Landslides are frequent in areas where there is high seismicity and steep slopes. Landslides associated with earthquakes may cause as much damage as the initial ground shaking. They may also occur long after the earthquake.

Some of the major earthquakes that have occurred during the past 15 years demonstrate the hazards of seismically triggered landslides. THe Hebgen Lake, Mont., earthquake of 1959 triggered a very large landslide (fig. 1) that killed and injured many people, formed a temporary lake, and blocked travel in the area. The Anchorage, Alaska, earthquake of 1964 triggered extensive subaerial (fig 2.) and submarine landslides; tsunamis (seismic sea waves) generated by the submarine landslides caused extensive damage and many fatalities in coastal areas. The earthquake in western Peru in 1970 triggered a massive debris avalanche (fig. 3) that destroyed the cities of Yungay and Ranrahirca; it probably caused about one-half of the 38,000 fatalities attributed to the earthquake. The San Fernando, Calif., earthquake of February 9, 1971, triggered more than 6000 individual landslides, most of which were small (fig. 4), in the surrounding upland areas. Fortunately, only a few of them damaged manmade structures because residential and industrial development had been restricted almost wholly to the relatively flat floor of the San Fernando Valley.

Each of the major earthquakes described above had magnitudes greater than 6.5. Although smaller earthquakes may cause less damage to manmade structures by ground shaking, they are capable of triggering slope failures, especially renewed movements of old, marginally stable landslide deposits (fig. 5), in hillside areas.