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Megatsunami (often hyphenated as mega-tsunami, also known as iminami or “wave of purification”) is an informal term used mostly by popular media and popular scientific societies to describe a very large tsunami wave beyond the typical size reached by most tsunamis (usually around 10 m).Template:Fact A megatsunami is associated with waves beyond the norm for tsunamis, ranging from over 40 metres (131 feet) to giants over 100 metres (328 ft) tall. Note that the waves are often much higher when they meet land, as the water often floods upwards from the force of impact.
Megatsunamis are caused by a very large impact or landslide into a body of water when the water cannot disperse in all directions. For this reason, they are usually a highly localized effect, either occurring when the origin of a tsunami is extremely close to the shore, or in deep, narrow inlets, lakes or other water passages.
 General information
The astounding heights quoted for megatsunami waves are caused by the displacement of a very large volume of water in a limited space in a very short time creating a single powerful surge.
Megatsunamis may be caused by landslide and rockfall phenomena, explosive volcanic events, or meteor impacts. Underwater earthquakes do not normally generate such large tsunamis; typically tsunamis caused by earthquakes (such as the 2004 Indian Ocean earthquake) have a height of less than ten metres at the shore (depending on how much water was displaced by the earthquake and on various natural factors such as tree cover and the general shore characteristics) but can affect thousands of kilometres of coastline and reach many kilometres inland.
 Discovery and confirmation of existence
Megatsunamis were first hypothesized by geologists searching for oil in Alaska in 1953. They observed that mature forestation did not extend to the shoreline as it did in nearly all other bays in the region. Rather, there were bands of younger trees closer to the shore. The surveyors called the boundary between these bands a trim line, resembling those caused by the advance and retreat of glaciers. Selected trees were cut in the areas just above and below each trim line. The trees just above these lines showed severe scarring, as if hit very hard by something that came from the coast. The only possible explanation, according to scientists, was that there were unusually large waves in the nearby deep inlet called Lituya Bay, Alaska. This is a recently deglaciated fjord with steep slopes and crossed by a major fault. The topology of the inlet is particularly suited to producing landslide generated tsunamis. They speculated that something had caused a huge wave in relatively recent times, but the cause of this hypothetical wave remained unknown.
On July 10, 1958 the speculation was confirmed, following a nearby earthquake of magnitude 7.7, which generated a landslide that sent water surging across Lituya Bay. The landslide hit the water fast enough to shoot water up the opposite bay to a height of 524 meters. A wave with initial heights of about 333 meters traveled along the bay, and flowed over the entrance to the bay with a height of about 350 meters. Howard Ulrich and his son, Howard Jr. were in the bay in their fishing boat when they saw the wave. Ulrich tried to get over the wave and he and his son amazingly survived the wave, and reported that it carried their boat "over the trees". Another boat actually rode over the tsunami in the bay, and one was destroyed by the tsunami.
 Known megatsunamis
On May 18, 1980, the upper 1,500 feet (460 m) including the former summit of Mount St. Helens, a volcano in Washington state, detached as a landslide. The avalanche slammed into Spirit Lake sending a tsunami surging around the lake basin as high as 820 feet (250 m) above lake level. Above the upper limit of the tsunami, trees lie where they were knocked down by a pyroclastic surge; below the limit, the downed trees and the surge deposits were removed by the tsunami and deposited in Spirit Lake.
The geological record suggests that megatsunamis generated by the collapse of flank of a volcanic island, their most common cause, may occur every few thousand years . Their size and power can produce devastating effects; travelling across oceans and reshaping entire coastlines. The most recent such event so far known occurred approximately 4,000 years ago on Réunion island, to the east of Madagascar.  The most recent collapse occurred on Ritter Island in 1888 but it only generated 12-15 metre waves, which, although they killed 3,000 people on surrounding islands, were not megatsunamis and did not cause widespread devastation.
The most recent megatsunamis, such as the one at Lituya Bay in 1958 and in the Vajont Dam in 1963, have occurred as a result of landslides in largely enclosed bodies of waters and their effects have been limited.
Other recent megatsunamis include the 40 metre high waves generated by the collapse of Krakatoa during its eruption in 1883 which killed 36,000 people on Java, Sumatra and the small islands around them; and the collapse of much of Santorini during its cataclysmic eruption around 3,500 years ago which produced a 100-150 metre wave that struck the north coast of Crete after travelling 70 kilometres. However, these megatsunamis did not propagate thousands of miles to cause more widespread damage, in part leading to the controversy about whether the waves produced by island collapses travel great distances in the same way that tsunamis do.
There is evidence of a megatsunami-type freshwater disaster that occurred 10,000 to 20,000 years ago can be seen at Seton Portage, British Columbia (not far north of Harrison Lake) where a huge chunk of the Cayoosh Range suddenly slid north into what had been a large lake spanning the area from Lillooet, British Columbia to near Birken, in the Gates Valley or Pemberton Pass to the southwest. The event has not been studied much in modern times but the proto-lake must have been at least as deep as its two present-day halves, Seton and Anderson Lakes, on either side of the Portage, suggesting that the wave created by the giant landslide must have been comparable to Lituya Bay.
