Friday, September 26, 2008

How does Tsunami occur?

Causes for occurance of Tsunami

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. A tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

As a tsunami approaches shore, it begins to slow and grow in height. Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a runup height, of 10, 20, and even 30 meters.

Tsunamis are formed as a result of earthquakes, volcanic eruptions, or landslides that occur under the sea. When these events occur under the water, huge amounts of energy are released as a result of quick upward bottom movement. For example, if a volcanic eruption occurs, the ocean floor may very quickly move upward several hundred feet. When this happens, huge volumes of ocean water are pushed upward and a wave is formed. A large earthquake can lift thousands of square kilometers of sea floor which will cause the formation of huge waves. The Pacific Ocean is especially prone to tsunamis as a result of the large amount of undersea geological activity.
In the open ocean tsunamis may appear very small with a height of less than 1 meter (3 feet). Tsunamis will sometimes go undetected until they approach shallow waters along a coast. These waves have a very large wavelength (up to several hundred miles) that is a function of the depth of the water where they were formed. Although these waves have a small height, there is a tremendous amount of energy associated with them. As a result of this huge amount of energy, these waves can become gigantic as they approach shallow water. Their height, as they crash upon the shore, depends on the underwater surface features. They can be as high as 30 m (100 feet) or more. In 1737 , a huge wave estimated to be 64m (210 feet) in height hit Cape Lopatka, Kamchatka (NE Russia). The largest Tsunami ever recorded occurred in July of 1958 in Lituya Bay, Alaska. A huge rock and ice fall sent water surging up to a high water mark of 500m (1640 feet). It's no wonder that these waves can cause such massive destruction and loss of life.

n order for a tsunami caused by a seaquakes to occur, three things have to be happen:
1: The Earthquake must measure at least 7,0 on the Richter scale. Only from this intensity upwards is there enough energy released to rapidly displace enough water to create the tsunami.
2: The sea bed must be lifted or lowered by the earthquake. If the sea bed is displaced sidewards, no tsunami will occur as, for example, happened during the earthquake of 28 March, 2005 off the western coast of Sumatra.
3: The epicentre of the earthquake must be near to the earth's surface.

The difference between tsunamis and normal waves or waves caused by strong wind is the extreme distances between wavelengths. This is the distance from one wave crest to the next wave crest, which can be between 100 and 300 km. A further feature of tsunamis is their relatively small wave height on the open sea - mostly between half a metre and one meter. Even though they can travel up to 1 000 km/h, these waves are generally not noticeable in deep waters. The wave itself only becomes dangerous once it reaches land. In coastal areas where water levels gradually become shallower, the wave will slow down but tower into a wave wall as much as 30 meters high. The reason for this is the mass of water and energy contained in the tsunami wave. Whereas only the upper water layers are being moved in wind created waves, with a Tsunami wave, an entire mass of water from the sea bed to the surface is in motion.

If a trough of a tsunami wave approaches the land first, the water will be pulled back into the sea by enormous currents. Vast stretches of the seabed are often drained as happened during the December 2004 tsunami. In this case, and if recognised, people on the beach and beachfronts have between a few minutes and half an hour to escape to higher ground. The time to escape depends on when the wave crest strikes.

The first wave, that can grow to be up to 30 meters high at the beach, will usually be followed by more waves that are sometimes even more dangerous. Not only the crests of waves are dangerous but also the troughs, since their currents can pull people and whole houses many miles into the sea.

The Tsunami Alarm warning time can be between a few minutes up to several hours, depending on distance from the earthquake's epicentre.

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