Tsunami is a Japanese word that is made of twocharacters: tsu and nami. The character tsu meansharbor, while the character nami means wave.Therefore, the original word tsunami describes largewave oscillations inside a harbor during a ‘tsunami'event. In the past, tsunami is often referred to as‘tidal wave', which is a misnomer. Tides, featuringthe rising and falling of water level in the ocean in adaily, monthly, and yearly cycle, are caused bygravitational influences of the moon, sun, and planets.Tsunamis are not generated by this kind ofgravitational forces and are unrelated to the tides,although the tidal level does influence a tsunamistriking a coastal area.The phenomenon we call a tsunami is a series ofwater waves of extremely long wavelength and longperiod, generated in an ocean by a geophysical disturbancethat displaces the water within a shortperiod of time. Waves are formed as the displacedwater mass, which acts under the influence of gravity,attempts to regain its equilibrium. Tsunamis areprimarily associated with submarine earthquakesin oceanic and coastal regions. However, landslides,volcanic eruptions, and even impacts of objects fromouter space (such as meteorites, asteroids, andcomets) can also trigger tsunamis.Tsunamis are usually characterized as shallowwaterwaves or long waves, which are different fromwind-generated waves, the waves many of us haveobserved on a beach. Wind waves of 5–20-s period(T¼time interval between two successive wavecrests or troughs) have wavelengths (l¼T2(g/2p)distance between two successive wave crests ortroughs) of c. 40–620 m. On the other hand, a tsunamican have a wave period in the range of 10 minto 1 h and a wavelength in excess of 200km in a deepocean basin. A wave is characterized as a shallowwaterwave when the water depth is less than 5% ofthe wavelength. The forward and backward watermotion under the shallow-water wave is feltedthroughout the entire water column. The shallowwater wave is also sensitive to the change of waterdepth. For instance, the speed (celerity) of a shallowwaterwave is equal to the square root of the product of the gravitational acceleration (9.81ms2) and thewater depth. Since the average water depth in thePacific Ocean is 5 km, a tsunami can travel at a speedof about 800kmh1 (500 mi h1), which is almostthe same as the speed of a jet airplane. A tsunami canmove from the West Coast of South America to theEast Coast of Japan in less than 1 day.The initial amplitude of a tsunami in the vicinity ofa source region is usually quite small, typically only ameter or less, in comparison with the wavelength. Ingeneral, as the tsunami propagates into the openocean, the amplitude of tsunami will decrease for thewave energy is spread over a much larger area. In theopen ocean, it is very difficult to detect a tsunamifrom aboard a ship because the water level will riseonly slightly over a period of 10 min to hours. Sincethe rate at which a wave loses its energy is inverselyproportional to its wavelength, a tsunami will loselittle energy as it propagates. Hence in the openocean, a tsunami will travel at high speeds and overgreat transoceanic distances with little energy loss.As a tsunami propagates…
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