What is a tsunami?

A tsunami (jap. harbour wave) is a water wave where the whole water colum is in motion.

The wave is generated by the sudden displacement of huge volumes of water. Strong earth­quakes could cause ruptures and faults at the ocean floors and are therefore often the triggering event for a tsunami. However, landslides and volcanic eruptions during which huge land masses slip into the water or even meteorite impacts can also trigger tsunami waves.

Propagation velocity of tsunami waves

The propagation velocity of tsunami waves greatly depends on the water depth. In deep waters the tsunami wave is very shallow, but with a speed of about 800 km/h it is very fast. In shallow waters the speed is considerably reduced, but for this reason the wave rises up vertically and can reach a height of more than 30 metres.

Tsunami hazard regions

Tsunami occur in particular along plate boundaries, where continental plates either shift past each other, drift apart from each other or, as in subduction zones, where the oceanic plate subducts beneath the continental plate. The two most devastating tsunami disasters in recent years were triggered by severe earthquakes in 2004 in the Indian Ocean off the coast of Indonesia and in 2011 in the Pacific off the coast of Japan along subduction zones.

However, tsunami were also triggered in the past by earthquakes and volcanic eruptions in the Atlantic and the Mediterranean.

Text: Dr. Jörn Lauterjung, Dr. Ute Münch, GFZ

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