Tsunamis: how they form and their dangers

In short: A tsunami is a series of very long ocean waves that form when large masses of water are suddenly displaced, usually by an undersea earthquake. At the coast they can cause devastating flooding.

In more detail: Unlike wind-driven waves, a tsunami moves the entire water column from the seabed to the surface. In the deep ocean it is barely noticeable, but in shallow coastal water it slows down and piles up high.

In detail

• Causes: Most tsunamis are caused by strong earthquakes under the sea in which the seabed suddenly rises or drops. Underwater landslides, volcanic eruptions and, very rarely, meteorite impacts can also trigger them.
• On the open sea: In the open ocean a tsunami can travel as fast as an airliner. Its height there is often less than a metre, while the distance between two crests is many kilometres.
• At the coast: As the water gets shallower, the wave slows and its energy is compressed. The water then rises sharply within minutes and pushes far inland.
• Warning signs: The sea often retreats unusually far before the first wave. A strong, long earthquake near the coast is also an alarm signal.
• Several waves: A tsunami consists of a series of waves arriving minutes to an hour apart. The first is not always the largest.
• Known disasters: The Indian Ocean tsunami of 26 December 2004 and the tsunami in Japan in March 2011 killed very many people.
• Early warning systems: Buoys, tide gauges and seismic stations detect tsunamis and trigger warnings. After 2004 such a system was also set up for the Indian Ocean.
• What to do: On a warning or warning sign, move inland or to higher ground immediately and only return once the all-clear has been given.
• Background: Tsunamis are closely linked to the movement of tectonic plates (erdbeben-und-vulkane, plattentektonik, meere-und-meeresstroemungen).