Megatsunami

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Mega-tsunami (also known as iminami or "wave of purification"[1]) is an informal term to indicate a tsunami which is much larger than a normal tsunami with an initial wave amplitude measured in several tens, hundreds or possibly thousands of metres.

Any wave when it starts to come ashore does two things: its base pushes the water column above it upwards and also starts to slow. However, water does not have the ability to slow in the same manner as solid objects and so the water column above the base is forced forward whilst the base lags.

So called "mega-tsunami" may possibly be caused by a very large impact or landslide into a body of water when the water cannot disperse in all directions. For this reason the "mega-tsunami" that are historically known, all occurred with the origin very close to the shore, or in deep, narrow inlets, lakes or other water passages.

Due to the immense energy levels involved it is impossible for a nuclear explosion to generate a mega-tsunami. There are not sufficient nuclear devices in the arsenals of the whole nuclear powers combined.[citation needed]

Contents

[edit] General information

Mega-tsunami is a media generated term and is not used by geologists etc; except as in the context of "... So called mega-tsunami ..." Those geologists etc., who do use the term in public do so to emphasise that they are talking of a a tsunami of incredible proportions.

The large amplitudes (wave height) quoted for "mega-tsunami" waves are caused by the displacement of a very large volume of water in a limited space in a very short time creating powerful surges.

Mega-tsunamis may be caused by landslide and rockfall phenomena, explosive volcanic events, or asteroid impacts. Underwater earthquakes do not normally generate such large tsunami. Tsunami caused by earthquakes (such as the 2004 Indian Ocean earthquake) have a wave amplitude (height) equal to the vertical height of the displaced floor of the ocean and this is typically 10m or less (depending on how much water was displaced by 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. You can calculate the wave height at the shore by the formula

X_{max} = 0.06 \left ( \frac{H_o^{4/3} }{n^6} \right )

where X is measure in metres, Ho is the height of the wave at the coast in metres and raised to the power 4/3, n is the roughness of the surface raised to the 6th power and is typically between 0.015 and 0.07.[2]

[edit] Discovery and confirmation of existence

"Mega-tsunamis" were first hypothesised by geologists searching for oil in Alaska in 1953[3]. They 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, resembling those caused by the advance and retreat of glaciers. Selected trees were cut in the areas just above and below each trim line. The trees just above the trim lines showed severe scarring on their seaward side, as if hit very hard by something that came from the sea, whilst those from below the trim line did not exhibit the same scarring. The only possible explanation, according to scientists, was that there had been an unusually large wave or waves in the deep inlet. This is a recently deglaciated fjord with steep slopes and crossed by a major fault. The topology of the inlet is particularly suited to producing landslide generated tsunamis. They speculated that something had caused a huge wave in relatively recent times, but the cause of this hypothetical wave remained unknown.

On 9th July, 1958 the hypothesis was strengthened. An earthquake of magnitude 7.7 (Richter scale), caused a block of rock and ice to drop into the deep water at the head of the fjord Lituya Bay. The block fell almost vertically and hit the water with sufficient force to cause a surge of water approximately 524 meters high. A wave with initial heights of about 333 metres travelled along the bay, and flowed over low hills at the entrance to the bay. Howard Ulrich and his son, Howard Jr. were in the bay in their fishing boat when they saw the wave. Ulrich tried to get over the wave and he and his son amazingly survived the wave, and reported that it carried their boat "over the trees". Another boat actually rode over the tsunami in the bay, and one was destroyed by the tsunami.

Examination of the rock face revealed that it is deeply scoured with slickenslides indicating that the block detached and fell en-masse. Slickenslides are indicative of brittle failure in blocks of rock, all fault planes exhibit them and they are caused by the two rock faces gouging into each other as they slide past each other. The marks made by landslides are different to those made by brittle failure.

[edit] Known Mega-tsunamis

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[edit] Recent

  • Lituya Bay Tsunami

On 9th July 1958, a landslide at the head of Lituya Bay in Alaska, caused by an earthquake generated a wave with an initial amplitude of 524m (1720 ft). The resultant "mega-tsunami" 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.

  • Vajont Dam Tsunami
The Vajont Dam as seen from Longarone today, showing approximately the top 60-70 metres of concrete. The 200-250 metre "mega-tsunami" that over-topped the dam would have obscured virtually all of the sky in this picture.
The Vajont Dam as seen from Longarone today, showing approximately the top 60-70 metres of concrete. The 200-250 metre "mega-tsunami" that over-topped the dam would have obscured virtually all of the sky in this picture.

