TSUNAMI

A tsunami is a series of waves that is created when a large volume of a body of water, such as an ocean, is rapidly displaced. The Japanese term is literally translated into "(great) harbor wave."

Earthquakes, volcanic eruptions and other underwater explosions (detonations of nuclear devices at sea), landslides, bolide impacts, and other mass movements above or below water all have the potential to generate a tsunami. Due to the immense volumes of water and energy involved, the effects of tsunamis can be devastating.

The Greek historian Thucydides was the first to relate tsunami to submarine quakes, but understanding of the nature of tsunami remained slim until the 20th century and is the subject of ongoing research.

Many early geological, geographical, and oceanographic texts refer to tsunamis as "seismic sea waves."

Some meteorological storm conditions such as deep depressions causing cyclones, hurricanes, can generate a storm surge which can be several metres above normal tide levels. This is due to the low atmospheric pressure within the centre of the depression. As these storm surges come ashore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge inundated Burma (Myanmar) in May 2008.

Causes : - A tsunami can be generated when converging or destructive plate boundaries abruptly move and vertically displace the overlying water. It is very unlikely that they can form at divergent (constructive) or conservative plate boundaries. This is because constructive or conservative boundaries do not generally disturb the vertical displacement of the water column. Subduction zone related earthquakes generate the majority of all tsunamis.

Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 mm above the normal sea surface. They grow in height when they reach shallower water, in a "shoaling" process described below. A tsunami can occur at any state of the tide and even at low tide will still inundate coastal areas if the incoming waves surge high enough.

On April 1, 1946 a Magnitude 7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 m high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.

Examples of tsunami being generated at locations away from convergent boundaries include Storegga during the Neolithic era, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). In the case of the Grand Banks and Papua New Guinea tsunamis an earthquake caused sediments to become unstable and subsequently fail. These slumped and as they flowed down slope a tsunami was generated. These tsunami did not travel transoceanic distances.

It is not known what caused the Storegga sediments to fail. It may have been due to overloading of the sediments causing them to become unstable and they then failed solely as a result of being overloaded. It is also possible that an earthquake caused the sediments to become unstable and then fail. Another theory is that a release of gas hydrates (methane etc.,) caused the slump.

The "Great Chilean earthquake" (19:11 hrs UTC) May 22, 1960 (9.5 Mw), the March 27, 1964 "Good Friday earthquake" Alaska 1964 (9.2 Mw), and the "Great Sumatra-Andaman earthquake" (00:58:53 UTC) December 26, 2004 (9.2 Mw), are recent examples of powerful megathrust earthquakes that generated a tsunami that was able to cross oceans. Smaller (4.2 Mw) earthquakes in Japan can trigger tsunami that can devastate nearby coasts within 15 minutes or less.

In the 1950s it was hypothesised that larger tsunamis than had previously been believed possible may be caused by landslides, explosive volcanic action e.g., Santorini, Krakatau, and impact events when they contact water. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls at a rate faster than the ocean water can absorb it. They have been named by the media as "mega-tsunami."

Tsunami caused by these mechanisms, unlike the trans-oceanic tsunami caused by some earthquakes, may dissipate quickly and rarely affect coastlines distant from the source due to the small area of sea affected. These events can give rise to much larger local shock waves (solitons), such as the landslide at the head of Lituya Bay 1958, which produced a wave with an initial surge estimated at 524 m. However, an extremely large gravitational landslide might generate a so called "mega-tsunami" that may have the ability to travel trans-oceanic distances. This though is strongly debated and there is no actual geological evidence to support this hypothesis.