1) ON 26TH DEC,2004, CHENNAI
2) CAUSES OF TSUNAMI

3) Widespread Damage of the coast of MeluabohSource

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4) THE DEVASTATED MARINA BEACH IN CHENNAI AFTER INDIAN OCEAN TSUNAMI

MEANING:

Tsunami is a series of water waves (called a tsunami wave train) that is caused when a large volume of a body of water, such as an ocean, is rapidly displaced. The Japanese term is literally translated into "harbor wave."

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. Due to the immense volumes of water and energy involved, the effects of tsunamis can be devastating.

The Greek historian Thucydides was the first to relate tsunami to submarine quakes, but understanding of the nature of tsunami 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 be several metres above normal tide levels. This is due to the low atmospheric pressure within the centre of the depression. As these storm surges come ashore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge inundated Burma (Myanmar) in May 2008.

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). This 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. Their passage usually goes unnoticed by ships.

As the tsunami approaches the coast and the waters become shallow, the wave is compressed due to wave shoaling and its forward travel 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 a wavelength on the order of several km (a few miles), the tsunami may take minutes to ramp up to full height, with victims seeing a massive deluge of rising ocean rather than a cataclysmic wall of water. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep breaking front.

Although there is no calculated volume of water that constitutes a standardized quantity that will generate a tsunami, the amount displaced must be significant enough to create waves underwater in the vast ocean. Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples on a pond. The larger the displacement, the larger the wave generated.

CAUSES OF TSUNAMI:

A tsunami can be generated by four general ways:

(1) an undersea earthquake;

(2) landslide;

(3) volcanic eruption;

(4) an extraterrestrial collision.

(1)An Undersea Earthquake :

It is the most common form of tsunami formation, typically generating the most destructive tsunamis. The earth is constantly moving on large tectonic plates. When these tectonic plates move past each other, collide and/or slide under one another (subduction), an earthquake results. This is what happened with the recent tsunami that devastated Southern Asia. Here, a massive earthquake in the Indian Ocean measuring 10.0 on the Richter scale jolted the seabed causing the sudden displacement of a very large volume of water.

The earthquake temporarily produces a fluctuation in the mean sea level of a specified area. Waves quickly form as the displaced water tries to recapture equilibrium by filling the vacuum that was created. It should be noted that not all earthquakes generate tsunamis. Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.

(2)Landslides:

Resulting in rockfalls, icefalls, or underwater (submarine) landslides or slumps can generate displacement of water to create a tsunami. More often than naught, submarine landslides are often caused by earthquakes, large and small, therefore strengthening the force of an earthquake induced tsunami. The most notable example of a landslide-induced tsunami can be traced to Southern France in the 1980’s where the movement of a significant amount of earth for the construction of an airport triggered an underwater landslide, which resulted in destructive tsunami waves hitting the harbor of Thebes.

(3)Volcanic Eruption:

Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. Volcanic disturbances can generate waves by the sudden displacement of water caused by a volcanic explosion, by a volcano's slope failure, or more likely by a phreatomagmatic explosion and collapse and/or engulfment of the volcanic magmatic chambers. The majority of tsunamis that occur in the Pacific Ocean happen around the “Ring of Fire” Area surrounding the Hawaiian Islands. The periphery has also been dubbed the 'Ring of Fire' because of the extraordinarily high number of active volcanoes and seismic activity located in the region.

Since 1819, over 40 tsunamis have struck the Hawaiian Islands. One of the largest and most destructive tsunamis ever recorded was generated in August 26, 1883 after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia. This explosion generated waves that reached 135 feet, destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing 36, 417 people.

(4)Extraterrestrial Collision :

Tsunamis caused by extraterrestrial collision (i.e. asteroids, meteors) are an extremely rare occurrence.

Although no meteor/asteroid induced tsunami have been recorded in recent history, scientists realize that if these celestial bodies should strike the ocean, a large volume of water would undoubtedly be displaced to cause a tsunami. Scientists have calculated that if a moderately large asteroid, 5-6 km in diameter, should strike the middle of the large ocean basin such as the Atlantic Ocean, it would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. On both sides of the Atlantic, coastal cities would be washed out by such a tsunami.

An asteroid 5-6 kilometers in diameter impacting between the Hawaiian Islands and the West Coast of North America, would produce a tsunami which would wash out the coastal cities on the West coasts of Canada, U.S. and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands.

