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Megatsunami (also known as iminami or "wave of purification"¹) is an informal term to indicate a tsunami that has initial wave heights that are much larger than normal tsunami. Unlike usual tsunamis, which originate from tectonic activity and the raising or lowering of the sea floor, most known megatsunamis have originated from large scale impact events such as landslides, devastating volcanic eruptions, or meteor impacts.

Concept

A megatsunami is meant to refer to a tsunami with an initial wave amplitude (wave height) measured in several tens, hundreds or possibly thousands of metres.

Normal tsunamis generated at sea have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long). They generally pass unnoticed at sea, forming only a slight swell usually of the order of 30 cm above the normal sea surface. When they reach land the wave height increases dramatically as the base of the wave pushes the water column above it upwards.

Because megatsunamis are defined as beginning with a high wave height, all historically known tsunamis awarded the megatsunami designation have originated very close to the shore, or in deep, narrow inlets, lakes or other water passages where the water had few options for dispersal^{citation needed}.

Megatsunamis may be caused by landslide and rockfall phenomena, explosive volcanic events, or asteroid impacts. Underwater earthquakes do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes do, since they cause a massive amount of displacement.

History of the hypothesis

Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, whilst those from below the trim line did not. The scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault, one possibility was a landslide-generated tsunami.²

On 9 July 1958, an earthquake of magnitude 7.7 (Richter scale), caused 90 million tonnes of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force, and caused a wave approximately 524 meters high (1724 feet). In comparison, this wave was higher than any skyscraper on Earth. Howard Ulrich and his son, Howard Jr. were in the bay in their fishing boat when they saw the wave. Amazingly, they both survived, and reported that the wave carried their boat "over the trees", and washed them back into the bay.

This event and the evidence of a potentially similar past event at the same location inspired the term megatsunami.

List of megatsunamis

Prehistoric

• The asteroid which created the Chicxulub Crater in Yucatan approximately 65 million years BP may have generated the largest megatsunami in the geological record^{citation needed}.

• A giant tsunami may have been generated by the bolide impact that created the Chesapeake Bay impact crater, about 35.5 million years BP^{citation needed}.

• At Seton Portage, British Columbia, Canada, a freshwater megatsunami may have occurred approximately 10,000 BP.³ A huge block of the Cayoosh Range suddenly slid northwards 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 in detail, but the proto-lake (freshwater fjord) may have been at least as deep as the two present-day halves, Seton and Anderson Lakes, which is to say of unknown depth, on either side of the Portage, suggesting that the surge generated by the giant landslide in the narrow mountain confines of the fjord valley may have been comparable in scale to Lituya Bay^{citation needed}. Another more recent landslide on the south shore of Anderson Lake dropped a large portion of high mountainside down a debris chut, creating a rockwall "fan" which cannot not have made a megatsunami-type wave, though not as large as the main one at the Portage.

• 8000 years BP, a massive volcanic landslide off of Mt. Etna, Sicily caused a megatsunami which devastated the eastern Mediterranean coastline on three continents.

• In the Norwegian Sea, the Storegga Slide caused a megatsunami approximately 7,000 years BP^{citation needed}.

• Approximately 4,000 BP, a landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.⁴

• Evidence for large landslides has been found in the form of extensive underwater debris aprons around many volcanic ocean islands which are composed of the material which has slid into the ocean. In recent years, five such debris aprons have been located around the Hawaiian Islands. The Canary Islands have at least 14 such debris aprons associated with the archipelago.

Historic

• 365 Crete tsunami

In 365, a Tsunami caused by the Crete earthquake, devastated Alexandria. It was more than a 100 ft high when it hit the coast.

• Unzen Volcano Collapse

In 1792 the Unzen Volcano, in Japan, erupted, causing part of the volcano to collapse into the sea. The landslide caused a megatsunami 330 ft (100 metres) high, that killed 15,000 people in the local fishing villages.

Recent

• Lituya Bay Megatsunami

On 9 July 1958, a giant landslide at the head of Lituya Bay in Alaska, caused by an earthquake, generated a wave with an initial amplitude of 524m (1720 ft). This is the highest wave ever recorded, and surged over the headland opposite, stripping trees and soil down to bedrock, and surged along the fjord which forms Lituya Bay, destroying a fishing boat anchored there and killing two people. Howard Ultrich and his son managed to ride the wave in their boat, and both survived.

Damage from the 1958 megatsunami can be seen in this oblique aerial photograph of Lituya Bay as the lighter-colored areas where trees have been stripped away.

• Vajont Dam Megatsunami

On 9 October 1963, a landslide above Vajont Dam in Italy produced a 250m (820 ft) megatsunami that overtopped the dam and destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing almost 2,000 people.

• Spirit Lake Megatsunami

On May 18, 1980, the upper 460 metres of Mount St. Helens, a volcano in Washington state, failed and detached in a massive landslide. This released the pressure on the magma trapped beneath the summit bulge which exploded as a lateral blast, which then released the over-pressure on the magma chamber and resulted in a plinian eruption. One lobe of the avalanche surged onto Spirit Lake causing a megatsunami which pushed the lake waters in a series of surges which reached a maximum height of 260 metres⁵, above the pre-eruption water level (~975 m asl). Above the upper limit of the tsunami, trees lie where they were knocked down by the pyroclastic surge; below the limit, the felled trees and the surge deposits were removed by the megatsunami and deposited in Spirit Lake.⁶

Potential future megatsunamis

Experts interviewed by the BBC think that a massive landslide on a volcanic ocean island is the most likely future cause of a megatsunami.⁷ The size and power of a wave generated by such means could produce devastating effects; travelling across oceans and inundating up to 25 kilometres (16 mi) inland from the coast.

