A (or rarely a) tsunami ( jap. 津波, literally 'harbor wave'), formerly called earthquake wave , is a sequence of particularly long water waves that can spread over very large distances and as such a displacement of water or sea in a row represent a displacement.
When advancing into areas of low water depth, the sea is compressed and thus piles up on the coasts to form several high tidal waves . These carry the water far beyond the shore line with great force and usually cause great destruction. During the subsequent receding, the material carried away on the flooded land, often including people and animals, is usually washed far out to sea.
Tsunamis occur as a result of sudden water displacement, e.g. B. when parts of the ocean floor are raised or lowered by an undersea earthquake or by large masses of earth and rock sliding into the water as well as by strong winds ( → Meteotsunami ), but also by artificially caused explosions or extremely rarely by the impact of a celestial body .
Tsunamis not only occur on the world's oceans, so-called inland tsunamis can also form on inland lakes .
The term tsunami (Japanese for: harbor wave) was coined by Japanese fishermen who returned from fishing and found everything in the harbor devastated, although they had not seen or felt any waves on the open sea. That's 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 word was on everyone's lips, especially after the severe earthquake in the Indian Ocean in 2004 , which triggered an extremely destructive tsunami.
The earliest known scientific description of this natural phenomenon with an exact cause analysis comes from the Austrian geoscientist Ferdinand von Hochstetter , who in 1868 and 1869 in several publications of the Imperial Academy of Sciences correctly associated the earthquake in Peru on August 13, 1868 with the tsunami waves on August 15, 1868 on the east coast of New Zealand and Australia in a causal relationship. From time-delayed registrations from observation stations, he calculated the wave speed at 325 to 464 nautical miles per hour and also found that water masses are influenced by the tidal waves down to great depths.
About 90% of tsunamis are caused by strong earthquakes under the ocean floor (so-called seaquakes ); the rest are caused by volcanic eruptions , submarine landslides, and in very rare cases by meteorite impacts . In addition, z. B. described by violent winds on a thunderstorm front triggered " Meteotsunamis ".
At around 80%, tsunamis occur most frequently in the Pacific : At the edge of the Pacific Ocean, in the subduction zone of the Pacific Ring of Fire , tectonic plates of the earth's crust ( lithosphere ) slide over one another. The interlocking plates create tension that suddenly discharges at an unforeseeable point in time, triggering earthquakes and seaquakes. The tectonic plates are shifted horizontally and vertically. The vertical displacement also raises or lowers the overlying water masses. Due to gravity, the water is distributed in all directions as a wave crest or wave trough; the deeper the sea area, the faster. A wavefront propagates in all directions. Usually the submarine fracture zone is not planar but line-shaped, then the wave front moves v. a. in two directions (perpendicular to the fault 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 sea at the bottom of the sea.
- It causes the sea floor to shift vertically, which causes the overlying water column to move.
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 referred to 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 unmoved. Thus, the propagation speed does not depend on the water depth. If such a wave moves into shallower water, it becomes a shallow water wave, i.e. it moves the entire water column and becomes slower. Due to their large wavelength, tsunamis are almost always flat water waves. In contrast to wind waves, they move the entire water column. Their speed is therefore almost always 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, gravity acts as a restoring force , which works towards a water surface that is as horizontal as possible. For this reason, tsunamis are counted among the gravity waves . In particular, a tsunami is not a pressure wave or a sound wave . Compressibility , viscosity and turbulence are not relevant. In order to understand the physics of a tsunami, it is sufficient to consider the potential flow of an ideal, i.e. frictionless, incompressible and turbulent liquid. Mathematically, tsunamis are described as solutions to the Korteweg-de-Vries equation .
The theory of gravity waves is simplified in the two limiting cases of deep and shallow water waves . Normal waves, which are caused, for example, by wind, moving ships or stones thrown into the water, are mostly deep-water waves because 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 the wave propagation can also be seen on the ocean floor. This corresponds to the fact that with tsunamis the wavelength (distance from one wave crest to the next) is much larger than the water depth. A much larger amount of water is moved.
A tsunami is described in simplified form by two basic parameters:
- its mechanical energy ;
- its wave period : the time that elapses for two consecutive wave crests to 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.
Tsunamis that are seismic in nature 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 sea depth 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. The speed of a tsunami wave (more precisely: its phase speed ) results from the square root of the product of gravitational acceleration 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. In the case of wind-generated waves, on the other hand, the speeds are between 8 km/h and 100 km/h. At low water depths, 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 wave height and finally to the breaking of the wave.
