A tsunami is a series of water waves that is caused by the displacement of a large volume of a body of water, such as an ocean by earthquakes, landslides, volcanic eruptions and large meteorite impacts.
A tsunami is a wave in the ocean caused by earthquakes or volcanic eruptions. A tsunami is a very long wave. It can be hundreds of kilometers long. It is a chain of fast moving waves caused by fast changes in the ocean. Usually, a tsunami starts suddenly. It will begin as normal waves and change to a very big wave very quickly. The waves travel at a great speed across an ocean with little energy loss. They can remove sand from beaches, destroy trees, damage houses and even destroy whole towns.
The water will draw back from the coast half of the period of the wave before it gets to the coast. If the slope of the coast is shallow, the water may pull back for many hundreds of metres. People who do not know of the danger will often remain at the shore. Tsunamis can not be prevented, but there are ways to help stop people from dying from a tsunami. Some regions with a high risk of tsunamis may use warning systems to warn the general population before the big waves reach the land. Because an earthquake that caused the tsunami can be felt before the wave gets to the shore, people can be warned to go somewhere safe.
The deadliest tsunami recorded was on December 26, 2004. It was caused by an earthquake. The earthquake was said to had magnitude 9.3 on the Richter scale. It was centered in the ocean near the coast of Sumatra, Indonesia. Over 215,000 people died from this disaster. The giant wave moved very quickly. Hundreds of thousands of people in Indonesia, Sri Lanka, Thailand, India, Somalia, and other nations, were killed or injured by it.
Tsunamis are often called tidal waves. This is misleading, because tsunamis are not related to tides.
Some tsunamis can be caused by the overlapping of tectonic plates.
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A tsunami is a series of water waves (called a tsunami wave train) that is caused by the displacement of a large volume of a body of water, such as an ocean. The original Japanese term literally translates as "harbor wave." Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions. Casualties can be high because the waves move faster than humans can run.
Earthquakes, volcanic eruptions and other underwater explosions (detonations of nuclear devices at sea), landslides and other mass movements, bolide impacts, 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 understanding of 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. Such a storm surge inundated Burma (Myanmar) in May 2008.
The term tsunami comes from the Japanese, meaning "harbor" (tsu) and "wave" (nami). (For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in the Japanese.)
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 a native word for this disastrous wave. In the Tamil language, the word is aazhi peralai.
Causes: Most tsunamis are generated by underwater earthquakes. A tsunami can be generated when convergent 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 tsunami.
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 due 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, producing a distinctly visible wave. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf 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.
About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They may be caused by landslides, volcanic explosions, bolides and seismic activity.
If the first part of a tsunami to reach land is a trough (called a drawback) rather than a wave crest, the water along the shoreline recedes dramatically, exposing normally submerged areas.
A drawback occurs because the tectonic plate on one side of the fault line sinks suddenly during the earthquake, causing the overlaying water to propagate outwards with the trough of the wave at its front. It is also for this reason that there would not be any drawback when the tsunami travelling on the other side arrives ashore, as the tectonic plate is "raised" on that side of the fault line.
Drawback begins before the wave's arrival at an interval equal to half of the wave's period. If the slope of the coastal seabed is moderate, drawback can exceed hundreds of meters. People unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, the sea withdrew and many people went onto the exposed sea bed to investigate. Pictures show people walking on the normally submerged areas with the advancing wave in the background. Few survived.
Tsunami intensity and magnitude scales: As with earthquakes, several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events. The first scales used routinely to measure the intensity of tsunami were the Sieberg-Ambraseys scale, used in the Mediterranean Sea and the Imamura-Iida intensity scale, used in the Pacific.
The first scale that genuinely calculated a magnitude for a tsunami, rather than an intensity at a particular location was the ML scale proposed by Murty & Loomis based on the potential energy. Difficulties in calculating the potential energy of the tsunami means that this scale is rarely used.
Warnings and predictions: Drawbacks can serve as a brief warning. People who observe drawback (many survivors report an accompanying sucking sound), can survive only if they immediately run for high ground or seek the upper floors of nearby buildings. In 2004, ten-year old Tilly Smith of Surrey, England, was on Maikhao beach in Phuket, Thailand with her parents and sister, and having learned about tsunamis recently in school, told her family that a tsunami might be imminent. Her parents warned others minutes before the wave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney.
A tsunami cannot be precisely predicted—even if the right magnitude of an earthquake occurs in the right location. Geologists, oceanographers, and seismologists analyse each earthquake and based upon many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and automated systems can provide warnings immediately after an earthquake in time to save lives. One of the most successful systems uses bottom pressure sensors that are attached to buoys. The sensors constantly monitor the pressure of the overlying water column.
Regions with a high tsunami risk typically use tsunami warning systems to warn the population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs indicate evacuation routes.
The Pacific Tsunami Warning System is based in Honolulu, Hawiʻi. It monitors Pacific Ocean seismic activity. A sufficiently large earthquake magnitude and other information triggers a tsunami warning. While the subduction zones around the Pacific are seismically active, not all earthquakes generate tsunami. Computers assist in analysing the tsunami risk of every earthquake that occurs in the Pacific Ocean and the adjoining land masses.
Signs of an approaching tsunami
The following have at various times been associated with a tsunami:
- An earthquake may be felt.
- Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling.
- The water in the waves may be unusually hot.
- The water may smell of rotten eggs (hydrogen sulfide) or of petrol or oil.
- The water may sting the skin.
- A thunderous boom may be heard followed by
- a roaring noise as of a jet plane
- or a noise akin to the periodic whop-whop of a helicopter,
- or a whistling sound.
- The sea may recede to a considerable distance.
- A flash of red light might be seen near the horizon.
Tsunami in history
Tsunami are not rare, with at least 25 tsunami occurring in the last century. Of these, many were recorded in the Asia–Pacific region—particularly Japan.
The 2004 Indian Ocean tsunami killed over 200,000 people with many bodies either being lost to the sea or unidentified.
According to an article in Geographical magazine (April 2008), the Indian Ocean tsunami of December 26, 2004 was not the worst that the region could expect. Professor Costas Synolakis of the Tsunami Research Center at the University of Southern California co-authored a paper in Geophysical Journal International which suggests that a future tsunami in the Indian Ocean basin could affect locations such as Madagascar, Singapore, Somalia, Western Australia, and many others.
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.
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.
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.
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