Since the 1960’s, the California Geological Survey (CGS) has produced numerous maps that show landslide features and delineate potential slope-stability problem areas. Preparation of these maps has been episodic, often driven by landslide disasters and subsequent legislative mandates. Many CGS landslide maps and related products have been produced for local or state agencies in response to their specific needs.
Please see our Landslide Map Index to find the landslide map that covers your area of interest. Refer to the explanation on that map and descriptions of the types of landslides and types of landslide maps for details on how landslides are mapped and described.
CGS has prepared a statewide map of landslide susceptibility based on mapped landslides, variations in rock strength and slope. That map has been published as CGS Map Sheet 58.
In the 1970’s, CGS prepared a series of “Geology for Planning” and “Environmental Geologic Analysis” reports and maps for local agencies in urbanizing areas. These products were designed to assist local agencies in evaluating hazards and developing policies that consider landslide hazards as residential development spread into landslide-prone terrain.
Following the 1982 El Nino storms in the San Francisco Bay area, the Landslide Hazard Mapping Act mandated new maps to show landslides and landslide hazards. Landslide Hazard Identification Maps were prepared by CGS for use by local government planners from 1986 to 1995. A set of three to four maps was prepared for each map study area, usually encompassing a USGS 7.5-minute topographic quadrangle map. The set of maps typically consisted of a geologic map, a landslide inventory map - showing the location and distribution of existing landslides – and one or two landslide relative susceptibility maps.
Also beginning in the early 1980’s, concerns about the long-term ecologic impacts of timber harvesting lead to the mapping of landslide features in forested lands. Maps developed under this program include landslide inventory maps, and depict other geomorphic features (e.g. debris-slide slopes and inner gorges) where shallow landsliding is a dominant mode of mass wasting.
Although the Geology for Planning, Environmental Geologic Analysis, and Landslide Hazard Identification programs are no longer in existence, their products are generally still available as paper prints. Work is in progress to convert these reports and maps into high-quality digital versions. Currently active landslide mapping programs at CGS, and their respective products are described in more detail below.
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The Forest and Watershed Geology Program provides maps and information on landslides, erosion, and sedimentation to help guide land-use decisions in California’s forested lands, and help preserve water quality and fish habitat.
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The Seismic Hazards Zonation Program maps existing landslides and delineates landslide zones of required investigation. The zone maps, which also identify liquefaction hazards, identify areas where a site-specific study must be completed before a building permit is approved. Landslide Inventory Maps prepared for Seismic Hazard Zonation are now available as part of a new CGS map publication series, the Landslide Inventory Map Series
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CGS has prepared a series of maps and reports for the California Department of Transportation (Caltrans) in the Caltrans Highway Corridor Mapping project that evaluate the severity of landslide hazards along highway corridors through mountainous and potentially unstable terrain.
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CGS regularly provides technical input and advice to the California Emergency Management Agency (CalEMA) during and immediately following major landsliding events, and immediately following major wildfires in anticipation of post-fire debris flows. CGS provides brief reports on individual emergency response tasks to CalEMA and local emergency response agencies to assist them in evaluating ongoing hazards and planning for recovery. Post-event reports and maps documenting landslide occurrences have been summarized as Open-File Reports, articles in California Geology magazine, and on CGS’s web pages.
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TYPES OF LANDSLIDES
The California Geological Survey classifies landslides with a two-part designation based on Varnes (1978) and Cruden and Varnes (1996). The designation captures both the type of material that failed and the type of movement that the failed material exhibited.
Material types are broadly categorized as either rock or soil, or a combination of the two for complex movements. Rock refers to hard or firm bedrock that was intact and in place prior to slope movement. Soil, either residual or transported material, is used in the engineering sense to mean unconsolidated particles or poorly cemented rock or aggregates. Soil is distinguished further on the basis of texture as debris (coarse fragments) or earth (fine fragments). The distinction between rock and soil, and the further distinction between debris and earth is most often based on interpretation of geomorphic characteristics within landslide deposits, but can also be inferred from geologic characteristics of the parent material described on maps or observed in the field.
Landslide movements are interpreted from the geomorphic expression of the landslide deposit and source area, and are categorized as falls, topples, spreads, slides, or flows. Falls are masses of soil or rock that dislodge from steep slopes and free-fall, bounce, or roll downslope. Topples move by the forward pivoting of a mass around an axis below the displaced mass. Lateral spreads, commonly induced by liquefaction of material in an earthquake, move by horizontal extension and shear or tensile fractures. Slides displace masses of material along one or more discrete planes. In rotational sliding the slide plane is curved and the mass rotates backwards around an axis parallel to the slope; in translational sliding the failure surface is more or less planar and the mass moves parallel to the ground surface. Flows mobilize as a deforming, viscous mass without a discrete failure plane. More than one form of movement may occur during a failure, in which case the movement is classified as complex if movements occur sequentially and composite if they do not.
