For more information

Web sites:

http://geohazards.cr.usgs.gov

http://www.usgs.gov/themes/landslid.html

Publications:

Godt, J.W., 1997, Digital representation of Landslide Overview Map of the conterminous United States: USGS Open-File Report 97-289.

U.S. Geological Survey, 1997, El Niño and the national landslide hazard outlook for 1997-1998: USGS Fact Sheet 180-97.

Alerting Nation to Landslides

Landslides are a nationwide hazard with direct and indirect costs estimated at $2 billion a year. Landslides threaten many metropolitan areas and communities, as well as transportation and communication lifelines. As development intrudes upon unstable terrain, the potential for landslide losses increases, and planners and engineers often lack information about the stability of terrain and the threat of landslides.

The USGS Landslide Hazards Program, in conjunction with many partners, is developing and communicating technical knowledge about landslides associated with unstable geologic formations, heavy precipitation, earthquake shaking, and volcanic activity. Geologic and engineering consultants and planners and decision makers use this knowledge to protect lives and safeguard property.

Long-range meteorological forecasts of heavy precipitation from the 1997-98 El Niño raised concern about possible increased landsliding in parts of the Nation. Characterized by warming of equatorial waters in the

Pacific Ocean, El Niño influenced weather patterns in the United States and elsewhere. The El Niño of the winter of 1982-83 was marked by widespread landsliding in various parts of the Western Hemisphere.

To alert the Nation to where landslide problems might be most abundant in 1997-98, in mid-November the USGS prepared National Landslide Hazard Outlook Maps for the ensuing winter and spring quarters. These maps superimpose precipitation forecasts by the National Oceanic and Atmospheric Administration on the USGS National Landslide Overview Map for the conterminous United States.

The maps, posted on the Internet, indicated broad regions of the United States where the potential for landsliding may be increased during this El Niño episode. Such information alerts government and private planners and decision makers to review and make ready contingency plans for responding to increased landslide activity. As the El Niño months progressed, the outlook maps were revised to incorporate records of actual precipitation.

Beyond the national hazard outlook, the USGS is cooperating with the National Weather Service and the California Office of Emergency Services and Division of Mines and Geology to inventory and understand El Niño-related landslide activity, including debris flows, in southern and central California and to enhance rapid communication of hazardous conditions. These efforts include the preparation of large-scale (1:24,000) demonstration hazard maps that can be used to guide development.

National Landslide Hazard Outlook Map

December 1997-February 1998

How To Interpret Precipitation Outlook:

These precipitation outlook maps show contours of the probability that a given area will experience above-, near-, or below-normal precipitation. These contours are based on measured precipitation from historical El Niño events and several models used by NOAA to characterize climate and weather. These maps do not show areas of below-normal precipitation because this condition does not generally increase landslide activity.

For example, the dark pink contours show areas for which there is a 63 to 67 percent chance for above-normal precipitation; a 33 percent chance for near-normal precipitation; and a 0 to 3 percent chance for below-normal precipitation. In a two-category system, there is approximately an 80 percent chance for above-normal precipitation and approximately a 20 percent chance for below-normal precipitation.

How To Interpret Landslide Incidence/Susceptibility:

The landslide incidence/susceptibility map was prepared by (1) classifying geographic areas as having high, medium, or low landslide incidence and (2) evaluating geologic formations or groups of formations in these areas as being of high, medium, or low susceptibility to landsliding.

Landslide incidence is defined by the number of landslides that have occurred in a given geographic area. Susceptibility to landsliding is defined as the probable degree of response of geologic formations to natural or artificial cutting, loading of slopes, or to unusually high precipitation. Generally, it is assumed that unusually high precipitation or changes in existing conditions can initiate landslide movement in areas where rocks and soils have experienced numerous landslides in the past.

The map units are split into three incidence categories according to the percentage of the area that has been involved in landslide processes. High incidence means greater than 15 percent of a given area has been involved in landsliding; medium incidence means that 1.5 to 15 percent of an area has been involved; and low incidence means that less than 1.5 percent of an area has been involved. High, medium, and low susceptibility are delimited by the same percentages used for classifying the incidence of landsliding. Susceptibility is not indicated where it is the same as or lower than incidence.