Soil Liquefaction is the process by which saturated, unconsolidated soil or sand is converted into a suspension. It is commonly observed in quicksand and as a result of earthquake shock in unconsolidated sediments. It can be caused when flowing water reduces the friction between sand particles (as from an underground spring), or when a sudden change in pressure or repeated shock acting on water saturated or supersaturated sediments (as in an earthquake). Although the effects of liquefaction had been observed and understood for years, it was dramatically brought to the attention of engineers and seismologists in 1964 during the Niigata, Japan and Alaska earthquakes. It was a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.

Another example of perfectly firm soil suddenly becoming liquid, can be observed with the phenonemon of quick clay. It becomes a geologic hazard if human habitation is located on zones of potential liquefaction. The ensuing landslide quickly consumes anything in it's path.

Liquefaction in earthquakes

The shock or repeated shock of earthquake waves can cause water-saturated soil to rearrange itself in such a way that it essentially becomes a suspension of solids in the liquid. Heavy structures on such areas can suddenly sink or shift. Buried objects can shift and relatively low density objects can float to the surface.

"Often during earthquakes, fine-grained water-saturated sediments may lose their former strength and form into a thick mobile mudlike material. The process is called liquefaction. The liquefied sediment not only moves about beneath the surface but may also rise through fissures and “erupt” as mud boils and mud 'volcanoes.'"

"... the ground shaking reduces the strength of earth material on which heavy structures rest. Parts of many major cities, particularly port cities, have been built on naturally occurring bodies of soft, unconsolidated clay-rich sediment (such as the delta deposits of a river) or on filled areas in which large amounts of loose earth materials have been dumped to build up the land level. These water-saturated deposits often experience a change in property known as liquefaction when shaken by an earthquake. The material loses strength to the degree that it becomes a highly fluid mud, incapable of supporting buildings, which show severe tilting or collapse."^{How quicksand forms}

• Soil Liquefaction
• detailed dicussion based on research conducted in 1936
• Niigata earthquake.
• Alaska earthquake.
• For a video showing liquefaction in action.