Five of the 20 possible material/movement combinations are commonly found when preparing a landslide inventory map. These are Rock Slides, Earth Flows, Debris Slides, Debris Flows and Rock Falls, and these are described in more detail below.
Diagram by J. Appleby, R. Kilbourne,
and T. Spittler after Varnes, 1978
ROCK SLIDE: A landslide involving bedrock in which the rock that moves remains largely intact for at least a portion of the movement. Rock slides can range in size from small and thin to very large and thick, and are subject to a wide range of triggering mechanisms. The sliding occurs at the base of the rock mass along one to several relatively thin zones of weakness, which are variably referred to as “slide planes,” “shear surfaces,” “slip surfaces,” “rupture surfaces,” or “failure surfaces.” The sliding surface may be curved or planar in shape. Rock slides with curved sliding surfaces are commonly called “slumps” or “rotational slides,” while those with planar failure surfaces are commonly called “translational slides,” “block slides,” or “block glides.” Rock slides that occur on intersecting planar surfaces are commonly called “wedge failures.”
Rock slides commonly occur on relatively steep slopes in competent rocks. Slope gradients are commonly from 35% to as steep as 70%. Movement of an intact rock mass along a curved slide plane leads to a steep, arcuate headscarp at the upper boundary of the slide. Immediately below the headscarp is a block that is commonly rotated so that it is less steep than the surrounding hill slopes. Below the bench, the slide mass may be intact with a similar gradient to the surrounding slopes or may have additional scarps and benches. The lower parts of the slopes may bulge outward and be steeper that the surrounding slopes.
Diagram by J. Appleby, R. Kilbourne, and
C. Wills after Varnes, 1978
EARTH FLOW: A specific type of Soil Flow landslide where the majority of the soil materials are fine-grained (silt and clay) and cohesive. The material strength is low through much of the slide mass, and movement occurs on many discontinuous shear surfaces throughout the landslide mass. This movement along numerous internal slide planes disrupts the landslide mass leading to cumulative movement that resembles the flow of a viscous liquid characterized by a lumpy, or “hummocky” slope morphology. The lower parts of an earth flow usually bulge outward and are steeper than adjacent slopes.
Earth flows commonly occur on moderately steep slopes. Slope gradients are commonly from 10% to as steep as 30%, although steeper slopes may be found in headscarp and toe areas.
Earth flows typically are initiated by periods of prolonged rainfall and sometimes don’t initiate until well after a storm or the rainy season has passed. They are characteristically slow moving, in the millimeters or centimeters per day range, and may continue to move for a period of days to weeks after initiating.
Diagram by J. Appleby, R. Kilbourne, and
C. Wills after Varnes, 1978
DEBRIS SLIDE: A slide of coarse-grained soil, most common in unconsolidated sandy or gravelly units, but also are common in residual soils that form from in-place weathering of relatively hard rock. Owing to the granular constituents, overall strength of the debris slide mass generally is higher than that of earth flows, but there may be a very low strength zone at the base of the soil or within weathered bedrock. Debris slides typically move initially as shallow intact slabs of soil and vegetation, but break up after a short distance into falls and flows. Movement of the slide mass as a shallow slab leads to a smooth, steep, commonly curved scar. The debris is deposited at the base as a loose hummocky mass, although the deposit may be rapidly removed by erosion.
Debris slides commonly occur on very steep slopes, as steep as 60% to 70%, usually in an area where the base of a slope is undercut by erosion. Debris slides form steep, un-vegetated scars which are likely to remain un-vegetated for years. Re-vegetated scars can be recognized by their steep slopes, and a shallow amphitheater morphology.
A single heavy rainstorm or series of storms may deliver enough rain to trigger debris slides. Individual debris slides may move at rates ranging from meters per day to meters per minute. Debris slide scars are extremely steep and therefore are very sensitive to renewed disturbance. Natural erosion at the base of debris slide scars may trigger additional slides. Cutting into the base of a debris slide scar may also trigger renewed slides. Even without additional disturbance, debris slide scars tend to ravel and erode, leading to small rock falls and debris slides from the same slope.
Diagram by J. Appleby and R. Kilbourne, CGS
DEBRIS FLOW: A Soil Flow where the majority of the materials are coarse-grained (fine sand to boulder size particles) and non-cohesive. Debris flows are most often triggered by intense rainfall following a period of less intense precipitation, or by rapid snow melt. High pore water pressures cause the soil and weathered rock to rapidly lose strength and flow downslope. Debris flows can move very rapidly, at rates ranging from meters per hour to meters per second and travel relatively long distances, making them a significant threat to life and property.
