What Is Liquefaction?

San Francisco home buyers often ask us: What is liquefaction?

Liquefaction takes place when the soil is a little softer. This could be caused by mud, underground streams or in San Francisco it could be caused by landfill. This liquefaction map shows the areas around San Francisco’s coastline are the most liquid.

But as you look at this map you’ll notice some areas in the central part of the city, like the Mission, also have liquefaction. This is because those areas have underground streams under them.

San Francisco liquefaction map

San Francisco liquefaction map

In this USGS liquefaction susceptibility map of San Francisco, areas of very high risk are marked in red, high risk in orange, moderate risk in yellow, low risk in green, and very low risk are white.

Should you buy a property on liquefaction?

The answer will vary for each buyer and their risk tolerance. Sometimes buyers will only buy a house on liquefaction if the house has been structurally upgraded or recently built with liquefaction in mind. For example, new construction in the Marina is built with mat foundations which are steel laid over the ground, kind of like a ship. It’s made to move as the ground moves. That’s how new construction is built on landfill.

In larger construction, you may see very, very, very large posts that are drilled super deep into the ground. You’ll see this on the high-rise buildings like 181 Fremont. I’m sure many of you have heard of Millennium Towers. Millennium Towers did not choose to drill down to bedrock and as a result, the building leans significantly. It’s our leaning tower in San Francisco. Another thing buyers may find concerning with liquefaction is soft story construction.

What is soft story construction?

A soft story is when you have a garage underneath a living space. If the garage is mostly open then that building is going to be more susceptible to movement. In these cases, buildings need a lot of retrofitting around the garage door. This usually means adding a lot of steel in the garage and a lot of seismic upgrading around it. If they have a mat foundation it should be stronger.

If you were buying on liquefaction the biggest area of concern would be a building built in the early 1900s, in the Marina, that had not been upgraded.

You have your buyers who are not willing to buy liquefaction at all, you have some buyers that feel it’s a case-by-case basis depending on the structural upgrades and the timing of those upgrades.

If you fall into the category where you wouldn’t buy on liquefaction at all we would eliminate many areas of the city like the Marina or South Beach that are liquefaction areas. If you’re looking in areas like the Mission you can simply ask the question at an open house – is this liquefaction? You could eliminate the property right then and there.

If you have additional questions about liquefaction please reach out to us. We want to make sure your home buying decisions are as informed as possible.

August 23, 2022
Buying a Home
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