Tsunami- Historical and Scientific Background
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded and the last was in March 11, 2011. Owing to the immense volumes of water and the high energy involved, tsunamis can distroy coastal regions. Earthquakes, volcanic eruptions , underwater explosions, landslides glacier calvings , other mass movements, etc can make a tsunami.
Etymology and history
The term tsunami comes from the Japanese, composed of the two words tsu meaning “harbor” and nami, meaning “wave”. 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 harbors, use of the term tidal wave is discouraged by geologists and oceanographers.
As early as 426 B.C. the Greek historian Thucydides referred about tsunami in his book History of the Peloponnesian War and was the first to argue that ocean earthquakes must be the cause. The Roman historian 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. Other devastating tsunamis include one that took place in 1883, after Krakatoa erupted. Waves up to 30 meters high caused some 36,000 deaths. In Japan, in 1896, a wave that reached a height of about 20 meters killed about 26,000. In 1755, Portugal was hit by a tsunami. More than 100,000 people were killed.
While Japan may have the longest recorded history of tsunamis, the sheer destruction caused by the 2004 tsunami was the life 2,30,000 people. In 2004 December 25, an earthquake shook the ocean floor in the Indian Ocean near Indonesia. The resulting tsunami killed more than 200,000 people in Indonesia, Sri Lanka, Thailand, and India. Waves reached a height of 20 meters.
On March 11, 2011, an earthquake off the coast of Japan, measuring 8.9 on the Richter scale, created a tsunami with waves of over 10 meters high. Millions of households were left without electricity or water, over 100,000 buildings were destroyed, and several nuclear reactors were damaged. The leakage from explosions within the nuclear reactor plants caused the evacuation of residents from within 17 kilometres from the area. It is considered the most costly natural disaster to date. Japan is not yet recovered from this attack.
Mechanism of Tsunami
The cause of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually due to earthquakes, landslides, volcanic eruptions, glacier calvings or nuclear tests. The waves formed in this way are then sustained by gravity.
Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. When earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Movement on normal faults will also cause displacement of the seabed, but the size of such events is normally too small to give rise to a significant tsunami. Tsunamis have a small amplitude in offshore, and a very long wavelength which is why they generally pass unnoticed at sea. They grow in height when they reach shallower water.
In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could be caused by giant landslides. 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. Their existence was confirmed in 1958, when a giant landslide in Lituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 1700 feet. The wave didnt travel far, as it struck land almost immediately. Scientists named these waves as mega tsunami.
Tsunamis cause damage by two mechanisms. (1) The smashing force of a wall of water travelling at high speed. (2) The destructive power of a large volume of water draining off the land by carrying everything with it.
While everyday wind waves have a wavelength of about 100 metres and a height of roughly 2 metres, a tsunami in the deep ocean has a wavelength of about 200 kilometres. Such a wave travels at well over 800 kilometres per hour. 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 . Its wavelength diminishes to less than 20 kilometers and its amplitude grows enormously. When the tsunamis 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. If the first part of a tsunami to reach land is a trough, the water along the shoreline recedes dramatically, exposing normally submerged areas. This is called Drawback
Intensity and Magnitude scales
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 Ocean. The latter scale was modified by Soloviev, who calculated the Tsunami intensity I according to the formula
[pic] Hav is the average wave height, along the nearest coast. This scale, known as the Soloviev-Imamura tsunami intensity scale, is used in the global tsunami.
The first scale that genuinely calculated a magnitude for a tsunami, at a particular location was the ML scale proposed by Murty & Loomis based on the potential energy. It is rarely used. Abe introduced the tsunami magnitude scale Mt, calculated from,
[pic] where h is the maximum tsunami wave amplitude (in m) measured by a tide gauge at a distance R from the epicenter, a, b & D are constants used to make the Mt scale match as closely as possible with the moment magnitude scale.
Warnings and predictions
A tsunami that is generated from close-by can reach the shore in less than ten minutes. This does not allow authorities time to issue a warning. The only warning might be movement in the ground, which could alert people close to the shore that a tsunami is imminent. Areas at greatest risk are usually within 1.6 km of the shoreline and less than 25 above sea level. Since the tsunami arrives as a series of waves, the danger exists even after the first wave hits. Often, subsequent waves may be more dangerous than the first one.
Drawbacks can serve as a brief warning. People who observe drawback can survive only if they immediately run for high ground A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, and seismologists analyse each earthquake and based on 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. In Japan, the community is well-educated about earthquakes and tsunamis, and along the Japanese shorelines the tsunami warning signs are reminders of the natural hazards together with a network of warning sirens, typically at the top of the cliff of surroundings hills. The Pacific Tsunami Warning System is based in Honolulu, Hawai. It monitors Pacific Ocean seismic activity. 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.
Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors relay information in real time. Based on these pressure readings and other seismic information and the seafloors shape and coastal topography, the models 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 believe that some animal species have ability to sense subsonic Rayleigh 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. It is possible that certain animals may have heard the sounds of the tsunami as it approached the coast. The elephants reaction was to move away from the approaching noise. By contrast, some humans went to the shore to investigate and many drowned as a result.
Stopping the tsunami is impossible, but proper warnings can save many life.
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