Soft Soil: The Untold Problem With 56% of San Francisco’s Properties

Deniz Kahramaner
Atlasa
Published in
8 min readMay 26, 2018

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Ever since I became a realtor, I’ve been advising my clients not to buy on very heavy liquefaction zones in San Francisco. It is time to explain what soft soil is, dive into why not to buy on soft soil and back my recommendations with seismic data.

What is Liquefaction?

A basic demonstration of liquefaction at work

Soil liquefaction is the phenomenon where soil loses its stiffness when an earthquake happens. Liquefaction usually happens in sandy soils or landfill, both of which are landmarks of San Francisco. When soil loses its stiffness, it shakes up to 10x more and/or sinks during an earthquake. I call liquefaction susceptible soil soft soil.

How in the World Does Soft Soil Shake 10x More Than Normal Soil During an Earthquake?

Watch this video and you will understand.

The ground essentially becomes jello and every disturbance is amplified greatly.

Does San Francisco Have Homes Built on Liquefaction?

Yes, a lot of them. Most of homes the Marina, SOMA, South Beach, Dogpatch, and Mission are in heavy liquefaction zones. This NYTimes article describes just how big the risk is.

Footage of Liquefaction at the 2011 Christchurch Earthqauake

This video shows the earthquake at Christchurch, which was only 6.2 in magnitude. The 1906 earthquake in San Francisco was 7.9 in magnitude, which is 50 times (!) more intense than the Christchurch earthquake.

Below is a seismic map of San Francisco, taken from our data partners at Temblor, founded by Ross Stein, a Consulting Professor of Geophysics at Stanford University, and Volkan Sevilgen, an engineer in seismology.

This scale below is used to measure liquefaction susceptibility.

Areas that are very highly susceptible to liquefaction were historically:

  • Marshes (e.g. SOMA)
  • Parts of the bay — in other words, water! (e.g. Marina)
  • Streams or Rivers.

Currently, all of these areas are landfill.

Areas that are moderately susceptible to liquefaction were historically:

Sand dunes at the edge of the Richmond District for San Francisco’s Golden Gate Park in the 1880s.

Most of the West Coast of San Francisco falls under this

category.

In this Temblor map, areas deemed by the USGS to have very high liquefaction susceptibility in northern San Francisco are sites of fill (artificial land); the South of Market (SoMa) area was formerly a marsh and Mission Bay. All suffered liquefaction in the 1906 and 1989 earthquakes.

As can be seen above, most of Marina, Financial District, South Beach, SOMA, Mission Bay, Dogpatch, Hunter’s Point and the Mission are on very heavy liquefaction zones.

Stats About San Francisco’s Liquefaction

In 2017, out of the 2,343 condos that were sold:

44.07% of San Francisco condos sold were on low liquefaction. 30.63% on moderate, 25.30% on heavy.

55.93% of San Francisco condos sold were on moderate or above liquefaction.

In conjunction with the liquefaction map above, the pie charts below represent the liquefaction rating per neighborhood.

A Case Study: The Leaning Tower of San Francisco

Millenium Tower Side by Side with Its Sister Tower in Italy

Facts about Millenium Tower:

The Millennium Tower was one of the most well known premium luxury buildings in all of San Francisco. Condos in the tower sold for millions of dollars when they went on sale. All was great in the real estate world until May 2016, when the residents were informed that the tower had begun sinking, and July 19, 2017 when the tower leaned some more! There are reports that claim that the tower is sinking 2 inches every year. This issue primarily happened because the tower was built on soft soil. As soon as the nearby construction of the Transbay Transit Center broke ground, the soft soil underneath the Millennium Tower moved and made the tower lean.

Price Per Square Feet has been normalized to reflect 2017 prices across all sold years observed

The condos have already lost 30% of their value, and I expect the trend to continue until they lose more than 50% of their value.

Furthermore, each condo currently sits on the market much longer. No buyer wants to buy a slice of the new Tower of Pisa.

Takeaway:

If you buy on soft soil, foundation settlement after the earthquake means you will lose at least 30% of the value of your home, if not much much more…

While 73 condos sold before the tilt, only 13 sold since mid-2016 after the tilt happened. This means that a lot of owners aren’t even selling, knowing that they will lose money, hopelessly waiting for the lawsuits to return some of their lost capital.

If I Buy On a Liquefaction Zone and Risk My Safety, Will It Be Cheaper for me?

We looked at many neighborhoods in the city, and tried to discern whether non-liquefaction zones had a price premium over risky zones in 2017:

When we look at 2017’s sales, we see a negligible price difference between properties on liquefaction and properties not on liquefaction. We looked at every condo sale between January-December 2017.

In 2017…

  • In the Marina, risky soil was only $60 per sqft cheaper than safe soil.
  • In South Beach/Mission Bay, risky soil was only $90 per sqft cheaper than safe soil.
  • In Potrero Hill, Financial District, Hayes Valley and the Mission, risky soil was the same price per sqft (!!!!) as safe soil.

