Landslide is a general term for mass movement. It implies a gradual movement rather than the more sudden movement of an avalanche. Landslides occur when a mass of soil or rock moves down an incline under the influence of gravity. The causes may be either natural or artificial. Natural landslides can be caused by the access of groundwater, the properties of the soil itself, the gradient of a slope, the height of a slope, or the erosion of a slope at its base. The causes of artificial landslides lie in the way soil has been cut or banked. The velocity of a landslide varies greatly, ranging from a slow of 1 cm a year to a speeds of several metres a day for volcanic landslides. The velocity can change over a period of time. In the case of a landslide covering a wide area, several slides may take place concurrently, each one having its own rate of slippage. The movement occurs not only in the direction of the slope but also upwards and downwards.
Even though landslides are generally not so spectacular or costly as earthquakes, major floods, hurricanes and some other natural catastrophes, they are more widespread and, over a period of years may cause more property loss than any other geologic hazard. Moreover, much of the damage and sometimes a considerable proportion of the loss of life attributed to earthquakes and intense storms is really due to landslides. In many developing regions landslides constitute a continuing and serious impact on the social and economic structure, of which the true measure is not in monetary units but rather in disruption and attendant misery of human lives.
In the May 1970 earthquake in Peru which cost about 70,000 lives, about 20,000 people perished in the debris avalanche from the north peak of Nevado Huascaran. During the period 1971-75, of some 19,000 lives lost in earthquakes, tsunamis, volcanic eruptions, landslides, and snow or ice avalanches, about 84% of the casualties were attributed to earthquakes and 14% to landslides. Annual direct or indirect costs of landslides in the USA have been estimated to exceed US$ 1,000 million. Annual losses in Italy have been estimated at US$ 1,140 million.
Two huge landslides, following heavy rains, struck a mining region in southwest Yunnan province, China, in 1996. The disaster buried 23 gold pits, killing at least 79 people and leaving another 148 missing.