Editing Tsunami (section)

==Characteristics==
[[File: Propagation du tsunami en profondeur variable.gif|thumb|upright=1.15|When the wave enters shallow water, it slows down and its amplitude (height) increases.]]
[[File:Tsunami2.JPG|thumb|upright=1.15|The wave further slows and amplifies as it hits land. Only the largest waves crest.]]

Tsunamis are caused by earthquakes, landslides, volcanic explosions, glacier calvings, and [[bolide]]s. They cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with waves that do not appear to be large.

While everyday [[wind wave]]s have a [[wavelength]] (from crest to crest) of about {{convert|100|m|ft}} and a height of roughly {{convert|2|m|ft}}, a tsunami in the deep ocean has a much larger wavelength of up to {{convert|200|km|mi}}. Such a wave travels at well over {{convert|800|km/h|mph}}, but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about {{convert|1|m|ft}}.<ref>[http://earthsci.org/education/teacher/basicgeol/tsumami/tsunami.html Earthsci.org], Tsunamis</ref> This makes tsunamis difficult to detect over deep water, where ships are unable to feel their passage.

The velocity of a tsunami can be calculated by obtaining the square root of the depth of the water in metres multiplied by the acceleration due to gravity (approximated to 10&nbsp;m/s<sup>2</sup>). For example, if the Pacific Ocean is considered to have a depth of 5000 metres, the velocity of a tsunami would be {{sqrt|5000 × 10}} = {{sqrt|50000}} ≈ {{convert|224|m/s|ft/s}}, which equates to a speed of about {{convert|806|km/h|mi/h}}. This is the formula used for calculating the velocity of [[Shallow water equations#Wave modelling by shallow water equations|shallow-water]] waves. Even the deep ocean is shallow in this sense because a tsunami wave is so long (horizontally from crest to crest) by comparison.

The reason for the Japanese name "harbour wave" is that sometimes a village's [[fishermen]] would sail out, and encounter no unusual waves while out at sea fishing, and come back to land to find their village devastated by a huge wave.

As the tsunami approaches the coast and the waters become shallow, [[wave shoaling]] compresses the wave and its speed decreases below {{convert|80|km/h|mph}}. Its wavelength diminishes to less than {{convert|20|km|mi}} and its amplitude grows enormously—in accord with [[Green's law]]. Since the wave still has the same very long [[frequency|period]], the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not [[Breaking wave|break]], but rather appears like a fast-moving [[tidal bore]].<ref name="Walrus">{{cite web|url=http://walrus.wr.usgs.gov/tsunami/basics.html|title=Life of a Tsunami|date=22 October 2008|work=Western Coastal & Marine Geology|publisher=United States Geographical Survey|access-date=2009-09-09}}</ref> Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed ''run up''. Run up is measured in metres above a reference sea level.<ref name="Walrus" /> A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run-up.<ref name="Tulane">{{cite web|url=http://www.tulane.edu/~sanelson/geol204/tsunami.htm|title=Tsunami|author=Prof. Stephen A. Nelson|date=28 January 2009|publisher=Tulane University|access-date=2009-09-09}}</ref>

About 80% of tsunamis occur in the Pacific Ocean, but they are possible wherever there are large bodies of water, including lakes. However, tsunami interactions with shorelines and the seafloor topography are extremely complex, which leaves some countries more vulnerable than others. For example, the Pacific coasts of the United States and Mexico lie adjacent to each other, but the United States has recorded ten tsunamis in the region since 1788, while Mexico has recorded twenty-five since 1732.<ref>{{cite web|url=https://www.worlddata.info/america/usa/tsunamis.php|website=WorldData|title=Tsunamis in the United States}}</ref><ref>{{cite web|url=https://www.worlddata.info/america/mexico/tsunamis.php|website=WorldData|title=Tsunamis in Mexico}}</ref> Similarly, Japan has had more than a hundred tsunamis in recorded history, while the neighbouring island of Taiwan has registered only two, in 1781 and 1867.<ref>{{Cite web|url=https://www.worlddata.info/asia/japan/tsunamis.php|title=Tsunamis in Japan|website=Worlddata.info}}</ref><ref>{{Cite web|url=https://www.worlddata.info/asia/taiwan/tsunamis.php|title=Tsunamis in Taiwan|website=Worlddata.info}}</ref>

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