How the seafloor moves?
Sometimes sediments that accumulate on seafloor of the continental shelf and slope, reach a thickness where they become unstable. When this happens an earthquake can trigger the sediment into an giant mud slide that flows down the continental slope into deeper water. These mud slides and flows can be massive, involving hundreds of cubic kilometres of sediment. One large sediment flow studied in the North Atlantic covered an area larger than Wales.
Tsunami are giant waves generated by these underwater landslides, earthquakes or volcanoes. They travel very quickly over deep water, up to 600 km/hour, but may be only a metre or so high. Once they reach shallow water, however, they slow down and increase in height, up to as much as 30m or more. The city of Lisbon in Portugal was hit by an earthquake in 1755. The earthquake happened out at sea, and generated a tsunami which swept across the city, causing further damage and many deaths. Tsunami deposits have been found in Britain, resulting from a tsunami generated by the Storegga Slides, when a huge area off the coast of Norway collapsed several thousand years ago.