San Francisco Bay Region Geology and Geologic Hazards
A cooperative project with the California Geological Survey
About This Section - Liquefaction
The Purposes of this liquefaction section include:
This Section is not a regulatory document and does not provide map information that is sufficient for site-specific decisions.
Construction of the Liquefaction Section of This Web Site:
The liquefaction section was written by Keith L. Knudsen (Senior Engineering Geologist with the California Geological Survey) and Carl M. Wentworth (Geologist Emeritus with the U. S. Geological Survey).
Illustrations and other materials were obtained from scientific and engineering literature, reports from post-earthquake investigations, and public information documents.
Technical help was provided by Scott R. Horvath (web development) and J. Luke Blair ("Zoomified" map) of the U.S. Geological Survey.
The liquefaction susceptibility map was compiled by Knudsen, Wentworth, and Robert C. Witter (then with William Lettis & Associates, Inc.) from the recently released report by Witter and others (2006) for the central San Francisco Bay Area, and from an earlier report by Knudsen and others (2000) for the remainder of the 9-county San Francisco Bay region. Susceptibility ratings for the latter were revised according to the method of Witter and others.
Preparation of these original reports and maps on liquefaction susceptibility was a cooperative effort between the U.S. Geological Survey, the California Geological Survey, and William Lettis & Associates, Inc. (funded by USGS NEHRP External Grant Program). Two principal steps were involved in producing the liquefaction susceptibility maps:
The maps of Quaternary deposits (included in the original reports) delineate about 40 units, each map unit being a unique combination of geologic age and depositional environment. The depositional environment is used to infer the composition of the deposit and the age is used to infer the density or consolidation of the sediment.
Liquefaction susceptibility was assigned to each Quaternary map unit on the basis of a limited set of geotechnical data (subsurface borings), the distribution of historical occurrences of liquefaction (primarily from the 1906 San Francisco and 1989 Loma Prieta earthquakes), and the likely depths to ground water (using deposit age and topographic setting as a guide).
As in all regional mapping, the sediment in each map unit may be heterogeneous. Thus, two areas that have been shown to have the same age and environments of deposition (and that have been assigned the same liquefaction susceptibility) may behave differently in an earthquake. This is particularly true for some of the units assigned MODERATE susceptibility and for artificial fill over estuarine mud. Artificial fill that has been deposited along the Bay margin (where there was once open water or marsh) is rated VERY HIGH, despite the fact that some fills are engineered to reduce their susceptibility.
The liquefaction susceptibility maps provide a regional depiction of the hazard using five units (Very High to Very Low). The maps are intended to be used as a guide for those concerned about the hazard for purposes ranging from land-use planning to site development. Where liquefaction may have significant impact, and certainly for site development, more detailed information and expert advice should be sought.