Megatsunami

An artist's rendering of wave shoaling of a tsunami

Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis. Unlike usual tsunamis – which originate from tectonic activity and the raising or lowering of the sea floor – known megatsunamis have originated from a large-scale landslide, collision, or volcanic eruption event.

Concept

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

Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long), and generally pass unnoticed at sea, forming only a slight swell usually of the order of 30 cm (12 in) 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.

By contrast, megatsunamis are caused by giant landslides and other impact events. This could also refer to a meteorite hitting an ocean. Underwater earthquakes or volcanic eruptions do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes can, since they cause a massive amount of displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result.

Two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger.

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.[1]

On 9 July 1958, an earthquake of magnitude 7.7–8.3[2] (on the 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 to create a wave up to 1720 feet (524 m) high. Howard Ulrich and his son, Howard Jr., were in the bay in their fishing boat when they saw the wave. They both survived and reported that the wave carried their boat "over the trees" on one of the initial waves which washed them back into the bay. [1]

List of megatsunamis

Prehistoric

Modern

1792: Mount Unzen, Japan

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

1958: Lituya Bay, Alaska, USA

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

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 up to 520 metres (1,710 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 Ulrich and his son managed to ride the wave in their boat, and both survived.[1]

1963: Vajont Dam, Italy

Main article: Vajont Dam

On 9 October 1963, a landslide above Vajont Dam in Italy produced a 250 m (820 ft) surge that overtopped the dam and destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing nearly 2,000 people.[12]

1980: Spirit Lake, Washington, USA

On May 18, 1980, the upper 460 meters (1,509 feet) of Mount St. Helens 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 meters (853 feet)[13] above the pre-eruption water level (~975 m asl/3,199 ft). Above the upper limit of the tsunami, trees lie where they were knocked down by the pyroclastic surge; below the limit, the fallen trees and the surge deposits were removed by the megatsunami and deposited in Spirit Lake.[14]

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.[15] 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 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, to be unstable enough to collapse into the lake, generating a megatsunami that might destroy the town of Harrison Hot Springs (located at its south end).[16]

Canary Islands

Geologists Dr. Simon Day and Dr. Steven Neal Ward consider that a megatsunami could be generated during a future eruption of Cumbre Vieja on the volcanic ocean island of La Palma, in the Canary Islands, Spain.[17][18]

In 1949, this volcano erupted at its Duraznero, Hoyo Negro and Llano del Banco vents, and there was an earthquake with an epicentre near the village of Jedy. The next day Juan Bonelli Rubio, a local geologist, visited the summit area and found that a fissure about 2.5 kilometres (1.6 mi) long had opened on the east side of the summit. As a result, the west half of the volcano (which is the volcanically active arm of a triple-armed rift) had slipped about 2 metres (6.6 ft) downwards and 1 metre (3.3 ft) westwards towards the Atlantic Ocean,[19]

Cumbre Vieja is currently dormant, but will almost certainly erupt again. Day and Ward hypothesize[17][18] that if such an eruption causes the western flank to fail, a mega-tsunami could 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.[17][18] However, the western half of the volcano has an approximate volume of 500 cubic kilometres (120 cu mi) and an estimated mass of 1.5 trillion metric tons (1.7×1012 short tons). If it were to catastrophically slide into the ocean, it could generate a wave with an initial height of about 1,000 metres (3,300 ft) at the island, and a likely height of around 50 metres (164 ft) at the Caribbean and the Eastern North American seaboard when it runs ashore eight or more hours later. Tens of millions of lives could be lost in the cities and/or towns of St. John's, Boston, Halifax, New York, Baltimore, Washington, D.C., Miami, Havana and the coasts of both South Carolina and North Carolina, as well many other cities on the Atlantic coast in Europe, South America and Africa.[17][18] The likelihood of this happening is a matter of vigorous debate.[20]

The last eruption on the Cumbre Vieja occurred in 1971 at the Teneguia vent 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.[21]

Geologists and volcanologists are in sharp disagreement about whether an eruption on the Cumbre Vieja would cause a single large gravitational landslide or a series of smaller landslides, or whether a slide is likely at all. There are also questions about the dynamics. Day and Ward have admitted that their original analysis of the danger was based on several worst case assumptions.[22][23]

Hawaii

Sharp cliffs and associated ocean debris at the Kohala Volcano, Lanai and Molokai indicate 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.[24][25][26] A future tsunami event is also possible, with the tsunami potentially reaching up to about 1 kilometre (3,300 ft) in height.[27][28] According to a documentary called National Geographic's Ultimate Disaster: Tsunami, if a big landslide occurred at Mauna Loa or the Hilina Slump, a 30 metres (98 ft) tsunami would take only thirty minutes to reach Honolulu, Hawaii. There, hundreds of thousands of people could be killed as the tsunami could level Honolulu and travel 25 kilometres (16 mi) inland.

