|SCIENCE & TECHNOLOGY|
killer called tsunami
The killer called tsunami
THE word "tsunami" might have been only a tongue- twister for most Indians till December 26, but today it is the most dreaded word for not only the victims of the killer wave but also everyone who cares about the misery of others.
It may have been a stranger for India but destructive tsunamis have occurred in all of the world’s oceans and seas. In the last half of the 20th century, Pacific-wide, destructive tsunamis occurred in 1946, 1952, 1957, 1960, and 1964. Many more tsunamis occurred in inland seas around the periphery of the Pacific. These were extremely destructive locally and claimed thousands of lives. Such localised tsunamis occurred in 1975, 1983, 1985, 1992, 1993, 1995, 1998, 1999 and 2001. But none of them matched the fury of the 2004 killer.
The International Tsunami Information Centre, based in Honolulu, Hawai, has been educating the public about the threat from tsunami for the past many years. The write-ups on this page are an abridged version of the information provided by the centre.
Tsunami is a series of large waves of extremely long wavelength and period usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the ocean. When a sudden displacement of a large volume of water occurs, or if the sea floor is suddenly raised or dropped by an earthquake, big tsunami waves can be formed by forces of gravity. The waves travel out of the area of origin and can be extremely dangerous and damaging when they reach the shore.
The word tsunami (pronounced tsoo-nah’-mee) is composed of the Japanese words "tsu" (which means harbor) and "nami" (which means "wave"). Often the term, "seismic or tidal sea wave" is used to describe the same phenomenon, however the terms are misleading, because tsunami waves can be generated by other, non-seismic disturbances such as volcanic eruptions or underwater landslides, and have physical characteristics different of tidal waves.
The tsunami waves are completely unrelated to the astronomical tides — which are caused by the extraterrestrial, gravitational influences of the moon, sun, and the planets. Thus, the Japanese word "tsunami", meaning "harbour wave" is the correct, official and all-inclusive term. It has been internationally adopted because it covers all forms of impulsive wave generation.
By far, the most destructive tsunamis are generated from large, shallow earthquakes with an epicentre or fault line near or on the ocean floor.
These usually occur in regions of the earth characterised by tectonic subduction along tectonic plate boundaries. The high seismicity of such regions is caused by the collision of tectonic plates. When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as a 1,000 km or more. The sudden vertical displacements over such large areas, disturb the ocean’s surface, displace water, and generate destructive tsunami waves. The waves can travel great distances from the source region, spreading destruction along their path.
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.
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. According to this mechanism, waves may be generated 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/engulfment of the volcanic magmatic chambers.
Less frequently, tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Such events may be caused impulsively from the instability and sudden failure of submarine slopes, which are sometimes triggered by the ground motions of a strong earthquake.
Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from the source and across the ocean, or within an enclosed or semi-enclosed body of water — such as a lake or a fjord.
However, It should be noted, that the largest tsunami wave ever observed anywhere in the world was caused by a rock fall in Lituya Bay, Alaska on July 9, 1958. Triggered by an earthquake along the Fairweather fault, an approximately 40 million cubic meter rock fall at the head of the bay generated a wave, which reached the incredible height of 520-meter wave (1,720 feet) on the opposite side of the inlet.
Tsunamis are disasters that can be generated in all of the world’s oceans, inland seas, and in any large body of water. Each region of the world appears to have its own cycle of frequency and pattern in generating tsunamis that range in size from small to the large and highly destructive events. Most tsunamis occur in the Pacific Ocean and its marginal seas. The reason is that the Pacific covers more than one-third of the earth’s surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire" - where most earthquakes occur (off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America). Many tsunamis have also been generated in the seas which border the Pacific Ocean.
Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the ocean’s depth. A tsunami is made up of a series of very long waves. The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond.
The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event. If the tsunami is generated from a large earthquake over a large area, its initial wavelength and period will be greater.
Fortunately, for mankind, it is indeed very rare for a meteorite or an asteroid to reach the earth. No asteroid has fallen on the earth within recorded history.
Most meteorites burn as they reach the earth’s atmosphere.
However, large meteorites have hit the earth’s surface in the distant past.
The fall of meteorites or asteroids in the earth’s oceans has the potential of generating tsunamis of cataclysmic proportions.
Scientists studying this possibility have concluded that the impact of moderately large asteroid, 5-6 km in diameter, in the middle of the large ocean basin such as the Atlantic Ocean, would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States.
Conceivably tsunami waves can also be generated from very large nuclear explosions.
However, no tsunami of any significance has ever resulted from the testing of nuclear weapons in the past. Furthermore, such testing is presently prohibited by international treaty.
In the deep ocean, tsunami wave amplitude is usually less than 1 m (3.3 feet). The crests of tsunami waves may be more than a hundred kilometers away from each other. Therefore, passengers on boats at sea, far away from shore where the water is deep, will not feel nor see the tsunami waves as they pass by underneath at high speeds. The tsunami may be perceived as nothing more than a gentle rise and fall of the sea surface.
For the same reason of low amplitude and very long periods in the deep ocean, tsunami waves cannot be seen nor detected from the air. From the sky, tsunami waves cannot be distinguished from ordinary ocean waves.
Tsunamis arrive at a coastline as a series of successive crests (high water levels) and troughs (low water levels) — usually occurring 10 to 45 minutes apart. As they enter the shallow waters of coastlines, bays, or harbours, their speed decreases to about 50-60 km/hr. For example, in 15 m of water the speed of a tsunami will be only 45 km/hr.
However 100 or more km away, another tsunami wave travels in deep water towards the same shore at a much greater speed, and still behind it there is another wave, travelling at even greater speed.
As the tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy is directed upward — thus increasing their heights considerably.
Just as with ordinary surf, the energy of the tsunami waves must be contained in a smaller volume of water, so the waves grow in height. Even though the wavelength shortens near the coast, a tsunami will typically have a wavelength in excess of 10 kilometers when it comes ashore.
Depending on the water depth and the coastal configuration, the waves may undergo extensive refraction — another process that may converge their energy to particular areas on the shore and thus increase the heights even more.
On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers. In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more — again depending on the generating source. Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be. For example, at the deepest ocean depths the tsunami wave speed will be as much as 800 km/hr, about the same as that of a jet aircraft.
1. All earthquakes do not cause tsunamis, but many do. When you hear that an earthquake has occurred, stand by for a tsunami emergency.
2. An earthquake in your area is a natural tsunami warning. Do not stay in low-lying coastal areas after a strong earthquake has been felt.
3. A tsunami is not a single wave, but a series of waves. Stay out of danger areas until an "all-clear" is issued by competent authority.
4. Approaching tsunamis are sometimes preceded by a noticeable rise or fall of coastal water. This is nature’s tsunami warning and should be heeded.
5. Small tsunami at one point on the shore can be extremely large a few miles away. Don’t let the modest size of one make you lose respect for all.
6. All tsunamis like hurricanes are potentially dangerous, even though they may not damage every coastline they strike.
7. Never go down to the shore to watch for a tsunami. When you can see the wave you are too close to escape it. Never try to surf a tsunami; tsunamis do not curl or break like surfing waves.
8. Homes and small buildings located in low-lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning.
9. Offshore reefs and shallow areas may help break the force of tsunami waves, but large and dangerous waves can still be a threat to coastal residents in these areas. Staying away from all low-lying coastal areas is the safest advice when there is a tsunami warning.
10. Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground.