Natural Disasters Project Landslides Landslides occur when a portion of a hillslope becomes too weak to support its own weight. This weakness is generally initiated when rainfall or some other source of water increases the water content of the slope, reducing the strength of the materials. Other causes of landslides include earthquakes and loud sounds. Many types of landslides move seasonally or periodically and may lie dormant for years. Landslides are generally classified into slides, falls and flows.
Slides move as large bodies by slipping along one or more failure surfaces.
Falls of rock or soil originate on cliffs or steep slopes. Large rockfalls can be catastrophic events. An earthquake off the coast of Peru in 1970 started a rockfall which accelerated to more than 170 miles per hour and buried more than 18,000 people. Flows are landslides that behave like fluids. Mudflows involve wet mud and debris.