Megatsunami

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A megatsunami is a rare tsunami more than 100 meters (325ft) high. The last one possibly occurred approximately 4,000 years ago. Geologists say it is usually caused by a very large landslide, such as a collapsing island, into a vast body of water such as an ocean or sea.

Megatsunami can rise to heights of hundreds of meters, travel at 890 km/h in mid-ocean and potentially reach 20 km inland in low-lying regions.

In deep ocean, a megatsunami is barely noticeable. It moves as a vertical shift of only a metre or so throughout the volume of water, with a crest to crest distance of hundreds of kilometers. However the huge amount of energy in the motion of this massive volume generates a much higher wave as it approaches shallow water.

Underwater earthquakes do not normally generate such large tsunamis unless they also trigger an underwater landslide — typically they have a height of less than ten metres.

Landslides that are large compared to the depth of water hit the water so fast that the displaced water cannot settle before the rocks hit the bottom. This means that the rocks displace the water at full speed all the way to the bottom. If the water is deep, the displaced volume is large and the lower parts are under high pressure. The resulting wave contains large amounts of energy.

Some have conjectured that historic megatsunamis underlie the deluge myths that are common to many cultures throughout the world. However this is unlikely, considering that megatsunami usually occured without any warning, only hit coastal areas, and does not necessarily occur after a rain.

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Historical megatsunamis

Megatsunami were first hypothesized by geologists searching for oil in Alaska. They observed evidence of unusually large waves in the nearby bay. Five years later, landslides were revealed to be the source of the Alaskan waves.

The geological record shows that megatsunamis are very rare, but devastate anything near the receiving shore. Some can devastate the coasts of entire continents. The last-known such event occurred approximately 4,000 years ago on [[R�union]] island, to the east of Madagascar.

A smaller megatsunami did occur in Lituya Bay, Alaska in 1958. This is an ice-scoured inlet 220 meters deep with an entrance only 10 meters wide. The topology of the inlet is particulary suited to producing local megatsunami. A nearby magnitude 7.5 earthquake on July 8 generated a landslide within the inlet which produced a wave that washed out trees 200 meters above normal sea level. Comparison with previous photographs indicated that several hundred feet of ice had been removed from the front of a nearby glacier by a 520 meter high wave.

Megatsunami threats

Volcanic Islands such as [[R�union]] and the Hawaiian Islands can cause megatsunamis because often they are structurally little more than large, unstable piles of loosely aggregated material heaped up by successive eruptions. Evidence for large landslides has been found in the form of extensive underwater debris aprons around them composed of the material which has slipped into the ocean. In recent years five such debris aprons have been found in the Hawaiian Islands alone.

Some geologists believe the most likely candidate for the source of the next megatsunami is the island of La Palma, in the Canary Islands. During the 1949 eruption the western half of the Cumbre Vieja ridge slipped several metres downwards into the Atlantic Ocean. It is believed that this process was driven by the pressure caused by the rising magma heating and vaporising water trapped within the structure of the island, causing the island's structure to be pushed apart. During an eruption that is anticipated to occur sometime within the next few thousand years the western half of the island, weighing perhaps 500 billion tonnes, will catastrophically slide into the ocean. This will inevitably generate a megatsunami which will travel across the Atlantic and strike the Caribbean and the Eastern American seaboard several hours later with a wave possibly 90 meters (300 feet) high, resulting in massive coastal devastation.

The aftermath would hold obvious implications for affected populations, governments and for the global economy. While potentially not as devastating as a supervolcano, a megatsunami would be an unprecedented disaster in whatever region of the world it occurred.

Besides fjords in Alaska, many locations face threats of localized, but still potentially dangerous, megatsunami-type waves. For example, some geologists speculate that an unstable rock face at the north end of Harrison Lake in the Fraser Valley in southwestern British Columbia could collapse into the lake, generating a large wave that might destroy the town and Harrison Hot Springs resort at the south end.

Megatsunamis are a favorite subject of many films, given their undoubted visual impact; these megatsunamis are often caused by bolide impacts or other extraterrestrial causes, rather than by landslides. Examples of this are the movies Deep Impact and the director's cut of The Abyss.

Further reading

  • 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.

External links


References

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