landslide is the gravity-controlled displacement of soil or rock. The rate of displacement can be slow or fast, but never very slow. Landslides can be shallow or deep. The material consists of a mass corresponding to a portion of the slope or the slope itself. Displacement occurs downhill and outward, to fall on a clear plane.
The term landslide is used in its broad sense, and includes the downward and outward displacement of the material that makes up a slope (bedrock and soil). Landslides can be caused by heavy rainfall, soil erosion or earthquakes, but they can also occur in areas covered by thick layers of snow.
It is difficult to judge landslides as independent phenomena, so it seems appropriate to associate them with other hazards, such as tropical cyclones, powerful local storms and river flooding.
Rockfall refers to rocks or stones falling freely from the wall of a vertical cut in the ground. It is caused by weakening or weathering of the ground, or by the degradation of permanently frozen ground.
Subsidence is the downward movement of the earth’s surface, relative to a plane of comparison (e.g., sea level). Subsidence (dry) can result from: geologic faulting, glacial or isostatic rebound, human activities (e.g., mining, natural gas extraction), etc. Subsidence (wet) can originate from: karst soils, changes in soil water saturation, degradation of permanently frozen ground (thermokarst), etc.
Avalanches refer to a quantity of debris/terrain/snow or ice that slides downslope by the force of gravity. It frequently gathers material that is underneath the snowpack, such as soil, rocks, etc. (debris avalanche).
The warning period may vary. When the displacement is due to an earthquake, this period may be small, or it may not be possible to warn. However, a general warning may be issued when there is a risk of landslides due to heavy and persistent rainfall. Sometimes small initial landslides may be considered a warning of subsequent large-scale landslides.
- Monitoring systems, where appropriate.
- Land use and building regulations.
- Public awareness programs.