The word tsunami originates from Japanese, literally meaning ‘harbour wave’. Tsunamis are giant tidal waves usually triggered at destructive margins, by either volcanic eruptions (Krakatoa, 1883) or by submarine earthquakes (Sumatra, 2004). Other causes can be underwater mass movements, glacier calvings and underwater explosions. They can travel across the ocean at speeds of up to 600km/hr. The height of a tsunami wave can increase tremendously and can sometimes exceed 15 m. Owing to the massive volumes of water that are displaced and the high energy involved, tsunamis can bring immense devastation to coastal regions.
They are generally triggered by the same process that incites an earthquake – basically the sudden deformation of sea floor which displaces the overlying water. This often happens after the abrupt movement of thrust faults at convergent or destructive plate boundaries. However, at normal faults displacement of the seabed can induce a small reaction but is not significant to cause a significant tsunami.
Below are pictures of the destruction caused by the recent Japanese tsunami. There are two mechanisms that allow tsunamis to cause such havoc: firstly the smashing force of the incoming high speed wall of water, followed by the destructive power of the colossal volume of water the drains the land on which it travels. The first picture is from the BBC and shows the moment the wave invaded Japanese land.
The two pictures below are from National Geographic. The first illustrates the wreckage caused at the site of the Japanese nuclear power station. The second picture shows an enormous whirlpool that took shape in front of the port in Oarai town.