Gravity waves are caused by the synchronous movement of large masses of water. Each individual partial volume of the water only moves by tiny amounts. For a shallow-water gravity wave with the amplitude in a body of water at depth , this can even be stated quantitatively: The speed with which the matter involved in the wave moves in a circle is smaller by a factor than the phase speed of the wave. For a large tsunami, this factor is of the order of : When a wave propagates in the open ocean , the water elements move only with . This is small compared to currents and wind waves and is not directly observable. At the same time, it explains the low energy loss of the gravity wave during its migration.
Tsunamis are so-called shallow water waves because their wavelength is much greater than the depth of the sea . Typical wavelengths 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 be over
be calculated to:
is the time that elapses before the second wave arrives.
The longer 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, to a first approximation, inversely proportional to the distance from the point of origin of the tsunami.
|depth (m)||Speed (km/h)||Wavelength (km)|
amplitude (wave height)
The wave height ( amplitude ) of the tsunami depends on the energy and the water depth . For long-wavelength tsunamis:
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 propagating to depth decrease by the factor ). This can be illustrated by throwing a stone into a shallow puddle. The amplitude of the water waves decreases only noticeably because the energy is distributed in a circular pattern over a larger wave crest. The energy loss due to the internal friction of the water is negligible and the impulse is passed on almost undiminished. The energy of a tsunami wave weakens in the open sea only through its geometric spread. Tsunami waves can therefore circle the globe several times. In the case of tsunamis of shorter wavelength – usually not caused by earthquakes – the amplitude can decrease much faster with distance.
On the open ocean, the amplitude is rarely more than a few decimeters . The water level is thus raised and lowered only slowly and only 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.
hitting the coast
Wave energy, which was widely distributed in the open ocean, is concentrated by nonlinear mechanisms as tsunamis approach shores. Then the waves are slowed down, compressed and stand up.
increase in amplitude
The water becomes shallow near the coast. As a result, wavelength and phase velocity decrease (see table). Due to the conservation of total energy (see law of conservation of energy ), the available energy is converted into potential energy , which increases the amplitude of the wave and the speed of the matter involved. This concentrates the energy of the tsunami wave 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 is independent of the wave crest height h in the above-mentioned approximation .
Typical amplitudes when a tsunami hits the coast are of the order of 10 m. On April 24, 1771, a record height of 85 m was reported in flat terrain near the Japanese island of Ishigaki . The amplitude can increase to about 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 the Lituya Bay in Alaska , waves have been detected which, although not exceeding 100 m in height, rolled over a 520 m high hill ( megatsunami ). However, these gigantic waves were not caused by an earthquake at a distance, but by 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 only happens due to the gradual flattening of the water, the resulting reduction in the propagation speed and thus the wavelengths, which must lead to an increase in the amplitudes of the water masses. If the coast is also bay-shaped, then there is also a lateral superimposition or focusing of the water masses, which can significantly further 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). The tsunami can build up to considerable surf heights on high cliffs on the mainland, but then usually does not penetrate far into the hinterland. Furthermore, atolls rising steeply from the deep sea with linear dimensions much smaller than the wavelength of the tsunami are hardly noticed in the open ocean and are only washed over shallowly.
The masses of water that the tsunami moves over the shoreline onto land are referred to as run-up . The maximum height above sea level that the water reaches is the run - up height .
The change in wave propagation speed as the tsunami approaches the coast depends on the depth profile of the sea floor. Depending on the local conditions, refraction effects can occur: Just as light changes direction when passing from air to water or glass, a tsunami also changes direction when it crosses a zone in which the sea depth changes at an angle. Depending on the place of origin of the tsunami and the underwater topography, the tsunami can focus on individual coastal areas. This effect cannot be clearly distinguished from the funnel effect of a fjord and can overlap with it.
receding 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 packet add up in different ways from place to place and from minute to minute. A tsunami can be observed at a point on the coast first as a wave crest or first as a wave trough. If the cause of the tsunami is a landslide or the collapse of a continental plate, water is accelerated towards the bottom . Water is displaced and initially a wave trough is formed. The water then moves back again and the wave crest is formed. When the wave hits the coast, the shoreline first recedes, possibly by several hundred meters. If the tsunami hits an unprepared population, people may be drawn in by the unusual spectacle of the receding sea instead of being drawn to it use the remaining minutes before the tidal wave arrives to escape to higher ground.
When the amplitude of a tsunami near the shore is no longer negligibly small compared to the water depth, part of the oscillation of the water converts to a general horizontal movement, called the Stokes current . In the immediate vicinity of the coast, this rapid horizontal movement is more responsible for the destruction than the rising water level.
