Posted: Tuesday, March 30, 2010 by minangka in Labels:

Less than two years after the devastating Indian Ocean tsunami in 2004, Java island in Indonesia was hit once again by a tsunami on July 17. The death toll quickly rose to more than 500, while more than 200 people were missing. Tens of thousands of people were displaced because of the damage to the infrastructure, while residents made for the hills in panic as memories of the deadly 2004 tsunami flashed in their minds.

The term tsunami comes from the Japanese language and means "harbor" (tsu) and "wave" (nami). A tsunami (pronounced tsoo-NAH-mee) is a series of waves generated when a body of water, such as an ocean, is rapidly displaced on a massive scale. Earthquakes, landslides, volcanic eruptions, and large meteorite impacts all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating.

Tsunamis, unlike normal wind-generated waves, are shallow-water waves. Tsunamis have a very long wavelength — up to 100 kilometers (62 miles) — and extremely long periods — up to several hours. This allows them to travel transoceanic distances with limited energy losses and at very high speeds.

Contrary to popular belief, the idea of the tsunami as a "huge tidal wave" is not true. A tsunami actually looks like an endless fast-coming wave that breaks its way through any obstacles. The real damage caused by a tsunami is due to the huge mass of water traveling behind it. The smashing water is enough to reduce buildings to their foundations, and cars and boulders can be tossed several miles inland before the water resides. It can erode beaches that took years to form.

Out in the open water, the height of a tsunami is around 1 meter (3.3 feet), which is unnoticeable to people on ships, but because of its incredible wavelength, the wave mobilizes the ocean beneath it. The wave travels at an average speed of 800 kilometers (497 miles) an hour. As it nears the shore and the water becomes shallow, the wave slows down, causing it to pile up and increase in size. That is why, while it traveled at a height of 1 meter in the ocean, by the time it reaches the coast, a tsunami can reach a height onshore of 30 meters (98 feet), which is more than ten stories high!

How Tsunamis Are Generated

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. This happens because of shifts in the layers of the crust. When this happens, the displaced mass of water forms waves, and if a large enough mass of water is displaced, a tsunami may be formed.

Submarine landslides, as well as submarine volcanoes, can also cause tsunamis by the disturbance of the overhead column of water thus leading to the displacement of large amounts of water. Meteorites or shoreline landslides can also cause disturbance from outside that can create a tsunami. This type, however, usually dissipates quickly and is not as damaging as one created by earthquakes or underwater disturbances.

Tsunami Warning System

Tsunamis cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami and there are many systems being developed and in use to reduce the damage from tsunamis. The earliest warning system was animals. Animals seem capable of detecting danger earlier and fleeing to high ground, a few minutes or even hours before a tsunami strikes. This has led scientists to speculate on whether animals can detect the subsonic Rayleigh waves, which are a type of surface wave produced by earthquakes and subterranean movement of magna.

Also, in tsunami-prone countries, several protection systems have been used to reduce the damage from the wave. Japan, for example, has been building an extensive system of tsunami walls up to 4.5 meters (14.8 feet) high to protect heavily populated coastal regions. The effectiveness of this, however, is argued because the wave can be much higher than that. While a tsunami wall may slow down the wave and break its height a little, it might not be enough to prevent major damage and loss of life. The effect of a tsunami can also be reduced drastically by nature itself. A wall of trees in the 2004 Indian Ocean tsunami saved the Tamil Nadu region in India from much damage because the trees broke the speed and force of the wave.

Now, in light of the 2004 tsunami in the Indian Ocean, the deadliest recorded tsunami ever, UNESCO and several other world bodies called for the establishment of a global tsunami warning system that would prevent loss of life through early detection of tsunamis. The system is divided into two parts. The first is concerned with a network of sensors and detectors to determine tsunamis early, while the second component is a communication infrastructure to spread warnings early enough to allow for evacuation.

As a first step towards an international early warning program, the UN meeting in Japan agreed to form the Indian Ocean Tsunami Warning System (IOTWS), which became active late June 2006. It consists of 25 seismographic stations relaying information to 26 national tsunami information centers, as well as three deep-ocean sensors. However, UNESCO warned that further coordination between governments and the ability of the different centers to relay information to civilians were critical for the success of the system.

The Pacific Ocean

The rim of the Pacific Ocean still remains the area most prone to tsunamis. This is because around the margins of the Pacific Ocean, denser oceanic plates tend to slip under continental plates in a process called subduction. This sometimes brings the lip of the continental plate with it. Eventually, the high pressure on the lip causes it to snap back, sending shockwaves through the earth's crust that cause an underground earthquake. The resulting subduction earthquakes are particularly effective in generating tsunamis.

The Pacific Ocean and Indian Ocean alone are recorded to be the site of around eight major tsunamis in the past century, in addition to several smaller-scale tsunamis that have not been as damaging or may have gone undetected. These include tsunamis in Asia, such as the 2004 Indian Ocean tsunami, or ones in the United States such as the infamous Chilean tsunami that struck Hawaii in 1960.

The 2004 Asia tsunami has brought the world's attention to the danger posed by this devastating natural phenomenon with a death toll in the 300,000 range. Much work has gone into early warning and control to prevent the repeat of such misery. But as with all natural disasters, no protective measure is 100 percent safe. Even with UNESCO's ambitious plan of protection, much will come down to the governments and the people to prevent as much loss as possible if events are repeated.m.yahia

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