What is liquefaction? Liquefaction may occur when water-saturated sandy soils are subjected to earthquake ground shaking. When soil liquefies, it loses strength and behaves as a viscous liquid (like quicksand) rather than as a solid. This can cause buildings to sink into the ground or tilt, empty buried tanks to rise to the ground surface, slope failures, nearly level ground to shift laterally tens of feet (lateral spreading), surface subsidence, ground cracking, and sand blows.

Because of extensive urban development in Northern California since 1906, the strong earthquakes expected in the coming decades may be very destructive. For example, a magnitude 7 earthquake occurring today on the Hayward Fault (a part of the San Andreas Fault system, along the densely populated eastern side of San Francisco Bay) would likely cause hundreds of deaths and almost $100 billion of damage. In 1990, the USGS reported that there is a 67% chance that one or more quakes of about magnitude 7 or larger will occur in the San Francisco Bay area before the year 2020.

Future strong temblors in Northern California are inevitable, but the damage they cause can be reduced significantly with adequate preparation. Studies of earthquake shaking, active faults, and the response of structures to shaking have already led to improved building codes and a better understanding of how to reduce the threat posed by earthquakes.

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The following information is contained in the DRAFT Guidelines for liquefaction studies under consideration by the California Division of Mines and Geology (CDMG). They are being provided here as a service. For more information on the CDMG Program, click here.

Liquefaction is a process by which water-saturated materials (including soil, sediment, and certain types of volcanic deposits) lose strength and may fail during strong ground shaking. Liquefaction is defined as "the transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore-water pressure" (Youd, 1973, p. 1).

Liquefaction occurs worldwide, commonly during moderate to great earthquakes. In California, liquefaction-related ground failures occurred in 1857 (Fort Tejon earthquake), 1906 (San Francisco earthquake), 1933 (Long Beach earthquake), 1971 (San Fernando earthquake), 1973 (Point Mugu earthquake), 1979 and 1981 (Imperial Valley earthquakes), 1989 (Loma Prieta earthquake), and 1994 (Northridge earthquake), and others.

Four kinds of ground failure commonly result from liquefaction: lateral spread, flow failure, ground oscillation, and loss of bearing strength.

Lateral Spread
Lateral displacement of surficial blocks of sediment as the result of liquefaction in a subsurface layer is called a lateral spread. Once liquefaction transforms the subsurface layer into a fluidized mass, gravity plus inertial forces that result from the earthquake may cause the mass to move downslope towards a cut slope or free face (such as a river channel or a canal). Lateral spreads most commonly occur on gentle slopes that range between 0.3° and 3°, and commonly displace the surface by several meters to tens of meters. Such movement typically damages pipelines, utilities, bridges, and other structures having shallow foundations. During the 1906 San Francisco earthquake, lateral spreads causing displacement of only a few feet damaged every major pipeline that broke. Thus, liquefaction compromised the ability to fight the fires that caused about 85 percent of the damage to San Francisco.
Flow Failure
The most catastrophic mode of ground failure caused by liquefaction, flow failure usually occurs on slopes greater than 3°. The flows are principally liquefied soil or blocks of intact material riding on a liquefied subsurface zone. Displacements are commonly tens of meters, but in favorable circumstances, has displaced material tens of miles at velocities of tens of miles per hour. The extensive damage to Seward and Valdez, Alaska, during the 1964 Alaska earthquake was caused by submarine flow failures.
Ground Oscillation
When liquefaction occurs at depth but the slope is too gentle to permit lateral displacement, the soil blocks that are not liquefied may decouple from one another and oscillate on the liquefied zone. The resulting ground oscillation may be accompanied by the opening and closing of fissures and sand boils, potentially damaging structures and underground utilities.
Loss of Bearing Strength
When a soil loses strength and liquefies, loss of bearing strength may occur beneath a structure, possibly causing the building to settle and tip. If the structure is buoyant, it may float upward. During the 1964 Niigata, Japan, earthquake, buried septic tanks rose as much as 3 feet and structures in the Kwangishicho apartment complex tilted as much as 60°.

Research into the process and consequences of liquefaction in past earthquakes have linked liquefaction to certain hydrologic and geologic settings, characterized by water-saturated, cohesionless, granular materials situated at depths of less than 40 feet. In simplified terms, the procedure used to delineate areas having significant potential for liquefaction requires development of a liquefaction susceptibility map and a liquefaction opportunity map. The former depicts areas where the geology and hydrology are favorable for liquefaction, and the latter summarizes information about the potential for strong earthquake shaking. When considered together, the two maps determine the liquefaction potential— the relative likelihood that an earthquake will cause liquefaction in an area. The following areas are those identified as being favorable for liquefaction:

  1. Areas known to have experienced liquefaction during historic earthquakes.
  2. Areas of uncompacted fills containing liquefaction susceptible material that are saturated, nearly saturated, or may be expected to become saturated.
  3. Areas where sufficient existing geotechnical data and analyses indicate that the soils are potentially liquefiable.
  4. Areas containing young (less than 15,000 years) soils where there is limited or no geotechnical data.

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