Water And Oceanography

How Tsunamis are Formed



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Tsunamis are a series of waves that result from seismic disturbances at the bottom the sea that cause the sea floor to move. These disturbances are most often earthquakes at the boundaries between tectonic plates, but they can also be volcanic eruptions or explosions, or even landslides or meteorite impacts.

In some of these disturbances the earth can move several hundred feet over many miles, and this movement displaces enormous volumes of water and releases enormous amounts of seismic energy. Together, this forms a tsunami.

The old term for tsunamis was tidal waves, but tsunamis have nothing to do with the tides and so the term became obsolete. Some scientists have referred to them as seismic sea waves, but this term is also falling out of use. The word tsunami is derived from two Japanese words, tsu (harbour) and nami (wave).

While at sea, tsunami waves are very small (usually a couple of feet high at the most) but they travel at incredible speeds (as fast as a jet aircraft) and contain enormous energy.

Tsunamis usually consist of several waves, and spread out from the epicentre of the disturbance, rather like ripples on a pond. Tsunamis have a massive wavelength (distance between waves) that can be several hundred miles in length, which means that the waves created by the disturbance can be hundreds of miles apart. The period (time between one wave and the next) can be up to 10 to 120 minutes.

The rate of energy loss of a wave is inversely proportional to the wavelength, so while at sea the tsunami loses very little of its massive store of energy because of its huge wavelength. If you were on a ship and a tsunami wave passed under you, you probably would not notice its passing.

By contrast the waves we normally observe on a beach are relatively low in energy and usually have a wavelength of about 100 to 200 yards and a period of about 5-20 seconds. These waves are wind generated and therefore can curl and break. Tsunami waves do not curl and break, and so the representations of them made by artists are usually incorrect. Tsunamis do not look like surfing waves and they are not big while out at sea.

Tsunami waves grow large only when they hit the shallows. The velocity of a wave in shallow water is proportional to the depth of the water, so as the wave reaches shallower water its velocity decreases, but the total energy of the wave (which is enormous) is unchanged. The period is also unchanged, so more water is forced between the wave crests, and this causes the wave to grow in height as the power of the wave overwhelms everything in front of it.

Tsunamis occur as a series of waves from 10 minutes to two hours apart, and each of the waves grows as it reaches shallow water. Just before the first wave hits, the water may be drawn back into the sea exposing the sea floor, and this is a classic sign that a tsunami is on its way.

Not all submarine earthquakes cause tsunamis, which tend to be caused by those that release their energy slowly, over minutes rather than those that suddenly release their energy over a period of seconds.

The largest tsunami ever recorded was in Lituya Bay, Alaska in 1958, which was caused by a landslide. This tsunami reached an amazing 60 metres in height. The largest in terms of loss of human life was the Boxing Day tsunami in 2004, which killed over 220,000 people, making the death toll at least four times greater than the previous worst tsunami

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