Mass movement occurs when the stresses exceed the internal strength. This condition is not always caused by an increase in stress. In some cases, the internal strength of the materials can be reduced over relatively short periods of time resulting in mass movement. Many factors can act as triggers for hillslope failure. One of the most common is prolonged or heavy rainfall.
Rainfall can lead to mass movement through three different mechanisms.
Often these mechanisms do not act alone. The saturation of soil materials increases the weight of slope materials which then leads to greater gravitational force. Saturation of soil materials can reduce the cohesive bonds between individual soil particles resulting in the reduction of the internal strength of the hillslope. Lastly, the presence of bedding planes in the hillslope material can cause material above a particular below ground level to slide along a surface lubricated by percolating moisture. Earthquakes are another common mechanism that can trigger mass movement.