A megatsunami is a tsunami—a large wave due to displacement of a body of water—with an initial wave amplitude (height) measured in several tens, hundreds, or possibly thousands of metres.
Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, are very long (often hundreds of kilometres), 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 large 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.
A way to visualize the difference, is that an ordinary tsunami is caused by sea floor changes, somewhat like pushing up on the floor of a large tub of water to the point it overflows, and causing a surge of water to "run off" at the sides. In this analogy, a megatsunami would be more similar to dropping a large rock from a considerable height into the tub, at one end, causing water to splash up and out, and overflow at the other end.
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.
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, a 7.8 Mw strike-slip earthquake in southeast Alaska 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 that surged up the opposite side of the head of the bay to a height of 1720 feet (524 m), and was still many tens of metres high further down the bay, when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat. They were washed back into the bay and both survived.
The mechanism giving rise to megatsunamis was analysed for the Lituya Bay event in a study presented at the Tsunami Society in 1999; this model was considerably developed and modified by a second study in 2010.
Although the earthquake which caused the megatsunami was considered very energetic, and involving strong ground movements, several possible mechanisms were not likely or able to have caused the resulting megatsunami. Neither water drainage from a lake, nor landslide, nor the force of the earthquake itself led to the megatsunami, although all of these may have contributed.
Instead, the megatsunami was caused by a massive and sudden impulsive impact when about 40 million cubic yards of rock several hundred metres above the bay was fractured from the side of the bay, by the earthquake, and fell "practically as a monolithic unit" down the almost vertical slope and into the bay. The rockfall also caused air to be "dragged along" due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater. The study concluded that:
"The giant wave runup of 1,720 feet (524 m.) at the head of the Bay and the subsequent huge wave along the main body of Lituya Bay which occurred on July 9, 1958, were caused primarily by an enormous subaerial rockfall into Gilbert Inlet at the head of Lituya Bay, triggered by dynamic earthquake ground motions along the Fairweather Fault.The large mass of rock, acted as a monolith (thus resembling high-angle asteroid
impact), struck with great force the sediments at bottom of Gilbert Inlet at the head of the bay. The impact created a large crater and displaced and folded recent and Tertiary deposits and sedimentary layers to an unknown depth. The displaced water and the displacement and folding of the sediments broke and uplifted 1,300 feet of ice along the entire front of the Lituya Glacier at the north end of Gilbert Inlet. Also, the impact and the sediment displacement by the rockfall resulted in an air bubble and in water splashing action that reached the 1,720 foot (524 m.) elevation on the other side of the head of Gilbert Inlet. The same rockfall impact, in combination with the strong ground movements, the net vertical crustal uplift of about 3.5 feet, and an overall tilting seaward of the entire crustal block on which Lituya Bay was situated, generated the giant solitary gravity wave which swept the main body of the bay.This was the most likely scenario of the event – the" PC model" that was adopted for subsequent mathematical modeling studies with source dimensions and parameters provided as input. Subsequent mathematical modeling at the Los Alamos National Laboratory (Mader, 1999, Mader & Gittings, 2002) supported the proposed mechanism – as there was indeed sufficient volume of water and an adequately deep layer of sediments in the Lituya Bay inlet to account for the giant wave runup and the subsequent inundation. The modeling reproduced the documented physical observations of runup."
A 2010 model examined the amount of infill on the floor of the bay, which was many times larger than that of the rockfall alone, and also the energy and height of the waves, and the accounts given by eyewitnesses, concluded that there had been a "dual slide" involving a rockfall, which also triggered a release of 5 to 10 times its volume of sediment trapped by the adjacent Lituya Glacier, as an almost immediate and many times larger second slide, a ratio comparable with other events where this "dual slide" effect is known to have happened.The asteroid linked to the extinction of dinosaurs, which created the Chicxulub crater in Yucatán approximately 66 million years ago, would have caused an over 100 metres (330 ft) tall megatsunami. The height of the tsunami was limited due to relatively shallow sea in the area of the impact; in deep sea it would be 4.6 kilometres (2.9 mi) tall.A series of megatsunamis were generated by the bolide impact that created the Chesapeake Bay impact crater, about 35.5 million years ago.During the Messinian the coasts of northern Chile were likely struck by various megatsunamis.A megatsunami affected the coast of south–central Chile in the Pliocene as evidenced by the sedimentary record of Ranquil Formation.The Eltanin 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.The northern half of the East Molokai Volcano suffered a catastrophic collapse and likely megatsunami 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.The existence of large scattered boulders in only one of the four marine terraces of Herradura Bay south of the Chilean city of Coquimbo has been interpreted by Roland Paskoff as the result of a mega-tsunami that occurred in the Middle Pleistocene.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).In the North Sea, the Storegga Slide caused a megatsunami approximately 8,200 years ago. It is estimated to have completely flooded the remainder of Doggerland.Approximately 8,000 years ago, a massive volcanic landslide off Mt. Etna, Sicily caused a megatsunami which devastated the eastern Mediterranean coastline on three continents. Wave heights on the coast of Calabria are estimated to have reached a maximum of 40m.Approximately 4,000 years ago, a landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.3,600 years ago, the Thera volcano erupted, the force of the eruption causing megatsunamis which affected the whole Aegean sea and the eastern Mediterranean Sea.
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.
The eruption of Krakatoa created pyroclastic flows which generated megatsunamis when they hit the waters of the Sunda Strait on 27 August 1883. The waves reached heights of up to 24 metres (79 feet) along the south coast of Sumatra and up to 42 metres (138 feet) along the west coast of Java.
On July 9, 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.
On October 9, 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.
On May 18, 1980, the upper 460 metres (1,509 feet) of Mount St. Helens collapsed, creating 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 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 metres (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.
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 an eruption of 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 Llano del Banco vents, and there was an earthquake with an epicentre near the village of Jedey. 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,
Cumbre Vieja is currently dormant, but will almost certainly erupt again. Day and Ward hypothesize 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. 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, Halifax, Boston, New York, Baltimore, Washington, D.C., Miami, Havana and the rest of the Eastern Coasts of the United States and Canada, as well 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 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.
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.
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 tsunami event is also possible, with the tsunami potentially reaching up to about 1 kilometre (3,300 ft) in height. According to the documentary National Geographic's Ultimate Disaster: Tsunami, if a big landslide occurred at Mauna Loa or the Hilina Slump, a 30-metre (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. Also, the West Coast of America and the entire Pacific Rim could potentially be affected.
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.