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Guidelines for Evaluation and Mitigation of Seismic Hazards in California

 

 


GEOLOGIC ENVIRONMENTS
LIKELY TO PRODUCE
EARTHQUAKE-INDUCED LANDSLIDES

(Anaheim Hills soils are noted in red.)
 
Landslide Type
Type of Material
Slope Angle
Remarks
Rock falls Rocks weakly cemented, intensely fractured, or weathered; contain conspicuous planes of weakness dipping out of slope or contain boulders in a weak matrix.
40º
1.7:1
Particularly common near ridge crests and on spurs, ledges, artificially cut slopes, and slopes undercut by active erosion.
Rock slides Rocks weakly cemented, intensely fractured, or weathered; contain conspicuous planes of weakness dipping out of slope or contain boulders in a weak matrix.
35º
1.4:1
Particularly common in hillside flutes and channels, on artificially cut slopes, and on slopes undercut by active erosion. Occasionally reactivate preexisting rock slide deposits.
Rock avalanches Rocks intensely fractured and exhibiting one of the following properties: significant weathering, planes of weakness dipping out of slope, weak cementation, or evidence of previous landsliding.
25º; 2.1:1
Usually restricted to slopes of greater than 500 feet (150 m) relief that have been undercut by erosion. May be accompanied by a blast of air that can knock down trees and structures beyond the limits of the deposited debris.
Rock slumps Intensely fractured rocks, preexisting rock slump deposits, shale, and other rocks containing layers of weakly cemented or intensely weathered material.
15º
3.7:1
 
Rock block slides Rocks having conspicuous bedding planes or similar planes of weakness dipping out of slopes.
15º; 3.7:1
 
Soil falls Granular soils that are slightly cemented or contain clay binder.
40º
1.7:1
Particularly common on stream-banks, terrace faces, coastal bluffs, and artificially cut slopes.
Disrupted soil slides Loose, unsaturated sands.
15º
3.7:1
 
Soil avalanches Loose, unsaturated sands.
25º
2.1:1
Occasionally reactivate preexisting soil avalanche deposits.
Soil slumps 

(Anaheim Hills soils types)

Loose, partly to completely saturated sand or silt; uncompacted or poorly compacted manmade fill composed of sand, silt, or clay, preexisting soil slump deposits.
10º
5.7:1
(A slope angle greater than this is unacceptably
dangerous.)
Particularly common on embankments built on soft, saturated foundation materials, in hillside cut-and-fill areas. 
(These dangerous conditions are present in Anaheim Hills.)
Soil block slides

(Anaheim Hills soils types)

Loose, partly or completely saturated sand or silt; uncompacted or slightly compacted manmade fill composed of sand or silt, bluffs containing horizontal or subhorizontal layers of loose, saturated sand or silt.
11:1
(A slope angle greater than this is unacceptably
dangerous.)
Particularly common in areas of preexisting landslides along river and coastal flood plains, and on embankments built of soft, saturated foundation materials.
(These dangerous conditions are present in Anaheim Hills.)
Slow earth flows Stiff, partly to completely saturated clay and preexisting earth-flow deposits.
10º
5.7:1
 
Soil lateral spreads Loose, partly or completely saturated silt or sand, uncompacted or slightly compacted manmade fill composed of sand.
0.3º
190:1
Particularly common on river and coastal flood plains, embankments built on soft, saturated foundation materials, delta margins, sand dunes, sand spits, alluvial fans, lake shores and beaches.
Rapid soil flow Saturated, uncompacted or slightly compacted manmade fill composed of sand or sandy silt (including hydraulic fill earth dams and tailings dams); loose, saturated granular soils.
2.3º
25:1
Includes debris flows that typically originate in hollows at heads of streams and adjacent hillsides; typically travel at tens of miles per hour or more and may cause damage miles from the source area.
Subaqueous landslides Loose, saturated granular soils.
0.5º
110:1
Particularly common on delta margins.
 
  

Copyright © California Department of Conservation,
Division of Mines and Geology, 1997.