Tsunamis: dangerous water masses

Why tsunamis are so dangerous

Tsunamis are powerful waves in the sea, usually caused by earthquakes, volcanic eruptions or landslides. These tidal waves displace large amounts of water in the ocean, but are still barely visible on the surface. A tsunami on the open ocean can reach a speed of up to 500 km/h with a wave height of only one meter. Only in shallow waters near the coast are the water masses slowed down and pile up to many meters.

Tsunamis on the open ocean move huge masses of water that spread out in waves over entire ocean basins. This makes them particularly dangerous when they hit land, because the size of the tidal waves causes them to travel far inland at high speed.

The large volume of water is not the only danger. Due to the enormous kinetic energy, debris, sediments and even buildings and vehicles are carried along on land and, in the case of strong events, are sometimes pushed miles inland. While a person could often swim and survive even in the current, most fatalities occur as a result of injuries from flotsam.

"Drawdown" - the negative wave

Before an approaching tsunami, the water retreats. This can be observed particularly clearly on shallow beaches and takes place about 5 to 10 minutes before the arrival of the first tidal wave.

Due to the large amount of water carried along, the tsunami wave is extremely long-waved, but at the same time very fast with only a small height. In shallow water, the wavelength now shortens while the wave height increases. Due to the wave characteristics, the wave trough reaches the coast first before the tsunami. This extracts water from the surrounding area, which accumulates in the subsequent wave crest. This becomes visible when the water line recedes.

Wave crest and wave trough have approximately the same size in physics - which is only partially correct in this case due to the onshore speed. Thus, the distance by which the water recedes indicates approximately how long the first tidal wave will be. In the case of severe tsunamis, this can be several kilometers.

The most disastrous tsunamis of the past 2,000 years

The largest floodwave measured so far reached a "megatsunami" in 1958 in Lituya Bay in Alaska. A massive rock slide on one side of the fjord caused a tidal wave that rose up to 524 meters on the other side. The fjord was only 3 km wide.

Unlike volcanoes and earthquakes, the effects of tsunamis are often spread over large areas. In the most famous tsunami, in December 2004, off the coast of Indonesia and Thailand, a tidal wave arose resulting in deaths in 17 countries. The wave reached a height of up to 50 meters and even reached the African continent.