A tsunami ( tsu , “port” and 波 nami , “vagueness”) is a Onde caused by a fast movement of a great volume of water (Océan or Mer). This movement is in general due to a seism, an underwater volcanic eruption of explosive type or a underwater Landslide of great scope. A meteoritic impact can also be the cause about it, just as a underwater atomic explosion. Thus, contrary to the waves, a tsunami is not created by the Vent.
Although the tsunamis can reach a speed of 800 km/h when the ocean floor is deep, they are unperceivable with broad because their amplitude seldom exceeds there the Mètre for one period (Temps between two successive waves) several minutes at several hours; they thus should not be confused with the vague scélérates which cause shipwrecks in open sea. On the other hand, they can cause enormous damage on the coasts where they appear by:
- a fall of the level of water and a retreat of the sea in the few minutes which precede them;
- a tidal wave , namely a fast rise in the level of water of to several tens of meters causing a powerful current able to penetrate deeply inside the grounds when the relief is flat. The wave (a height which can reach 60 meters in height - that depends on various parameters, mainly of a geometrical nature: bathymetry, presence of a bay, a river, etc) slow down close to the coasts and take height. Then, it can all devastate on several kilometers.
In certain rather rare cases, the tsunami can take the form of a vague beachcomber or, on a river, of a Mascaret. 75% of the tsunamis occur in the Pacific Ocean, the majority of the others in the Indian Ocean, because of the strongest tectonic activity and seismic.
According to the intensity of the mechanical action which generates them and of the geometry of the ocean, they are propagated on thousands even ten thousands of kilometers and can touch several Continent S, in zones where the seism or the volcanic eruption was not detected. At the time of a strong earthquake in coastal area, they are generally more fatal and destructors that the jolt itself.
The term tsunami (Kanji: 津波 ) is a Japanese word composed of tsu ( 津 ), “port”, “ford”, and of nami ( 波 ), “Vague”; it means literally “vague harbor”. It was named thus by the fishermen who, not having perceived anything abnormal with broad, found their devastated port city. The word is francized, it thus takes a S in the plural (of the tsunamis).
In the French expression “tidal wave”, the term “strong current” indicates a fast current. It is a word of origin Viking which was imported during the invasion of the Normandy, then passed in the Breton before passing in French. He also gave the name to the Pointe of the Strong current, and the word English race (Course), which also evokes the speed, with the same etymology.
The problem of the term “tidal wave” is that the phenomenon has nothing to do with the Marée S, which are caused by the attraction of the the Moon and the Sun; the tidal wave is caused by events of terrestrial origin. Association with the tides refers to its appearance, like an extremely fast rising of the sea level, rather than like a giant wave. In addition the term of tidal wave remains vague because he does not prejudge a seismic origin of the phenomenon: the passage of a hurricane can also raise the level of the water of to deux meters and to cause similar floods (example of the Katrina hurricane in New-Orleans); certain bays or certain ports having a particular configuration can react to the passage of a depression while being emptied and/or while filling quickly: one will speak about a weather tsunami, rather frequent phenomenon in the Mediterranean (Balearic Islands, Adriatic Sea) which can involve damage.
To avoid false association with the tides and to mitigate the inaccuracy of the term of tidal wave, the scientists prefer the word tsunami, officialized in 1963. The term passed in addition in the current language.
Sources: to see Bibliography set of themes: etymology .
Creation, propagation and surgeA tsunami is created when a great water mass is moved. That can be the case at the time of an important seism, a magnitude of 7 or more, when the level of the ocean floor along a Faille drops or rises brutally (see fig. 1), at the time of a coastal or underwater landslide, or at the time of an impact by a meteorite. It is notable that a strong seism does not produce necessarily a tsunami: all depends on the way in which the level of the ocean floor in the neighborhoods of the fault changes and whose deformation is transmitted to the water column.
The displacement of water is propagated gradually and created a movement of large Wavelength (generally a few hundred kilometers) and great period (a few tens of minutes). When the cause of the tsunami takes place close to a coast, this one can be reached in less than one hour; one speaks then about local tsunami.
