JCP Reports - Hazard Information

Hazard Information - Liquefaction

By JCP Geologists

Since the 1994 Northridge earthquake, a lot of attention has been directed to a geologic hazard called "liquefaction." There was widespread evidence of liquefaction occurring as a result of that quake in Los Angeles County, but it appears to have had little impact on homes and other structures. Although liquefaction is not common, areas where this phenomena could potentially occur have been mapped for areas throughout California. These liquefaction zones are found as either Seismic Hazard Zones mapped pursuant to the Seismic Hazard Mapping Act or in the Seismic Safety Element of the General Plan for many cities and counties as well as on Seismic Hazard Maps. This information is included in the JCP Natural Hazard Disclosure Statement.

What is liquefaction?

Just what is liquefaction? Well, the following should answer just about every question you may have about this uncommon geologic event. Liquefaction is a soil phenomenon that can occur during intense earthquake shaking under certain conditions. The types of soils that are most susceptible to liquefaction are loose, water saturated, fine-grained sands and silty sands that lie within 50 feet of the ground surface.

"Liquefaction" is when soil temporarily becomes soft and liquid-like during very strong ground shaking. As a result, the soil may be unable to fully support something on the surface. "Cyclical loading" is the characteristic of earthquake shaking that causes this to happen. What we feel as ground shaking during an earthquake is really waves of energy passing through the ground. These waves are much like the waves that radiate outward from a stone thrown into a pool of water.

If liquefaction-susceptible soil is water saturated, strong earthquake shaking causes the water pressure in the soil to increase as the waves of energy pass through it (this is called "loading"). If the ground shakes long enough, the water pressure may increase until it equals the weight of the overlying soil. What this means is that the water pressure pushing up against the soil equals the weight of soil pushing down. When these forces are equal, the soil "floats." The consequence is the soil becomes mobile enough to move vertically or sideways if there isn't anything to hold it in place. For example, liquefied soils will "boil" up through cracks in the ground or pavement. Logically enough, these are called "sand boils."

During the 1994 Northridge earthquake in Los Angeles County, sand boils were observed in localized areas along the coast as far north as Mugu Lagoon and as far south as the Port of Los Angeles. In the inland areas, sand boils were seen in the Santa Clara River Valley, Simi Valley, Potrero Canyon, and at Hansen and Los Angeles Dams. In other areas, liquefaction is suspected to have caused some of the damage that occurred, but there was no direct evidence of it.

What Does this Mean to Buyers?

In discussing liquefaction or any other type of earthquake risk, two aspects of that risk need to be taken into account. The first is the risk of earthquakes occurring. The second aspect is the risk of damage. Most everyone understands that the risk of earthquakes occurring in California is higher than most of the country. This is because most of the developed land lies near one of the most active faults in the country - the San Andreas fault. Many other active and potentially active faults also exist throughout California.

At present, it is not possible to predict when large magnitude earthquakes will occur except in very general terms. Techniques for accurately forecasting earthquakes are currently being researched, but, at present, no reliable methods are available - yet.

Much of the earthquake damage we see from California earthquakes results from poor quality construction practices, buildings not built according to current standards (older structures), or poor maintenance (no earthquake bolting, termite damage, etc.). All of these factors are very important when considering earthquake risk. A quality home inspection is a good investment and helps to determine if any of these adverse conditions exist. In many cases, the construction practices are of greater importance than local geology. However, if you or your clients have any questions about liquefaction or other geologic hazards, please give us a call at JCP and one of our geologists will be happy to help.