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

  • Spirit Lake Tsunami

On May 18, 1980, the upper 460 metres - including the former summit 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 mega-tsunami which pushed the lake waters in a series of surges which reached a maximum height of 260 metres (Voight et al., 1983), 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 "mega-tsunami" and deposited in Spirit Lake.[4]

[edit] Historical

Day et al; (1999)[5], Ward and Day (2001)[6], hypothesise that a future "mega-tsunami" may be generated by the failure of the western flank of the Cumbre Vieja, which is the volcanically active arm of a triple-armed rift on La Palma a volcanic ocean island in the Canary Island archipelago. Edifice failure of a volcanic ocean island is considered to be the most theoretically common cause of the generation of a "mega-tsunami". [7] Their size and power means that they can produce devastating effects; travelling across oceans and inundating up to 25 km inland from the coast. The most recent such event so far known occurred approximately 4,000 years ago on Réunion island, to the east of Madagascar. [8] The most recent collapse occurred on Ritter Island in 1888 but it only generated 12-15 metre waves, which, although they killed 3,000 people on surrounding islands, did not cause widespread devastation.

These recent "mega-tsunami", such as the one at Lituya Bay in 1958 and in the Vajont Dam in 1963, have occurred as a result of landslides in largely enclosed bodies of waters and their effects were as a consequence rather limited.

Other recent "mega-tsunami" include the 40 metre high waves generated by the collapse of Krakatau during its eruption in 1883 which killed 36,000 people on Java, Sumatra and the small islands around them; and the collapse of much of Santorini during its cataclysmic eruption approximately 3,615 BP (1615 BC). It is estimated that it produced a wave that surged to 100-150 metres and devastated much of the north coast of Crete after travelling 70 kilometres. However, these "mega-tsunami" did not propagate thousands of miles to cause more widespread damage, in part leading to the controversy about whether the waves produced by island collapses travel great distances in the same way that tsunamis do. Recent work by members of the "Tsunami Society" including Pararas-Carayannis, (2002)[9] contradicts the claims by some geologists e.g. Day et al., (1999)[5], Ward and Day (2001)[6], of "mega-tsunami" travelling trans-oceanic distances. However, it maybe that the confining geography of the Mediterranean Sea restricted the devastation.

Some recent work by scholars (Egyptologists and Israeli plus others), geologists and oceanographers (including Dr Iain Stewart of University of Plymouth, UK), indicates that the Santorini eruption (about 1615 BC) may have caused the destruction of the Egyptian armies that is mentioned in the Exodus. The Exodus is dated as occurring between 1290 and 1340 BC. It is unlikely that the details were recorded in the immediate aftermath and there was probably a delay in the writing of the account. Whilst there is a discrepancy of about 300 years, given the circumstances surrounding the dating of the Santorini eruption it is possible that the two events did coincide. Further support for this is that the southeastern corner of the Mediterranean was marshland prior to the construction of the Suez Canal and was a known source of reeds. Is the "Red Sea" a wrong interpretation of the "Reed Sea," and did the Santorini or Minoan eruption coincide with the Exodus? Critics note that the Bible was not written in English: the English name of the sea is a direct translation from other languages, in which the word for "red" sounds nothing like the word for "reed". There is ongoing research into this including drilling boreholes to look for tsunamite - the deposit left by tsunamis and other evidence to support or disprove this theory.

These were not mega-tsunami and the heights quoted refer to the surge heights which occurred when the tsunami inundated the coastlines.

[edit] Prehistoric

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

There is evidence that a freshwater "mega-tsunami" occurred approximately 10,000 to 20,000 years ago. It can be seen at Seton Portage, British Columbia (north of Harrison Lake) where 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 may have been at least as deep as the two present-day halves, Seton and Anderson Lakes, on either side of the Portage, suggesting that the surge generated by the giant landslide may be comparable to Lituya Bay.

They have also been generated by bolide impacts. 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.

The asteroid which created the Chicxulub Crater in Yucatan approximately 65 million years ago may have generated the largest "mega-tsunami" in the geological record.

[edit] Threats and Speculation

[edit] Megatsunami threats

  • Parts of volcanic ocean islands (such as the Canary Islands, Cape Verde, La Réunion and the Hawaiian Islands) may fail and generate "mega-tsunami." The "mega-tsunami" may inundate other islands in the archipelago. Volcanic ocean islands are unstable due to the manner that they develop. They develop over-steepened flanks which have poor horizontal support. They are constructed of poorly consolidated materials piled up by successive eruptions from central vents and rifts, bisected by faults and stress lines created by ongoing volcanic activity.
  • 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.

