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tsunami (tso͝onäˈmē), series of catastrophic ocean waves generated by submarine movements, which may be caused by earthquakes, volcanic eruptions, landslides beneath the ocean, or an asteroid striking the earth. Tsunamis are also called seismic sea waves or, popularly, tidal waves.
In the open ocean, tsunamis may have wavelengths of up to several hundred miles and travel at speeds up to 500 mi per hr (800 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 the crests of a tsunami's waves varies from 5 min to about 1 hr. When tsunamis 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. Tsunamilike waves can also occur on lakes and within inlets and small bays as a result of large landslide into the water or an underwater landslide.
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 tsunamis 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 and pressure monitors, such as those emplaced as part of the U.S. Tsunami Warning System; established in 1949 and originally confined to the Pacific region, the system has been expanded to the Caribbean and the W North Atlantic. An early warning system for the Indian Ocean began operating in 2006. Measurement of sudden sea level changes from satellites are also used to warn of a potential tsunami.
One of the most destructive tsunamis to occur during historical times followed 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. Its passage was traced as far away as Panama. On Dec. 26, 2004, a 9.1 earthquake off NW Sumatra, Indonesia, caused a tsunami with waves as high as 65 ft (20 m) nearest the epicenter. Some 230,000 people are believed to have died. The waves devastated many areas in the E Indian Ocean basin, particularly the nearby coast of N Sumatra, the Andaman and Nicobar Islands, and the E and S coasts of Sri Lanka. Areas of SE India and SW Thailand were also hard-hit. The 9.0 earthquake off NE Honshu, Japan, on Mar. 11, 2011, caused a tsunami that devastated nearby areas on the Honshu coast. The water overtopped 33 ft (10 m) seawalls and in some locations reached places as far as 5 mi (8 km) inland. Most of the nearly 18,500 killed or missing as a result of the earthquake were lost to the tsunami. 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 a tsunami that reached heights of more than 425 ft (130 m).
(also seismic sea wave), a very high gravitational sea wave that arises as a result of upward or downward (vertical) dislocations of protracted segments of the ocean bottom during strong submarine and coastal earthquakes or, sometimes, as a result of volcanic eruptions and other tectonic processes. Because of the low compressibility of water and the rapidity of deformation of the ocean bottom, the column of water directly above is also shifted without having time to spread, which causes a certain rise or drop on the surface of the ocean. The resulting disturbance is transformed into oscillatory movements of the water known as tsunamis. Such waves travel at great speeds (50–1,000 km/hr), which are proportional to the square root of the depth of the sea. The distance between two adjacent crests varies from 5 to 1,500 km. The height of the waves in the vicinity of the tsunami’s origin varies between 0.01 and 5 m, sometimes reaching 10 m along the coast; in places where the topography is unfavorable, for example, embayments and river valleys, the waves can rise to more than 50 m. The maximum velocity of the changing currents that accompany tsunamis is more than 20 km/hr.
About 1,000 occurrences of tsunami are known, of which more than 100 had disastrous consequences, causing total destruction and carrying away structures and the soil and vegetation, for example, the tsunamis along the coast of Japan in 1933 and Kamchatka in 1952.
Eighty percent of the tsunamis arise on the periphery of the Pacific Ocean, including the western slope of the Kuril-Kamchatka Trench. Coastal regions are zoned according to the degree of tsunami danger based on the regularities of occurrence and propagation of tsunamis. Partial protection against tsunamis can be achieved by the construction of coastal structures, such as breakwaters and embankments, and by the planting of wooded belts along the coast. To alert the public of approaching tsunamis, the United States, Japan, and the USSR formed tsunami warning systems in the 1940’s and 1950’s based on the early detection of earthquakes by coastal seismographs.
REFERENCESPoniavin, I. D. Volny tsunami. Leningrad, 1965.
Problema tsunami. [Collection of articles.] Moscow, 1968.
Volny tsunami. Iuzhno-Sakhalinsk, 1972–73. (Trudy Sakhalinskogo kompleksnogo n.-i. in-ta, fases. 29–32.)
Solov’ev, S. L., and Ch. N. Go. Katalog tsunami na Zapadnom poberezh’e Tikhogo okeana. Moscow, 1975.
Tsunamis in the Pacific Ocean. Honolulu, 1970.
Tsunami Research Symposium 1974. Wellington-Paris, 1976.
S. L. SOLOV’EV
What does it mean when you dream about a tsunami?
Because water often represents the emotions, a tidal wave may indicate a billowing emotional situation that must be faced and handled.