Near the coast, the Stokes current has a theoretical velocity of
with the phase velocity of the tsunami and the gravitational acceleration , i.e.:
The Stokes flow thus reaches several tens of km/h.
dangers and protection
Tsunamis are among the most devastating natural disasters that humans can face, because a powerful tsunami can carry its destructive energy thousands of kilometers or even around the globe. Without protective coastal rocks, waves just a few meters high can penetrate several hundred meters inland. The damage caused by a tsunami as it advances is magnified when the water masses flow away again. The peak height of a tsunami only has limited information about its destructive power. Especially at low land heights, a low wave height of just a few meters can cause destruction similar to that of a large tsunami of dozens of meters.
On December 26, 2004, the great tsunami in Southeast Asia killed at least 231,000 people. The wave was triggered by one of the strongest earthquakes since records began. The devastating effect here was mainly due to the large volume of water that hit land per kilometer of coastline, while the wave height was comparatively low at mostly just a few meters.
Due to its geographic location, Japan has had the highest number of deaths from tsunamis in the last thousand years. Over 160,000 people died during this period. Traditionally , tsunami stone plaques pointed to past catastrophes and thus warned against careless settlements near the coast. Today Japan has an effective early warning system . Regular training programs are held for the population. Many Japanese coastal cities are protected by levees . An example is the 105 m high and 25 m wide wall on Okushiri Island .
In Indonesia , on the other hand, half of the tsunamis still have a catastrophic effect. Most coastal residents are unaware of the signs that herald a tsunami. Most of the land is also very flat and the water masses flow inland. See also: 2004 Indian Ocean earthquake and July 2006 seaquake off Java .
Tsunamis also occur on the European coasts, albeit much less frequently. Because the Adriatic , Aegean , and African plates subduct under the Eurasian plate at certain locations, tsunamis from Mediterranean and Atlantic earthquakes can occur at these locations . The earthquake on the Montenegrin coast in 1979 (Mw 7.2) triggered a tsunami that swept away houses along 15 km of coast.
A meteorite impact can also trigger a tsunami. The celestial body is more likely to hit the sea than it will hit the ground, since seas make up most of the Earth's surface. To trigger a tsunami, however, very large meteorites are needed.
Compared to direct damage from earthquakes, volcanic eruptions, landslides or rock avalanches, which usually only occur locally or in relatively narrow spatial areas, tsunamis can still wreak havoc on coasts thousands of kilometers away and claim human lives.
Reefs off the coast, sandbanks or shallow water areas can reduce the destructive power of tsunami waves, sometimes also special breakwater structures, such as those built on some of Japan's particularly endangered sections of coast. However, there are also examples of the necessary passage areas in such protective structures dangerously increasing the flow rate and wave height of the tsunami locally and thus also increasing the damage in the area actually to be protected.
Experiences from Japan indicate that tsunami amplitudes of less than 1.5 m generally do not pose a danger to people or buildings. But there are cases, such as the night-time tsunami of 1992 in Nicaragua, where children in particular, sleeping on the floor in fisherman's huts on the beach, drowned in the water, which in some places rose by only 1.5 m. With waves higher than 2 m, lightweight structures made of wood, sheet metal or clay, and with waves higher than 3 m, buildings made of concrete blocks are usually completely destroyed. With waves over 4 m high, the number of fatalities increases drastically. Solid reinforced concrete structures, on the other hand, can withstand tsunami waves of up to 5 m in height. Therefore, 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 low chances of escaping outdoors.
Tsunamis often penetrate hundreds of meters, particularly high waves even several kilometers, into flat coastal areas and not only devastate human settlements there, but also make agricultural land and wells unusable through salinization and silting. As the water masses advance and return several times, the flooded areas are littered with mud and sand, smashed objects and parts of buildings. Ships in ports are thrown ashore, roads are blocked, railroad tracks are undermined and thus unusable. Low-lying harbor areas and fishing settlements are often submerged for a long time and have become uninhabitable. There are also dangers from leaking barrels of fuel and chemicals, flooding of sewage treatment plants or sewage pits and dead bodies of people and animals. In tropical regions in particular, this increases the acute risk of drinking water poisoning, outbreaks of epidemics, etc. The direct damage caused by the tsunami is often compounded by the outbreak of fires caused by ruptured gas lines and electrical shorts, often associated with fuel spills from stranded ships and vehicles or leaking tanks in ports. Consequential damage can result from the complete accident of coastal industrial plants, as in 2011 in 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 take advantage of the fact that certain information about the possible occurrence of a tsunami can be obtained before the tsunami itself can develop its destructive power. 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 a possible tsunami risk can thus be predicted. GPS stations measure the displacement of the earth's surface with centimeter precision, which can be extrapolated to the seabed and enables a precise forecast of the tsunami risk. Buoys measure the tsunami wave directly on the high seas, so that there is advance warning.
