Landslides are natural geologic phenomena that range from slow moving, deep-seated slumps to rapid, shallow debris flows. Landslide risk can be exacerbated by development. Grading for roads, home construction and landscaping can decrease hillside stability by adding weight to the top of a slope, destabilizing the bottom of a slope, and/or increasing water content of the underlying materials.
Landslides are most frequently triggered in periods of high rainfall. The hazard is greater in steeply-sloped areas, although slides may occur on slopes of 15 percent or less if the conditions are right. Slope steepness and underlying soils are the most important factors affecting the landslide hazard. However, surface and subsurface drainage patterns also affect the landslide hazard, and vegetation removal can increase the likelihood of a landslide.
Rainfall-Induced Landslides :: Existing Landslides
Source: USGS, updated 1998. San Francisco Bay Region Landslide Information: Summary Distribution of Slides and Earth Flows
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. This map shows areas where landslides have occurred in the past.
Rainfall-Induced Landslides :: Debris Flow Source Areas
Source: USGS, 1997. San Francisco Bay Region Landslide Information: Map Showing Principal Debris-Flow Source Areas
This map identifies the principal areas in the San Francisco Bay region that are likely to produce debris flows, which are also called “mudslides.” Debris flows that occur in the bay region are fast-moving downslope flows of mud that may include rocks, vegetation, and other debris. These flows begin during intense rainfall as shallow landslides on steep slopes. The rapid movement and sudden arrival of debris flows pose a hazard to life and property during and immediately following the triggering rainfall. More information.
Source: California Geologic Survey (CGS) Seismic Hazards Zonation Program
This map is derived from State-mandated regulatory maps that show “Zones of Required Investigation” for landslide (and liquefaction) hazard. They do not depict different degrees of hazard, rather they identify zones within which site-specific studies will be required for new construction. These zone maps also are used in real estate transactions – sellers of properties within a “Zone of Required Investigation” must disclose that fact to prospective buyers.