A tsunami or a tsunami ( Japanese 津 波 , literally 'harbor wave'), formerly called earthquake waves in German , is a sequence of particularly long water waves that can spread over very large distances and as such represent a displacement of water or the sea as a result of displacement .
When penetrating into areas of shallow water, the sea is compressed and piles up in several high tidal waves on the coasts . These carry the water with great force far over the shoreline and usually cause great damage. During the subsequent retreat, the material carried away on the flooded land, often also humans and animals, is mostly washed far out into the ocean.
Tsunamis occur as a result of sudden water displacement, e.g. E.g. when parts of the ocean floor are raised or lowered during an undersea earthquake or when large masses of earth and rock slide into the water as well as due to strong winds ( → Meteotsunami ), but also due to artificially caused explosions or, very rarely, due to the impact of a celestial body .
Tsunamis occur not only on the high seas, even on inland lakes called to inland tsunamis form.
The term tsunami (Japanese for: harbor wave) was coined by Japanese fishermen who returned from fishing and found everything devastated in the harbor, although they had not seen or felt a wave on the open sea. That is why they called the mysterious waves Tsu-nami, which means “wave in the harbor”.
A series of devastating tsunamis between 1945 and 1965 made this natural phenomenon known worldwide and formed the basis for scientific work, as a result of which the Japanese term internationalism prevailed.
The earliest known scientific description of this natural event with an exact cause analysis comes from the Austrian geoscientist Ferdinand von Hochstetter , who correctly described the earthquake in Peru on August 13, 1868 with the tsunami waves on August 15, 1868 in several publications by the Imperial Academy of Sciences in 1868 and 1869 on the east coast of New Zealand and Australia in a causal context. From time-delayed registrations from observation stations, he calculated the wave speed to be 325 to 464 nautical miles per hour and also found that the tidal waves affect water masses at great depths.
Around 90% of the tsunamis are triggered by strong earthquakes below the ocean floor (so-called seaquakes ); the rest arise as a result of volcanic eruptions , submarine landslides and, in very rare cases, meteorite impacts . In addition, z. B. described " Meteotsunamis " triggered by strong winds on a storm front .
Tsunamis occur most frequently in the Pacific with about 80% : At the edge of the Pacific, in the subduction zone of the Pacific Ring of Fire , tectonic plates of the earth's crust ( lithosphere ) push one another. The interlocking plates create tensions that suddenly discharge at an unforeseeable point in time, triggering earthquakes and seaquakes. The tectonic plates are shifted horizontally and vertically. The vertical shift also raises or lowers the water masses above. Due to gravity, the water is distributed as a wave crest or wave trough in all directions; the deeper the ocean, the faster. A wave front spreads in all directions. Most of the time, the submarine fracture zone is not flat, but linear, then the wave front moves v. a. in two directions (at right angles away from the break line).
An earthquake can only cause a tsunami if all three of the following conditions are met:
- The quake reaches a magnitude of 7 or more.
- Its hypocenter is near the surface of the earth on the sea floor.
- It causes a vertical shift in the sea floor, which sets the water column above in motion.
Only one percent of the earthquakes between 1860 and 1948 caused measurable tsunamis.
Tsunamis are fundamentally different from waves caused by storms. The latter are known as shallow water waves or deep water waves, depending on the water depth in relation to the wavelength. With deep water waves, the wave has no contact with the bottom and the deeper water layers remain motionless. Thus the speed of propagation does not depend on the water depth. If such a wave moves in shallower water, it becomes a shallow water wave, moving the entire water column and thereby slowing down. Because of their long wavelength, tsunamis are shallow waves almost everywhere. In contrast to wind waves, they move the entire water column. Their speed is therefore practically everywhere dependent on the water depth.
Tsunamis are gravity waves
Wave propagation is always possible when a deflection from an equilibrium position , in this case a rise or fall in the water level, results in an opposing restoring force . In the case of ocean waves, the force of gravity acts as a restoring force and works towards a water surface that is as horizontal as possible. For this reason, tsunamis are counted as gravity waves . In particular, a tsunami is neither a pressure wave nor a sound wave . Compressibility , viscosity and turbulence are not relevant. To understand the physics of a tsunami, it is sufficient to consider the potential flow of an ideal, i.e. frictionless, incompressible and eddy-free liquid. Mathematically, tsunamis are described as solutions to the Korteweg-de-Vries equation .
