What Is Liquefaction?

An automobile lies crushed under the third story of this apartment building in the Marina District. The ground levels are no longer visible because of structural failure and sinking due to liquefaction. Credit: J.K. Nakata, USGS. Public domain.

One of the most important things we look at when considering properties is the Natural Hazards Disclosure Statement (NHDS), which outlines any potential naturally-occurring dangers associated with the property. This includes flooding/dam inundation, fire risk, and of course fault zones and seismic hazards. Within the seismic hazards, there are two zones: landslide and liquefaction. Both are cases where the land is altered by earth movement caused by quakes, but each is different.

Landslides occur when a quake destabilizes the land, causing earth to cascade downhill. Landslide zones tend to be on steep slopes, and occur in a relatively small amount of San Francisco’s total footprint, as shown in the map below.

Liquefaction occurs when earth movement causes the soil to loosen, allowing water to saturate the soil and effectively turning it liquid, or like quicksand. Should this happen to the earth under your property, the foundation can become unstable. This was most dramatically illustrated during the 1989 Loma Prieta earthquake, when tracts of the Marina and some other neighborhoods were destabilized, causing massive damage.

Liquefaction zones are pretty common in San Francisco, as illustrated in the map below:

Why is there so much liquefaction in San Francisco?

One thing to keep in mind is that the shoreline of San Francisco today was not always the way it is today. North Beach was actually a beach, Montgomery Street was the bay shoreline, and lagoons and creeks used to reach pretty deep into the city. The original Mission Dolores, on what is now Albion between 16th and 17th Streets, was built on the shore of Dolores Lagoon, on the site of an Ohlone Indian village.

Over time, lagoons were filled and shorelines expanded as the city’s population grew dramatically beginning with the Gold Rush. Hundreds of abandoned boats from prospectors choked up the bay, many ultimately rotting and sinking in place, and ultimately getting buried in new land. And after the devastating 1906 quake and fire, much of the debris was simply dozed into the bay. Part of the city’s plan to demonstrate its ability to rebound from the disaster involved hosting the Panama-Pacific International Exposition of 1915. This was the birth of the Marina District, which was created by filling in an area called Harbor Cove.

However, relatively loosely packed soil on a high water table are exactly the kinds of conditions that make liquefaction likely. So, when the 1989 quake hit, many homes in the Marina crumbled, the upper span of the then-present Embarcadero Freeway collapsed, and more devastation occurred.

Should I buy a home in liquefaction?

This is really a question of risk tolerance. Some buildings have been reinforced with the intent to retain integrity in the event of liquefaction, and in many new construction projects, such as in Dogpatch and Mission Bay, buildings are constructed on pylons that go down to bedrock, making them stable. However, that does not necessarily mean that the streets and sidewalks around them will fare as well, so while your property may be safe, you may be isolated by impassable avenues.

Of course, it’s entirely possible that you could live in a property for decades and never experience an earthquake significant enough to cause serious liquefaction. More than 80 years passed between the two biggest ones in recorded history — but the most recent one was more than 30 years ago. Quakes do not operate on a predictable cycle, so the next big one could happen tomorrow, or a hundred years from now, or any other time.

If you choose to buy in liquefaction, it’s important that you do so knowingly, and be prepared should a major seismic event occurs.