Tsunamis: dangerous water massesEarthquakes and volcanoes not only disturb land masses, but often water masses as well. The more violent the displacement is, the stronger the tsunamis, which are hardly visible in the open sea. But in shallow waters, the waves surge to dozens of meters and sometimes cause the most severe damage on landfall.
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Why tsunamis are so dangerousTsunamis 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 waveBefore 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 yearsThe 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.
|Date||Origin||Region||Wave height||Deaths||Total damage|
12/26/2004: Off W. Coast Of Sumatra
|Off W. Coast Of Sumatra||50 m||227,899|
03/11/2011: Honshu Island
|Honshu Island||39 m||18,431|
05/21/1792: Shimabara Bay, Kyushu Island
|Shimabara Bay, Kyushu Island||55 m||15,000|
05/22/1960: Southern Chile
|Southern Chile||25 m||2,226|
12/07/1944: Off Southeast Coast Kii Peninsula
|Off Southeast Coast Kii Peninsula||10 m||1,223|
12/12/1992: Flores Sea
|Flores Sea||26 m||1,169|
04/24/1771: Ryukyu Islands
|Ryukyu Islands||85 m||13,486|
08/05/2018: Bali Sea
|Bali Sea||2 m||560|
12/30/1703: Off Sw Boso Peninsula
|Off Sw Boso Peninsula||11 m||5,233|
09/20/1498: Enshunada Sea
|Enshunada Sea||10 m||5,000|
12/28/1908: Messina Strait, Ionian Sea
|Messina Strait, Ionian Sea||13 m||2,000|