The International Information Center for Geotechnical Engineers

Landslides: Slope stability, triggers, failure dynamics, and morphology - Landslide triggers


            A landslide trigger decreases the factor of safety to less than one. When the factor of safety is less than one, driving forces are greater than resisting forces, and failure will occur. Triggers include both natural and human-caused events.  Human induced triggers include removal of the toe of the landslide through excavation, loading of the head of the landslide (addition of mass), and artificial vibration.  Natural triggers include toe removal through erosion, changes in water pressure, and earthquakes. Any of these potential triggers can also combine to cause failure (Waltham, 1994).

Water Pressure

            Increasing water levels is the most common trigger of landslides. Increased water pressure decreases the effective stress and the factor of safety of a slope. The Oso landslide in Washington was likely triggered by increased precipitation in the weeks before the slide occurred (Henn et al, 2015). Precipitation can also trigger landslides destabilized by a previous event such as an earthquake, as seen in the landslides triggered by a rainstorm after the Wenchuan earthquake (Tang et al, 2011).

Earthquake triggers

            Earthquakes cause failure in two different manners. The vibration from an earthquake can cause liquefaction in uniformly graded, fine-grained, sediments due to loss of effective stress.  Earthquakes can also increase the shear stress on a slope, decreasing the factor of safety to below one (De Blasio, 2011). According to Newmark analysis (Newmark, 1965), displacement and landsliding occurs when a critical acceleration is reached. The critical acceleration for failure can be calculated using the following equation:AccEqnwhere ac is the critical acceleration, g is acceleration due to gravity and alpha is local slope.

            Large, shallow earthquakes frequently trigger landslides. Work from Keefer (1984) suggests that an earthquake as small as a magnitude 4.0 can trigger failures. The smallest earthquakes (ML (Richter local magnitude) = 4.0) can trigger rock falls, rockslides, soil falls, and soil slides. The largest earthquakes (Ms (Richter surface wave magnitude) 6.5 can trigger rock and soil avalanches. The area affected by landsliding is also dependent on earthquake magnitude (see figure 2). As magnitude increases, the extent of landsliding increases. An example of the extensive area that can be effected by landsliding during an earthquake is shown in figure 3 below. Figure 3 shows the distribution of landslides from the magnitude 6.7 1994 Northridge earthquake. Around 10,000 km2 was effected by landsliding, matching predictions made by Keefer (see figure 2).


Figure 2: The relation between earthquake magnitude and the area affected by landslides. Figure is from Keefer (1984). 

NR ls
Figure 3: An example of the distribution of earthquake triggered landslides after the 1994 Northridge earthquake. Landslides (Harp and Jibson, 1996) are blue polygons and active quaternary faults (USGS and CGS, 2006) are the red lines.

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