The term ‘tsunami’ (harbour wave) comes from Japan where the phenomenon frequently occurs. When an underwater quake or any other cause suddenly sets a great depth of water into motion, it causes a gigantic wave to form. In the open sea, the undulation cannot be seen. However, when it hits shore, its height increases, submerging the coastline and destroying everything it encounters.
The main events preceding the Tsunami are illustrated here:
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The tsunami wave can travel at a speed of several hundred kilometres an hour over thousands of kilometres in the open sea without being spotted (the wave may be only a few metres high for up to an hour or more).
On reaching the coast, it slows down and its wavelength decreases. The energy is transformed into amplitude, resulting in a tidal wave that can crest to heights of 30 metres. How devastating a tsunami is depends on the amount of water set into motion and the type of coastline (flat or mountainous) it encounters.
Warning centres like the one in Hawaii in the Pacific Ocean are needed all over the world to monitor underwater quakes and alert the ever-growing coastal populations.