Answer: A landslide is defined as, the movement of a mass of rock, debris, or earth
down a slope. (Cruden, 1991). Landslides are a type of “mass wasting”
which denotes any down slope movement of soil and rock under the direct influence
of gravity. The term “landslide” encompasses events such as rock
falls, topples, slides, spreads, and flows (Varnes, 1996). Landslides can be
initiated by rainfall, earthquakes, volcanic activity, changes in groundwater,
disturbance and change of a slope by man-made construction activities, or any
combination of these factors. Landslides can also occur underwater, causing
tidal waves and damage to coastal areas. These landslides are called submarine
landslides.
Failure of a slope occurs when the
force that is pulling the slope downward (gravity) exceeds the strength of the
earth materials that compose the slope. They can move slowly, (millimeters per
year) or can move quickly and disastrously, as is the case with debris-flows.
Debris-flows can travel down a hillside of speeds up to 200 miles per hour (more
commonly, 30 – 50 miles per hour), depending on the slope angle, water
content, and type of earth and debris in the flow. These flows are initiated
by heavy, usually sustained, periods of rainfall, but sometimes can happen as
a result of short bursts of concentrated rainfall in susceptible areas. Burned
areas charred by wildfires are particularly susceptible to debris flows, given
certain soil characteristics and slope conditions. More information can be found
in USGS Fact Sheet numbers FS-071-00, “Landslide Hazards” (English
Version), and FS-072-00, “Peligros de Deslizamientos” (Spanish Version.).
English Version: http://greenwood.cr.usgs.gov/pub/fact-sheets/fs-0071-00/
Spanish Version: http://greenwood.cr.usgs.gov/pub/fact-sheets/fs-0072-00/
Information on debris flows can be
found in our “Publications” section.
Debris Flows: http://landslides.usgs.gov/html_files/nlic/nlicpub.html
Sources of Information for
FAQ #1:
(1). Cruden, D.M., 1991. A Simple
Definition of a Landslide. Bulletin of the
International Association of Engineering Geology, No. 43, pp. 27-29.
(2). Varnes, D.J., 1996. Landslide Types and Processes, in Turner, A. K.,
and R.L. Schuster, Landslides: Investigation and Mitigation, Transportation
Research Board Special Report 247, National Research Council, Wasington, D.C.:
National Academy Press.
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