Liquefaction (Earthquake Trigger)

Primary reference(s)

AGI, 2017. Liquefaction [soil]. American Geosciences Institute (AGI). Accessed 14 October 2002.

Additional scientific description

For liquefaction to occur, the shear strength of the soil volume (e.g., the strength due to contact between individual soil grains) must be reduced to near-zero. In the case of earthquakes, strong shaking applies a cyclic load to the soil body. If the soil body compresses under this load, the pore-water pressure will increase, causing the grains to separate thus reducing soil strength (Kramer, 1996).

Soil compression increases the pore-water pressure, causing the water to move toward the Earth’s surface where pressure is lower. Under typical loading (e.g., from temperature changes, increased groundwater), the water then drains, and contact between grains retain their strength. However, when loading cycles occur rapidly, such as during an earthquake, intermittent drainage is prohibited, and liquefaction may initiate (Kramer, 1996).

The following characteristics are common to deposits most susceptible to liquefaction (Kramer, 1996):

• Loose, sandy soils (but liquefaction has occasionally been observed in gravels and coarse silts)
• Rounded, well-sorted grains (e.g., uniform grain size); these compact most easily
• Recently deposited, especially of Holocene age (<11.7 ky), uncompacted soils including human-made deposits
• Soils that are saturated, below sea level, or within a few meters of groundwater.

Some of the most common landforms in which liquefaction occurs are marshlands, riverbanks, beaches, and floodplains. Post-earthquake field studies have shown that earthquake-triggered liquefaction often recurs at the same locations (Kramer, 1996). Earthquake-induced liquefaction can have varied effects on the surrounding built environment. Buildings, infrastructure, and utilities normally supported by the soil may sink, or undergo cracking or other structural damage; pile foundations may buckle or tilt; and lightweight, buried masses such as pipelines may become buoyant and float to the surface. Liquefaction can also cause rapid settling of sediments, flooding (including breaches of earthen embankments or other retaining structures), and lateral spreading of soils (Kramer, 1996).

In general, sites closer to an earthquake’s epicentre are more likely to liquefy, while the distance at which sites are susceptible to liquefaction increases with moment magnitude (MW) and the duration (or number of cycles) of ground motion.

The smallest earthquake for which liquefaction records exist was MW ~ 5, with the most distant observed liquefaction reaching only ~2 km; by contrast, the most distant liquefaction for an earthquake of MW >7, may exceed 100 km (Ambraseys, 1988). During the 2011 MW 9.0 Tohoku earthquake, damage due to liquefaction occurred at least 250 km from the epicentre (Yamaguchi et al., 2012).

Liquefaction susceptibility can be assessed in advance of earthquakes (e.g., Lirer at al., 2019). Often, this is based on a simplified indication of a site’s likelihood to liquefy. A common approach is the liquefaction potential index (LPI), which considers a factor of safety against liquefaction, the layers of earth that might liquefy, and the proximity of these layers to the ground surface (Iwasaki et al., 1984). While several methods are available for determining the factor of safety, they generally reflect the ability of the soil to resist the power of an earthquake. Soil resistance is either measured in situ or estimated based on the surficial deposits and hydrological conditions (Kramer, 1996; Witter et al., 2006). The comparison to earthquake power can be deterministic for the worst-case scenario earthquake (Orhan et al., 2013), or probabilistic for the range of possible earthquakes that could occur (Witter et al., 2006).

Metrics and numeric limits

Liquefaction susceptibility maps (also called liquefaction hazard maps) are currently not available on a global scale but are often provided by the geological agencies in a region. See USGS (no date) for an example of a liquefaction map for the San Francisco Bay Area.

Examples of drivers, outcomes and risk management

In-situ testing of liquefaction resistance using standard penetration tests, cone penetration tests, shear wave velocity recordings, and dilatometer tests (Kramer, 1996); land microzonation via LPI or another assessment parameter that prohibits building on susceptible deposits (Lirer et al., 2019); and soil stabilisation via compaction methods or injection of grout, such as vibro stone columns and dynamic compaction (Shenthan et al., 2004).