Certain tsunamis are able to be propagated on distances from several thousands of kilometers and to reach the whole of the coasts of an ocean in less than one day. These tsunamis of great extent are generally of tectonic origin, because the landslides and the explosions volcanic generally produce waves shorter wavelength which are dissipated quickly: one will speak about dispersion of the waves.
It is necessary to keep in mind that it is not mainly the height of the tsunami which in fact its destroying force but duration of the rise in the level of water and quantity of water moved with its passage: so waves of plusieurs meters height, even ten de meters, are legion on the peaceful coasts, it do not transport enough of water to penetrate in the grounds. On the contrary, a tsunami a height of one or deux meters can prove to be devastating, because the quantity of water which it transports makes it possible him to break until several hundreds de meters inside the grounds if the relief is flat and without natural obstacles (trees). One can see the phenomenon under another angle: a traditional wave, one period of at more the one minute, does not raise the level of water sufficiently a long time so that it penetrates deeply, while the level of water rises with the top of its normal level during 5 to 30 minutes at the time of the passage of a tsunami.
Sources: to see Bibliography set of themes: general files .
Dangers related to the tsunamisThe dangers related to the tsunamis are due to the flood which results from it, with the force of the current which they generate so much at the time of flow than backward flow and with its capacity to grab the people with the broad one.
Human lossesThe victims carried by a tsunami can receive various blows by the carted objects (pieces of destroyed dwellings, boats, cars, etc) or be projected violently of the terrestrial objects against (urban furniture, trees, etc): these blows can be mortals or cause a loss of the capacities leading to the drowning. Certain victims can also be trapped under the debris of dwellings. Lastly, the backward flow of the tidal wave is able to take along people to the broad one, where they derive and, without help, die of drowning by exhaustion or thirst.
In the days and weeks following the event, the assessment can be weighed down, in particular in the poor countries. After tsunami than the wave itself can be mortal. The diseases related on the putrefaction of corpses, the contamination of drinking water and the time limitation of food are likely to make their appearance. The hunger can occur in the event of destruction of harvests and food stocks.
For example, the Tsunami of December 26th, 2004 made more than 300.000 died.
DamageThe tsunamis are likely to destroy dwellings, infrastructures and flora in reason:
- of the fort running which carries the structures little anchored in the ground (see the photograph opposite);
- of the flood which weakens the foundations of the dwellings, sometimes already reached by the earthquake preceding the tidal wave;
- of degradations due to the shocks of objects carted at high speed by the rising.
- of the Pollution S induced by the destruction of dangerous installations and toxic dispersion of , Pathogenic S starting from these installations (underwater factories, discharges.) or by dispersion of polluted sediments (estuaries, ports, downstream from industrial emissary, underwater or littoral discharges). At the time of the Tsunami of December 26th, 2004, a deposit of immersed ammunition for example was dispersed on sea-beds at long distances. There exist several hundreds of underwater discharges in the world, in particular containing nuclear waste and military or industrial waste highly toxic.
PreventionThe presence of an alarm system allowing to alert the population a few hours before occurred of a tsunami, the sensitizing of the coastal populations to the risks and the gestures of survival, and the security of the habitat make it possible to save the majority of the human lives.
Alarm systemIt is generally enough to move away from a few hundreds de meters with a few kilometers of the coasts or to reach a headland of quelques meters with a few tens de meters to be saved. The setting with the shelter thus takes only a few minutes with fifteen minutes, also a Alarm system to the tsunami makes it possible it to avoid the majority of the human losses.
A system of buoys adapted to the reception of the movements (pressure pick-ups laid out on the oceanic funds) can be installed along the coasts and thus prevent danger.
A surveillance device and of alarm, using a sub-oceanic mesh of probes and tracking the seisms potentially releases of tsunamis, makes it possible to alert the populations and the plagists of the arrival of a tsunami in the countries giving on the Pacific Ocean: the peaceful Center of alarm of tsunami, based on the beach of Ewa to Hawaii, not far from Honolulu.
Sources: to see Bibliography set of themes: prevention .
