IFRC

Geophysical hazards: Mass movement dry


Definition and characteristics

A landslide is the movement of soil or rock controlled by gravity and the speed of the movement usually ranges between slow and rapid, but not very slow. It can be superficial or deep, but the materials have to make up a mass that is a portion of the slope or the slope itself. The movement has to be downward and outward with a free face.

The term landslide is used in its broad sense to include downward and outward movement of slope forming materials (natural rock and soil). It is caused by heavy rain, soil erosion and earth tremors and may also happen in areas under heavy snow

Landslides are difficult to estimate as an independent phenomenon. It seems appropriate, therefore, to associate landslides with other hazards such as tropical cyclones, severe local storms and river floods.

Rockfall refers to quantities of rock or stone falling freely from a cliff face. It is caused by undercutting, weathering or permafrost degradation.

Subsidence is the motion of the Earth's surface as it shifts downward relative to a datum (e.g. the sea level). Subsidence (dry) can be the result of: geological faulting, isostatic rebound, human impact (e.g. mining, extraction of natural gas) etc. Subsidence (wet) can be the result of: karst, changes in soil water saturation, permafrost degradation (thermokarst) etc.

Mass movement describes a quantity of debris/land/snow or ice that slides down a mountainside under the force of gravity. It often gathers material that is underneath the snowpack like soil, rock etc (debris avalanche).

Warning period may vary. Little or no warning may be available if the cause is earthquake. However, some general warning may be assumed in the case of landslides arising from continuous heavy rain. Minor initial landslips may give warning that heavy landslides are to follow.

How to prevent

  • Monitoring systems, where applicable.
  • Land-use and building regulations.
  • Public awareness programs.

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