What Causes a Landslide During an Earthquake?

A landslide occurs if surface material, such as rocks, gravel and soil, move down a slope or hill. In the United States, landslides average more than 25 fatalities and $1 to 2 billion in damages each year. Landslides and earthquakes are tied together as quakes frequently are the cause of landslides.

Landslide Due To Strong Shaking

• In the aftermath of an earthquake, a landslide can occur due to the strong shaking of the quake. A 4.0 magnitude earthquake can cause landslides on susceptible slopes, and a 6.0 magnitude has the potential to trigger widespread landslides. Earthquake-induced landslides usually result in rock falls and slides of rock fragments from steep slopes.

Types of Landslides

• The most dangerous types of earthquake-induced landslides are from large rock avalanches, soil avalanches and underwater landslides. One of the worst landslides occurred in Peru in 1970 after an earthquake of 7.9 to 8.0 on the Richter scale led to the death of over 18,000 people.

Factors for Judging Size of Landslides

• The size of the landslide depends on the magnitude of the earthquake, the duration of tremors or ground shakes caused by the quake, the stability and angle of the soil on the slope and the vegetation in the area. Of these factors, the magnitude of the earthquake and its vicinity to the slope have the greatest impact on the severity of the landslide.

Predicting Landslides

• There are methods of predicting landslide susceptibility for the purpose of improving and designing slopes that are less susceptible to earthquake-induced landslides. However, it is difficult to predict with certainty the areas that are most susceptible because a major factor is the magnitude of the earthquake. The type of soil can also influence the susceptibility of a landslide.