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Tsunamis - General
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Tsunami is a Japanese word meaning harbor wave. It is a water wave or a series of waves generated by an impulsive vertical displacement of the surface of the ocean or other body of water. This slide set depicts advancing waves, harbor damage, and structural damage from seven tsunami events which have occurred since 1946 in the Pacific region. The set includes before-and-after views of Scotch Cap Lighthouse (the Aleutian Islands).Tsunami is a Japanese word meaning "harbor wave." It is a water wave or a series of waves generated by an impulsive vertical displacement of the surface of the ocean or other body of water. Tsunamis are commonly called seismic sea waves or incorrectly, tidal waves. The term "tidal wave" is frequently used in the older literature and in popular accounts, but is now considered incorrect. Tides are produced by the rotational attraction of the sun and moon and occur predictably, with twelve hour periods. The effects of a tsunami may be increased or decreased depending on the level of the tide, but otherwise the two phenomena are independent.Major tsunamis occur in the Pacific Ocean region only about once per decade. These major events, such as that in Prince William Sound, Alaska, in March 1964, and the tsunami generated off the coast of Chile in 1960, have been devastating over large distances. Tsunamis have been responsible for thousands of fatalities, especially in Japan and Indonesia.Most tsunamis are caused by a rapid vertical movement along a break in Earth’s crust (i.e., their origin is tectonic). A tsunami is generated when a large mass of earth on the bottom of the ocean drops or rises, thereby displacing the column of water directly above it. This type of displacement commonly occurs in large subduction zones, where the collision of two tectonic plates causes the oceanic plate to dip beneath the continental plate to form deep ocean trenches. Shallow tsunamigenic earthquakes occur offshore in these trenches. Subduction occurs along most of the island arcs and coastal areas of the Pacific, except for the west coast of the United States and Canada. Such trench areas off continental coasts and island arcs are generating areas for major tsunamis affecting the entire Pacific Basin.Volcanic eruptions have also generated significant tsunamis, resulting in death tolls as high as 30,000 people from a single event, as in the Krakatoa eruption of 1883. Tsunamis effectively transmit energy to areas outside the reach of the volcanic eruption itself. The most efficient methods of tsunami generation by volcanoes include disruption of a body of water by the collapse of all or part of the volcanic edifice, subsidence, an explosion, a landslide, a glowing avalanche, and an earthquake accompanying or preceding the eruption. Roughly one-half of all volcanic tsunamis are generated at calderas or at cones within calderas. Submarine eruptions may also cause minor tsunamis.The largest tsunamis are caused by meteorite impact in ocean basins. While there have been none recorded in historical times, tsunamis from the Chixulub, Yucatan, crater (66 million years b.p.) were up to 5 km high. Tsunami deposits have been found in west Texas nearly 1,000 kilometers inland. Modelling shows that a moderate size meteor impact in the Atlantic Ocean would have devastating results along the entire Atlantic seaboard of the United States. Such impacts are expected on an average of once in about 15,000 years.Subaerial and submarine landslides into bays or lakes have generated locally destructive tsunamis. Other possible but less efficient methods of tsunami generation also exist. More than one mechanism commonly is involved in the generation of a tsunami including vertical movement of the crust by a seismic impulse (an earthquake) or a submarine landslide.Tsunamis have been reported since ancient times. They have been documented extensively, especially in Japan and the Mediterranean areas. One of the first recorded tsunamis may have occurred off the coast of Syria in 2000 B.C. Since 1900 (the beginning of instrumentally-located earthquakes), most tsunamis have been generated off the coast of Japan, Peru, and Chile. The only regions that have generated remote-source tsunamis affecting the entire Pacific Basin are the Kamchatka Peninsula, the Aleutian Islands, the Gulf of Alaska, and the coast of South America. Hawaii, because of its location in the center of the Pacific Basin, has experienced tsunamis generated in all parts of the Pacific.The Atlantic Ocean has less tsunamis than the Pacific. Most tsunamis in the Atlantic Ocean are generated by great earthquakes west of Portugal. These can be destructive on both sides of the Atlantic. In the Atlantic Ocean, there are no subduction zones at the edges of plate boundaries to spawn tsunamis except small subduction zones under the Caribbean and Scotia arcs. The Caribbean and Mediterranean Seas both have histories of locally destructive tsunamis. In the Indian Ocean, the Indo-Australian plate is being subducted beneath the Eurasian plate at its east margin. Most tsunamis generated in this area are propagated toward the southwest shores of Java and Sumatra, rather than into the Indian Ocean. However, only a few tsunamis have been recorded as originating in the Indian Ocean.Our knowledge of tsunamis is incomplete. The generation phenomenon has not been observed nor measured directly. Large tsunami events are somewhat rare and therefore difficult to capture on high quality slides or videos. However, photographs can contribute significantly to our understanding of tsunamis by recording permanently the heights and the damaging effects of the waves of past tsunami events.

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