Landslides are caused by gravity acting on slopes. When gravitational stresses exceed the strength of rock or soil, slope failure occurs. Many landslides are initiated by triggering factors that increase stress and weaken slope materials. These triggers include:
• Heavy rains, rapid snowmelt, or irrigation that load slopes with water
• Natural erosion or human activities that increase slope angles or undercut the toes of slopes
• Shaking by earthquakes
• Removal of vegetation by wildfire, logging, agriculture, or overgrazing
• Loading of slopes with piles of rock, ore, or mining waste