Name: Landslide Probability

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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.

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