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Earthquake liquefaction, often referred to simply as liquefaction, is the process by which saturated, non-cohesive soil (sand and silt) loses shear strength during seismic shaking and behaves like a liquid, rather than a solid. The effect on structures and buildings can be devastating, and is a major contributor to urban seismic risk.

Sand Compaction

Liquefaction occurs when a saturated sand formation is subject to cyclic shaking. The shaking causes increased pore water pressure which reduces the effective stress, and therefore reduces the shear strength of the sand. Soils most prone to liquefaction are loose sands between layers of lower permeability soil that prevent rapid dissipation of cyclic pore pressures. As seen in the figure, uniform sand grains can be packed either in a loose or a compact (dense) formation. Loose sand has usually been deposited gently underwater, either naturally, or sluiced into what is called hydraulic fill. The loose grains can support considerable weight, as they are in contact with each other in a statically stable formation.

Once strong earthquake shaking begins, the grains are separated by high pore water pressure and are no longer resting on each other. Eventually, the grains will settle into a more compact arrangement. However, this transition is not immediate, and requires excess water to leave the formation. For a short period of time, depending how long it takes for the water to drain from the formation, the grains float in a liquid slurry. The excess water is squeezed out which causes the quicksand condition at the surface. If there is a dry soil crust or impermeable cap, the excess water will sometimes come to the surface through cracks in the confining layer, bringing liquefied sand with it, creating sand boils, colloquially called "sand volcanos".

A liquefaction susceptibility map - excerpt of USGS map for the San Francisco Bay Area. Many areas of concern in this region are also densely urbanized.

Soil liquefaction can cause damage to structures in several ways [1]. Buildings whose foundations bear directly on sand which liquefies will experience a sudden loss of support, which will result in drastic and irregular settlement of the building. Liquefaction causes irregular settlements in the area liquefied, which can damage buildings and break underground utility lines where the differential settlements are large. Sand boils can erupt into buildings through utility openings, and may allow water to damage the structure or electrical systems. Soil liquefaction can also cause slope failures. Areas of land reclamation are often prone to liquefaction because many are reclaimed with hydraulic fill, and are often underlain by soft soils which can amplify earthquake shaking. Soil liquefaction was a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.

Study of past liquefaction events in geologic formations can provide information about the strength of prehistoric earthquakes, a vital component of paleoseismology [2].

Liquefaction primarily occurs in sands, but there are formations of quick clay, which can fail in a very similar way.

Mitigating potential damage from liquefaction is part of the field of geotechnical engineering.

• Soil liquefaction
• Quick clay
• Geotechnical engineering

• Shaking, Liquefaction on Harbor Island. One of the few known live observations of an earthquake liquefaction event by a seismologist.
• Soil liquefaction web page at University of Washington
• Earthquakes and the New Madrid Fault Line