Security of the habitatOn Hawaii, where the phenomenon is frequent, the payments of town planning impose that constructions close to the shore are built on Pilotis.
With Male, the capital of Maldives, a line of exceeding concrete tétrapodes of 3 meters the sea level is designed to decrease the impact of the tsunamis.
SensitizingSensitizing with the phenomenon and its dangers is also a determining factor to save human lives, because all the coasts do not have a warning system - the coasts of the Atlantic Oceans and Indien are in particular deprived by it. Moreover, certain tsunamis cannot be detected in time (local tsunamis).
Two indices announcing the occurred possible one of a tsunami are to be recognized and imply that it is necessary to go in sure place:
- fast and unexpected withdrawal of the sea, because he announces occurred of a tidal wave;
- earthquake, even of weak density, because it can be a question of a distant major seism causing a tsunami.
If one is surprised by the tidal wave, to climb on the roof of a dwelling or the summit of a solid tree, to try to cling to a floating object that the tsunami carts are solutions of last recourse. To in no case, it is not sure to return near the coasts in the hours following the tidal wave, because this one can be made up of several waves spaced of a few tens of minutes to several hours.
Sources: to see Bibliography set of themes: prevention .
Natural barriersA report published by PNUE suggests that the tsunami of December 26th, 2004 because less damage in the zones where natural barriers, such as the Mangrove S, the coral reefs or the coastal vegetation, were present.
Frequency and localization of the phenomenonAt the 20th century, ten tsunamis per annum were recorded, including one and half per annum caused damage or human losses. Over this one century period, seven caused more than one thousand of deaths, that is to say less than one every ten years.
80% of the recorded tsunamis are it in the Pacific Ocean; among the eight tsunamis successor in title more than one thousand of victims since 1900, only the tsunami of December 26th, 2004 did not take place in the Pacific Ocean.
Sources: to see Bibliography set of themes: statistics on the tsunamis .
Propagation in open seaOn the open sea, the tsunami behaves like the Houle: it is a Onde with elliptic propagation, i.e. the water particles are actuated by an elliptic movement to its passage. There is (almost) no total displacement of water, a particle finds its initial position after the passage of the tsunami. Figure 2 illustrates the displacement of the water particles to the passage of vagueness.
But, contrary to the swell, the tsunami as well causes an oscillation of water on the surface (a floating object is actuated by an elliptic movement to its passage, cf not red top on the fig. 2) as in-depth (water is animated of a horizontal oscillation in the direction of the wave propagation, to see the red point of bottom on the fig. 2). This fact is related to the big wavelength of the tsunami, typically a few hundred kilometers, which is much higher than the depth of the ocean - ten kilometers at most. It results from it that the quantity of water put moving quite higher than what the swell is produced; as the tsunami transports it much more energy as the swell.
Fundamental characteristicsA tsunami has two fundamental parameters:
- mechanical energy released;
- to simplify, its period , i.e. the time passed between two successive peaks (In practice, a tsunami is a short wave train which is characterized by its spectrum of periods - to see Transformée of Fourier for a detailed explanation).
These parameters are appreciably constant during the propagation of the tsunami, whose loss of energy by friction is weak because of its big wavelength.
The tsunamis of tectonic origin have long periods, generally between ten minutes and more than one hour. The tsunamis generated by landslides or the collapse of a volcano often have shorter periods, of a few minutes to fifteen minutes.
The other properties of the tsunami like the height of vagueness, the wavelength (distance between the peaks) or the propagation velocity are variable quantities which depend on bathymetry and/or the fundamental parameters and .
WavelengthThe majority of the tsunamis have a wavelength higher than the hundred kilometers, quite higher than the depth of the oceans which hardly exceeds 10 km, so that their propagation is that of a wave in “not very major” medium. The wavelength depends then on the period and the depth of water according to the relation:
Propagation velocityFor the tsunamis of sufficiently long period, typically ten minutes, is the majority of the tectonic tsunamis of origin, the speed of displacement of a tsunami is function only depth of water :
Variability of this propagation velocity, it results a Réfraction from vagueness in the not very major zones. Thus, the tsunami seldom has the pace of a circular wave centered on the point of origin, as the fig. 5 shows it. However, the hour of arrival of a tsunami on different the east coasts foreseeable since the bathymetry of the oceans is well-known. That makes it possible to organize the evacuation as well as possible when a monitoring system and of alarm is in place.