Cape Verde Islands

Steep cliffs on the Cape Verde Islands have been caused by catastrophic debris avalanches. These have been common on the submerged flanks of ocean island volcanoes and more can be expected in the future.[29]

See also

References

  1. ↑ 1.0 1.1 1.2 Miller, Don J. "Giant Waves in Lituya Bay, Alaska". uwsp.edu. p. 3. Archived from the original on 13 October 2014.
  2. ↑ "Juneau, Alaska profile". City-Data.com.
  3. ↑ "Armageddon" Episode 2.3 (2007) of Lost Worlds, The History Channel, original air date: 15 August 2007.
  4. ↑ Poag, C. W. (1997). "The Chesapeake Bay bolide impact: A convulsive event in Atlantic Coastal Plain evolution". Sedimentary Geology 108 (1–4): 45–90. Bibcode:1997SedG..108...45P. doi:10.1016/S0037-0738(96)00048-6.
  5. ↑ "Hawaiian landslides have been catastrophic". mbari.org. Monterey Bay Aquarium Research Institute.
  6. ↑ Culliney, John L. (2006) Islands in a Far Sea: The Fate of Nature in Hawaii. Honolulu: University of Hawaii Press. p. 17.
  7. ↑ Gardner, J.V. (July 2000). "The Lake Tahoe debris avalanche". 15th Annual Geological Conference. Geological Society of Australia.
  8. ↑ "SetonPortage.ca website".
  9. ↑ Pareschi, M. T.; Boschi, E.; Favalli, M. (2006). "Lost tsunami". Geophysical Research Letters 33 (22): L22608. Bibcode:2006GeoRL..3322608P. doi:10.1029/2006GL027790.
  10. ↑ Bondevik, S.; Lovholt, F.; Harbitz, C.; Mangerud, J.; Dawsond, A.; Svendsen, J. I. (2005). "The Storegga Slide tsunami—comparing field observations with numerical simulations". Marine and Petroleum Geology 22 (1–2): 195–208. doi:10.1016/j.marpetgeo.2004.10.003.
  11. ↑ "Mega-tsunami: Wave of Destruction". BBC Two. 12 October 2000.
  12. ↑ http://www.uwsp.edu/geo/projects/geoweb/participants/Dutch/VTrips/Vaiont.HTM Vaiont Dam photos and virtual field trip (University of Wisconsin), retrieved 2009-07-01
  13. ↑ Voight et al. 1983
  14. ↑ USGS Website. Geology of Interactions of Volcanoes, Snow, and Water: Tsunami on Spirit Lake early during 18 May 1980 eruption
  15. ↑ Mega-tsunami: Wave of Destruction. Transcript. BBC Two television programme, first broadcast 12 October 2000
  16. ↑ Evans, S.G.; Savigny, K.W. (1994). "Landslides in the Vancouver-Fraser Valley-Whistler region" (PDF). Geological Survey of Canada. Ministry of Forests, Province of British Columbia. pp. 36 p. Retrieved 2008-12-28.
  17. ↑ 17.0 17.1 17.2 17.3 Day et al. 1999
  18. ↑ 18.0 18.1 18.2 18.3 Ward & Day 2001
  19. ↑ Bonelli 1950
  20. ↑ Pararas-Carayannis 2002
  21. ↑ As per Bonelli Rubio
  22. ↑ Ali Ayres (2004-10-29). "Tidal wave threat 'over-hyped'". BBC NEWS.
  23. ↑ Pararas-Carayannis, George (2002). "Evaluation of the threat of mega tsunami generation from postulated massive slope failures of the island volcanoes on La Palma, Canary Islands, and on the island of Hawaii". Science of Tsunami Hazards 20 (5): 251–277. Retrieved 7 September 2014.
  24. ↑ McMurtry, Gary M.; Fryer, Gerard J.; Tappin, David R.; Wilkinson, Ian P.; Williams, Mark; Fietzke, Jan; Garbe-Schoenberg, Dieter; Watts, Philip (1 September 2004). "Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa". Geology 32 (9): 741. Bibcode:2004Geo....32..741M. doi:10.1130/G20642.1.
  25. ↑ McMurtry, Gary M.; Fryer, Gerard J.; Tappin, David R.; Wilkinson, Ian P.; Williams, Mark; Fietzke, Jan; Garbe-Schoenberg, Dieter; Watts, Philip (September 1, 2004). "A Gigantic Tsunami in the Hawaiian Islands 120,000 Years Ago". Geology, Volume 32. SOEST Press Releases. Retrieved 2008-12-20.
  26. ↑ McMurtry, G. M.; Tappin, D. R.; Fryer, G. J.; Watts, P. (December 2002). "Megatsunami Deposits on the Island of Hawaii: Implications for the Origin of Similar Deposits in Hawaii and Confirmation of the 'Giant Wave Hypothesis'". AGU Fall Meeting Abstracts 51: 0148. Bibcode:2002AGUFMOS51A0148M.
  27. ↑ Pararas-Carayannis, George (2002). "Evaluation of the threat of mega tsunami generation from postulated massive slope failures of island volcanoes on La Palma, Canary Islands, and on the island of Hawaii". drgeorgepc.com. Retrieved 2008-12-20.
  28. ↑ Britt, Robert Roy (14 December 2004). "The Megatsunami: Possible Modern Threat". LiveScience. Retrieved 2008-12-20.
  29. ↑ Le Bas, T.P. (2007). "Slope Failures on the Flanks of Southern Cape Verde Islands". In Lykousis, Vasilios. Submarine mass movements and their consequences: 3rd international symposium. Springer. ISBN 978-1-4020-6511-8

Further reading

External links