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Megatsunami is a term used by the popular media to describe very large tsunamis. There is no scientific definition of a megatsunami, but informally the term has been used for tsunamis with waves of height from 40 m to over 100 m.
They are a highly local effect, either occurring on shores extremely close to the origin of a tsunami, or in deep, narrow inlets. The largest waves are caused by a very large landslide, such as a collapsing island, into a body of water. They can potentially reach 20 km inland in low-lying regions.
The astounding heights quoted for megatsunami waves are due to the displacement of a very large volume of water movement in a very short time near a shoreline.
Megatsunamis are caused by impact, explosive volcanic, or landslide phenomena. Underwater earthquakes do not normally generate such large tsunamis; typically tsunamis caused by earthquakes have a height of less than ten metres at the shore but can affect thousands of kilometres of coastline.
Megatsunamis were first hypothesized by geologists searching for oil in Alaska in 1958. They observed evidence of unusually large waves in the nearby deep inlet called Lituya Bay, Alaska. This is an ice-scoured inlet 220 m deep with an entrance only 10 m wide. The topology of the inlet is particularly suited to producing local megatsunamis. A nearby magnitude 7.5 earthquake on July 8 generated a landslide within the narrow inlet which produced a wave that washed out trees 200 m above normal sea level. Comparison with previous photographs indicated that several hundred feet of ice had been removed from the front of a nearby glacier by a 520 m high wave. However this quoted figure was not the height of the open water wave, but the height it tore up the mountainside due to its force of impact.
In 1963, a man-made megatsunami occurred as a result of human destabilisation of a mountain valley. An enormous slab from the side of Mount Toc , in the mountains north of Venice, Italy, became destabilised as a result of reservoir filling, and slid into the Vajont Dam reservoir at 110 km/h, emptying 50% of the water within 10 minutes. This produced waves some 250 m high which destroyed several villages, and killed nearly 2000 people. Remarkably, most of the dam survived, although it was rendered almost useless by the infill of the reservoir and structural damage to the I-beams and mechanisms of its interior.
The geological record suggests that megatsunamis are rare, but due to their size and power, can produce immensely devastating effects. However as with Lituya bay, this is often localized; the most recent megatsunami known to have a widespread impact which reshaped an entire coastline occurred approximately 4,000 years ago on Réunion island, to the east of Madagascar. 
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.
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 structurally little more than large, unstable piles of loosely aggregated material heaped up by successive eruptions. 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, although further research has dismissed the threat. During the 1949 eruption the western half of the Cumbre Vieja ridge slipped several metres downwards into the Atlantic Ocean. 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. During an eruption that is anticipated to occur sometime within the next few thousand years the western half of the island, weighing perhaps 500 billion tonnes, may catastrophically slide into the ocean in a single event. Were this to happen it could in theory generate a megatsunami, causing local wave heights of hundreds of metres and a likely height of around 10–25 m at the Caribbean and the Eastern American seaboard coast several hours later.
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 the north end 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 and Harrison Hot Springs resort at the south end.
Megatsunamis are a favorite subject of many films, given their undoubted visual impact; these megatsunamis are often caused by bolide impacts or other extraterrestrial causes, rather than by landslides. Examples of this are the movies Deep Impact, the director's cut of The Abyss and The Day After Tomorrow.
- 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.
- 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
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