Secondary School Level

Tsunami

Outline: Explaining the term and describing the brief condition of the calamity. Early history of the oceanic earthquakes causing tsunami. Generation of tsunami. Attempts to detect tsunami. Measure to reduce damages caused onshore during tsunami.

The term ‘tsunami', also referred as 'tidal waves', comes from the Japanese language. Tsu means, “Harbour” and name means, “Wave”. Tsunami has a wavelength ranging from 10 to 500 kilometers. Thus, it is a series of ocean waves with very long wavelength. Tsunami produces waves of water that moves inshore. The inland movement of water is much greater and lasts for a longer period, giving the impression of an incredibly high tide.

However, the term tsunami is no more accurate because tsunami is not limited to harbours.

As early as 426 BC the Greek historian Thucydides investigated in his book 'History of the Peloponnesian War' about the causes of tsunami. He was the first to argue that ocean earthquakes must be the cause of tsunami. Japan has the longest record of tsunamis. Absolute destruction caused by the tsunami wave marks it as the most devastating natural calamity.

Tsunami generates when the sea floor abruptly deforms and vertically dislodges the overlying water. Tectonic earthquakes are particular kind of earthquakes that are associated with the earth's crustal deformation. When these earthquakes occur beneath the sea, the water above displaces from its position.

About 80% of tsunamis occur in the Pacific Ocean. However, there are possibilities that it can occur where there are large bodies of water. Earthquakes, landslides and volcanic outbursts cause tsunami. A tsunami in the deep ocean has a wavelength of about 200 kilometers. Such a wave travels at nearly 800 kilometers per hour. Ships can rarely notice their passage. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave.

Several attempts are made to set up scales to detect tsunami. The first scales used to measure the intensity of tsunami were, the Sieberg- Ambraseys scale in the Mediterranean Sea and the Imamura-lida intensity scale in the Pacific Ocean. The latter scale was modified to calculate the intensity of tsunami.

In some tsunami-prone nations, measures are taken to reduce the damages caused onshore. Japan, where tsunami science and response measures first began, has produced elaborate counter-measures and plans. To protect populated coastal areas many tsunami walls of up to 15 feet are constructed. Channels to redirect the water from tsunami are also built.

However, their effectiveness has been questioned as tsunami often overtops the barriers. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it has not prevented major destructions and loss of life.


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