In recent decades, many countries have set up technical early warning systems that can detect tsunamis as they form by recording seismographic plate movements, so that the endangered coastal areas can be evacuated thanks to the time advantage gained. This is especially true in the Pacific Ocean . Between 1950 and 1965, a network of sensors on the sea floor and other important locations was set up there, which continuously measures all relevant data and reports it via satellite to the Pacific Tsunami Warning Center (PTWC) in Honolulu , Hawaii . This continuously evaluates the data and can issue a tsunami warning within 20 to 30 minutes. Since the affected countries have an effective communication system and regional emergency plans, there is a good chance that rescue measures can be initiated in good time 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. Coastal protection also monitors sea level changes with cameras . 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 affects the Indian Ocean . It also happens that authorities do not pass on tsunami warnings for fear of losing tourism as a source of income.
After the tsunami 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 on behalf of the German Federal Government by the Geoforschungszentrum (GFZ) Potsdam and seven other institutions, which went into test operation in November 2008 and has been in operation since March 2011 is. Thanks to seismic sensors and GPS technology, this complex system allows even more precise forecasts than the PTWC. In the beginning, buoys that floated on the sea surface were also used. 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. For 2012, the shortening to ten minutes was announced.
Taiwan commissioned an undersea seismic observation system on November 14, 2011. The components of the early warning system, which are attached to an undersea cable at a depth of around 300 m under sea, are distributed over a distance of 45 km and are intended to further extend the advance warning time for tsunamis and earthquakes.
The coordination of the existing systems into a global system has been promoted since mid-2005. The seismological evaluations of the UNO , which are normally used for the monitoring of the complete nuclear test ban treaty CTBT , are also used for the detection of earthquakes. To do 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 nuclear test ban treaty organization CTBTO .
Since 2007, 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 set up in the Atlantic and Mediterranean regions .
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.
Behavior 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 state that information and warnings from the local authorities should be heeded and passed on to other people in the area. When staying on the open sea, it is recommended to keep a sufficient distance from the coast and never enter the port. When staying on land, the GFZ recommends fleeing to elevated places as far away from the coast as possible on foot, since panicking fleeing drivers often lead to traffic jams. In the event of a very short warning period, it may be safer to go to one of the highest floors in a stable, newer building than to try to flee inland. The danger of further, possibly higher waves after the first tidal wave has subsided is expressly pointed out.
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 also occur in all other oceans and sea areas. Although tsunamis are rare, they are a major hazard. Safe protection from tsunamis is not achievable unless one avoids settlement and development in low-lying areas (less than 30 m above sea level) in potentially tsunami-prone areas.
- Within a few minutes, tsunamis can wreak havoc on the coasts near their source and claim many lives. However, strong tsunamis also have an effect on distant coasts, since they can spread over entire ocean basins in the course of hours.
- The speed at which tsunamis spread depends on the depth of the water. 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 tens of minutes up to more than 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 sea and are reduced to about 10 km in shallow water areas.
- The wave heights are low in deep open sea, usually less than 1 m and due to the large wavelengths they are 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 pile up to more than 10 m, in extreme cases more than 30 m to 50 m high, flood flat land behind the coast up to several kilometers inland and cause devastation.
- People on land do not necessarily perceive an approaching tsunami as a wave, but as a sudden drop or rise in sea level that is much faster than the ebb and flow. You notice e.g. For example, water suddenly runs over the ground that was previously dry, a few moments later you might already be waist-deep in water and cars are being swept away like matchboxes. The sea level may continue to rise rapidly by several meters and flood low-lying coastal areas. The water then drains back to the sea in the opposite direction, transporting destroyed buildings and debris for miles out to sea as it drains.
Tsunamis not only occur on the world's oceans, so-called inland tsunamis can also form on inland lakes . Inland tsunamis are caused either by earthquakes or by landslides that reach the lake surface or occur below the water surface.
Several tsunami events have been documented in Switzerland through historical documents or through sediment deposits, such as the Tauredunum event of 563. At that time, a landslide occurred at the eastern end of Lake Geneva . This triggered a 13 meter high tsunami. Similar inland tsunamis are known from Lake Lucerne (1601 and 1687) and Lake Lauerz (1806).
In 2009, a rather small tsunami triggered by a landslide in a flooded open-cast mining lake washed a pleasure boat onto the opposite bank of Lake Concordia in the municipality of Seeland in Saxony-Anhalt / Germany.
In the night from July 23 to 24, 2014, a landslide occurred in the Askja area in Iceland , in which a 1 km wide piece of the crater wall came loose; an estimated 50 million m³ of rock slid off and triggered several tsunamis about 50 m high in Öskjuvatn . Destabilization of the subsoil due to strong thaw is assumed to be the trigger.
See: List of tsunamis
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