A tsunami consists of gigantic water waves caused by the abrupt displacement of a large body of water. Typically the displacement is created by an earthquake, but can also be attributed to volcanic eruptions, landslides, glacier calvings and meteorite impacts. These are for the most part all fairly common events, with a tsunami only occurring if the resulting energy is substantial and is transferred to the water at a rate faster than the water can absorb. Tsunami waves are different that tidal waves in that, although they may have a tidal motion, they are completely unrelated to tides. Initially a typical tsunami wave will have a relatively small amplitude (height) of only a metre or so, compared to normal wind waves that regularly have amplitudes of two metres. However, normal sea waves have a typical wavelength (from crest to crest) of 100 metres whereas a tsunami's wavelength can extend to over 200 kilometres. This makes tsunamis difficult to detect in the open ocean and would actually suggest that the water is very calm. When the tsunami reaches shallower waters, generally closer to shore, a wave shoaling process occurs. Shallow water forces the wave velocity to slow, causing the energy density to increase in order to maintain energy flux. This results in a reduced but still very large wavelength and a tremendously greater wave height.
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| Water Shoaling (Régis Lachaume, 2005) |
The 2004 Indian Ocean Tsunami was the sixth most destructive natural disaster in all of recorded history. Caused by an undersea megathrust earthquake, the resulting tsunami killed over 230 000 people in over 14 costal South Asian countries. The destructive power of tsunamis is attributed to the sheer wave impact and sudden flooding. The properties of this destruction is largely dependant on which part of the wave meets the shore first. If the positive peak of the wave, also called the ridge, leads the tsunami the first effects seen on land will be flash flooding or a colossal breaking wave. When the negative peak, or trough, hits shore a drawback will occur. Massive drawbacks are one of the most recognizable warnings associated with tsunami's. The water will seem to recede, in the case of the 2004 Indian Ocean tsunami for several kilometres, exposing the seabed. If the first drawback occurs before the rest of the tsunami, anyone who realizes a tsunami is happening can seek safety inland. The breaking wave of the ridge and the drawback of the through can repeat themselves with every wavelength, adding to the destruction with repetition.
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| 2004 Indian Ocean tsunami in Thailand |
Tsunami is a japanese word combining the words tsu, meaning harbour, and nami, meaning wave. It literally translates to harbour-wave because historically japanese fisherman would sail out for work and encounter no unusual waves, only to sail back to a tsunami devastated port-village. The kanji character 津 (tsu) is made from two other characters; 氵, meaning water, and 聿, a kind of liquid or saliva. The kanji 波 (nami) is a phono-semantic compound of the semantic 氵 (water) and the phonetic 皮, meaning waves of water.
Sources
Tsunami — Wikipedia, the free encyclopedia. November 17, 2012.
NOAA Tsunami Website. Nov 17. 2012.
The Deadliest Tsunami in History? — National Geographic. November 18, 2012.
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