Debris flows commonly begin as a slide of a shallow mass of soil and weathered rock. Their most distinctive landform is the scar left by the original shallow slide. The path of the debris flow may be marked by a small drainage that has been stripped of vegetation. The debris flow may not leave any deposit if it flows directly into a larger creek and is immediately eroded away. Many debris flow deposits are ephemeral, but in some cases successive debris flows may deposit material in the same area thereby forming a debris fan, which resembles a small, steep alluvial fan.
Individual debris flows typically are small in areal extent and their deposits are relatively thin. Evidence of past debris flow movements often is masked by vegetation growth which can cover the surface rapidly, sometimes within a few years, making them difficult to identify using aerial photographic and field reconnaissance methods. Therefore, only the larger and more recent debris flows typically are identified and included on landslide inventory maps
Diagram modified after Colorado Geological Survey
ROCK FALL: A landslide where a mass of rock detaches from a steep slope by sliding, spreading or toppling and descends mainly through the air by falling, bouncing or rolling. Intense rain, earthquakes or freeze-thaw wedging may trigger this type of movement.
Rockfalls occur on steep slopes of hard, fractured rock. The scar left by a rockfall on the slope may be no more apparent than an area of rock that is less weathered than the surrounding rocks. Rockfall deposits are loose piles of rubble that may be easily removed by erosion. Because neither the scar nor the deposit are distinctive, and because the most frequently occurring rockfalls are typically small, individual rock falls are usually not shown on regional-scale (1:24,000 and smaller) landslide maps.
Though infrequent, moderate- to large-volume rockfalls can be extremely dangerous and sometimes fatal. Large slabs of rock impacting a hard ledge after a long drop can rapidly break apart, leading to air entrainment and long runouts, induced airblasts, airborne projectiles (flyrock) and severe dust clouds.
Cited References
CRUDEN, D.M., and VARNES, D.L. 1996, Chapter 3: Landslide types and processes, pp. 36-75 in A.K. Turner and R.I. Schuster (eds.), Landslides: Investigation and Mitigation, Special Report 247, Transportation Research Board, National Research Council, Washington D.C.: National Academy Press.
VARNES, D.J., 1978, Slope movement types and processes. In, Special Report 176: R.L. Schuster and R.J. Krizek (eds.), Landslides: Analysis and Control, TRB, National Research Council, Washington, D.C. pp. 11-33.
Four principal types of information describing the various classes of landslides are portrayed by different landslide maps prepared by the California Geological Survey: (1) inventories of existing landslides, (2) landslide hazard—expressed as landslide susceptibility or landslide potential maps, (3) landslide risk maps, and (4) landslide zone maps. The maps can be either qualitative or quantitative in their preparation.
- Landslide-inventory maps, the most basic landslide maps, portray the location of prior failure. Because one clue to the location of future landsliding is the distribution of past movement, maps that show existing landslides are helpful in predicting the hazard. Inventory maps do not necessarily distinguish fresh movements, but in any one year some of the mapped slides—or more frequently, portions of them—may become active. A landslide inventory reveals the extent of past movement and thus the probable locus of some future activity within those landslides, but it does not indicate the likelihood of failure for the much larger area between mapped landslides. For this, hazard, risk or zone maps are required.
- Landslide-hazard maps describe an unstable condition arising from the presence or likely future occurrence of slope failure. There are two general types of landslide-hazard maps, each of which provides a different level of information and detail:
- Landslide-susceptibility maps describe the relative likelihood of future landsliding based solely on the intrinsic properties of a locale or site. Prior failure (from a landslide inventory), rock or soil strength, and steepness of slope are the three site factors that most determine susceptibility.
- Landslide-potential maps describe the likelihood of landsliding (susceptibility) jointly with the occurrence of a triggering event (opportunity). Potential commonly is based on the three factors determining susceptibility plus an estimate or measure of the probability (likelihood of occurrence) of a triggering event such as earthquake or excessive rainfall.
- Landslide-risk maps describe landslide potential jointly with the expected losses to life and property if a failure was to occur. The potential for landslide damage to a road system, for example, can be evaluated by considering the exposure of the roads to different levels of landslide hazard and the vulnerability of the roads to consequent damage. Similarly, the risk of excessive sedimentation in streams and other ecological damage can be evaluated by considering the landslide hazard jointly with the properties of streams and their sensitivity.
- Landslide-zone maps depict areas with a higher probability of landsliding, within which specific actions are mandated by California law prior to any development. These maps typically are binary in nature (a given site is either in or out of the zone) and are designed for use as planning tools by non-geoscientists. Zone maps may be derived from landslide potential or susceptibility, but some have been based simply on slope gradient or landslide-inventory maps.
Landslide hazard, risk and zone maps are prepared in many ways, increasingly involving complex manipulations of multiple criteria by computer. Because the value of landslide maps can be judged only by whether they correctly predict the locations of future failures, effectiveness of the different approaches to constructing them is difficult to evaluate.
