Tsunami: Fact File

A powerful Pacific Ocean earthquake today triggered giant tsunami waves that devastated areas of the Samoan Islands.

A look at other earthquakes that have triggered tsunamis in previous years:

- September 2007: A 7.8-magnitude earthquake rattles Sumatra island, triggering regional tsunami alerts and damaging scores of buildings.

- September 2007: An earthquake measured at a magnitude of 8.4 near Sumatra triggers a wave in the coastal city of Padang. The tremor kills at least 25 people and injures around 50.

- April 2007: At least 28 people in the Solomon Islands die in a tsunami and earthquake measured at a magnitude of 8.1.

- July 2006: A magnitude-6.1 earthquake triggers a tsunami off Java island's southern coast, killing at least 600 people.

- December 2004: An Indian Ocean tsunami, triggered by a magnitude-9.0 earthquake, kills 230,000 in a dozen countries.

- August 1976: A magnitude-8.0 earthquake hits near the islands of Mindanao and Sulu in the Philippines, generating a tsunami and leaving at least 5,000 dead.

- March 1964: An 9.2-magnitude earthquake in Prince William Sound, Alaska, kills 131 people,
including 119 from a tsunami.

- April 1946: An earthquake measured at a magnitude of 8.1 near Unimak Islands, Alaska, triggers a tsunami, killing 165 people, mostly in Hawaii.

- April 1868: A 7.9-magnitude earthquake strikes the Big Island, Hawaii, leaving 77 people dead, including 46 from a tsunami.

Tsunami: from the Japanese tsu' (harbour) and nami' (wave).

How a Tsunami forms: Vertical movements from seismic activity such as powerful, shallow sub-sea earthquakes cause the motion of the entire water column from the surface to the seabed. This formation makes them very different to and more powerful than regular waves since a tsunami wave is motion of the sea from top to bottom with vast amounts of energy, whereas a wind wave is more like a ripple on the surface. Although tsunamis are often formed by earthquakes they can be caused by volcanic eruptions, land-slides and other mass movements.

The Tsunami offshore: While travelling offshore tsunami waves have much bigger wavelengths (distant from crest to crest) than regular wind generated waves and as such only have an amplitude (height) of about a metre, making them very difficult to detect and ships rarely notice them. Due to the energy generated at the source of the tsunami they travel at 500-600 miles per hour.

The Tsunami as it reaches land: When the waves reach land and the depth of the seabed decreases the wave slows to around 50 miles per hour. This compresses the wavelength from around 200km to around 20km causing a massive growth in the amplitude. This compression can be seen from the shore and the wave will appear to grow as it comes closer to land. The wave itself appears like a large tidal bore and not a breaking wave, making it look like a great wall of water, the height and shape is also influenced by the shape of the seafloor close to the shore. Despite looking like a tidal bore tsunamis are not related to tides although their impact can be influenced by them.

Tsunamis on land: Once the waves reach land, their vast energy means they can travel over land quicker than people can run. This means the drowning is the most common cause of death associated with tsunamis because people can be cut off rapidly. The power of the waves means they can also cause severe damage to buildings, especially in the run-up where it is at its full height.

Tsunami occurrences: Major Tsunamis happen roughly once a decade. 59% of the world's tsunamis have occurred in the Pacific Ocean, 25% in the Mediterranean Sea, 12% in the Atlantic Ocean and 4% in the Indian Ocean. The largest recorded earthquake, of 9.5 magnitude in 1960 in Chile generated a tsunami that killed many people around the Pacific Ocean. The 2004 tsunami was caused by a 9.15 magnitude earthquake and killed 230,000 people around Asia and even on the African coast.

This is a collection of videos from the 2004 tsunami which clearly shows the speed and power of the water and the destruction it can cause.