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The best available predictor of where movement of slides and earth flows might occur is the distribution of past movements. 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.

This data provides a summary of the distribution of landslides evident in the landscape of the San Francisco Bay Region. Original identification and delineation of these landslides required detailed analysis of the topography by skilled geologists, a task generally accomplished through the study of aerial photographs. In general, landslide maps are now available for most of the region at scales of 1:24,000 - 1:62,500. The data modifies and improves the earlier compilations of landslide data, which was prepared from sources available in the mid-1970's. The generalized landslide distribution shown then has here been improved in areas where the 1970's sources were notably deficient 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 this data to regional considerations. For more detailed information, reference the original 1:24,000 - 1:62,500 maps, if available in local libraries, or consult local officials or private consultants.

FLATLANDS

Slides and earth flows do not occur on nearly flat ground -- they require slopes that are steep and long enough to permit failure. Thus this data excludes gently sloping ground from principal consideration. A slope boundary of 15 percent for this purpose. A similar criterion is the boundary between hillsides and areas of recent alluvial deposition. This boundary typically occurs at a slope of about 15 percent. This criterion has the advantage over slope alone of being limited to the lowland areas and excluding such other areas of low slope as hilltops and sidehill benches.

LANDSLIDE CATEGORY

The principal source of information used to define the distribution of slides and earth flows in the region is category 5 (landslides) of a 1979 regional slope stability map. The category 5 areas are a generalization of the distribution of mapped landslide deposits recognizable in the terrain, consisting principally of slumps, translational slides, and earth flows.

An important limitation of the earlier map is the varied character of the landslide mapping used in its compilation. For many areas, landslide inventory maps of various kinds, and even some detailed engineering geologic maps, were available, whereas elsewhere only general geologic maps were available. Some of the landslide inventory maps delineated only the most obvious landslides in the landscape, whereas others represented a thorough effort to identify all recognizable landslides.

From these varied sources, a generalization of the distribution of the landslides was determined by drawing envelopes around areas containing any type or size of mapped landslide that was within 1,000-1,500 feet of another landslide. Envelopes were also drawn around groups of landslides in such topographic settings as the same hillslope or creek bank, under the logic that such groups of landslides have a common local cause. Isolated landslides were represented directly where large enough, and inversely, inlying areas larger than 1,000-1,500 feet in diameter that lacked landslides were similarly delineated.

The result was subdivision of the hillside terrain of the region into two categories, one that contained scattered landslides together with intervening ground typically as wide as 1,000-1,500 feet, and a second that contained no mapped landslides. (These two categories are described as 'Mostly Landslide' and Few Landslides', respectively in the data.) Although generally consistent, in detail the content of each category depends on the type of landslide mapping represented by each compilation source. Where the landslide mapping was thorough, for example, the non-landslide category contains few if any mappable landslides, whereas in the areas for which geologic maps were used as sources, the non-landslide category may contain numerous landslides not deemed important in depicting the areal geology.

ADDITION OF OTHER LANDSLIDE INFORMATION

Additions to the earlier compilations focused on those areas lacking any information and those which used general geologic maps as their source.

In northern Sonoma County, landslide mapping was done at a scale of 1:62,500. Rather than drawing envelopes around these landslides -- a task for which time was not available -- envelopes were drawn around areas lacking mapped landslides. The result is similar to category 5 of earlier compilations, but includes more areas of non-landslide. This category is combined with the 'Few Landslides' category in the data.

In southeastern Sonoma County, more tractable information was provided. Envelopes were drawn around these landslides and groups of landslides. Other, local additions were made in southern Sonoma County. In the northeastern part of the Bay region, 1:24,000 landslide maps were similarly used to draw envelopes around landslides and groups of landslides, although numerous small landslides and questionably identified larger landslides were excluded on a case-by-case basis.

A large patch of northwestern Marin County is not addressed by any available landslide mapping. In this area, a map of terrain types was used in concert with a digital slope map (30-meter resolution) to interpret landslide distribution. Areas mapped as old erosion surface or 'Hard terrain' together with other areas with slopes less than about 30 percent were categorized as having Few Landslides. Those areas mapped as 'Soft terrain' where slopes are greater than 30 percent were categorized as being Mostly Landslide, and intervening areas of 'Intermediate terrain' steeper than 30 percent were categorized as having Many Landslides.

The original United States Geological Survey report and data are from Open-File Report 97-745 C.