Three examples of landslides triggered by earthquakes have been examined to consider the natural variability of the slope forming materials, and the uncertainties surrounding input ground motions. These were the landslides at Villa Del Monte, California, Tachia Gorge, Taiwan and Las Colinas, El Salvador which were triggered by the Loma Prieta, (MW=7.0, 1989), Chi Chi, (MW=7.6, 1999) and El Salvador (MW=7.6, 2001) earthquakes respectively. The results of these analyses show a large scatter in the calculated factors of safety for earthquake conditions, some of which indicated stability when failure actually occurred.
The models used in the calculation of seismic slope stability yield acceptable results. However, it is clear that the natural variability of slope forming materials has a strong influence on the range of results reported for the analysis of slopes subjected to strong shaking. This variability is exacerbated by limited knowledge about the interaction with seismic waves and slopes resulting in topographic amplification. Such problems are often further complicated by uncertainty and error associated with attenuation relationships.