Landsliding refers to the downward shifting of the ground on a slope. The slope can naturally exist and/or be manmade. The reason for the shifting is either there has been added force to the slope or the slope has been weakened. There are a number of ways downward force can be added to the slope. Some of the more common applications which can cause landslides are:
The slope is steepened and/or heightened with fill soil
Slope becomes water logged and thus made heavier
A structure is constructed on or close to the slope
The water level in the waterway at the bottom of the slope dropped significantly causing downward seepage forces.
More typical causes for landsliding from slope weakening are: waterway bank erosion; man-made undercutting or excavations along the slope; removal of root reinforcement from vegetation; and weathering of the soil mantle. Landsliding can occur slowly to abruptly with little warning. More typical tell-tale signs of slope instability are ground cracking along the slope which is most commonly towards the upper portion of the slope; trees, poles, fences, etc. which are leaning downslope and ground surface bulging of heaving near the bottom of the slope.
Diagram of an idealized landslide showing commonly used nomenclature for its parts. Courtesy of the Utah Geological Survey.