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Understanding Liquefaction Zones
The ABAG report "The REAL dirt on Liquefaction" and web site provides the following quote:
"The 1989 Loma Prieta earthquake caused a total of $5.9 billion in property damage. Most of the damage was due to ground shaking. However, approximately $100 million of that (1.6%) was due to liquefaction."
The Loma Prieta earthquake occurred at the end of the dry season in October of a relatively dry year.
Three factors are required for liquefaction to occur
- loose, granular sediment - typically beach and stream deposits or land fill.
- saturation of the sediment by ground water - water fills the spaces between sand and silt grains
- strong shaking - all portions of the San Francisco Bay Area have the potential for susceptible areas to be shaken hard enough for liquefaction to occur.
The San Francisco Bay Area has 77 square miles of land created since 1845 by fill being poured into the bay and wetlands. This and much of the naturally occurring land at low elevations is susceptible to liquefaction. Additionally loose sediment along existing and filled streams may be susceptible. The immediate question is how large an area is impacted? In San Jose the area extends from the San Francisco Bay to approximately Stevens Creek Blvd. In Palo Alto most of the area between the bay and Alma Street is impacted.
The Santa Clara County Government Geologic Hazard Maps provide excellent detail. A screen shot of one for Palo Alto is shown below.
Sources for additional information
- California Department of Conservation
- Santa Clara County Planning Office
- Association of Bay Area Government - Liquefaction Hazards
- USGS - About Liquefaction