AmplitudeFor tsunamis of long period, which present little dissipation of energy even at long distances, the amplitude of the tsunami is given by the relation:
- , i.e. the amplitude increases when water becomes less deep, in particular with the approach of the coasts (see fig. 4) and when energy is higher. It decreases with the distance, typically in because energy is distributed on a larger wave front.
For the tsunamis of weak period (often those of nonseismic origin) the decrease with the distance can be much faster.
Surge on the coasts
Horizontal movement of waterWhen the tsunami approaches the coasts its period and its speed decrease, its amplitude increases. When the amplitude of the tsunami becomes considerable compared to the depth of water, part of the speed of oscillation of water is transformed into a total horizontal movement, called current of Stokes. On the coasts, it is more this horizontal and fast movement (typically several tens of km/h) which is the cause of the damage that the rise in the level of water.
With the approach of the coasts, the current of Stokes of a tsunami has as a theoretical speed
Complexity of the effects of a tsunami in coastal areasHowever, contrary to the propagation in open sea, the effects of a tsunami on the coasts are difficult to envisage because many phenomena can take place. Against a cliff, for example, the tsunami can be strongly considered; with his passage one observes a standing wave in which water has primarily a vertical movement.
- According to the angle of attack of the tsunami on the coast and the geometry of this one, the tsunami can interfere with its own reflection and cause a series of standing waves with nonflooded coastal areas (“nodes”) and particularly touched neighbouring zones (“bellies”).
- a tsunami with the approach of an island is able to circumvent this one because of the phenomenon of Diffraction related to its large Wavelength; in particular the coast opposed to the direction of arrival of the tsunami can also be touched. At the time of the tsunami of December 26th, 2004, the town of Colombo to the Sri Lanka flooded although was protected from the direct effects of the tsunami by the remainder of the island (see the fig. 5).
- In the narrow Fjord S and estuaries, the amplitude of vagueness can be amplified, as it is the case for the tides (the latter can reach dix meters of amplitude on certain coasts as with the Mount Saint-Michel whereas it does not reach a meter on islands like Madeira). For example the Baie of Hilo has one period of typical oscillation of 30 min and was more devastated that the remainder of the island at the time of the passage of the tsunami of 1946, which had one period of 15 min: the first vagueness of the tsunami constructivement interfered with the third, and so on.
List tidal wave of great importanceThe magnitudes of the seisms in the list below are given only for the recent events. The number of victims of the tsunamis is round; they are estimates for the catastrophes of before the 20th century.
The tsunamis are deferred below having made more 1 000 estimated victims, like some others less fatal, but of amplitude or extent considerable:
- Antiquity and the Middle Ages
- approximately Crete: the eruption of the volcano of the Greek island of Santorin causes a tsunami of a hundred de meters in Crete which contributes to the disappearance of the Minoan Civilization.
- July 21st 365 apr. J. - C., seism and tidal waves felt in all Eastern Mediterranean and in particular in Alexandria.
- 1570, Chile, 2 000 victims.
- 17th century
- 1605, Japan, 5 000 victims.
- 1611, Japan, 5 000 victims.
- 1674, Indonesia, 2 500 victims.
- 1692, Jamaica, 3 000 victims.
- 18th century
- 1703, Japan, 5 000 victims.
- 1707, Japan, 30 000 victims.
- October 17th 1737, Kamchatka and islands Kouriles: a consecutive tsunami with the seism of Kamchatka reaches 50 m height in the north of the Kouriles islands.
- 1746, Peru, 4 000 victims, primarily with Lima.