EFFECTS OF TSUNAMI ON ENVIRONMENT:

Precious coral reefs and mangrove areas would have been crushed by the huge tsunami waves that have devastated southern Asia, an environmental and economic setback that could take years to reverse.

The reefs around Sri Lanka and Phuket have been severely damaged due to them having to bear the brunt of the forceful walls of water. When the waves get close to shore, their height is amplified and they release all their energy, decimating everything in their paths. The atolls of the alluring Maldives and the southern Thai islands (including Mangrove areas that act as nursery habitats to fish and shrimp) were also destroyed by the strong waves.

According to scientists, reef-forming coral grows only about 0.5 cm, or 1/5 inch a year, thus for the seaside resorts on the numerous affected islands to regain their previous splendor could take several years to a decade. The worst marine damage was likely to have been concentrated 100m to 1km from shore. Fortunately, large sea mammals such as whales and dolphins probably suffered little impact.

According to Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO), dolphins can feel disturbances happening in the water and would have most likely headed for deep water where they would be safe. Also, they mostly inhabit the areas far offshore, where the tsunami has the least damaging capacity.

WARNING & PREVENTION:

A tsunami cannot be prevented or precisely predicted—even if the right magnitude of an earthquake occurs in the right location. Geologists,Oceanographers and Seismologist 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 there are many systems being developed and in use to reduce the damage from tsunami.

One of the most important systems that is used and constantly monitored are bottom pressure sensors. These are anchored and attached to buoys. Sensors on the equipment constantly monitor the pressure of the overlying water column—this can be deduced by the simple calculation of:

P = pgh
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 andh = the height of the water column in metres.
Hence for a water column of 5,000 m depth the overlying pressure is equal toor about 5.7 Million tonnes per metre square.
In instances where the leading edge of the tsunami wave is the trough, the sea will recede from the coast half of the wave’s period before the wave’s arrival. If the slope of the coastal seabed is shallow, this recession can exceed many hundreds of meters. People unaware of the danger may remain at or near the shore out of curiosity, or for collecting fish from the exposed seabed.

During the Indian Ocean tsunami of December 26, 2004, the sea withdrew and many people then went onto the exposed sea bed to investigate. Pictures taken show people on the normally submerged areas with the advancing wave in the background. Most people who were on the beach were unable to escape to high ground and died.

Regions with a high risk of tsunami may use tsunami warning systems to detect tsunami and warn the general population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs advise people of evacuation routes.

The Pacific Tsunami Warning System is based in Honolulu. It monitors all sesimic activity that occurs anywhere within the Pacific. Based up the magnitude and other information a tsunami warning may be issued. It is important to note that the subduction zones around the Pacific are seismically active, but not all earthquakes generate tsunami and for this reason computers are used as a tool to assist in analysing the risk of tsunami generation of each and 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 of 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—observations have shown that predicted arrival times are usually within minutes of the actual time. Bottom pressure sensors are able to relay information in real time and based upon the readings and other information about the seismic event that triggered it and the shape of the seafloor (bathymetry) and coastal land (topography), it is possible to estimate the amplitude and therefore the surge height, of the approaching tsunami.

All the countries that border the Pacific Ocean collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures to prepare people for the inevitable tsunami. In Japan such preparation is a mandatory requirement of government, local authorities, emergency services and the population.
Some zoologists hypothesise that animals may have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. Some animals seem to have the ability to detect natural phenomena and if correct, careful observation and monitoring could possibly provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and has not been proven scientifically.

There are some unsubstantiated claims that animals before the Lisbon quake were restless and moved away from low lying areas to higher ground. Yet 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 (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—inland. 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 some earthquake engineering measures have been taken to reduce the damage caused on shore. Japan has implemented an extensive programme of building tsunami walls of up to 4.5 m (13.5 ft) high in front of 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 surge higher than the barriers.

For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake on July 12, 1993 created waves as much as 30 m (100 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.

The effects of a tsunami may be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami’s energy being absorbed by trees such as coconut palms and mangroves. In one striking example, the village of Naluvedapathy in India’s Tamil Nadu region suffered minimal damage and few deaths as the wave broke up on a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records. Environmentalists have suggested tree planting along stretches of seacoast which are prone to tsunami risks. It would take some years for the trees to grow to a useful size, but such plantations could offer a much cheaper and longer-lasting