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Southern California Geology


Slope-failure hazards in Southern California

The geologic setting of southern California locally is conducive to slope failures and slope-failure deposits (landslides) that can be a hazard to human life and property. These hazards are created when geologic materials are displaced down a topographic slope under the influence of gravity. Factors that determine slope-failure occurrence include:


  • Slope angle
  • Geologic materials (substrate)
  • Climatic conditions
  • Earthquake shaking

Table 1 (modified from Varnes, 1978) summarizes one way of naming slope-failure deposits and for interpreting their emplacement mechanism:


Predominantly coarse

Predominantly fine
Rock fall
Debris fall
Earth fall
Rock topple
Debris topple
Earth topple
Slides rotational few units
Rock slump
Debris slump
Earth slump

Rock block slide

Rock slide

Debris block slide

Debris slide

Earth block slide

Earth slide

translational many units
Lateral Spreads
Rock spread
Debris spread
Earth spread

Rock flow

(deep creep)

Debris flow

(soil creep)

Earth flow
Combination of two or more principal types of movement

Table 1


Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., eds., Landslides: analysis and control: Washington, D.C., Transportation Research Board, National Academy of Sciences, Special Report 176, p. 11-33.

Landslide Studies by the U.S. Geological Survey (USGS)

The USGS Landslide Program is charged with mapping and describing landslides and evaluating landslide susceptibility on a regional basis. In southern California, typical USGS studies include:


The USGS also maintains a National Landslide Information Center that distributes information about landslides to the lay public and to various Federal, State, and County agencies.

Landslide Studies by the California Geological Survey (CGS)

The California Geological Survey Seismic Hazards Evaluation and Zoning Program carries out a systematic program of seismically-induced landslide studies in southern California. This program is described in Chapter 5 of CDMG Special Publication 117 that identifies State guidelines for evaluation and mitigation of seismic hazards. A facts sheet provides a brief overview of CDMG's seismic-hazards mapping Program. Useful CDMG reports on landslides include:


An index of CDMG seismic-hazards evaluations in southern California shows the extent of CDMG landslide evaluations, including released and ongoing investigations. The index points to 1:24,000-scale quadrangle maps where landslide susceptibility is displayed online using small-scale graphics. Some evaluations include down-loadable .pdf files of reports that contain chapters describing landslide hazards (for example, see seismic-hazard evaluations for the Pasadena 7.5-minute quadrangle and the San Fernando 7.5-minute quadrangle).

Selected References
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This site last updated September 3, 2004 (ps)

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