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Liquefaction zone shown in greenish color on this mapThe Silicon Valley liquefaction zones cover much of the Bay Area and Santa Clara County, but the risks are often not well understood or investigated. We know that this is earthquake country and tremblers are to be expected. But what difference does it make where you live or work – won’t the whole valley be shaking equally?

I’m a Realtor, not a geologist or geotech engineer, but I’d like to share some resources that may help answer some questions and provide avenues for further research on this topic.

What is liquefaction?

Liquefaction refers to the ground becoming liquified, or becoming fluid, during severe shaking. In 2010 and 2011, New Zealand experienced this and it made worldwide news. The Science Learning Hub website states that “During the Canterbury earthquakes of September 2010 and February 2011, liquefaction caused silt and fine sand to boil up and bury streets and gardens and caused buildings and vehicles to sink.”

Silty, sandy soil will respond very differently to bedrock in the case of extreme shaking. So no, the valley won’t all be shaking equally in the case of a large tremor.  Liquefaction soils areas will get the worst of it. That’s why this designation matters so much.

What is a liquefaction zone?

After the Loma Prieta earthquake in 1989, the Department of Conservation, California Geological Survey created maps to make residents aware of areas in which there are increased risks from earthquake shaking due to landslides or liquefaction and to make sure that construction in those zones have extra investigation required. The liquefaction zones are noted by the state to be more dangerous due to the risk of the ground becoming liquid.  You can learn more about these and related issues at the California Department of Conservation’s website.

When you buy or sell a home in California, one of the reports shared by sellers is a Natural Hazard Disclosure, which provides info on whether or not a property is in a liquefaction zone.

There’s no nuance in these zones: the property is either in or not in the zone of required investigation.

For people buying a home outside of the liquefaction zone, they might be surprised to learn that there is still some liquefaction risk. Or if buying within that zone, there are more and less risky areas. If you visit the U.S.G.S. website with info on liquefaction risk and soil types, the 2nd map (liquefaction risk) presents colorfully with various levels of danger.

Below is a snippet of the second map on that page from the U.S.G.S. with a few annotations from me, and I moved the color coding bar into the image to make the point that here it’s not just in or out of a risk zone. The zones’ risk, from highest to lowest, is red, orange, yellow, green, and white.

It’s a bit of a pain, as you need to download and then extract compressed files, but worth it: visit the U.S.G.S. site to download maps of soil types in the bay and seismic susceptibility. Or just click on the image directly below to visit that page. The major challenge, after all of that, is finding landmarks. I wish they had included some street names, but no such luck.

 

Liquefaction risk map with various colors and code

 

Silicon Valley liquefaction zones: is there a history of the ground turning to liquid from quakes?

We did not see the ground liquify in Santa Clara County during the Loma Prieta Earthquake of 1989, which registered a 6.9 on the Richter Scale and was considered a strong quake – but not a major one, amazing as that may seem to anyone like me who was here for it.

Larger earthquakes have happened nearby, though, and could again. In April 1906, the famed San Francisco Earthquake hit a magnitude of 7.9, a major event, which was about 16 times larger per the U.S.G.S than the Loma Prieta.  That one did have liquefaction and it was disastrous. The San Francisco one continued for much longer, too: 45-60 seconds for the San Francisco quake and just 15 seconds for the Loma Prieta (it felt longer!). Bigger still have been recorded in North America in Alaska at 9.2 and in other regions of the world as high as 9.5 per the U.S.G.S.

Would we get liquefaction if another 7.9 quake hit? What about an 8.5 or higher? That’s what we do not know. In Christchurch, New Zealand, there was liquefaction with a quake smaller in magnitude than our 1989 shaker, but it was close to the surface.

Where are the Silicon Valley liquefaction zones?

Most of the areas are low lying (low elevation) areas that many moons ago made up the bottom of the San Francisco Bay. The liquefaction areas also tend to follow rivers and creeks in small bands close to the banks of the water. Much of San Jose, and I believe all of Santa Clara are in liquefaction zones.

As you move closer to the hills on the east and west sides of the valley – especially on the west – there are more pockets which are out of the official zones.  For example, the elevation in Santa Clara is 72′. In Campbell, which is mostly out of the zone, it’s 200′.

The Silicon Valley liquefaction zones are shaped almost like a funnel stretching toward South County – but they stop before getting to Morgan Hill, San Martin, and Gilroy.

There are many natural hazard zones to consider in addition to the liquefaction zones.  The mountains may not have this issue, but they may have earthquake faults, landslide areas, or high fire danger from wildfires.  Other parts of the county may be in 100 year flood plains or in areas likely to be inundated should a dam fail. A great resource that I’d encourage you to check and bookmark is the California My Hazards Awareness site. Please just remember that like most sites, it will report the liquefaction issue as “in or not in” the zone.

FAQs on the Silicon Valley liquefaction zones

  • Over the years, many clients have asked me if they should worry about a proximity to this or that natural or environmental hazard. Most of the time, home buyers will need to do some research so that they can make a decision. Every home purchase involves compromises, so the question goes back to the buyer: is this a compromise you’re willing to make? I had one client who was so worried about liquefaction zones that she and her family moved to Sacramento. For others, the risk elicits a shrug with the sense that if most of the valley is in that zone, what could be done?
  • Does being in a liquefaction zone impact resale value? Not that I have seen.
  • Are homes in the Silicon Valley liquefaction zones built to withstand the shaking? The map act of 1990 may have created stronger regulations for homes built after that time for those regions, but houses and condos built pre-1990 do not seem to be differentiated in their construction whether in or out of those zones. Newer construction of course has better engineering for seismic safety than older homes do unless the older ones have been retrofitted.
  • Can you tell if a home is in the liquefaction zone by looking at or touching the soil? No, it’s the soil deeper than what you can see which is the issue.