A price premium, when it exists, is tiny compared to the risk. Paying 0–9% more to be significantly safer in an earthquake is a great investment decision.

If There is No Huge Price Difference, Why Would Anyone Buy On Liquefaction Zones?

In a market with such tight inventory, some uninformed buyers make this unfortunate leap. I believe it is every realtor’s duty to at least share the risks in great detail and educate their buyers. All will be fine until an earthquake hits.

What Will Happen To Prices in the Said Neighborhoods If a Big Earthquake Hits?

  1. Some condos will lose up to 90% of their value, because the building will become uninhabitable and will need to be rebuilt.
  2. Some condos will lose more than 30-50% of their value, because they will have settlement or large cracks.
  3. Some condos will lose up to 30% of their value if they are habitable and undamaged, because buyers will actually start caring about liquefaction.

Given that 56% of San Francisco’s inventory is on moderate or high liquefaction zones, the owners of the remaining 44% will be the beneficiaries of one of the biggest supply constrained markets in California history.

Conclusions for Sellers

What Do I Do If My Home Is On Liquefaction?

There are two reasonable strategies:

  1. Strategy 1: Sell it while you can and buy on bedrock! If you sell now, you will be able to buy on bedrock for almost the same price.
  2. Strategy 2: Get your home seismically retrofitted and/or get an earthquake insurance policy with a high deductible. Please reach out to me if you would like recommendations for earthquake insurance companies and seismic retrofitters.

If you’d like to explore either strategy with me, reach out to me at deniz@deniz.io or 650–770–3100

What Do I Do If My Home Isn’t On Liquefaction?

Good job getting a house on bedrock!

Just because you aren’t on liquefaction doesn’t mean you are safe. You should consider getting a seismic retrofit depending on how and when your home was built.

I’d be happy to assist you if you reach out to me at deniz@deniz.io

Conclusions for Buyers

What Not to Buy

I advise against buying on very heavy liquefaction zones, because:

  1. You risk your life by buying on an unsafe earthquake zone. San Francisco’s building codes aren’t as good as Japan’s, and even if the builder in San Francisco claims that the building is built to code, that doesn’t mean much.
  2. You risk your money, because if the big earthquake hits, your property’s value will decrease drastically for an indeterminate amount of time. For most people, a home is the biggest investment they’ll ever make. Make sure your investment is sound.

Moderate Liquefaction

Like everything else, soft soil has a spectrum. If you reach out to me, I’d be happy to point you to resources that can teach you more or introduce you to experts.

Additional Considerations

If you buy an older home in a non-liquefaction zone, you should further look into

  1. Has it been seismically retrofitted?
  2. When was it built and how were the building codes back then?
  3. What is it built out of? Is it wood frame, brick, concrete or something else?
  4. How many floors is the building? Are you on the top floor or a lower floor?
  5. If you get earthquake insurance, how much will it cost?

A good realtor (like me) will help you assess all of these different factors when you make a decision. Reach out to me at deniz@deniz.io or 650–770–3100 if you’d like to chat more about what else you should look out for when buying a home.

How Do I Check If a Home is On Liquefaction

  1. You can use apps like Temblor before you check out an open house or visit a property to know whether it is on a liquefaction zone.
  2. In disclosure packages given out when you are interested in a property in San Francisco, there is a Natural Hazards Report, which mentions whether the property is on a liquefaction zone.

Authors

Deniz Kahramaner

Deniz Kahramaner is a Luxury Property Specialist at Compass. He formerly led data efforts and was an Advisor at Accompany Inc, which was acquired by Cisco for 270 Million Dollars in May 2018. He holds a BS in Electrical Engineering and an MS in Computer Science from Stanford University.

Email: deniz@deniz.io

Phone: 650–770–3100

Jeremie Young

Jeremie Young is a Real Estate Marketing Professional at Pacific Union. He is a marketer by day and a big data analyst who seeks to uncover useful insights in real estate by night. He previously held positions at Redfin and ZeroCater.

FAQ

As we get questions from our readers, we answer them below. If you have any questions, reach out to Deniz at deniz@deniz.io

My realtor told me that if this property survived the 1989 Loma Prieta Earthquake, it should be fine during a future earthquake

This is likely misleading.

The 1989 Loma Prieta Earthquake was magnitude 6.9, one tenth the intensity of the 1906 earthquake in San Francisco.

Furthermore, Loma Prieta, the center of the earthquake, was 60 miles away from San Francisco, so it lost some of its intensity when it reached the city.

Will heavy liquefaction zones in San Francisco actually liquefy?

Yes, very likely if the earthquake is big enough. Here is a photo of the Loma Prieta earthquake that shows some liquefaction.

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