U.S. Geological
Survey Fact Sheet 180-97

El Niño and the National Landslide Hazard Outlook
for 1997-1998

The U.S. Geological Survey (USGS) has prepared national maps of landslide
hazard outlook for 1997 and 1998 by combining forecast information for precipitation
from the National Oceanic and Atmospheric Administration (NOAA) with a USGS
map showing landslide incidence and susceptibility for the conterminous
United States. These predictive maps show contours of precipitation anomalies
and zones of landslide susceptibility and incidence for large areas of the
country.

The World Wide Web address for the Landslide Hazard Outlook map and
additional information on El Niño is: www.usgs.gov/elnino.html.

Derailment of California Zephyr
near Granby, Colorado. Landslide triggered by anomalously high precipitation
in the spring of 1985. (Photo: USGS/R.L. Schuster)



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Abnormally high precipitation that may accompany the current El Niño
climatic event is causing mounting concern that late 1997 and 1998 may be
years of exceptional landslide activity. El Niño events are characterized
by a warming of equatorial waters in the western Pacific Ocean that spreads
eastward to the Western Hemisphere. El Niño events occur every few
years, and some events are much larger than others. The El Niño of
the winter of 1982-83 was marked by widespread landsliding in different
parts of the Western Hemisphere. According to NOAA, the current El Niño
may be the largest of this century. Although long-term forecasting is very
generalized, a strong possibility exists for increased precipitation with
the current El Niño that could lead to increased landsliding.

As part of the USGS response to El Niño, the Landslide Hazards
Program is combining the newly digitized USGS National Landslide Overview
Map with NOAA's National Climate Outlook Maps. The digital landslide map
delineates areas where large numbers of landslides have occurred and areas
that are susceptible to landsliding in the conterminous United States. NOAA's
National Climate Outlook Maps show contours of probability that El Niño-induced
precipitation will be above, near, or below normal for 1997-98. We have
not included the below-normal contours in the combined maps, since dry conditions
are not conducive to landsliding. The resulting maps of the conterminous
United States appear on the World Wide Web and indicate where there may
be above-normal precipitation and potential landsliding during late 1997
and 1998.

In the digital National Landslide Overview Map, landslides are characterized
as any downward and outward movement of earth materials on a slope. The
term landslide includes downward movement of rigid blocks on thin surfaces
of slippage and earth and debris flows, where movement is more fluid. Landslide
movement can be slow to rapid.

Susceptibility to landsliding is (1) defined as the probable degree of
response of geologic formations to natural or artificial cutting, loading
of slopes, or to above-normal precipitation and (2) classified as being
high, medium, or low. Because individual landslides could not be shown at
the small scale of these maps, the percentages used for classifying the
incidence of landsliding (number of landslides) are defined according to
the percentage of the area observed to have been involved in landslide processes.



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Outlook as of November 13, 1997

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.



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Outlook as of November 13, 1997

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.



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Abnormally high precipitation that may accompany the current El Niño
climatic event is causing mounting concern that late 1997 and 1998 may be
years of exceptional landslide activity. El Niño events are characterized
by a warming of equatorial waters in the western Pacific Ocean that spreads
eastward to the Western Hemisphere. El Niño events occur every few
years, and some events are much larger than others. The El Niño of
the winter of 1982-83 was marked by widespread landsliding in different
parts of the Western Hemisphere. According to NOAA, the current El Niño
may be the largest of this century. Although long-term forecasting is very
generalized, a strong possibility exists for increased precipitation with
the current El Niño that could lead to increased landsliding.

As part of the USGS response to El Niño, the Landslide Hazards
Program is combining the newly digitized USGS National Landslide Overview
Map with NOAA's National Climate Outlook Maps. The digital landslide map
delineates areas where large numbers of landslides have occurred and areas
that are susceptible to landsliding in the conterminous United States. NOAA's
National Climate Outlook Maps show contours of probability that El Niño-induced
precipitation will be above, near, or below normal for 1997-98. We have
not included the below-normal contours in the combined maps, since dry conditions
are not conducive to landsliding. The resulting maps of the conterminous
United States appear on the World Wide Web and indicate where there may
be above-normal precipitation and potential landsliding during late 1997
and 1998.

In the digital National Landslide Overview Map, landslides are characterized
as any downward and outward movement of earth materials on a slope. The
term landslide includes downward movement of rigid blocks on thin surfaces
of slippage and earth and debris flows, where movement is more fluid. Landslide
movement can be slow to rapid.

Susceptibility to landsliding is (1) defined as the probable degree of
response of geologic formations to natural or artificial cutting, loading
of slopes, or to above-normal precipitation and (2) classified as being
high, medium, or low. Because individual landslides could not be shown at
the small scale of these maps, the percentages used for classifying the
incidence of landsliding (number of landslides) are defined according to
the percentage of the area observed to have been involved in landslide processes.

Aerial view of 1983 Thistle,
Utah, landslide associated with high El Niño precipitation, resulting
in $400 million loss. (Photo: USGS/R.L. Schuster)

High incidence means greater than 15 percent of a given area is involved
in landsliding; medium incidence means that 1.5 to 15 percent of an area
is involved; and low incidence means that less than 1.5 percent of an area
is 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. The susceptibility
categories are largely subjective because the data needed to make precise
determinations were not available.

Where more detailed information becomes available, we generally find
that the landslide hazard has been underestimated. The level of detail shown
for some areas was the result of greater availability of documented landslide
hazards. Recent experience with major landslide events in the Western United
States confirms that parts of western Washington and Idaho as well as the
Sierra Nevada have more extensive landslide hazards than shown on the map.

The NOAA National Climate Outlook Maps used to prepare the national landslide
hazard outlook maps are based on (1) precipitation patterns obtained from
a composite of past strong El Niño events and (2) results from several
forecast models. Data from strong El Niño episodes over the past
102 years were examined in 3-month intervals for above- and below-normal
precipitation. In coastal southern California, for example, seven of 11
El Niño years were among the wettest third, three were among the
near-normal third, and only one of the eleven was among the driest third.
These observations, combined with modern forecast methods, determine the
precipitation outlook for the winter of 1997. While the probability of above-normal
precipitation is very strong from a climate prediction standpoint (as much
as 25 percent above normal in some areas), the probability that precipitation
will exceed normal in most of the map area is no greater than about 65 percent.

Because these highly generalized maps for national landslide hazard outlook
were prepared at a small scale using limited landslide and climate information,
they are not intended for local planning or actual site selection. These
maps will indicate broad regions of the United States that may have increased
potential for landsliding during the coming El Niño climatic episode.
The national landslide hazard outlook maps will be updated frequently during
1997 and 1998 and will appear at the following internet website: http://geohazards.cr.usgs.gov/.



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To help prepare for landslides and lessen their impacts,
scientists from the USGS Landslide Hazards Program conduct research on landslide
processes. An example of this research is the assessment of rainfall thresholds
for landslide initiation. These thresholds are used for local and regional
landslide warning systems. Additional Landslide Program activities include
evaluation of landslide susceptibility at National and regional scales,
and emergency response to landslide emergencies and disasters, both in the
USA and in cooperation with foreign countries.

Jonathan W. Godt, Lynn M. Highland, and William Z. Savage



For more information, contact:

U.S. Geological Survey

National Landslide Information Center

Federal Center, Box 25046, MS 966

Denver, CO 80225-0046 USA

Tel: 1-800-654-4966

email: [email protected]

http://geohazards.cr.usgs.gov/





http://nic.fb4.noaa.gov/products/analysis_monitoring/