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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.
- 1 Concept
- 2 History of the hypothesis
- 3 List of megatsunami
- 4 Potential future megatsunami
- 5 See also
- 6 References
- 7 Further reading
- 8 External links
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 washes 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.
On 9 July 1958, an earthquake of magnitude 7.7–8.3 (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 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. 
List of megatsunami
- The asteroid which created the Chicxulub crater in Yucatán approximately 65 million years ago would have generated megatsunamis as high as 5 kilometres (3.1 mi); enough to completely inundate even large islands such as Madagascar.
- A series of megatsunamis were generated by the bolide impact that created the Chesapeake Bay impact crater, about 35.5 million years ago.
- An asteroid impact in the southeast Pacific Ocean 2.5 million years ago caused a megatsunami that was over 200 m (660 ft) high in southern Chile and the Antarctic Peninsula, the wave swept across much of the Pacific Ocean.
- A massive collapse of the western edge of the Lake Tahoe basin, which formed McKinney Bay around 50,000 years ago, is thought to have generated a tsunami/seiche wave with a height approaching 330 ft (100 m).
- At Seton Portage, British Columbia, Canada, a freshwater megatsunami may have occurred approximately 10,000 years ago. 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.
- Approximately 8,000 years ago, 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 8,000 years ago.
- Approximately 6,000 years ago, a landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.
- The northern half of Molokai suffered a catastrophic collapse and likely megatsumami about 1.5 million years ago and now lies as a debris field scattered northward across the ocean bottom, while what remains on the island are the highest sea cliffs in the world.
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
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 300 feet (91 m). 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.
1963: Vajont Dam, Italy
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.
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) 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.
Potential future megatsunami
Experts interviewed by the BBC think that a massive landslide on a volcanic ocean island is the most likely future cause of a megatsunami. 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.
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).
Geologists Dr. Simon Day and Dr. Steven Neal 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, Spain.
In 1949, this volcano erupted at its Duraznero, Hoyo Negro and San Juan vents. During the eruption, an earthquake occurred with an epicentre near the village of Jedy. The following day Rubio Bonelli, a local geologist, visited the summit area and discovered that a fissure about 2.5 kilometres (1.6 mi) long had opened on the eastern side of the summit. As a result, the western 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,
The Cumbre Vieja volcano is currently in a dormant stage, but will almost certainly erupt again. Day and Ward hypothesize that if such an eruption causes the western flank to fail, a megatsunami will 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. However, the western half of the volcano has an approximate volume of 500 cubic kilometres (120 cu mi) and an estimated mass of 1,500,000,000,000 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 many other cities on the Atlantic coast in Europe, South America and Africa. The likelihood of this happening is a matter of vigorous debate.
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.
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. A future tsunami event is also possible, with the tsunami potentially reaching up to about 1 kilometre (3,300 ft) in height. 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.
- Historic tsunamis
- La Palma
- Cumbre Vieja
- Minoan eruption
- 2004 Indian Ocean Earthquake
- Tsunamis in lakes
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