Voyager: I know that landslides can happen when storms rain on a cliff or hill, but how do landslides happen underwater?

A: When landslides occur on land, the biggest threats could be to houses, roads, electric and gas lines that lie below the slide. The moving mass of earth can cause much damage to these structures.  A landslide is a mass of earth that moves as a glob over top of the earth below it.  The face or plane along which the glob moves is called the interface. 

Two things are important to determine if a landslide will occur. The first is how steep the slope of the interface is and second is how sticky the interface is.  You can demonstrate this to yourself by laying two books sideways on top of each other. Pick up one end of the lower book and raise it, eventually the upper book will slide off.  If you try different combinations of books you can see how the angle and stickiness between the books control the “bookslide.” 

Landslides can also happen on the ocean floor and can be triggered by nearby earthquakes or other events.   Usually the only man-made structures that can be damaged by the mass of seafloor are underwater cables. Unlike a slide on land, however, an underwater landslide can also cause a strong water wave called a tsunami. This wave can travel much farther than the seafloor mass. Where this wave runs up on shore, it can cause great damage to houses, businesses and even entire cities. In 2004, an earthquake in the Indian Ocean caused an underwater landslide that triggered such a wave. The wave struck several countries and caused the deaths of some 200,000 people. 

— David Chadwell, geophysicist, Marine Physical Laboratory