Widely spaced, fast-moving ocean wave(s) most commonly initiated by sudden displacements of the sea floor during earthquakes or submarine landslides. Volcanic eruptions can also cause tsunamis if unconsolidated volcanic sediment flows rapidly or falls into the water as in a catastrophic slope failure from a steep-sided volcanic cone or edifice, or if explosive eruptions occur at or near sea level. Tsunamis are capable of inundating significant portions of the coastline, especially if the wave energy is focused by narrowing of inlets and bays.
Tsunami is a Japanese word meaning "harbor wave." A tsunami is a wave or series of waves that are generated in a body of water by a sudden disturbance that displaces water. They are typically caused by earthquakes and landslides in coastal regions. Volcanic eruptions, nuclear explosions, and even impact of meteorites, asteroids, and comets from outer space can generate a tsunami.
Photograph courtesy of the Pacific Tsunami Museum in Hilo, Hawai`i
People run from an approaching tsunami in Hilo, Hawai'i, on 1 April 1946; note the wave just left of the man's head in right center of image (click on image for larger size).
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The powerful explosive eruption of Krakatau in Indonesia in 1883 generated many lethal tsunami with each major explosive event. More than 34,000 Javans and Sumatrans were killed by the tsunami. The largest wave reached as high as 40 m and washed ashore coral blocks weighing as much as 600 tons. Tide gauges also recorded the sea wave's passage far from the volcano—the wave reached a distance of 3800 nautical miles in 12 hours.
The most destructive recent volcanic event in Japan occurred in 1792 at Unzen volcano. A landslide from the eastern flank of Mae-yama cone crashed into the Ariake Sea, which produced a tsunami that killed about 14,5000 people along a 77-km stretch of the Shimabara Peninsula and across the Ariake Sea.
Tsunami are sometimes referred to as "tidal waves" and "seismic sea waves." The term "tidal wave" is a misnomer. The impact of a tsunami upon a coastline is partially dependent upon the tidal level at the time it strikes, but it's generation is unrelated to ocean tides. The term seismic sea wave is also misleading. Seismic sea wave implies that a tsunami is generated by an earthquake, but a tsunami can also be caused by landslides, volcanic eruptions, and meteorite impacts.
Tsunami may go unnoticed by ships on the open ocean, but as they approach land and the water shallows, they can grow to great heights and rush far inland.
Tsunami travel at speeds up to 800 km/hour across the ocean.
Simkin, T., and Fiske, R.S., 1983, Krakatau 1883—the volcanic eruption and its effects: Smithsonian Institution Press, Washington D.C., p. 464.
Seibert, L., 1984, Large volcanic debris avalanches: characteristics of source areas, deposits, and associated eruptions: Journal of Volcanology and Geothermal Research, v. 22, p. 163-197.

Glossary of volcanic terms. - University of Wisconsin Oshkosh. . 2001.

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