The theory of gravity waves is simplified in the two borderline cases of deep and shallow water waves . Normal waves, caused for example by wind, moving ships or stones thrown into the water, are mostly deep water waves , as their wave base is usually above the bottom of the water, i.e. where the wave no longer has any effect. A tsunami, on the other hand, is a shallow water wave even in the deepest ocean, since the entire water column is moved and a slower movement in the direction of wave propagation can also be determined on the ocean floor. This corresponds to the fact that in tsunamis the wavelength (distance from one wave crest to the next) is much greater than the water depth. A much larger amount of water is moved here.
A tsunami is described in a simplified way by two basic parameters:
- its mechanical energy ;
- its wave period : the time that passes in which two successive wave crests pass the same point.
During the propagation of a tsunami, these two parameters remain largely constant, since the energy losses due to friction are negligible due to the large wavelength .
Seismic tsunamis have long wave periods ranging from ten minutes to two hours. Tsunamis generated by events other than earthquakes often have shorter wave periods ranging from a few minutes to a quarter of an hour. Other properties such as wave height and length or the speed of propagation depend only on the depth of the sea in addition to the two basic parameters.
The speed of a tsunami depends on the depth of the sea: the deeper the sea, the faster the tsunami is. The speed of a tsunami wave (more precisely: its phase speed ) results from the root of the product of acceleration due to gravity and water depth
The speed of propagation in oceans (water depth approx. 5000 m) is approx. 800 km / h. This is comparable to the cruising speed of an airplane. Tsunamis can therefore cross entire oceans within a few hours and spread up to 20,000 km without being noticed immediately. With waves generated by the wind, on the other hand, the speeds are between 8 km / h and 100 km / h. When the water depth is low, i.e. near the coast, the tsunami slows down, as can be seen in the adjacent animation. This also reduces the wavelength, which leads to an increase in the height of the wave and ultimately to the breaking of the wave.
Gravity waves are caused by the simultaneous movement of large masses of water. Each individual partial volume of the water only moves tiny amounts. This can even be stated quantitatively for a shallow water gravity wave with the amplitude in a body of water : The speed at which the matter involved in the wave moves in a circular manner is a factor smaller than the phase speed of the wave. This factor is of the same order of magnitude for a large tsunami : If a wave propagates in the open sea , the water elements only move with it . This is small compared to currents and wind waves and cannot be directly observed. At the same time, it explains the low energy loss of the gravity wave during its migration.
Because their wavelength is much greater than the depth of the ocean , tsunamis are so-called shallow water waves. Typical wave lengths for tsunamis are between 100 km and 500 km. The wavelengths of wind-generated waves, on the other hand, only reach between 0.2 km and 1 km. In general, the relationship applies to waves
between speed , wavelength and wave period .
With the tsunami speed from above and the indication of the wavelength, typical wave periods can exceed
can be calculated as:
is the time that elapses before the second wave arrives.
The larger the wavelength, the lower the energy losses during wave propagation. In the case of circular propagation, the energy with which a wave hits a coastal strip is, in a first approximation, inversely proportional to the distance from the point of origin of the tsunami.
|Depth (m)||Speed (km / h)||Wavelength (km)|
Speed and wavelength of a tsunami as a function of the water depth
Amplitude (wave height)
The wave height ( amplitude ) of the tsunami depends on the energy and the water depth . The following applies to tsunamis with a long wavelength:
This means that the amplitude increases with shallower water depth . In the open sea, it only decreases by the factor with increasing distance (spherical waves that propagate into the depth decrease by the factor ). This can be illustrated by throwing a stone into a shallow puddle. The amplitude of the water waves only decreases noticeably, as the energy is distributed in a circle over a larger crest of the waves. The loss of energy due to the internal friction of the water is negligible and the impulse is passed on almost without weakening. The energy of a tsunami wave is only weakened in the open sea by its geometric expansion. Tsunami waves can therefore circle the globe several times. In the case of tsunamis of smaller wavelengths - usually not caused by earthquakes - the amplitude can decrease significantly faster with distance.
