Liquefaction

Soil liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. During strong shaking, water-logged sediments at the Earth’s surface temporarily behave like a liquid, causing structures and other objects to shift or sustain major damage. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

Liquefaction occurs in saturated soils, where the space between individual soil particles is completely filled with water. Prior to an earthquake, the water pressure within the soil is relatively low and the weight of the buried soil rests on the framework of grain contacts that comprise it. However, earthquake shaking can disrupt the soil structure, so that the soil particles no longer support all the weight and the groundwater pressure begins to rise. The soil particles can move farther, become entrained in the water, and the “liquefied” soil begins to flow. Liquefied soil will force open ground cracks in order to escape to the surface. The ejected material often results in sand boils or flooding and may leave cavities in the soil. Once shaking stops, the soil re-solidifies, locking tilted structures into place.

Whether and where liquefaction will take place depends on many factors. These include: the degree of soil saturation; local grain size distribution and consistency at a site; the strength, duration, and frequency content of the shaking; and even the grain shape and depth of soil. Liquefaction susceptibility is generally highest in places where the soil is less consolidated or more highly saturated, such as reclaimed land (landfill) or river valleys. 

Areas with a high to moderate susceptibility to liquefaction are often mapped in urban areas. This mapping has been done by various agencies in the Pacific Northwest. Relevant links can be found below.

WASHINGTON: Liquefaction susceptibility maps are available for Washington through the Washington Geologic Survey's Geologic Information Portal, under Earthquakes > Ground Response > Liquefaction Susceptibility.

OREGON: Liquefaction susceptibility maps are available for Oregon through the Oregon Department of Geology and Mineral Industries (DOGAMI) through their Statewide Geohazards Viewer, under Earthquake Hazard > Liquefaction Susceptibility Map.

The following videos show examples of liquefaction and resulting damage from the magnitude 6.2 Christchurch, NZ, earthquake on February 22, 2011.