders the release of pressure that builds up in the surrounding rock as well as in the frozen soil below. The pressure may increase until the rock and soil fracture under the stress. The sudden release of energy that results may be strong enough to produce audible booming sounds (called cryoseismic booms) and seismic waves (which may be recorded by seismographs).

Cryoseisms occur in regions subject to rapid changes in air temperature, specifically in areas subject to incursions of cold air and under conditions characterized by little or no snow cover. Under these circumstances, temperatures can fall below freezing very quickly. Such sudden deep-freeze events may take place in Canada and in several of the Midwestern and Northeastern states of the United States. The cracking, shaking, and sounds associated with cryoseisms often occur between midnight and dawn—typically the coldest part of the day.