In the open ocean, the amplitude is rarely more than a few decimeters . The water level is therefore only raised and lowered slowly and by a small amount, which is why the occurrence of a tsunami on the open sea is usually not even noticed.
The destructive power of a tsunami is not fundamentally determined by its amplitude, but by the wave period and the amount of water transported.
Hit the coast
The energy of the waves, which was still widely distributed in the open ocean, is concentrated by non-linear mechanisms when the tsunamis approach the coasts. Then the waves are braked, compressed and stand up.
Increase in amplitude
The water becomes shallow near the coast. As a result, the wavelength and phase velocity decrease (see table). Due to the conservation of the total energy (see the law of conservation of energy ) the available energy is converted into potential energy , whereby the amplitude of the wave and the speed of the matter involved increase. The energy of the tsunami wave is concentrated more and more until it hits the coast with full force. The energy content of a wave train is proportional to the cross section times the wavelength times the square of the particle velocity and, in the approximation mentioned above, is independent of the wave crest height h .
Typical amplitudes when a tsunami hits the coast are in the order of 10 m. On April 24, 1771, near the Japanese island of Ishigaki, a record height of 85 m in flat terrain was reported. The amplitude can rise to around 50 m near the shore of a deep sea cliff. If a tsunami runs into a fjord , the wave can build up to well over 100 m.
In Lituya Bay in Alaska , waves were detected that did not exceed 100 m in height, but rolled over a 520 m high hill ( megatsunami ). These gigantic waves, however, did not arise as a long-range effect of an earthquake, but rather through water displacement in the fjord itself: violent earthquakes caused mountain slopes to slide into the fjord and suddenly caused it to overflow.
The piling up of the water masses happens only through the gradual flattening of the water, the resulting reduction in the speed of propagation and thus the wavelengths, which must lead to an increase in the amplitudes of the water masses. If the coast is also in the shape of a bay, the water masses are also laterally superimposed or focused, which can significantly increase the amplitude increase caused by the vertical water profile, especially when resonances occur (wavelengths in the order of magnitude of the linear bay dimensions). On the high cliffs of the mainland, the tsunami can build up to considerable surf heights, but then usually does not penetrate far into the hinterland. Furthermore, atolls rising steeply out of the deep sea with linear dimensions much smaller than the wavelength of the tsunami are barely perceived in the open ocean and only flooded shallowly.
The water masses that the tsunami moves over the coastline to the land is known as run-up . The maximum height above sea level, which reaches the water, the run-up height ( run-up height ).
The change in the speed of wave propagation when the tsunami approaches the coast depends on the depth profile of the seabed. Depending on the local conditions, refraction effects can occur: Just as light changes its direction when it passes from air to water or glass, a tsunami also changes its direction when it runs diagonally through a zone in which the sea depth changes. Depending on the place of origin of the tsunami and the underwater topography, the tsunami may focus on individual coastal areas. This effect cannot be clearly separated from the funnel effect of a fjord and can be superimposed with it.
Retreat of the sea
Like an acoustic signal, a tsunami does not consist of a single wave, but of a whole package of waves with different frequencies and amplitudes. Waves of different frequencies propagate at slightly different speeds. Therefore, the individual waves of a package add up in a different way from place to place and from minute to minute. A tsunami can first be observed as a wave crest or first as a wave trough at one point on the coast. If the cause of the tsunami is a slope slide or a breakdown of a continental plate, then water is accelerated towards the bottom . Water is displaced and initially a trough of waves is created. Then the water moves back again and the wave crest is created. When the wave arrives at the coast, the coastline initially retreats, possibly by several 100 m. If the tsunami hits an unprepared population, the unusual spectacle of the receding sea may lure people instead of using the minutes remaining until the arrival of the tidal wave to escape to higher ground.
