What Is Liquefaction?
Liquefaction is what happens when strong earthquake shaking turns loose, water-soaked sand and silt from a solid into something that behaves like a thick liquid. For a short time the ground loses its strength and acts like quicksand. Buildings resting on it can sink or tilt, and the ground itself can slide.
It is a form of ground failure, separate from the shaking itself, and the Utah Geological Survey counts the valleys of the Wasatch Front among the places most vulnerable to it.
What Causes Liquefaction?
Three things have to line up:
- The right soil: loose sand and silt, usually laid down in the last 10,000 years, within about 30 feet of the surface.
- Water: groundwater within about 30 feet of the surface, so the soil is saturated.
- Strong shaking: strong enough to make those soils liquefy. Liquefaction is rare in earthquakes below magnitude 5.
Rare isn't never. The magnitude 4.5 Randolph earthquake of April 15, 2010, in Rich County, was one of the smallest earthquakes recorded with modern instruments to cause liquefaction.
What Liquefaction Does to the Ground
- Sinking and tilting: buildings can settle into the ground or lean.
- Lateral spreading: nearly level ground, on slopes of only 0.3 to 3 degrees, can slide sideways by as much as tens of feet, tearing apart whatever sits on it.
- Flow failures: on slopes steeper than 3 degrees, liquefied ground can flow downhill. These are the most catastrophic kind.
- Ground cracks and settling: the surface can crack open and drop.
- Sand boils: small eruptions of liquefied sand onto the surface.
Where Liquefaction Is a Risk in Utah
The valleys of the Wasatch Front are especially vulnerable, the Utah Geological Survey says, because they combine susceptible soils, shallow groundwater and a relatively high chance of moderate to large earthquakes. The most susceptible ground is generally along rivers, streams and lake shorelines. Around the Great Salt Lake, sand and silt sit over shallow groundwater.
In a magnitude 7 Wasatch Fault earthquake, the Survey's scenario maps show large ground displacements are most likely in the densely populated northern Salt Lake Valley, the west Bountiful area and the northern end of Utah Valley.
Its liquefaction maps sort ground into four categories by the chance of an earthquake strong enough to liquefy the soil within 100 years:
| Category | Chance in 100 Years |
|---|---|
| High | More than 50% |
| Moderate | 10% to 50% |
| Low | 5% to 10% |
| Very Low | Less than 5% |
Liquefaction in Utah Earthquakes
2020 Magna
The magnitude 5.7 Magna earthquake produced sand boils, tension cracks, lateral spreading and patches of sinking ground near the epicenter, along the Jordan River and along the shore of the Great Salt Lake. Near Saltair, liquefaction showed up in the fill around the Interstate 80 and State Route 202 interchange and in the Great Saltair parking area, with many sand boils underwater. What geologists found confirmed the existing hazard maps, which already showed high liquefaction potential there.
1901, 1934 and 1962
In the 1901 earthquake near Richfield, fine white sand and water spouted from cracks in the ground. The 1934 Hansel Valley and 1962 Cache Valley earthquakes also caused liquefaction. Their stories are in our guide to Utah earthquake history.
2010 Randolph
The magnitude 4.5 Randolph earthquake caused sand boils despite its small size, a reminder that loose, wet ground can fail in shaking far below the level of a major earthquake.
How to Check Your Property
The Utah Geological Survey publishes liquefaction maps for the Wasatch Front, including county liquefaction-potential maps, and shows them on its Utah Geologic Hazards Portal. Search for either by name to look up an address.
A map category describes an area, not a single lot. To know how the ground under a specific building would behave, the Survey says a site-specific geotechnical investigation by a qualified professional is needed. Then follow the steps in our guide to what to do during an earthquake.