Click on the arrow on this animation to see how an undersea landslide can cause tsunami.
The amount of sediment and depth of the sea floor determine whether tsunami are generated. The sediment on the Continental Shelf, off the east coast of Australia, could be dislodged by a seismic event. Gravity could also cause the sediment to slide of its own accord if the slope of the shelf is steep enough. Either of these could have the potential to generate tsunami. Undersea landslides associated with smaller earthquakes can also generate destructive tsunami.
The most likely cause of the tsunami that devastated the northwest coast of Papua New Guinea in 1998 was an undersea landslide. This landslide was the result of a 7.1 magnitude earthquake which caused movement of the sea bed. Three waves, more than seven meters high, struck a ten kilometre stretch of coastline within ten minutes of the earthquake. Three coastal villages were completely swept away by the tsunami, killing over 2,000 people.
Tsunami can also be caused by a landslide into a body of water. It is usually a highly localised effect occurring when the origin of the tsunami is extremely close to the shore, or in deep, narrow inlets, lakes or other water passages.
Lituya Bay, on the Alaskan south-east coast, was the site of the largest tsunami ever recorded. On the night of 7 July 1958, a magnitude 8.0 earthquake occurred, causing a large landslide into the bay. This produced a localised tsunami measuring 524 metres high.