When the amplitude of a tsunami near the coast is no longer negligibly small compared to the water depth , part of the oscillation of the water is converted into a general horizontal movement, known as the Stokes current . In the immediate vicinity of the coast, this rapid horizontal movement is more responsible for the destruction than the rise in the water level.
Near the coast the Stokes current has a theoretical speed of
with the phase velocity of the tsunami and the gravitational acceleration , i.e.:
The Stokes flow thus reaches several dozen km / h.
Hazards and protection
Tsunamis are among the most devastating natural disasters that humans can confront, because a powerful tsunami can carry its destructive energy over thousands of kilometers or even carry it around the globe. Without protective coastal rocks, waves a few meters high can penetrate several hundred meters into the country. The damage caused by a tsunami as it penetrates is increased when the water masses drain away again. The summit height of a tsunami has only limited informative value about its destructive power. Especially at low land heights, even a low wave height of only a few meters can cause destruction similar to a large tsunami of tens of meters.
On December 26, 2004, at least 231,000 people were killed in the great tsunami in Southeast Asia . The wave was triggered by one of the strongest earthquakes since records began. The devastating effect was mainly due to the large volume of water that hit the land per kilometer of coastline, while the wave height was comparatively low, mostly only a few meters.
Due to its geographic location, Japan has suffered the most deaths from tsunamis over the past thousand years. Over 160,000 people died during this time. Traditionally, tsunamist information boards pointed to past disasters and warned against frivolous settlements near the coast. Today Japan has an effective early warning system . There are regular training programs for the population. Many Japanese coastal cities are protected by levees . One example is the 105 m high and 25 m wide wall on the island of Okushiri .
In Indonesia, however, half of the tsunamis are still catastrophic today. Most coastal residents are unaware of the signs that a tsunami will occur. Most of the country is also very flat and the water masses flow inland. See also: 2004 Indian Ocean earthquake and tsunami and earthquake off Java in July 2006 .
Tsunamis also occur on the European coasts, albeit much less frequently. Since the Adriatic , Aegean and African plates subduct below the Eurasian plate at certain points, earthquakes in the Mediterranean and Atlantic can cause tsunamis at these points . The earthquake on the Montenegrin coast in 1979 (Mw 7.2) triggered a tsunami that carried away houses along a 15 km coastline.
A meteor strike can also trigger a tsunami. The celestial body is more likely to hit the ocean than it will hit the ground, since oceans make up most of the earth's surface. In order to trigger a tsunami, however, very large meteorites are necessary.
Compared to direct damage as a result of earthquakes, volcanic eruptions, landslides or rock avalanches, which usually only occur locally or in relatively spatially limited areas, tsunamis can wreak havoc on coasts thousands of kilometers away and claim human lives.
Reefs, sandbars or shallow water areas off the coast can reduce the destructive power of tsunami waves, sometimes special breakwater structures, such as those built on some particularly endangered coastal sections of Japan. However, there are also examples that the necessary passage areas in such protective structures locally dangerously increased the flow velocity and wave height of the tsunami and thus also increased the damage in the area actually to be protected.
Experience from Japan shows that tsunami amplitudes below 1.5 m generally do not pose a threat to people or structures. But there are cases like the nightly onset of the tsunami in Nicaragua in 1992, where mainly children who slept on the floor in fishermen's huts on the beach drowned in the water, which in some places only rose by 1.5 m. With wave heights over 2 m, lightweight structures made of wood, sheet metal, clay, and with waves over 3 m high, structures made of concrete blocks are usually completely destroyed. With wave heights over 4 m, the death toll increases dramatically. Solid reinforced concrete structures, on the other hand, can withstand tsunami waves up to 5 m high. This is why the upper floors of reinforced concrete high-rise buildings or hotels can also be used as places of refuge in the event of very short warning times and little chance of escape outdoors.