They have also been generated by bolide impacts. There are indications that a giant tsunami was generated by the bolide impact that created the Chesapeake Bay impact crater, a shallow-water near-shore impact off the eastern North American coastline about 35.5 million years ago, in the late Eocene Epoch.
 Megatsunami threats
Volcanic islands (such as Réunion and the Hawaiian Islands) can cause megatsunamis to hit other nearby islands in the same chain because often they are comparatively steep sloped structures with little horizontal support made up of often loosely aggregated material heaped up by successive eruptions from a central vent area and bisected by faults and stress lines created by ongoing vulcanism. Evidence for large landslides has been found in the form of extensive underwater debris aprons around them composed of the material which has slipped into the ocean. In recent years five such debris aprons have been found in the Hawaiian Islands alone.
Some geologists speculate that the most likely candidate for the source of the next large-scale megatsunami is the island of La Palma, in the Canary Islands. Reports say that during the 1949 eruption the western half of the Cumbre Vieja ridge slipped four metres downwards into the Atlantic Ocean, though this is disputed. It is believed that this process was driven by the pressure caused by the rising magma heating and vaporising water trapped within the structure of the island, causing the island's structure to be pushed apart. The island is still considered active, though quiescent at present, but it is expected to erupt again some time in the next few hundred years. Were this to happen it is speculated that a megatsunami would be created as the western half of the island, weighing perhaps 500 billion tonnes, catastrophically slides into the ocean in a single event, causing local wave heights of hundreds of metres and a likely height of around 10–25 m at the Caribbean and the Eastern North American seaboard coast several hours later. However, this is speculative since there is disagreement whether it would in fact happen, when, or how likely it is.
There is also disagreement among scientists about whether an eruption of Cumbre Vieja would cause a single large landslide or a series of smaller landslides and if such a landslide would generate a tsunami capable of crossing the Atlantic. The Tsunami Society issued a statement in 2003 that such collapses are rare and occur at intervals of thousands or millions of years, that the risk of La Palma collapsing was over-dramatized, and that although the catastrophic collapse of the islands of Krakatoa and Santorini produced megatsunamis in the local region, huge waves did not propagate across oceans to cause similar devastation on more distant coasts, adding that evidence (including computer simulations and experiments with models) suggests this type of wave does not travel great distances in the same way that normal tsunamis do. 
Besides fjords in Alaska, many locations face threats of localized, but still potentially dangerous, megatsunami-type waves. Some geologists speculate that an unstable rock face at Mount Breakenridge above the north end of the giant fresh-water fjord of Harrison Lake in the Fraser Valley in southwestern British Columbia could collapse into the lake, generating a large wave that might destroy the town of Harrison Hot Springs at the south end.
Fictional megatsunamis are a favourite subject of many films, given their undoubted visual impact; these megatsunamis are often caused by bolide impacts, extraterrestrial causes and other dramatic causes, rather than by landslides. Examples of this are the movies Deep Impact and the director's cut of The Abyss.
- ↑ Don J. Miller, Giant Waves in Lituya Bay, Alaska
- ↑ USGS Website. Geology of Interactions of Volcanoes, Snow, and Water: Tsunami on Spirit Lake early during 18 May 1980 eruption
- ↑ Mega-tsunami: Wave of Destruction. Transcript. BBC Two television programme, first broadcast 12 October 2000
- ↑ Mega-tsunami: Wave of Destruction. Background article. BBC Two television programme first broadcast 12 October 2000
- ↑ Mega Tsunami hazards, The Tsunami Society, 15 January 2003
 See also
 Further reading
- Evaluation of the Threat of Mega Tsumami Generation from Postulated Massive Slope Failure of Island Stratovolcanoes on La Palma, Canary Islands, and on The Island of Hawaii, George Pararas-Carayannis, Science of Tsunami Hazards, Vol 20, No.5, pages 251-277, 2002.
- Ward, S.N. and Day, S. 2001. Cumbre Vieja Volcano — Potential collapse and tsunami at La Palma, Canary Islands. Geophysical Research Letters, 28, 17 pp. 3397–3400.
 External links
- Mader, Charles L. Mega-Tsunamis Description of the Lituya Bay event.
- Ward, S.N. and Day, S. 2001. Cumbre Vieja Volcano — Potential collapse and tsunami at La Palma, Canary Islands. Online version in Adobe PDF format.
- Benfield Hazard Research Centre
- Science of Tsunami Hazards A more skeptical view from The Tsunami Society.
- BBC — Mega-tsunami: Wave of Destruction BBC Two program broadcast 12 October 2000
- La Palma threat "over-hyped", BBC News, 2004-10-29
- [http://www.envirtech.org/envirtech_tsunameter.htm Envirtech Tsucs:Megatsunami