tsunami - Tsunami EENS 204 Tulane University Natural...

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This page last updated on 16-Feb-2005 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Tsunami What is a Tsunami A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water. Tsunami are sometimes called "seismic sea waves", although, as we will see, they can be generated by mechanisms other than earthquakes. Tsunami have also been called "tidal waves", but this term should not be used because they are not in any way related to the tides of the Earth. Because tsunami occur suddenly, often without warning, they are extremely dangerous to coastal communities. Physical Characteristics of Tsunami All types of waves, including tsunami, have a wavelength, a wave height, an amplitude, a frequency or period, and a velocity. z Wavelength is defined as the distance between two identical points on a wave (i.e. between wave crests or wave troughs). Normal ocean waves have wavelengths of about 100 meters. Tsunami have much longer wavelengths, usually measured in kilometers and up to 500 kilometers. z Wave height refers to the distance between the trough of the wave and the crest or peak of the wave. z Wave amplitude - refers to the height of the wave above the still water line, usually this is equal to 1/2 the wave height. Tsunami can have variable wave height and amplitude that depends on water depth as we shall see in a moment z Wave frequency or period - is the amount of time it takes for one full wavelength to pass a stationary point. z Wave velocity is the speed of the wave. Velocities of normal ocean waves are about 90 km/hr while tsunami have velocities up to 950 km/hr (about as fast as jet airplanes), and thus move much more rapidly across ocean basins. The velocity of any wave is equal to Tsunami 2/16/2005 Page 1 of 8
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the wavelength divided by the wave period. V = λ /P Tsunami are characterized as shallow-water waves. These are different from the waves most of us have observed on a the beach, which are caused by the wind blowing across the ocean's surface. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength of 100 to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and wavelengths greater than 500 km. A wave is characterized as a shallow-water wave when the ratio of the water depth and wavelength is very small. The velocity of a shallow-water wave is also equal to the square root of the product of the acceleration of gravity, g, (10m/sec 2 ) and the depth of the water, d. The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wavelength, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy. For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific
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tsunami - Tsunami EENS 204 Tulane University Natural...

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