The soils on most hillsides are in a stable state. If they were unstable a landslide would have taken them to the bottom of that hillside a long time ago. The landslides that we see today usually occur when natural or human actions disturb a stable slope. The actions that cause landslides are frequently referred to as "triggers." Although people trigger a few landslides most are triggered by natural processes.

A summary of landslide triggering conditions is given in the table below. The columns of this table separate the two landslide triggering categories (natural and human). The rows of the table are the different landslide triggering processes. Each cell within the table gives examples of situations that can caused landslides.

Removal of Support:   Excavation or erosion at the base of a slope (or on a slope) can cause an unstable situation. The removed material was frequently supporting the soil directly upslope from the disturbed area. This loss of support can immediately or eventually result in landsliding.

Removal of Vegetation:   Vegetation does two important things to reduce landslide incidence. 1) the vegetation removes water from the soil; and, 2) the root systems support the soil and provide a stabilizing effect. Areas that experience forest fire or clear-cut timbering are subject to landsliding for many years after the vegetation is removed.

Addition of Moisture:   any soils, especially clays, are hard when dry but transform into soft muds when a small amount of moisture is added. The addition of water reduces the shear strength of the soil and can result in landsliding. Also, water pressure within the porosity of a soil has the effect of "inflating" the pore spaces and reducing the frictional forces between soil particles. This loss of friction can result in a rapid landslide. The addition of moisture is the most common source of landslide problems.

Addition of Weight:   Adding weight to the top of a slope has the same effect as increasing the force of gravity. If the added weight exceeds the shear strength or increases the pore pressure of the soils below landsliding can result.

Oversteepening:   The angle of repose is the maximum angle that a material can be stacked and remain stable. If soil is piled at an angle that exceeds the angle of repose, landsliding can result. This term is sometimes used to describe situations in which erosion, excavation, or grading results in slopes that exceed the angle of repose.

Vibrations:   Sudden movements can cause the particles in a soil to briefly lose contact with one another. When this happens, the frictional forces that enable that material to remain on a slope are lost and landsliding can be triggered. Vibrations from earthquakes, blasting, heavy equipment, and loud noises have been known to trigger landslides.