Tsunamis

Tsunami - tsoonäˈmē, series of catastrophic ocean waves generated by submarine movements, which are caused by earthquakes, volcanic eruptions, landslides beneath the ocean, or an asteroid striking the earth. In the open ocean, tsunamis may have wavelengths of up to several hundred miles and travel at speeds up to 450 mi per hr (720 km per hr), yet have wave heights of less than 3 ft (1 m), which pass unnoticed beneath a ship at sea. The period between crests of tsunami waves varies from 5 min to about 1 hr. When tsunami approach shallow water along a coast, they are slowed, causing their length to shorten and their height to rise sometimes as high as 100 ft (30 m). When they break, they often destroy piers, buildings, and beaches and take human life. The wave height as they crash upon a shore depends almost entirely upon the submarine topography offshore. Waves tend to rise to greater heights along gently sloping shores, along submarine ridges, or in coastal embayments. There is little warning of approach; when a train of tsunami waves approaches a coastline, the first indication is often a sharp swell, not unlike an ordinary storm swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast. After several minutes, the first huge wave crest strikes, inundating the newly exposed beach and rushing inland to flood the coast. Generally, the third to eighth wave crests are the largest. Since tsunami principally occur in the Pacific Ocean following shallow-focus earthquakes over magnitude 6.5 on the Richter scale, one of the best means of prediction is the detection of such earthquakes on the ocean floor with a seismograph network (see seismology). Tsunamis may be detected by wave gauges, such as those emplaced as part of the Tsunami Warning System operating in the Pacific regions. Measurement of sudden sea level changes from satellites are also used to warn of potential tsunami. Probably the most destructive tsunami occurred following the explosive eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883, when over 36,000 people were killed as a result of the wave. Waves were up to 100 ft (30 m) high with speeds between 350 and 450 mi per hr (560–720 km per hr). Its passage was traced as far away as Panama. It is believed that a 0.6-mi-wide (1-km-wide) asteroid that struck the ocean SW of New Zealand about a.d. 1500 created tsunamis that reached heights of more than 425 ft (130 m).

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