Tsunamis often penetrate hundreds of meters, particularly high waves even a few kilometers, into flat coastal areas and not only devastate human settlements there, but also render agricultural areas and wells unusable due to salinization and silting up. As the water masses penetrate and flow back several times, the floodplains are littered with mud and sand, smashed objects and parts of buildings. Ships in harbors are thrown onto the land, roads are blocked, railway tracks are washed away and thus unusable. Low-lying port areas and fishing settlements are often under water for a long time and have become uninhabitable. There are also dangers from leaking barrels with fuel and chemicals, flooding of sewage treatment plants or sewage pits and corpses of people and animals. In tropical regions in particular, this increases the acute risk of drinking water poisoning, outbreaks of epidemics and the like. Ä. The direct tsunami damage is often exacerbated by the outbreak of fire as a result of broken gas lines and electrical short circuits, often in connection with leaked fuel from stranded ships and vehicles or leaked tanks in ports. Consequential damage can result from the complete disaster of industrial facilities close to the coast, as in 2011 at the Japanese nuclear power plant Fukushima , where there was a partial core meltdown with an uncontrolled release of radioactive substances. Coastal biotopes (mangrove forests, coral reefs, etc.) can also be severely damaged and permanently disturbed by tsunamis.
Early warning systems
Tsunami early warning systems make use of the fact that certain information about the possible occurrence of a tsunami can be obtained before the tsunami itself can develop its destructive force. Seismic waves propagate much faster than the tsunami wave itself. If, for example, a sufficiently dense network of seismic stations is available, precise conclusions can be drawn about the location and strength of an earthquake after just a few minutes, and thus a possible tsunami risk can be forecast. GPS stations measure the displacement of the earth's surface with centimeter precision, which can be extrapolated to the sea floor and enables a precise forecast of the tsunami risk. Buoys measure the tsunami wave directly on the high seas, so there is an advance warning time.
In the last few decades, many countries have set up technical early warning systems which, by recording seismographic plate movements, can recognize tsunamis as they arise, so that the endangered coastal areas can be evacuated with the time advantage gained. This is especially true for the Pacific Ocean . Between 1950 and 1965, a network of sensors was set up there on the sea floor and other important points, which continuously measure all relevant data and report it to the Pacific Tsunami Warning Center (PTWC) in Honolulu , Hawaii via satellite . This continuously evaluates the data and can broadcast a tsunami warning within 20 to 30 minutes. Since the affected states have an effective communication system and regional emergency plans, there is a good chance that rescue measures can be initiated in the event of a disaster.
Some coastal cities in Japan protect themselves with dikes up to 10 m high and 25 m wide, the gates of which can be closed within a few minutes. The coastal protection department also uses cameras to monitor sea level changes. An early warning system automatically issues a tsunami alarm in the event of a magnitude 4 earthquake , so that the residents can be evacuated .
Unfortunately, some states affected by the danger do not yet have these systems, and their information network is so poorly developed that advance warning is only possible to a limited extent or not at all. This particularly applies to the Indian Ocean . It also happens that authorities do not forward tsunami warnings for fear of losing their source of income for tourism.
After the flood disaster in South Asia in 2004, the states on the Indian Ocean decided to set up a tsunami early warning system.
Indonesia has ordered a German early warning system - the German Indonesian Tsunami Early Warning System (GITEWS) - which was developed by the Geoforschungszentrum (GFZ) Potsdam and seven other institutions on behalf of the German Federal Government, which went into test operation in November 2008 and has been in operation since March 2011 is. With seismic sensors and GPS technology, this complex system allows even more precise predictions than the PTWC. Initially, buoys were also used that floated on the surface of the sea. However, these proved to be unreliable.
Malaysia has set up the Malaysian National Tsunami Early Warning System ( MNTEWS ), which currently enables the population to be alerted within twelve minutes of the event. It was announced that the alarm time would be reduced to ten minutes in 2012.
Taiwan put an undersea seismic observation system into operation on November 14, 2011. The components of the early warning system, which are attached to a submarine cable at a depth of around 300 m, are distributed over a distance of 45 km and are intended to further shorten the advance warning time for tsunamis and earthquakes.
The coordination of the existing systems into a global system has been advanced since mid-2005. For the detection of earthquakes, the seismological evaluations of the UN , which are normally used for the monitoring of the complete nuclear test ban treaty , are also used. For this, only the reporting systems have to be integrated into the national alarm systems, since the detection options are already available. The reports of these artificial or natural earthquakes caused by nuclear explosions converge in Vienna at the CTBTO .