British Columbia

Some geologists consider 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 of southwestern British Columbia, Canada, could collapse into the lake, generating a large wave that might destroy the town of Harrison Hot Springs (located at its south end).⁸

Canary Islands

Geologists S. Day and S. Ward consider that a megatsunami could be generated during a future eruption involving the Cumbre Vieja on the volcanic ocean island of La Palma, in the Canary Islands.⁹¹⁰

In 1949, the Cumbre Vieja volcano erupted at its Duraznero, Hoyo Negro and San Juan vents. During this eruption, an earthquake with an epicentre near the village of Jedy occurred. The following day Rubio Bonelli, a local geologist, visited the summit area and discovered that a fissure about 2.5 km long had opened on the eastern side of the summit. As a result, the western half of the Cumbre Vieja (which is the volcanically active arm of a triple-armed rift) had slipped about 2 m downwards and 1 m westwards towards the Atlantic Ocean^{citation needed}.

The Cumbre Vieja volcano is currently in a dormant stage, but will almost certainly erupt again in the future. Day and Ward hypothesize⁹¹⁰ that if such an eruption causes the western flank to fail, a megatsunami may be generated.

La Palma is currently the most volcanically active island in the Canary Islands Archipelago. It is likely that several eruptions would be required before failure would occur on Cumbre Vieja.⁹¹⁰ However, the western half of the volcano has an approximate volume of 500 km³ (5 x 10¹¹ m³) and an estimated mass of 1.5 x 10¹⁵ kg. If it were to catastrophically slide into the ocean, it could generate a wave with an initial height of about 1,000 metres (3,281 ft) at the island, and a likely height of around 580 metres (1,903 ft) at the Caribbean and the Eastern North American seaboard when it runs ashore eight or more hours later. The likelihood of this happening is a matter of vigorous debate.¹¹

The last Cumbre Vieja eruption occurred in 1971 at the southern end of the sub-aerial section without any movement. The section affected by the 1949 eruption is currently stationary and does not appear to have moved since the initial rupture.¹²

Geologists and volcanologists also disagree about whether an eruption on the Cumbre Vieja would cause a single large gravitational landslide or a series of smaller landslides^{citation needed}.

Experts also doubt whether a large gravitational landslide could even generate a tsunami capable of crossing the Atlantic. According to the Tsunami Society,¹³ such collapses are rare and occur at intervals of thousands or millions of years, and that although the catastrophic collapse of the islands of Krakatoa and Santorini produced tsunamis in the local region, huge waves did not propagate across oceans to cause similar devastation on more distant coasts. The Society also says that computer simulations and experiments with models do not support the claim that a megatsunami wave (or one that begins with a high wave height) will propagate great distances in the same way that normal tsunamis do.¹³

Hawaii

Prehistoric sedimentary deposits on the Kohala Volcano, Lanai and Molokai controversially indicates that landslides from the flank of the Kilauea and Mauna Loa volcanoes in Hawaii may have triggered past megatsunamis, most recently at 120,000 BP.¹⁴¹⁵¹⁶ A future tsunami event is also possible, with the tsunami potentially reaching up to about a kilometer in height.¹⁷¹⁸

See also

• Supervolcano
• La Palma
• Cumbre Vieja
• Minoan eruption
• Hypothetical future disasters

References

Further reading

• BBC 2 TV; 2000. Transcript “Mega-tsunami; Wave of Destruction”, Horizon. First screened 21.30 hrs, Thursday, 12 October 2000.
• Carracedo, J. C; 1994. The Canary Islands: an example of structural control on the growth of large oceanic-island volcanoes. J. Volcanol. Geotherm Res. 60, 225-241.
• Carracedo, J. C; 1996. A simple model for the genesis of large gravitational landslide hazards in the Canary Islands. In McGuire, W: Jones, & Neuberg, J. P. (eds). Volcano Instability on the Earth and Other Planets. Geological Society, London. Special Publication, 110, 125-135.
• Carracedo, J. C; 1999. Growth, Structure, Instability and Collapse of Canarian Volcanoes and Comparisons with Hawaiian Volcanoes. J. Vol. Geotherm. Res. 94, 1-19.
• Day, S. J; Carracedo, J. C; Guillou, H. & Gravestock, P; 1999. Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: volcanic rift zone re-configuration as a precursor to flank instability. Journal of Volcanology and Geothermal Research 94, 135-167.
• Moore, J. G; 1964. Giant Submarine Landslides on the Hawaiian Ridge. US Geologic Survey Professional Paper 501-D, D95-D98.
• Pararas-Carayannis, G; 2002. 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, Science of Tsunami Hazards, Vol 20, No.5, pp 251-277.
• Pinter, N., and S.E. Ishman, 2008, Impacts, mega-tsunami, and other extraordinary claims PDF version, 304 KB. GSA Today. vol. 18, no. 1, pp. 37-38.
• Rihm, R; Krastel, S. & CD109 Shipboard Scientific Party; 1998. Volcanoes and landslides in the Canaries. National Environment Research Council News. Summer, 16-17.
• Siebert, L; 1984. Large volcanic debris avalanches: characteristics of source areas, deposits and associated eruptions. J. Volcanol. Geotherm Res. 22, 163-197.
• Vallely, G. A; 2005. Volcanic edifice instability and tsunami generation: Montaña Teide, Tenerife, Canary Islands (Spain). Journal of the Open University Geological Society, 26(1), 53-64
• Voight, B; Janda, R; Glicken, H. & Douglas, P. M; 1983. Nature and mechanics of the Mount St Helens rockslide-avalanche of 18 May 190. Géotechnique. 33, 243-273.
• 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.
• World's Biggest Tsunami: The largest recorded tsunami with a wave 1720 feet tall in Lituya Bay, Alaska.
• 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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