Tsunami
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For other uses, see Tsunami (disambiguation).
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A tsunami (津波) (pronounced /(t)suːˈnɑːmi/) is a series of water waves (called a tsunami wave train[1]) 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.[2] 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), landslidesand 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,[3][4] 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 lowatmospheric 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.
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Etymology
The term tsunami comes from the Japanese, meaning "harbor" (tsu, 津) and "wave" (nami, 波). (For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in the Japanese.[5])
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"[6] or "having the form or character of"[7] 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 geologistsand 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. In the Acehnese language, it is ië beuna or alôn buluëk [8] (Depending on the dialect. Note that in the fellow Austronesian language of Tagalog, a major language in the Philippines, alon means "wave".) On Simeulueisland, off the western coast of Sumatra in Indonesia, in the Defayan language the word is semong, while in the Sigulai language it is emong.[9]
Causes
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 all tsunamis.
Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
On April 1, 1946, a magnitude-7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 metres (46 ft) high surge. The area where theearthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.
Examples of tsunami at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before traveling transoceanic distances.
The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates (methane etc.)
The 1960 Valdivia earthquake (Mw 9.5) (19:11 hrs UTC), 1964 Alaska earthquake (Mw 9.2), and 2004 Indian Ocean earthquake (Mw 9.2) (00:58:53 UTC) are recent examples of powerful megathrust earthquakes that generated tsunamis (known as teletsunamis) that can cross entire oceans. Smaller (Mw 4.2) earthquakes in Japan can trigger tsunamis (called local and regional tsunamis) that can only devastate nearby coasts, but can do so in only a few minutes.
In the 1950s, it was hypothesised[who?] that larger tsunamis than had previously been believed possible may be caused by landslides, explosive volcanic eruptions (e.g., Santorini and Krakatau), and impact events when they contact water. These phenomena rapidly displace large water volumes, as energy from falling debris or expansion transfers to the water at a rate faster than the water can absorb. The media dub them megatsunami.
Tsunamis caused by these mechanisms, unlike the trans-oceanic tsunami, may dissipate quickly and rarely affect distant coastlines due to the small sea area affected. These events can give rise to much larger local shock waves(solitons), such as the landslide at the head of Lituya Bay 1958, which produced a wave with an initial surge estimated at 524 metres (1,720 ft). However, an extremely large landslide might generate a megatsunami that can travel trans-oceanic distances, although there is no geological evidence to support this hypothesis.
Earthquake-generated tsunami
An earthquake may generate a tsunami if the quake:
- occurs just below a body of water,
- is of moderate or high magnitude, and
- displaces a large-enough volume of water.
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Characteristics
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).[10] 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 shoalingcompresses 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. [11] 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.[11] 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.[12]
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.
Drawback
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.
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.
In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other eastern coasts it reached. This was because the wave moved downwards on the eastern side of the fault line and upwards on the western side. The western pulse hit coastal Africa and other western areas.
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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. This is deduced through the calculation:
where
P = the overlying pressure in Newtons per metre square,
ρ = the density of the seawater= 1.1 x 103 kg/m3,
g = the acceleration due to gravity= 9.8 m/s2 and
h = the height of the water column in metres.
P = the overlying pressure in Newtons per metre square,
ρ = the density of the seawater= 1.1 x 103 kg/m3,
g = the acceleration due to gravity= 9.8 m/s2 and
h = the height of the water column in metres.
Hence for a water column of 5,000 m depth the overlying pressure is equal to
or about 5500 tonnes per metre square.[citation needed]
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.
As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat for all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is currently being installed in the Indian Ocean.
Computer models can predict tsunami arrival—predicted arrival times are usually within minutes of the actual time. Bottom pressure sensors relay information in real time and based upon the pressure readings and other seismic information and the seafloor's shape (bathymetry) and coastal topography, the modesl estimate the amplitude and surge height of the approaching tsunami. All Pacific rim countries collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures. In Japan such preparation is mandatory for government, local authorities, emergency services and the population.
Some zoologists hypothesise that some animal species have an ability to sense subsonicRayleigh waves from an earthquake or a tsunami. If correct, monitoring their behavior could provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and is not widely accepted. There are unsubstantiated claims about the Lisbon quake that some animals escaped to higher ground, while many other animals in the same areas drowned. The phenomenon was also noted by media sources in Sri Lanka in the 2004 Indian Ocean earthquake.[13][14] It is possible that certain animals (e.g., elephants) may have heard the sounds of the tsunami as it approached the coast. The elephants reaction was to move away from the approaching noise. Some humans, on the other hand, went to the shore to investigate and many drowned as a result.
It is not possible to prevent a tsunami. However, in some tsunami-prone countries someearthquake engineering measures have been taken to reduce the damage caused on shore. Japan built many tsunami walls of up to 4.5 metres (15 ft) to protect populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunami. However, their effectiveness has been questioned, as tsunami often overtop the barriers. For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island ofHokkaidō within two to five minutes of the earthquake on July 12, 1993 created waves as much as 30 metres (98 ft) tall—as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it did not prevent major destruction and loss of life.[15]
Natural factors such as shoreline tree cover can mitigate tsunami effects. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed because trees such as coconut palms and mangroves absorbed the tsunami's energy. In one striking example, the village of Naluvedapathy in India's Tamil Nadu region suffered only minimal damage and few deaths because the wave broke against a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records.[16] Environmentalists have suggested tree planting along tsunami-prone seacoasts. Trees require years to grow to a useful size, but such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than artificial barriers.
Tsunami in history
Main article: Historic tsunami
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.
2004 Indian Ocean tsunami
Main article: 2004 Indian Ocean earthquake
The 2004 Indian Ocean tsunami killed over 300,000 people with many bodies either being lost to the sea or unidentified. Some unofficial estimates have claimed that approximately 1 million people may have died directly or indirectly solely as a result of the tsunami.[citation needed]
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.
Tsunami in ancient history
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.[3][4]
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.[17]
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.[18][19]
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