Earthquake-induced landslides and their effects on alluvial fans

Earthquakes as small as magnitude 4.0 may dislodge landslides from susceptible slopes, and larger earthquakes can generate tens of thousands of landslides throughout areas of hundreds of thousands of square kilometers, producing billions of cubic meters of loose, surficial sediment. These landslides can have significant geomorphic effects that vary depending on the landslide characteristics and materials, and on the settings in which the landslides occur. A review of data from historical earthquakes indicates that the landslides they generated can be classified into 14 different types. These include highly disaggregated and fast-moving falls, disrupted slides, and avalanches; more coherent and slower-moving slumps, block slides, and earth slides; and lateral spreads and flows that involve partly to completely liquefied material. Rock falls, disrupted rock slides, and disrupted slides of earth and debris are the most abundant types of earthquake-induced landslides, whereas earth hows, debris flows, and avalanches of rock, earth, or debris typically transport material the farthest, Because of their abundance or long distances of transport, these landslides generally have the greatest effects on the landscape during and after earthquakes. Landslide effects on alluvial fans include direct deposition of material on fan surfaces; fissuring and displacement of fan materials; alterations in drainage basins such as devegetation, denudation, and changes in channel networks; and generation of large amounts of sediment that may be transported to fans by post-earthquake water flows or debris flows.