Description: The Liquefaction dataset presents a map and database of Quaternary deposits and liquefaction susceptibility areas the urban core of the San Francisco Bay region] within the County of Sonoma. It supersedes the equivalent area of U.S. Geological Survey Open-File Report 00-444 (Knudsen and others, 2000), which covers the larger 9-county San Francisco Bay region. The report consists of (1) a spatial database, (2) two small-scale colored maps (Quaternary deposits and liquefaction susceptibility), (3) a text describing the Quaternary map and liquefaction interpretation (part 3), and (4) a text introducing the report and describing the database (part 1). All parts of the report are digital; part 1 describes the database and digital files and how to obtain them by downloading across the internet. See below Defining Liquefaction Susceptibility The nine counties surrounding San Francisco Bay straddle the San Andreas fault system, which exposes the region to serious earthquake hazard (Working Group on California Earthquake Probabilities, 1999). Much of the land adjacent to the Bay and the major rivers and streams is underlain by unconsolidated deposits that are particularly vulnerable to earthquake shaking and liquefaction of water-saturated granular sediment. This new map provides a consistent detailed treatment of the central part of the 9-county region in which much of the mapping of Open-File Report 00-444 was either at smaller (less detailed) scale or represented only preliminary revision of earlier work. Like Open-File Report 00-444, the current mapping uses geomorphic expression, pedogenic soils, inferred depositional environments, and geologic age to define and distinguish the map units. Further scrutiny of the factors controlling liquefaction susceptibility has led to some changes relative to Open-File Report 00-444: particularly the reclassification of San Francisco Bay mud (Qhbm) to have only MODERATE susceptibility and the rating of artificial fills according to the Quaternary map units inferred to underlie them (other than dams ? adf). The two colored maps provide a regional summary of the new mapping at a scale of 1:200,000, a scale that is sufficient to show the general distribution and relationships of the map units but not to distinguish the more detailed elements that are present in the database. The report is the product of cooperative work by the National Earthquake Hazards Reduction Program (NEHRP) and National Cooperative Geologic Mapping Program of the U.S. Geological Survey, William Lettis and & Associates, Inc. (WLA), and the California Geological Survey. An earlier version was submitted to the U.S. Geological Survey by WLA as a final report for a NEHRP grant (Witter and others, 2005). The mapping has been carried out by WLA geologists under contract to the NEHRP Earthquake Program (Grant 99-HQ-GR-0095) and by the California Geological Survey.For detailed information about the map the USGS has an open report, "Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California. U.S. Geological Survey Open File Report 2006-1037 Version 1.1. http://pubs.usgs.gov/of/2006/1037/Defining Liquefaction Susceptibility USGS, San Francisco Bay Region Geology & Geologic Hazards Compilation of the Liquefaction Susceptibility MapLiquefaction defined as a phenomenon where saturated sand and silt take on the characteristics of a liquid during the intense shaking of an earthquake.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:1.Mapping of Quaternary sedimentary deposits;2.Assessment of the liquefaction susceptibility of Quaternary map units.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. Hence, 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.
Copyright Text: Geology by Robert C. Witter, Keith L. Knudsen, Janet M. Sowers, Carl M. Wentworth, Richard D. Koehler, and Carolyn E. Randolph. Digital database by Carl M. Wentworth, Suzanna K. Brooks, and Kathleen D. Gans