Landslides

Annually, landslides affect large numbers of people around the world due to both catastrophic and smaller, chronic events. According to a 2006 United Nations University report, annual losses due to landslides in Japan amount to $4 to 6 billion; for the United States, annual losses are estimated at $1 to 2 billion. Annually, 940 persons were killed in the decade from 1993 to 2002 due to catastrophic landslides, compared to 25,000 annual deaths due to flooding. Yet, landslide statistics are frequently underestimated, as many landslides worldwide are often triggered by another natural hazard, such as earthquakes or flooding; they also occur at many different scales, often not captured by official statistics. As an example, the 2001 El Salvador earthquake (magnitude 7.7) killed 844 persons, 585 of which were due to the landslide of Las Colinas.

Many hilly and mountainous regions around the world suffer from chronic and frequent landslides, greatly impacting rural populations in developing countries. Changes in slope morphology, groundwater level, or material properties can affect slope stability and decrease its shear strength (or the inherent force of a slope against gravity). Topography, lithology, geology, hydrology, and weathering processes are main causal or underlying factors for landslide-prone areas. Under such conditions, landslides can be triggered or reactivated by natural factors, especially rainfall, earthquakes, snowmelt, or rivers; or by human-induced activities, such as deforestation, grazing, road building, and mining. Mitigation solutions chosen to stabilize a landslide will depend closely on the risk it poses, economic means available for stabilization measures, and local risk cultures.

Usually, landslides affecting the top 16 feet of slopes are called shallow landslides. They are the most common, and are easily triggered without any warning by heavy rainfall or due to human activities such as land use changes, affecting infrastructure, downstream water quality, dam safety, and riverbed height. Deep-seated landslides affecting entire slopes can cut rivers, create temporary lakes, and cause extremely dangerous floods. They can occur on ocean bottoms or into lakes, provoking tsunamis.