TSUNAMI

also called Seismic Sea Wave, or Tidal Wave, catastrophic ocean wave, usually caused by a submarine earthquake occurring less than 50 km (30 miles) beneath the seafloor, with a magnitude greater than 6.5 on the Richter scale. Underwater or coastal landslides or volcanic eruptions also may cause a tsunami. The term tidal wave is more frequently used for such a wave, but it is a misnomer, for the wave has no connection with the tides. After the earthquake or other generating impulse, a train of simple, progressive oscillatory waves is propagated great distances at the ocean surface in ever-widening circles, much like the waves produced by a pebble falling into a shallow pool. In deep water, the wavelengths are enormous, about 100 to 200 km, and the wave heights are very small, only 0.3 to 0.6 m (1 to 2 feet). The resulting wave steepness, or ratio of height to length, ranges between 3/2,000,000 and 6/1,000,000. This extremely low steepness, coupled with the waves' long periods that vary from five minutes to an hour, enables normal wind waves and swell to completely obscure the waves in deep water. In any progressive oscillatory wave, the actual water motion at the surface consists of a vertical orbit with a diameter equal to the wave height, coming full circle during the period of the wave. Thus, a surface-water particle or a ship in the open ocean experiences the passage of a tsunami as an insignificant rise and fall of only 0.3 to 0.6 m, lasting from five minutes to an hour. The surface orbital motion of any progressive oscillatory wave is transmitted diminishingly downward through the water, becoming insignificant at a depth below the surface equal to approximately half the wavelength. Tsunamis, however, being enormously longer than even the greatest ocean depths, experience significant retardation of orbital motion near the seafloor and behave as shallow-water waves regardless of the depth of the ocean the waves are propagated across. The velocity of shallow-water waves is controlled by this friction with the bottom, obeying the formula in which c is the wave velocity, g is the acceleration of gravity, and D is water depth. This relationship was used to determine the average depth of the oceans in 1856, long before many deep-sea soundings had been taken. Assuming an average velocity for seismic sea waves of about 200 m per second (450 miles per hour), an average oceanic depth of about 4,000 m is obtained; this figure compares very well with the modern estimate of 3,808 m. The relationship has enormous practical value, enabling seismologists to issue warnings to endangered coasts immediately after an earthquake and several hours before the arrival of the tsunamis. As the waves approach the continental coasts, friction with the increasingly shallow bottom reduces the velocity of the waves. The period must remain constant; consequently, as the velocity lessens, the wavelengths become shortened and the wave amplitudes increase, coastal waters rising as high as 30 m in 10 to 15 minutes. By a poorly understood process, the continental shelf waters begin to oscillate after the rise in sea level. Between three and five major oscillations generate most of the damage; the oscillations cease, however, only several days after they begin. Tsunamis are reflected and refracted by nearshore bottom topography and coastal configurations as any other water waves. Thus, their effects vary widely from place to place. Occasionally, the first arrival of tsunami at a coast may be a trough, the water receding and exposing the shallow seafloor. Such an occurrence in Lisbon, Port., on Nov. 1, 1755, attracted many curious people to the bay floor; and a large number of them were drowned by the succeeding wave crest that arrived only minutes later. Perhaps the most destructive tsunami was the one that occurred in 1703 at Awa, Japan, killing more than 100,000 people. The spectacular underwater volcanic explosions that obliterated Krakatau (Krakatoa) Island on Aug. 26 and 27, 1883, created waves as high as 35 m in many East Indies localities, killing more than 36,000 people.