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A 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 (a normal tsunami is typically no more than 10–15 m). They are rare, and usually highly localized. The largest waves are caused by a very large landslide or impact, into a body of water. They can potentially reach 20 km inland in low-lying regions.
General information
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. Unlike an earthquake, where a large area moves by a few meters, a megatsunami is more usually caused by a far bigger impact in a far more localized area, causing one or a few massive waves of displaced water.
The Lituya Bay megatsunami, the first for which surviving eye-witnesses and before/after photographs exist, reached 524 metres (1700 feet) 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. The open water wave has been quoted as between 50–150 metres (150–500 feet), and was one of a short series of single waves (solitons). By comparison, a strong tsunami may reach typically around 10–15 metres above normal sea level, or up to 30 metres on occasion, and often come in trains of multiple waves.
The reason for the differences is that normal tsunamis are caused by subduction and similar earthquake phenomena. the earth's crust does not usually slip more than 10–15 metres vertically in an earthquake, and in the absence of any effect of focussing and sea floor shape, this is the maximum size most tsunamis will reach. Megatsunamis appear to be caused by impact, explosive volcanic, or landslide phenomena. Since there is no theoretical limit to how much material may impact or how fast or forcefully the water is displaced, the limiting factors (sea floor rise and fall distance) affecting normal tsunamis do not apply.
Underwater earthquakes do not normally generate such large tsunamis unless they also trigger an underwater landslide — typically they have a height of less than ten metres.
Some have conjectured that historic megatsunamis underlie the deluge legends that are common to many cultures throughout the world.
Known megatsunamis
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 particulary 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 meters 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.
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 resevoir 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 meters 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 resevoir 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. [1] (http://www.bbc.co.uk/science/horizon/2000/mega_tsunami.shtml)
In the Norwegian Sea, the Storegga Slide caused a megatsumani 7,000 years ago. Extensive geological investigations indicate that the risk of a re-occurance is minimal.
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 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 meters and a likely height of around 10–25 meters 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.
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.
Further reading
- 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.
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