EarthWorks Geospatial Catalog


Slides and earth flows are landslides that can pose serious hazard to property in the hillside terrain of the San Francisco Bay region. They tend to move slowly and thus rarely threaten life directly. When they move -- in response to such changes as increased water content, earthquake shaking, addition of load, or removal of downslope support -- they deform and tilt the ground surface. The result can be destruction of foundations, offset of roads, and breaking of underground pipes within and along the margins of the landslide, as well as overriding of property and structures downslope. The best available predictor of where movement of slides and earth flows might occur is the distribution of past movements (Nilsen and Turner, 1975). These landslides can be recognized from their distinctive topographic shapes, which can persist in the landscape for thousands of years. Most of the landslides recognizable in this fashion range in size from a few acres to several square miles. Most show no evidence of recent movement and are not currently active. Some small proportion of them may become active in any one year, with movements concentrated within all or part of the landslide masses or around their edges. These maps and databases provide a summary of the distribution of landslides evident in the landscape of the San Francisco Bay region. Original identification and map delineation of these landslides required detailed analysis of the topography by skilled geologists, a task generally accomplished through the study of aerial photographs. Such original landslide maps are now available for most of the region at scales of 1:24,000 - 1:62,500 (Pike, 1997). The summary map presented here makes selected use of these original maps and the 9-county compilation by Nilsen, Wright, and others (1979) to provide a basis for initial evaluation of areas vulnerable to slumps, translational slides, and earth flows in the region. The summary map modifies and improves the compilation by Nilsen and Wright, which was prepared from sources available in the mid-1970's. The generalized landslide distribution shown on that map has here been improved in areas where the 1970's sources were notably deficient (Figure 1), has been extended to include Santa Cruz County, and includes the distribution of surficial deposits that define landscape not generally vulnerable to these kinds of landslides. The method of compilation and resolution of 1:125,000 (1 inch = 2 miles) limits use of the map to regional considerations. For more detailed information, see the maps listed by Pike (1997) or consult local officials or private consultants.
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