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.
California Landslide Inventory
is an ongoing project to make the CGS's landslide information publicly accessible.
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
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
The California Landslide Inventory
The California Geological Survey (CGS) 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.
Click here to view the California Landslide Inventory.
A Brief History of Landslide Mapping at the 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
and most of the landslides from those maps have been included in the CGS digitial landslide inventory. Currently active landslide mapping programs at CGS, and their respective products are described in more detail below.
Landslide Programs at CGS
Forest and Watershed Geology
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 Zonation Program
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 the CGS
Landslide Inventory Map Series
Caltrans Highway Corridor Mapping
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.
Technical Reports and Disaster Response
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
magazine, and on CGS’s web pages.
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.
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.