Tsunamis

Tsunamis are giant sea waves generated by shallow focus underwater earthquakes, volcanic eruptions, underwater debris slides and large landslides into the sea.

Tsunamis have a very long wavelength (sometimes over 100km) and a low wave height (under 1 metre) in the open ocean, and they travel quickly at speeds of over 700km per hour but, when reacing shallow water bordering land, increase rapidly in height.

Around 90% of tsunamis are generated within the Pacific Basin, associated with the tectonic activity taking place. Most are generated at convergent plate boundaries where subduction is taking place.

The first warning given to coastal communities is the wave trough in front of the tsunami which results in a reduction in sea level, known as a drawdown. Behind this comes the tsunami itself, which can reach heights in excess of 25m.

Earthquakes near the coast can cause tsunamis and so this would warn people that a tsunami may happen.

The effects of a tsunami when it reaches the land will depend on:

• the height of the waves and the distance they have travelled
• the length of the event
• the extent to which warnings could be given
• coastal physical geography, both offshore and on the coastal area
• coastal land use and popuation density

The wave will wash boats and wooden coastal structures inland, and the backwash may carry them out to sea. People are drowned or injured by the tsunami because of the water and debris. Buildings, roads, bridges, trees and even soil are washed away.

- Tsunamis generated by the explosion of the volcano Krakatoa in 1883 have been estimated to have drowned over 35,000 people and produced waves that travelled around the world, the highest being over 40m.

- 'The Boxing Day' tsunami, December 2004, east Indian Ocean off Sumatra: generated by a powerful magnitude 9.0 earthquake 25km below the Indian Ocean floor. 300,000 people were killed and hundreds of communities were devastated. Damage was done to bridges, roads and local economies which left people unable to buy food. However, a warning system was set up among the countries that border the Indian Ocean. The disaster cost less than $5million.

- April 2007, Solomon Islands: generated by a 8.0 magnitude earthquake, the tsunami swept aross the islands killing at least 15 people.