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A megatsunami is an informal term used by popular media and popular science for very large tsunami-like waves significantly beyond the size reached by tsunamis (typically around 10 meters). For this reason, there is no scientific definition of a megatsunami. Informally, the term generally refers to waves beyond the norm for tsunamis, ranging from over 40 metres (131 feet) to giants over 100 metres (328 feet) tall. Note that megatsunamis often reach higher than their wave height when they meet land, as the water often floods upwards from the force of impact.

Megatsunamis are related to tsunamis in name only: they do not usually have the same cause or appearance, the only connection being that both are very large scale water movements caused by point events. They are also not the same as freak waves which appear in ocean waters and are often up to 30 meters tall.

Unlike tsunamis, which are primarily due to seismic activity in the earth's crust raising or lowering the sea bed by a few meters over a large area, 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 are typically caused by meteorite impacts, explosive volcanic events, or landslide phenomena. 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 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.

Known megatsunamis

Megatsunamis were first hypothesized by geologists searching for oil in Alaska in 1953. They observed evidence of unusually large waves in the nearby deep inlet called Lituya Bay, Alaska. This is an ice-scoured inlet 220m deep with an entrance only 10m wide. The topology of the inlet is particularly suited to producing local megatsunamis. A nearby magnitude 7.7 earthquake on July 10, 1958 generated a landslide within the narrow inlet creating a giant splash, which sent water surging across the point opposite the inlet washing out trees 524m/1,720ft above normal sea level. 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 a surge-wave some 250 m high which destroyed several villages and killed about 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, reshaping an entire coastline, occurred approximately 4,000 years ago on Réunion island, to the east of Madagascar. [1]

In the Norwegian Sea, the Storegga Slide caused a megatsunami 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. During the 1949 eruption the western half of the Cumbre Vieja ridge slipped four 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. The island is still considered active, though quiescent at present, but it is expected to erupt again some time in the next few thousand 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 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 amongst scientists as to if an eruption would cause a single large landslide (or a series of smaller landslides) and even if such a landslide would generate a tsunami capable of crossing the Atlantic.

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.

Evidence of a megatsunami-type freshwater disaster that occurred ten to twenty thousand 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.

Movies

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. The film Armageddon also mentions megatsunamis, but only a theoretical one which is expected to be "three miles high".

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

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 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]]. 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.  The island is still considered active, though quiescent at present, but it is expected to erupt again some time in the next few thousand 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&ndash;25 m at the [[Caribbean]] and the Eastern [[United States| 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 amongst scientists as to if an eruption would cause a single large landslide (or a series of smaller landslides) and even if such a landslide would generate a tsunami capable of crossing the Atlantic.
+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]]. During the [[1949]] eruption the western half of the [[Cumbre Vieja]] ridge slipped four 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.  The island is still considered active, though quiescent at present, but it is expected to erupt again some time in the next few thousand 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&ndash;25 m at the [[Caribbean]] and the Eastern [[United States| 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 amongst scientists as to if an eruption would cause a single large landslide (or a series of smaller landslides) and even if such a landslide would generate a tsunami capable of crossing the Atlantic.
 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, British Columbia|Harrison Hot Springs]] at the south end.