- November 1st 1755, Portugal and Madeira, 90 000 victims: a violent seism with Lisbon causes a tsunami and 85% of the city are devastated. The American review Science off Tsunamis Hazards , published by the Tsunami Society based in Hawaii, quoted the case of the captain of a British ship wetting off Barbados, in the Antilles (with more 4 000 km of distance from Portugal), which foot-note in its log book, on November 1st, 1755, the surge of a wave of more than trois meters in height on the beaches of the island. Other comparable testimonys bring back the effects of the tsunami in the other islands of the Antilles in the afternoon of the same day, the European seism having taken place several hours before.
- 1766, Japan, 1 500 victims.
- 1782, Southeast Asia, 40 000 victims: a tsunami touches the Southeast Asia, mainly in China.
- 1792, Japan, 15 000 victims.
- 19th century
- 1854, Japan, 3 000 victims.
- 1868, Chile, 25 000 victims.
- August 27th 1883, Indian Ocean, 40 000 victims: a tsunami associated with the eruption with the Krakatoa is detected on the majority of the coasts of the sphere, with a rise in the sea level of 40 meters close to the zone of origin.
- 1896, Japan, 25 000 victims.
- 1899, Indonesia, 3 500 victims.
- 20th century
- 1908, Metz-native and Calabria, 95 000 victims, of which 80 000 in Messine on a population of 140 000 inhabitants.
- 1923, Japan, 2 000 victims.
- 1933, Japan, 3 000 victims.
- April 1st 1946, Pacific Ocean, 2 000 victims: a seism magnitude 8,6 off Alaska causes a tsunami which reaches 30 m in Alaska, 12 m in Hawaii, and touches Japan as well as the west coast of the United States.
- July 9th 1958, Alaska, 2 victims: a consecutive landslide to a strong seism in bay of Lituya in Alaska causes the largest known tsunami - it devastates the vegetation on one of the sides until a 500 m height - but the geography of bay prevents it from being propagated in the Pacific Ocean.
- May 22nd 1960, Chile and Pacific Ocean, 5.000 victims: a seism magnitude 9,5 with the Chile causes a fatal tidal wave a height going up to 25 m in Chile, 10 m with Hawaii and 3 m in Japon.
Voir detailed article: Earthquake of 1960 in Chile .
- March 27th 1964, West of the the United States, 100 victims: a seism magnitude 9,3 off Alaska causes there a tsunami of 15 m, which touches California where the level of water rises of 6 Mr.
- 1976, Indonesia, 8.000 victims in the island of Célèbes.
- 1992, Indonesia, 2.200 died in the island of Flora.
- July 17th 1998, New Guinea-News-Guinea, 2 000 victims: a seism magnitude 7,0 with 20 km of the coasts causes a local tsunami a height from approximately 10 Mr.
- 21e century
- December 26th 2004, Indian Ocean, at least 285 000 victims (official results at the 1/30/2005): a seism magnitude 9,1 to 9,3 off Indonesia causes a tsunami which touches the countries of South Asia (Indonesia, Malaysia, Thailand, India, Sri Lanka) and to a lesser extent the Eastern coasts of the Africa.
See detailed article: tsunami of December 26th, 2004 .
- July 17th 2006, 668 dead: a seism of 7,7 with broad of the southern part of Java causes a tsunami making 668 dead, 287 missings, 878 wounded and approximately 100.000 disaster victims (at July 22nd). The alarm system set up after the tsunami of December 26th, 2004 appeared defective.
Sources: to see Bibliography set of themes: statistics on the tsunamis .
See also: Mégatsunami
One defines as mégatsunami a tsunami of which the height on the level of the coasts exceeds hundred meters. A mégatsunami, if it is propagated freely in the ocean, is able to cause major damage on a whole continent scale. The seisms being unable a priori to generate such cataclysmic waves, only events, a such meteoritic impact of great width or the collapse of a mountain in the sea, are the possible cause. Beyond the phantasm, one will note the following facts:
- No mégatsunami not room was brought back in the history of humanity. In particular, the explosion of the Krakatoa in 1883 and the collapse of the volcano of Santorin in the Antiquité did not produce any.
