TSUNAMI

     The word “tsunami” comes from the Japanese language meaning Great Waves (Tsu: port and Nami: wave). Adapan agreed definition many people is a tsunami is a natural disaster caused by rising sea waves to the mainland at high speeds due to the earthquake centered under the sea. The quake could have been caused by soil erosion, the plates were shifting, volcanic eruptions and who experienced meteor that crashed in the ocean. This usually happens when the tsunami earthquakes exceeding magnitude 7 on the Richter scale. Tsunami is quite dangerous, especially for those who live in Area beach. With great power, he will sweep everything in its path.

     When talking about the tsunami, then we would have to start from the cause, namely the earthquake in the ocean. Tsunami always preceded a powerful movement that we commonly call the earthquake. Although it is known that there are various types of this earthquake, but 90% of the tsunami caused by the movement of the plates in the earth that happened to be located in areas of the oceans. It should be also mentioned, the history of tsunamis ever recorded due to the eruption of Krakatoa.

     An earthquake in the bowels of the earth will result in the emergence of pressure to the vertical direction so that the bottom of the sea will rise and fall in a short time span. This will then lead to an imbalance in the ocean water which is then pushed into a large wave that moves reached the mainland.

     With great power there in the water waves, naturally if the buildings on the mainland can be washed away easily. The tsunami waves propagate at speeds unimaginable. He could reach 500 to 1000 kilometers per hour at sea. And when it reaches the shore, the speed is reduced to 50 to 30 kilometers per hour. Although reduced rapidly, but the pace was able to cause severe damage to humans.

     If we look at the tsunami, of course we understand that there is no human intervention in it. Thus, we do not have control to prevent these causes. However, the preparation and maximum vigilance, we can minimize the impact of the tsunami itself. A good example has been shown in Japan. Although prone to tsunamis, but the awareness of people able to reduce the number of victims of the disaster.

CHARACTERISTICS

     Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with waves that do not appear to be large.

     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 much larger wavelength of up to 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but owing 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).^[33] This makes tsunamis difficult to detect over deep water, where ships are unable to feel their passage.

     The velocity of a tsunami can be calculated by obtaining the square root of the depth of the water in meters multiplied by the acceleration due to gravity (approximated to 10 m sec²). For example, if the Pacific Ocean is considered to have a depth of 5000 meters, the velocity of a tsunami would be the square root of √5000 x 10 = √50000 = ~224 meters per second (735 feet per second), which equates to a speed of ~806 kilometers per hour or about 500 miles per hour. This formula is the same as used for calculating the velocity of shallow waves, because a tsunami behaves like a shallow wave as it peak to peak value reaches from the floor of the ocean to the surface.

     The reason for the Japanese name "harbour wave" is that sometimes a village's fishermen would sail out, and encounter no unusual waves while out at sea fishing, and come back to land to find their village devastated by a huge wave.

     As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its speed decreases below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore. 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. 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.

     About 80% of tsunamis occur in the Pacific Ocean, but they are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, glacier calvings, and bolides.

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 east-facing coasts that 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.

     A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, andseismologists 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, attached to buoys, which 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.^[40]

     The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi. 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 being installed in the Indian Ocean.

 One of the deep waterbuoys used in the DARTtsunami warning system

Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors can relay information in real time. Based on these pressure readings and other seismic information and the seafloor's shape (bathymetry) 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 hypothesise that some animal species have an ability to sense subsonic Rayleigh wavesfrom 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 (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. By contrast, some humans went to the shore to investigate and many drowned as a result.

Along the United States west coast, in addition to sirens, warnings are sent on television and radio via the National Weather Service, using the Emergency Alert System.