Important Historical Events
Megatsunami, Seiche Tsunamis
Related to a tsunami is a seiche, an underwater, irregular fluctuation or rhythmic rocking of the water level of a lake. Often large earthquakes produce both tsunamis and seiches at the same time and there is evidence that some seiches have been caused by tsunamis.
The highest tsunami wave ever recorded was very localized: caused by a landslide in Lituya Bay, Alaska in 1958, a tsunami more than 500 m high stripped trees and soil from the steep walls of a fjord. By the time the wave reached the open sea, however, it dissipated quickly. The height of the waves was determined more by the topography of the inlet than by the energy generated by the landslide.
Seismic waveA seismic wave is a wave that travels through the Earth, often as the result of an earthquake or explosion. Seismic waves are studied by seismologists, and measured by a seismograph.
Body waves travel through the interior of the Earth. They follow curved paths because of the varying density and composition of the Earth's interior. This effect is similar to the refraction of light waves. Body waves transmit the preliminary tremors of an earthquake but have little destructive effect. Body waves are divided into two types: primary (P) and secondary (S) waves.
Surface waves are analogous to water waves and travel over the Earth's surface. They travel more slowly than body waves. Because of their low frequency, they are more likely than body waves to stimulate resonance in buildings, and are therefore the most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and Love waves.
MegatsunamiA megatsunami is a rare tsunami more than 100 meters (325ft) high. Aside from some large tsunamis in Alaska, including one 520 m high, the last megatsunami to hit a populated area is believed to have occurred 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.
Megatsunamis 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 legends that are common to many cultures throughout the world. However this is unlikely, considering that megatsunamis usually occur without any warning, only hit coastal areas, and do not necessarily occur after a rain.
Historical megatsunamisMegatsunamis 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 Réunion island, to the east of Madagascar.  (http://www.bbc.co.uk/science/horizon/2000/mega_tsunami.shtml)
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 threatsVolcanic islands such as Réunion 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 with local wave heights of hundreds of meters. After the tsunami traveled across the Atlantic it would likely have a wave height of 10 to 25 meters at the Caribbean and the Eastern American seaboard coast several hours later.
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, in 1963, an enormous slab from the side of Mount Toc, in the mountains north of Venice, Italy, slid into the reservoir held by Vajont Dam, producing waves some 250 meters high, destroying several villages, and killing nearly 2000 people. 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.
In the Norwegian Sea, the Storegga Slide caused a megatsumani 7000 years ago. Extensive investigations has shown that the risk of a re-occurance is minimal.
Further readingWard, 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 Tsunami and Megatsunami LinksPotential collapse and tsunami at La Palma, Canary Islands
Ward, S.N. and Day, S. 2001. Cumbre Vieja Volcano (.pdf format)
Science of Tsunami Hazards
BBC Mega-tsunami: Wave of Destruction
Dyer, Gwynne. Unstoppable Gee-Gees
La Palma threat "over-hyped"