- the possible causes of a mégatsunami are rare phenomena , spaced scales of geological times - with the low word several tens of thousands of years, if they are not million years. Certain scientists however estimate that a mégatsunami recently would have been caused by the collapse of the Piton of the Furnace on itself, at the Meeting: the event would go back to 2.700 before Jesus-Christ approximately.
- the landslides produce tsunamis of short period which cannot be propagated on several thousands of kilometers without dissipating their energy. For example, at the time of the landslides in Hawaii in 1868 on the Mauna leasing and in 1975 on the Kilauea, of the important local tsunamis were generated, without the distant coasts American or Asian not being worried.
The risk of mégatsunami remains however mediatized and overestimated. Discussed models indeed predict two possible sources of mégatsunami in the next millenia: are considered a collapse along the sides of the Cumbre Vieja with the the Canaries (putting the east coast of the American continent in danger) and another with the Kilauea with Hawaii (threatening the west coast of America and those of Asia). More recent studies call into question the risk of collapse on the sides of these volcanos, on the one hand, and the nonlocal character of the generated tsunamis, on the other hand.
Sources: Bibliography set of themes: mégatsunamis .
Comparable phenomenaOne should not confuse the phenomenon with that of the Vague scélérate created locally and fugitively on the open sea by the wind by a phenomenon of resonance and amplification of the Houle accompanied by the refraction of the waves on terrestrial obstacles, the whole creating of the interferences whose amplitude can temporarily exceed very largely that of the only swell.
However, some tidal waves caused by the digging and the brutal attenuation of cyclones more the violent ones can have a behavior similar to the tsunami (including in its intensity, its propagation in the form of wave at long distances, or its devastators effects on the coasts).
In certain cases, the seismic or cyclonic origin of the tsunami cannot be completely given with certainty, such as the giant waves who touched the islands of the Réunion and Maurice about on May 13rd, 2007. Indeed, the wave of swell came from the South of the Indian Ocean, in the area of the islands Kerguelen where a violent one subtropical cyclone prevailed, but also an area known for its frequent seismic activity. It is not excluded that the two phenomena combined, although no important seism could be detected in this area where one has very few measuring instruments.
- Wave S in the Ocean S
- Vague scélérate
- possible causes of a tsunami
- Seism coastal or underwater
- meteoritic Impact
- underwater Explosion
- outstanding recent Tsunamis
- Tsunami of December 26th, 2004 in the Indian Ocean
- consecutive tsunami with the earthquake of 1960 in Chile in Pacific Ocean
- Tidal wave (soundtrack), France Inter, chronicle the word of the end of Alain Rey, December 27th 2004
- Tsunami or tidal wave? , Release n° 7352, December 30th 2004
- Statistical on the tsunamis , governmental site of the United States
Watchdog committees and of alarm
- international Bodies
- ITIC, international center of information on the tsunamis, body of UNESCO
- PTWC, peaceful center of alarm
- Recurring questions (FAQ) on the alarm system
- national Bodies of alarm
- WC/ATWC, center of alarm of the West coast of the United States and Alaska
- poseidon, center of alarm of Puerto Rico
- SHOA tsunami, Chilean system of alarm (in Castilian)
- Which is what a tsunami? , site futura-sciences.com
- the tsunamis , site notre-planete.info
- Glossary on the tsunamis , site of UNESCO
- Recurring questions (FAQ), on the site of the ITIC
- the large waves, booklet of the ITIC
- To survive a tsunami , testimonys collected by US Geologic Survey accompanied by councils
- Tsunami: fact sheet , on the site of the agency states-unienne FEMA
- Recurring questions (FAQ), on the site of Pacific Tsunami Museum (in English)
- Evaluation of the risk of mégatsunami , study detailed with references on the site of Dr. Pararas-Carayannis
- Photographs of tsunami on futura-sciences.com
- Photographs: Tsunami Penang, Malaysia
- Photographs: Tsunami - Kota Kuala Muda, Kedah, Malaysia
Be-X-old: Цунамі Map-bms: Tsunami Nds-nl: Vleuigolve Simple: Tsunami Zh-min-nan: Hái-tiòng
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