en.wikipedia.org/wiki/Tsunami
A tsunami (pronounced tsoo-NAH-mee) is a wave train, or series of waves, generated in a body of water by a pulsating or abrupt disturbance that v ertically displaces the water column.
coastlines, causing devastating property damage and loss of life. Tsunamis are often called "tidal waves" as they may resemble a non-lunar- tidal rush of rapidly rising water, rather than big cresting waves reach ing the shore. However, the term is discouraged by oceanographers since tsunamis are not related to tides.
Enlarge Schema of a Tsunami Tsunamis are unlike wind-generated waves on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths.
wave length of 150 m A tsunami, on the other hand, c an have a wavelength in excess of 100 km and period on the order of one hour. As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the wa ter depth and its wave length gets very small.
gravity (98 m/s/s) and the water depth - let's se e what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. Tsunamis can be generated when the sea floor abruptly deforms and vertica lly displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crust deformati on; when these earthquakes occur beneath the sea, the water above the de formed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor e levate or subside, a tsunami can be created.
Around the margins of the Pacific Ocean, for example, denser oceanic plates slip u nder continental plates in a process known as subduction. Subduction ear thquakes are particularly effective in generating tsunamis. A tsunami can be generated by any disturbance that displaces a large wate r mass from its equilibrium position. In the case of earthquake-generate d tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthq uakes, as well as collapses of volcanic edifices, can also disturb the o verlying water column as sediment and rock slump downslope and are redis tributed across the sea floor. Similarly, a violent submarine volcanic e ruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speakin g, tsunamis generated from these mechanisms, unlike the Pacific-wide tsu namis caused by some earthquakes, dissipate quickly and rarely affect co astlines distant from the source area.
Si nce the speed of a shallow-water wave is {\sqrt{g\cdot d}} , where g is the gravitational acceleration and d is the water depth, a tsunami in th e open ocean can obtain a speed of about 700 km/h.
Tsunamis can cause severe destruction on c oasts and islands, even at distances where the earthquake or other event that caused it is itself not even noticable without instruments.
Typically, tens of minutes before a tsunami, the sea will recede from the coast, exposing part of the seabed. If the slope is shallow, this reces sion can exceed 800 m People unaware of the danger may stay at the shor e, due to curiosity, but this may be a warning sign of a coming tsunami. There can be several waves, at two- to 45-minute intervals.
buoys as communication link are used t o detect waves which would not be noticed by a human observer on deep wa ter. The first rudimentary system to alert communities of an impending t sunami was attempted in Hawaii in the 1920s.
epicent er of a large underwater quake and the probable tsunami arrival times ca n be quickly calculated, it is almost always impossible to know whether massive underwater ground shifts have occurred, resulting in tsunami wav es.
Okushiri lost their lives, and hundreds more were missing or inju red. This tsunami struck just three to five minutes after the quake and most victims were caught while fleeing for higher ground and secure plac es after surviving the earthquake. While there remains the potential for sudden devastation from a tsunami, warning systems can be effective.
Japan, for example, would have a little more than 12 hours (and likely warnings from warning systems in H awaii and elsewhere) before any tsunami arrived, giving them some time t o evacuate areas likely to be affected.
Indonesia facing the brunt of the d evastation with 45,268 feared dead. The number of deaths caused by this tsunami is expected to double due to wide spread illness.
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