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|Causes of tsunamis. Tsunamis are ocean waves produced by earthquakes. Tsunamis are often incorrectly referred to as "tidal waves." Not all earthquakes produce tsunamis, but when they do, the waves may sweep ashore causing damage locally and at places thousands of miles from the earthquake epicenter. There are several ways tsunamis are produced. One way is by regional uplift or subsidence (as discussed in the previous section) of the seafloor during an earthquake. Tsunamis started this way can travel long distances and cause destruction thousands of miles from where the wave was generated. Underwater landslides are another cause of tsunamis. Above water landslides can also cause local tsunamis if they enter a body of water. Volcanoes can also cause tsunamis. Tsunamis started by this process are uncommon.|
|how they are created?|
Tsunamis are created when the relative flatness of the ocean's surface is suddenly disturbed, typically by some huge, underwater upheaval: an earthquake the ocean floor either shifts upwards (or downwards), the water above it must move with it, creating a bulge (or indentation) on the surface that can run for hundreds of kilometers in all directions.. The deeper the earthquake, the more massive the wave and the faster it moves.
|people at sea.|
Curiously, a ship at sea wouldn't even notice the wave as it passed by. That's because the wave is usually so wide that it would take as long as an hour to pass.
Tsunamis act very differently from typical surf swells; they propagate at high speeds and can travel great transoceanic distances with little energy loss. A tsunami can cause damage thousands of miles from its origin, so there may be several hours between its creation and its impact on the coast, more time than it takes for seismic waves to arrive. Tsunamis have extremely long periods; 2 minutes to over one hour, and long wavelengths, in excess of 100 km.by a faraway storm and rhythmically roll in, one wave after another, with a period of about 10 seconds and a wavelength of 150 m.) Typically undersea earthquakes give rise to between 3 and 5 distinct waves (crests), the second or third of which are usually the largest. In instances where the leading edge of the tsunami is its trough, the sea will recede from the coast half the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed 800 m. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the dry sea bottom. In instances where the leading edge of the tsunami is its first peak, low-lying coastal areas are flooded before the higher second wave reaches them. Again, being educated about a tsunami is important, to realize that when the water level drops the first time, the danger is not yet over. A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Since a tsunami has a large wavelength, tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s2) and the water depth. A tsunami travels at about 200 m/s (about 712 km/hr or 442 mi/hr) with little energy loss even for far distances, In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. While in deep water a person at the surface of the water would probably not even notice the tsunami, the wave can increase to a height of 30 m and more as it approaches the coastline and compresses. Tsunamis can cause severe destruction on coasts and islands, even at locations remote to the source event, where that event itself is not even noticable without instruments. Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe., tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.
|Megatsunamis and seiches|
Evidence shows that megatsunamis, a tsunami more than 100 meters (325ft) high, are possible. Related to a tsunami is a seiche, an underwater, irregular fluctuation or rhythmic rocking of the water level of a lake. Often large earthquakes produce both tsunamis and seiches at the same time and there is evidence that some seiches have been caused by tsunamis. The highest tsunami wave ever recorded was very localized: caused by a landslide in Lituya Bay, Alaska in 1958, a tsunami more than 500 m high stripped trees and soil from the steep walls of a fjord. By the time the wave reached the open sea, however, it dissipated quickly. The height of the waves was determined more by the topography of the inlet than by the energy generated by the landslide. A very large ocean wave caused by an underwater earthquake or volcanic eruption
Tsunamis are very dangers by flooding and destroying cities, towns, and homes. Millions get killed or badly injured. They also can move islands and/or put them under water. They burned the dead bodies and people from around the world helped them by giving them food, shelter, and clean water. The tsunami happened on December, 26, 2004. tsunami means in jappaness terms is "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
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