[edit] Speculation

[edit] Canary Islands

  • In 1949, the Cumbre Vieja volcano erupted at its Duraznero, Hoyo Negro and San Juan vents on the ocean island of La Palma. 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 of the Cumbre Vieja and discovered that a fissure about 2.5 km long - about 10% of the total length of the Cumbre Vieja, had opened (the fissure is actually 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 - slipped about 2 m downwards and 1 m westwards towards the Atlantic Ocean [verification required - factually incorrect]. It is claimed that this process was driven by the pressure generated by the rising magma[5] which heated ground waters trapped within the edifice of the volcano causing the waters to become super-heated. This then pushed apart and slipped apart causing the fissure rift and a potential failure plane within the edifice. The volcano is currently in a dormant stage, but will almost certainly erupt again at some as yet unknown future date.
  • La Palma is currently the most volcanically active island in the Canary Islands Archipelago. It is postulated that a future eruption may cause catastrophic failure of the western flank of the Cumbre Vieja volcano, although it is likely that several eruptions would be required before failure would occur[5][6]. Were this to happen, it is hypothesised that a mega-tsunami would be generated if the western half of the Cumbre Vieja, with an approximate volume of 500 km3 (5 x 1011 m3) and an estimated mass of 1.5 x 1015 kg, catastrophically slid into the ocean in a massive gravitational landslide, generating a wave with an initial amplitude (height) estimated to be about 1,000 metres at the island, and a likely height of around 25 metres at the Caribbean and the Eastern North American seaboard when it runs ashore about 5 or more hours later. However, this is hypothetical and is strongly debated. [9].
  • It should be noted that: the Cumbre Vieja is approximately 25 km long, the last eruption occurred in 1971 at the southern end of the sub-aerial section without any movement. In addition, the 1949 eruption occurred over approximately 2.5 km - or about 10% of the total exposed section of the Cumbre Vieja. The length is verifiable by walking the Ruta de Volcanes and is the only section that currently shows any sign of movement. Observations of this section indicate that the section is currently stationary and would not appear to have moved since the initial rupture in 1949 as the dimensions agree with those recorded at the time.[10]

[edit] Other locations

  • There is also disagreement among geologists and volcanologists about whether an eruption on the Cumbre Vieja would cause a single large gravitational landslide or a series of smaller landslides, or whether a large gravitational landslide would even generate a tsunami capable of crossing the Atlantic. The Tsunami Society claim (2003) that such collapses are rare and occur at intervals of thousands or millions of years, that the risk of La Palma collapsing was over-dramatized, 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, adding that evidence --- including computer simulations and experiments with models --- does not support the claim that this type of wave will propagate great distances in the same way that normal tsunamis do. [11]
  • Besides fjords in Alaska, many other locations face threats of localised, but still potentially dangerous, "mega-tsunami"-type waves. Some geologists consider that an unstable rock face at Mount Breakenridge above the north end of the giant fresh-water fjord (i.e. Harrison Lake in the Fraser Valley of southwestern British Columbia) could collapse into the lake, generating a large wave that might destroy the town of Harrison Hot Springs (located at its south end).

[edit] Popular Culture

Fictional mega-tsunamis are a favourite subject of many films, given their undoubted visual impact; these mega-tsunamis are often caused by: bolide impacts, extraterrestrial causes and other dramatic causes, rather than by landslides. Examples of this are the movies Deep Impact and the director's cut of The Abyss.

In Michael Crichton's novel, State of Fear, a group of ecoterrorists attempt to trigger a landslide that would produce a mega-tsunami large enough to strike the western (Pacific) coast of the United States.

In an episode of the series CSI Miami, which it titled Crime Wave, mega-tsunamis are used as part of its plot. The cause of the mega-tsunamis is said to be collapse of a minor part of the Canary Islands' Cumbre Vieja volcano.

[edit] See also

[edit] References

  1. ^ http://iis-db.stanford.edu/pubs/21261/No_70_Skaaning.pdf
  2. ^ http://www.nerc-bas.ac.uk/tsunami-risks/html/Phy3Impact.html
  3. ^ Don J. Miller, "Giant Waves in Lituya Bay, Alaska"
  4. ^ USGS Website. Geology of Interactions of Volcanoes, Snow, and Water: Tsunami on Spirit Lake early during 18 May 1980 eruption
  5. ^ a b c d e 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.
  6. ^ a b c d 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
  7. ^ Mega-tsunami: Wave of Destruction. Transcript. BBC Two television programme, first broadcast 12 October 2000
  8. ^ Mega-tsunami: Wave of Destruction. Background article. BBC Two television programme first broadcast 12 October 2000
  9. ^ a b 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
  10. ^ as per Bonelli
  11. ^ Mega Tsunami hazards, The Tsunami Society, 15 January 2003

[edit] 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.

[edit] External links