Megatsunami (also known as iminami, “purification wave”)) 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 large scale landslides or impact events.
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. 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.
[Image at right right: Historic partial volcano collapse on El Hierro Island in Canary Islands.]
[Current volcanic activity at El Hierro is occurring at about 1300 ft, or 1/4 mile deep and a mile or so off shore. In otherwards, the volcano is venting through the up slope side of the mountain, potentially weakening the mountain’s side structure for a replay of the previous collapse-seen above. Mr. Larry]
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.
The asteroid which created the Chicxulub crater in Yucatan approximately 65 million years ago would have generated megatsunamis as high as 3 kilometers (1.9 mi); enough to completely inundate even large islands such as Madagascar.
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. 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, 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. Another more recent landslide on the south shore of Anderson Lake dropped a large portion of high mountainside down a debris chute, creating a rockwall “fan” which may have made a megatsunami-type wave, though not as large as the main one at the Portage.
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 7,000 years ago.
Approximately 6000 years ago, a landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.
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. The island of Molokai had a catastrophic collapse over a million years ago; this underwater landslide likely caused large tsunamis. In recent years, five such debris aprons have been found around the Hawaiian Islands. The Canary Islands have at least 14 such debris aprons associated with the archipelago
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, Spain.
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 kilometers (1.6 mi) 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 meters (6.6 ft) downwards and 1 meter (3.3 ft) westwards towards the Atlantic Ocean.
The Cumbre Vieja volcano is currently in a dormant stage, but will almost certainly erupt again in the future. 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 would be lost as New York, Boston, Baltimore, Washington, D.C., Miami, Havana, and many other cities near the Atlantic coast are leveled. 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.
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.
[What’s the worst that could happen?]