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    Tsunami 2

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    ben

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    Tsunami 2

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    ben
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    Tsunami 2

    Uploaded by

    ben

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    © All Rights Reserved
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    of 2

    What is a tsunami?

    A tsunami is a catastrophic ocean wave, usually caused by a submarine earthquake, an


    underwater or coastal landslide, or a volcanic eruption. The term tidal wave is frequently
    used for such a wave, but it is a misnomer, for the wave has no connection with the tides.

    Origin and development

    After an earthquake or other generating impulse occurs, a train of simple, progressive


    oscillatory waves is propagated great distances over the ocean surface in ever-widening
    circles, much like the waves produced by a pebble falling into a shallow pool. In deep
    water, a tsunami can travel as fast as 800 km (500 miles) per hour. The wavelengths are
    enormous, sometimes exceeding 500 km (about 310 miles), but the wave amplitudes
    (heights) are very small, only about 30 to 60 cm (1 to 2 feet). The waves’ periods (the
    lengths of time for successive crests or troughs to pass a single point) are very long,
    varying from five minutes to more than an hour. These long periods, coupled with the
    extremely low steepness and height of the waves, enable them to be completely obscured
    in deep water by normal wind waves and swell. A ship on the high seas experiences the
    passage of a tsunami as an insignificant rise and fall of only half a meter (1.6 feet), lasting
    from five minutes to an hour or more.
    As the waves approach the coast of a continent, however, friction with the rising sea
    bottom reduces the velocity of the waves. As the velocity lessens, the wavelengths
    become shortened, and the wave amplitudes (heights) increase. Coastal waters may rise
    as high as 30 meters (about 100 feet) above normal sea level in 10 to 15 minutes. The
    continental shelf waters begin to oscillate after the rise in sea level. Between three and
    five major oscillations generate most of the damage, frequently appearing as powerful
    “run-ups” of rushing water that uproot trees, pull buildings off their foundations, carry
    boats far inshore, and wash away entire beaches, peninsulas, and other low-lying coastal
    formations. Frequently the succeeding outflow of water is just as destructive as the run-
    up or even more so. In any case, oscillations may continue for several days until the
    ocean surface reaches equilibrium.
    Much like any other water waves, tsunamis are reflected and refracted by the topography
    of the seafloor near shore and by the configuration of a coastline. As a result, their effects
    vary widely from place to place. Occasionally, the first arrival of a tsunami at a coast may
    be the trough of the wave, in which case the water recedes and exposes the shallow
    seafloor. Such an occurrence took place in the bay of Lisbon, Portugal, on November 1,
    1755, after a large earthquake; many curious people were attracted to the bay floor, and
    a large number of them were drowned by the wave crest that followed the trough only
    minutes later.
    What have been some of the worst tsunamis in history?
    Perhaps the most destructive tsunami in recorded history was the Indian Ocean Tsunami
    of 2004. A 9.1-magnitude earthquake occurred off the coast of Sumatra in Indonesia.
    Waves as high as 30 feet (9 meters) struck the eastern coasts of India and Sri Lanka—
    some 750 miles (1,200 km) away—and traveled more than 1,800 miles (3,000 km) to East
    Africa. The final death toll was at least 225,000, mostly in Indonesia, Thailand, India, and
    Sri Lanka. The affected countries also reported extensive economic and infrastructural
    damage.

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