Tsunami 27
Tsunami 27 : A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600-800 kilometers per hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive. Tsunamis are large, rapidly moving ocean waves triggered by a major disturbance of the ocean floor, which is usually caused by an earthquake but sometimes can be produced by a submarine landslide or a volcanic eruption. (Heliker, 1990). Damaging earthquakes and sea waves (tsunami) may also be closely realted to volcanoes and volcanic activity. Large earthquakes related to intrusion of magma into Hawaii's active rift zones of Mauna Loa and Kilauea have caused extensive damage on land and also triggered tsunami in 1868 and 1975 that devastated low-lying coastal areas. Large landslides from Alaskan volcanoes near the sea have also genereated tsunami that destroyed coastal villages. (Wright and Pierson, 1992). Major earthquakes occurring along subduction zones are especially hazardous, because they can trigger tsunami (from the Japanese word tsunami meaning "harbor wave") and pose a potential danger to coastal communities and islands that dot the Pacific. Tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes. During a mojor earthquake, the seafloor can move by several meters and an enormous amount of water is suddenly set into motion, sloshing back and forth for several hours. Thes result is a series of waves that race across the ocean at speeds of more than 800 kilometers per hour, comparable to those of commercial jetliners. The energy and momentum of these transoceanic waves can take them thousands of kilometers from their origin before slamming into far-distant islands or coastal areas. The 1883 eruption of Krakatau Volcano, located in the Sunda Straits between the islands of Sumatra and Java, Indonesia, provides an excellent example of an eruption-caused tsunami. A series of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000 people. (Kious and Tilling, 1996)
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