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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..

Megatsunamis are caused by impact, explosive volcanic, or landslide phenomena. 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. 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 the magnitude of the earthquake) but can affect thousands of kilometres of coastline.

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 geological record suggests that megatsunamis are rare, but due to their size and power, can produce immensely devastating effects. 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.

In the Norwegian Sea, the Storegga Slide caused a megatsumani 7,000 years ago. Extensive geological investigations indicate that the risk of a re-occurrence is minimal.

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 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.