Silicon Valley liquefaction zones

Silicon Valley Liquefaction Zones 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?

Well, we’re Realtors, not geologists or geotechnical engineers, but we can share some resources that may help answer these questions and provide avenues for further research on this topic.

What is liquefaction?

Liquefaction refers to the ground becoming liquified, or fluid. It takes 3 ingredients to liquefy land: loose sediment, water, and strong shaking. This loose, saturated soils when shaken to a certain point no longer behave like a solid and can slide, open, and swallow anything above it.

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.”

But I never heard about that happening during the 1906 San Francisco quake, or the 1989 Loma Prieta quake in Santa Clara County – that means it’s not a risk here, right? Wrong. The USGS Liquefaction and Sea Level Rise explains that a relatively dry rainy season in 1906 lowered the liquefaction risk, and the 1989 quake happened near the end of the dry season when groundwater levels were at their low point.

Silty, sandy soil will respond very differently to bedrock and clay in the case of extreme shaking. So no, the valley won’t all be shaking equally in the case of a large tremor. Liquefaction hazard zones will likely 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 investigational requirements to build safely. The liquefaction zones are noted by the state to be more susceptible to dangerous liquefaction in the event of an earthquake. You can learn more about these and related issues at the California Department of Conservation’s website. (more…)