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. Summary Distribution of Slides and Earth Flows
This map shows areas where landslides have occurred in the past.
Rainfall-Induced Landslides :: Debris Flow Source Areas
Source: USGS, 1997. Map Showing Principal Debris Source Flow Areas in the San Francisco Bay Region
This map shows areas that are likely to produce debris flows (mudslides). Debris flows are fast moving, down-slope flows that may include rocks, vegetation, and other debris.