|  | | | | | | For many people, tsunami is a strange word. Until 26, 2004, when the South Asian coast experienced tsunamis, most people in the region were ignorant about it. The high level of unfamiliarity is illustrated in a story. On that fatal day morning, an overseas government informed a minister in a South Asian nation that tsunami is coming to his country. He hurried off his staff to the airport to receive Ms. TSunami!! As the staff waited there, tsunamis washed off the coasts of his nation. Tsunami (pronounced soo-nah-mee) is a Japanese word, meaning harbor wave. It is composed to two characters “tsu” meaning harbor and “nami” meaning wave. Tsunamis are a series of highly powerful sea waves produced by sudden displacement of the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies can generate tsunamis. Tsunamis can devastate cost lines, damage property, kill people and wash off all that stands on its way.
| | | | Not Just Waves | | In the past, people called tsunamis "tidal waves" and scientific community termed it as "seismic sea waves." But both are incorrect in relation to a tsunami. Tides result from gravitational influences of the moon, sun, and planets. A tsunami has no relation to such tides. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event such as a landslide. Tsunamis are not just huge waves. Wind-generated waves and tsunamis are vastly different in their nature and power. Wind-generated waves, you can see in any beach rhythmically rolling in, one after another. The time gap between each of them might be roughly 10 seconds and have wavelengths of 150 meter. A tsunami, on the other hand, can have a wavelength of more than 100 kilometers and can happen after long gaps some times even an hour. In other words, tsunamis not only move at high speeds; they can also travel great distances with limited energy losses. | | | | How can earthquakes generate tsunamis? | | We all are now familiar with the facts of Dec. 26 tsunamis that hit Indian Ocean coast. It started after an under sea earthquake near Indonesia. But how can earthquake generate such devastating tsunamis on coastlines thousands of kilometers away. Tsunamis can happen when the seafloor suddenly caves in and displaces the overlying water. Certain kind of earthquakes called tectonic earthquakes can deform earth crust. When such quake happens beneath the sea, water balance changes. Imagine this. Kilometers of seafloor suddenly change shape, throwing up or pulling down large area of seawater. The water mass then acts to gain balance because of the gravitational influence. When all these happen in a matter of seconds, huge energy is released into water creating tsunamis. In the same way landslides, volcanic eruptions, and cosmic collisions could generate tsunamis. Landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column creating tsunamis. In other words, any disturbance that abruptly changes the shape seafloor and displaces water in a large area could cause tsunamis. | | | | What happens to a tsunami as it approaches land? | | As a tsunami moves from deep water to shallower water near the coast, it transforms. The speed of a tsunami is related to the water depth. Therefore, as the water depth decreases, the tsunami slows. But its energy remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into beach, its height grows. A tsunami may not be even noticeable at deep sea but it will grow as high as 30 meters or more when it nears the coast. As a tsunami approaches shore, part of the energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis can strip beaches of sand that may have taken years to accumulate, uproot trees and other coastal vegetation. They are capable of inundating, or flooding, hundreds of meters inland. The unexpected and fast moving water can crush homes and other coastal structures creating great loss of life.
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