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A megatsunami is a term used for a very large wave created by a large, sudden displacement of material into a body of water. Megatsunamis have quite different features from other, more usual type of tsunamis. Most tsunamis are caused by underwater tectonic activity (movement of the earth's plates) and therefore occur along plate boundaries and as a result of earthquake and rise or fall in the sea floor, causing water to be displaced. Ordinary tsunamis have shallow waves out at sea, and the water piles up to a wave height of up to about 10 metres (33 feet) as the sea floor becomes shallow near land. By contrast, megatsunamis can occur in locations where there is a very large amount of material that suddenly falls into water, or anywhere in water (for meteor impact), or may be caused by volcanic activity, and can have extremely high initial wave heights of hundreds and possibly thousands of metres, far beyond any ordinary tsunami, as the water is "splashed" upwards and outwards by the impact or displacement. As a result, two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger. Modern megatsunamis include the one associated with the 1883 eruption of Krakatoa (volcanic eruption), the 1958 Lituya Bay megatsunami (landslide into a bay), and the wave resulting from the Vajont Dam landslide (caused by human activity destabilizing sides of valley). Prehistoric examples include the Storegga Slide (landslide), and the Chicxulub, Chesapeake Bay and Eltanin meteor impacts. == Overview == A megatsunami refers to a tsunami - a large wave due to displacement of a body of water, with an initial wave amplitude (height) measured in several tens, hundreds, or possibly thousands of meters. Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, are very long (often hundreds of kilometers), and generally pass unnoticed at sea, forming only a slight swell usually of the order of above the normal sea surface. When they reach land, the wave height increases dramatically as the base of the wave pushes the water column above it upwards. By contrast, megatsunamis are caused by giant landslides and other impact events. This could also refer to a meteorite hitting an ocean. Underwater earthquakes or volcanic eruptions do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes can, since they cause a massive amount of displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result. A way to visualize the difference, is that an ordinary tsunami is caused by sea floor changes, somewhat like pushing up on the floor of a large tub of water to the point it overflows, and causing a surge of water to "run off" at the sides. In this analogy, a megatsunami would be more similar to dropping a large rock from a considerable height into the tub, at one end, causing water to splash up and out, and overflow at the other end. Two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Megatsunami」の詳細全文を読む スポンサード リンク
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