Description: This report presents a map and database of Quaternary deposits and liquefaction susceptibility for the urban core of the San Francisco Bay region. It supercedes 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. 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/
Service Item Id: 407553100ad64f879be1762f179c8a4e
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
Description: The distribution of landslides evident in the landscape -- most of which are slumps, translational slides, and earth flows -- is of interest both for evaluation of hazard and risk and for use in further study of landslides. Future movement of such landslides is most likely to occur within and around the places where they have previously occurred. A map showing the generalized distribution of these landslides was published by Nilsen and Wright for the 9-county San Francisco Bay region in 1979. Original sources available at the time of that compilation were incomplete for the region. Landslide mapping that has since become available provides a basis for revision and extension of that work, and modern procedures allow this to be done in digital form. We have digitized category 5 (landslides) from the Nilsen and Wright map, added equivalent information for Santa Cruz County, revised and filled in the principal deficiencies of their original map, and added Quaternary surficial deposits (to delimit areas largely invulnerable to these types of landslides). The result is reasonably complete coverage for the 10-county region, available in digital form as (1) vector polygon databases and (2) map-image files for the whole region and separately for each of the ten included counties. Nomenclature for landslides is complex (Varnes, 1978). For present purposes, we use the term slide to include both slumps and translational slides, and earth flow to represent flows of clayey earth. These kinds of landslides typically move slowly, in contrast to the rapid movement of debris flows (see companion report by Ellen and others, 1997). Slides and earth flows deform the ground surface when they move and remain in the landscape as recognizable landslide masses, whereas debris flows run down slope and form separate deposits lower in the landscape.Official Full Description of Landslide Susceptibility Value - From USGS Open File Report 97-745E, 1997 http://pubs.usgs.gov/of/1997/of97-745/of97-745e.htmlMostly Landslides - Consists of mapped landslides, intervening areas typically narrower than 1500 feet, and narrow borders around landslides; defined by drawing envelopes around groups of mapped landslides.Many Landslides - Consists of mapped landslides and more extensive intervening areas that in 'Mostly Landslides'; defined by excluding areas free of mapped landslides; outer boundaries are quadrangle and county limits to the areas in which this unit was defined.Few Landslides - contains few, if any, large mapped landslides, but locally contains scattered small landslides and questionably identified larger landslides; defined in most of the region by excluding groups of mapped landslides but defined directly in areas containing the 'Many Landslides' unit by drawing envelopes around areas free of mapped landslides.Very Few Landslides - (no additional information provided)Surficial Deposits - Areas of gentile slope at low elevation that have little or no potential for the formation of slumps, translational slides, or earth flow except along stream banks and terrace margins; defined by the distribution of surficial deposits.