What is Tsunami
A tsunami is a giant wave (or series of waves) created by an undersea earthquake, volcanic eruption or landslide. Tsunamis are often called tidal waves, but this is not an accurate description because tides have little effect on giant tsunami waves. Far out in the ocean, tsunami waves don’t get very high, but they move very fast. In fact, the National Oceanic and Atmospheric Administration (NOAA) reports that tsunami waves can travel as fast as a jet plane.
As a tsunami gets closer to land and the ocean depth decreases, the speed of the tsunami wave slows down and the height of the tsunami wave increases dramatically—along with its potential for destruction. One thing is certain about tsunamis: they are unpredictable. Once a tsunami makes landfall, the waves can last from five to 15 minutes and do not follow a set pattern. NOAA warns that the first wave may not be the largest, not all undersea earthquakes or other seismic events create tsunamis, which is why tsunamis are difficult to predict.
Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.
The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes, but the understanding of a tsunami's nature 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 conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called a meteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land.
How Tsunami is generated:The term tsunami comes from the Japanese composed of the two words tsu meaning "harbor" and nami, meaning "wave". Tsunami are sometimes referred to as tidal waves. In recent years, this term has fallen out of favor, especially in the scientific community, because tsunami actually have nothing to do with tides. The once-popular term derives from their most common appearance, which is that of an extraordinarily high tidal bore.
Tsunami and tides both produce waves of water that move inland, but in the case of tsunami the inland movement of water is much greater and lasts for a longer period, giving the impression of an incredibly high tide. Although the meanings of "tidal" include "resembling" or "having the form or character of" the tides, and the term tsunami is no more accurate because tsunami are not limited to harbours, use of the term tidal wave is discouraged by geologists and oceanographers. There are only a few other languages that have an equivalent native word.
In the Tamil language, the word is aazhi peralai. In the Acehnese language, it is ië beuna or alôn buluëk (Depending on the dialect. Note that in the fellow Austronesian language of Tagalog, a major language in the Philippines, alon means "wave".) On Simeulue island, off the western coast of Sumatra in Indonesia, in the Defayan language the word is smong, while in the Sigulai language it is emong.
Historic tsunami
As early as 426 B.C. the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and was the first to argue that ocean earthquakes must be the cause.[5][6]
The cause, in my opinion, of this phenomenon must be sought in the earthquake. At the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation. Without an earthquake I do not see how such an accident could happen.[12]
The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15-19) described the typical sequence of a tsunami, including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave, after the 365 A.D. tsunami devastated Alexandria.
While Japan may have the longest recorded history of tsunamis, the sheer destruction caused by the 2004 earthquake and tsunami event mark it as the most devastating of its kind in modern times, killing around 230,000 people. The Sumatran region is not unused to tsunamis either, with earthquakes of varying magnitudes regularly occurring off the coast of the island.
Specifications:
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying all with it, even if the wave did not look large.
While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft). This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.
As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.
When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed run up. Run up is measured in metres above a reference sea level. A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.
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How Tsunami is Detected:
To help identify and predict the size of a tsunami, scientists can look at the size and type of the underwater earthquake that precedes it. That is often the first information they receive, because seismic waves travel faster than tsunamis.
This information is not always helpful, however, because a tsunami can arrive within minutes after the earthquake that triggered it. And not all earthquakes create tsunamis, so false alarms can and do happen.
That’s where special open-ocean tsunami buoys and coastal tide gauges can help—by sending real-time information to tsunami warning centers in Alaska and Hawaii. In areas where tsunamis are likely to occur, community managers, educators and citizens are being trained to provide eye-witness information that is expected to aid in the prediction and detection of tsunamis.
