Landslides

In this section:

• Types of Landslide Maps
  
• Categories of Landslide Activity
  
• Confidence of Interpretation
  
• Literature Cited
  

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.

To find the landslide map that covers your area of interest, please see our Landslide Map Index . 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.

Types of Landslide Maps

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.

1. 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.
2. 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:
   1. 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.
   2. 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.
3. 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.
4. 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.

Categories of Landslide Activity

Each landslide is classified based on the recency of activity into one of four categories based on the system of Keaton and DeGraff, (1996). The landslide activity categories are graphically portrayed with the colors used to delineate landslide activity on the maps. The designation of activity shows an estimate of how recently the landslide moved, but also suggests the type of hazard represented. More recently active landslides are more likely to continue to fail, or to fail completely. Older landslides are less likely to move as single slide masses, but may be the source of smaller slides.

Confidence of Interpretation

Each landslide is classified according to a "confidence" that the geologic interpreter assigns to it, and can be regarded as a measure of the likelihood that the landslide actually exists.  Landslides are mapped on the basis of characteristic landforms, and the confidence of interpretation is based on the distinctness of those landforms. As a landslide ages after its last movement, erosional processes remove or cover the landforms that formed by landsliding. With time, these distinctive landforms become so subtle that they resemble landforms produced by geologic processes and conditions unrelated to landsliding.  The confidence of interpretation is depicted on a map by the use of different line styles (solid, dash, and dot).

Definite: Landslide exhibits many of the diagnostic landforms, including, but not limited to, prominent scarps, open cracks, rounded toes, offset streams, well-defined mid-slope benches, closed depressions, springs, and irregular or hummocky topography; or has clear records of prehistoric, historic or ongoing activity from reports, aerial photography or instrumental monitoring.

Probable: Landslide exhibits several of the diagnostic landforms commonly associated with landslides.  These landforms may be modified by erosion or obscured by vegetation such that other explanations are possible.  However, the preponderance of evidence strongly suggests that a landslide does exist.

Questionable: Landslide exhibits only one or a few of the diagnostic landforms associated with landslides. The landforms may be heavily modified by erosion, altered by grading, obscured by dense vegetation, or formed by other geologic processes such as differential erosion of lithologic and structural features in the underlying bedrock.

Translation of landslides from some legacy maps requires a geologist to interpret the original author's confidence from the graphics on those maps. Most commonly, older maps have two types of symbols indicating confidence: a landslide polygon is either shown with a query or with arrows indicating direction of movement. Where CGS compiled older mapping for this project, we reviewed aerial photos and discussed procedures with the original authors, if possible. Based on this review we have classified landslide areas shown with a query as "questionable" and others as "definite".

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.

Keaton, J.R., and DeGraff, J.V., 1996, Surface observation and geologic mapping, in Turner, A.K., and Schuster, R.L., editors, Landslides Investigation and Mitigation: National Research Council Transportation Research Board Special Report 247, p.178-230.

Varnes, D.J., 1978, Slope movement types and processes. In, R.L. Schuster and R.J. Krizek (eds.), Landslides: Analysis and Control, National Research Council Transportation Research Board Special Report Special Report 176, National Research Council, Washington, D.C. pp. 11-33.

In this section:

• History of Landslide Mapping at CGS
• Forest and Watershed Geology Program
• Seismic Hazards Program
• Highway Corridor Landslide Hazard Mapping
  
• Technical Reports and Disaster Response
  

History of Landslide Mapping at CGS

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 or pdfs and most of the landslides from those maps have been included in the California Landslide Inventory, discussed in the next section.

Currently active landslide mapping programs at CGS, and their respective products, are summarized below.

Forest and Watershed Geology Program

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.

Seismic Hazards Program

The Seismic Hazards 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 the CGS  Landslide Inventory Map Series

Highway Corridor Landslide Hazard Mapping

CGS has prepared a series of maps and reports for the California Department of Transportation (Caltrans) in the Highway Corridor Landslide Hazard Mapping project that evaluate the severity of landslide hazards along highway corridors through mountainous and potentially unstable terrain.

Technical Reports and Disaster Response

CGS regularly provides technical input and advice to the California Office of Emergency Services (CalOES) 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 CalOES 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.

The California Landslide Inventory is an ongoing project to collect and display landslide data for the State of California. These data include both the most recently mapped landslides and data developed over the past thirty years by the California Geological Survey and its collaborators. Multiple programs at the CGS create landslide data. Historically, each program created its own data standard to meet its specific needs. Between 2012 and 2015, the landslide mapping programs at CGS developed a common data standard for developing landslide data. These data have now been standardized into the Landslide Inventory, and new data will be developed in accordance with the Landslide Inventory's data standard.

The California Geological Survey is in the process of digitizing maps of landslides and has prepared a statewide landslide map database that is available online. The database shows many of the landslides mapped by CGS and others over the past 50 years. Mapping of landslides reflects the standards of the project and time the map was prepared. Many maps show landslide source areas (scarps) separate from landslide deposits, other maps combine scarps and deposits into a single feature. The amount of information recorded about each landslide has increased over time, so more information is available for more recently mapped landslides. Updates to the database are continuing, both to include more existing maps and to add current landslides as they occur. The landslide inventory, in combination with the map of susceptibility to deep-seated landslides (CGS Map Sheet 58) can give local planners, infrastructure owners, and the public a perspective on where landslides are most likely to be triggered by winter storms or by earthquakes in California.  View the California Landslide Inventory.

• Types of Landslides
• Burned Watershed Geohazards
• 2023 California Landslides Response
• Highway Corridor Landslide Hazard Reports
• CGS Information Warehouse: Landslide Maps and Reports
• Deep-Seated Landslide Susceptibility and Recent Fire Perimeters
• Map Sheet 58: Susceptibility to Deep-Seated Landslides in California
• CGS Note 33: Hazards from Mudslides...Debris Avalanches & Debris Flows in Hillside & Wildfire Areas
• CGS Note 40: Watershed Mapping
• CGS Note 45: Guidelines for Geologic Timber Harvesting Plans
• CGS Note 50: Factors Affecting Landslides in Forested Terrain
• CGS Note 52: Guidelines for Preparing Geologic Reports for Regional Scale Environmental and Resource Management Planning
• Special Publication 117A: Guidelines for Evaluating and Mitigating Seismic Hazards [including landslides] in California
• Site search results matching Geology and Landslides
• Other Geology-Related Web Sites page