A tsunami early warning system, the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, the Mediterranean and connected seas ( NEAMTWS ), has been in place in the Atlantic and the Mediterranean region since 2007 .
The problem with all early warning systems is that false alarms in the event of an unnecessary evacuation can result in high costs and undermine people's confidence in the forecasts.
What to do in the event of acute tsunami danger and tsunami warning
The German Research Center for Geosciences Potsdam (GFZ) gives advice in the event of a tsunami. These essentially mean that information and warnings from the local authorities should be observed and passed on to other people in the area. When staying on the open sea, it is recommended that you keep a sufficient distance from the coast and never enter the port. When staying on land, the GFZ recommends fleeing to elevated locations that are as far away from the coast as possible, as motorists fleeing in panic often lead to traffic jams. In the case of a very short warning time, it might be safer to go to one of the highest floors in a stable, newer building than to try to escape inland. Express reference is made to the risk of further, possibly higher waves after the first tidal wave has subsided.
Typical phenomena of tsunamis
- Tsunamis consist of a series of consecutive, very long-period ocean waves. These are mostly caused by strong submarine earthquakes, but also by volcanic eruptions or landslides.
- Most tsunamis occur in the Pacific Ocean, but they are also found in all other oceans and marine areas. Although tsunamis are rare, they represent a great danger. Reliable protection from tsunamis is not achievable unless settlement and building in low-lying areas (less than 30 m above sea level) are avoided in areas that are at risk of tsunamis.
- Within a few minutes, tsunamis can wreak havoc on the coasts near their source and claim many lives. Strong tsunamis also have their effect on distant coasts, as they can spread over entire ocean basins in the course of hours.
- The speed at which tsunamis spread depends on the water depth. In deep oceans it is over 800 km / h, in shallow water only 30 to 50 km / h.
- A tsunami usually consists of several wave crests, which follow each other at intervals of a few ten minutes to over an hour and often only reach maximum heights on the coast in later wave crests.
- The distances between the wave crests are a few 100 km on deep open seas and are shortened to about 10 km in shallow water areas.
- The wave heights are low on deep open seas, usually less than 1 m and, due to the large wavelengths, not dangerous for ships and can only be determined using special buoys or satellite altimetry . When approaching the coast, especially in shallow bays, the water masses can tower over 10 m, in extreme cases more than 30 to 50 m high, flood flat land behind the coast up to several kilometers inland and cause devastating devastation.
- People on land do not necessarily perceive an approaching tsunami as a wave, but rather as a sudden drop or rise in the sea level that is much faster than the ebb and flow of the tide. You notice e.g. For example, water suddenly runs over the previously dry ground, a few moments later you may already be waist-deep in the water and cars are washed away like matchboxes. The sea level may continue to rise rapidly by several meters and flood lower-lying coastal areas. The water then runs off in the opposite direction to the sea and, as it drains, transports destroyed buildings and debris for kilometers out into the open sea.
Tsunamis occur not only on the high seas, on inland lakes is called to inland tsunamis form. Inland tsunamis arise either from earthquakes or landslides that reach the surface of the lake or occur below the surface of the water.
Several tsunami events have been proven in Switzerland by historical documents or by sediment deposits, such as the Tauredunum event of 563. At that time, a landslide occurred on the eastern end of Lake Geneva . This triggered a 13 meter high tsunami. Similar inland sunamis are known from the Vierwaldstättersee (1601 and 1687) and from the Lauerzersee (1806).
A rather small tsunami triggered by a landslide in a flooded open pit lake in 2009 washed an excursion boat onto the opposite bank of Lake Concordia in the municipality of Seeland in Saxony-Anhalt / Germany.
On the night of July 23rd to 24th, 2014, a landslide occurred in the Askja area in Iceland , in which an approx. 1 km wide section of the crater wall detached; an estimated 50 million m³ of rock slipped off and triggered several 50 m high tsunamis in Öskjuvatn . Destabilization of the subsoil due to a strong thaw is suspected to be the trigger.
See: List of tsunamis
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