Tsunami

A tsunami (Japanese津 波, literally, harbor wave ' ), German formerly called Erdbebenwoge, is a particularly long water wave, which can propagate over very long distances, is compressed to penetrate other areas of shallow water, thereby one on a flat coast to high tidal wave piling up and so carries the water far beyond the shoreline; during the subsequent retreat of the entrained on the flooded land material is often washed out into the sea. A tsunami is created as a result of sudden elevation or reduction of parts of the ocean floor in a submarine earthquake or by the slipping of large amounts of earth into the water, in extremely rare cases by the impact of a celestial body.

• 4.1 increase in the amplitude
• 4.2 refraction effects
• 4.3 receding of the sea
• 4.4 Stokes flow

• 5.1 Risk Zones
• 5.2 Impact
• 5.3 Early Warning Systems
• 5.4 behaviors in acute tsunami hazard and tsunami warning

Etymology

The term tsunami (Japanese for: harbor wave) was coined by Japanese fishermen who returned from fishing in the harbor and all vorfanden devastated, although they had seen or noticed any wave in the open sea. That's why they called the mysterious waves tsunami, meaning " harbor wave ".

A series of devastating tsunamis 1945-1965 made ​​this natural phenomenon known throughout the world and formed the basis for scientific work, the Japanese term prevailed in consequence of which as internationalism. The previously earliest known scientific description of this natural phenomenon with exact root cause analysis comes from the Austrian geoscientist Ferdinand von Hochstetter, of it in 1868 in his essay About the earthquake in Peru on August 13, 1868, which thereby led Fluthwellen in Pacifischen ocean, especially on the coast of represented Chili and New Zealand.

Formation

Tsunamis are usually excited ( about 90% ) by strong earthquake under the ocean floor; the rest arise as a result of volcanic eruptions, submarine landslides or in very rare cases by meteorite impacts.

Tsunamis occur at about 79 % 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 slide ( lithosphere ) above the other. By hooking into each other plates produces stresses that discharged abruptly to an unforeseeable time, whereby earth and tsunamis are triggered. The tectonic plates are moved horizontally and vertically. The vertical displacement raises or lowers the overlying water masses. Due to the gravity, the water wave crest or trough distributed in all directions; the deeper the sea area, the faster. Thus, a wave front propagates in all directions. Most undersea fracture zone is not area- but linear, then moves the wavefront esp. in two directions ( at right angles from the line of fracture away).

An earthquake can cause a tsunami only when all the following three conditions are met:

• The quake reached a magnitude of 7 or more.
• Its hypocenter is located near the Earth's surface to the seabed.
• It causes a vertical displacement of the seafloor, which causes the overlying water column on the move.

Only one percent of the 1860-1948 earthquake caused measurable tsunamis.

Spread

Tsunamis differ fundamentally from waves caused by storms. The latter are referred to as a function of water depth versus wavelength as shallow water wave or deep water wave. For deep-water waves, the wave has no contact with the ground and the deeper water layers remain unmoved. Thus, the speed of propagation does not depend on the water depth. Moves such a wave in shallower waters, it becomes a shallow-water wave, that moves the entire water column and is thereby slowly. Due to their large wavelength tsunamis are almost everywhere shallow water waves. So you move in contrast to wind waves the whole water column. Your speed is therefore practically everywhere depending on the water depth.

Tsunamis are gravity waves

Propagation is always possible, when a displacement from an equilibrium position, in this case an increase or decrease of the water level, an opposite return force to the sequence has. When ocean waves acts as a restoring force of gravity, which is working towards a horizontal water surface as possible. For this reason, tsunamis are considered as gravity waves. A tsunami is thus in particular no pressure and no sound wave. Compressibility, viscosity and turbulence are not relevant. To understand the physics of tsunamis, it is sufficient to consider the potential flow of an ideal, ie, inviscid, incompressible and irrotational fluid. Mathematically, tsunamis described as solutions of the Korteweg -de Vries equation.

The theory of gravity waves is simplified in two limiting cases of the low and the shallow water wave. Normal waves, caused for example by wind, vessels or into the water thrown stones, are usually deep-water waves, since their wave base is usually located on the bottom of the water, ie where the wave has no effect. A tsunami, however, is also in the deepest ocean a shallow water wave, because the entire water column is moved and can notice a slower movement in the direction of wave propagation and on the ocean floor. This corresponds to that in the tsunami wavelength (distance from one wave crest to the next) is much larger than the water depth. In this case, a much larger amount of water is moved.

A tsunami is simplified by two basic parameters described:

• Its mechanical energy;
• Its wave period: the time that elapses happen in the two wave crests same point.

During the propagation of a tsunami, these two parameters remain substantially constant, since the energy losses are negligible by friction due to the large wavelength.

Tsunamis seismic nature have long wave periods, the move of between ten minutes and two hours. By other events as earthquakes generated tsunamis often have shorter wave periods ranging from several minutes to a quarter of an hour. Other properties, such as wave height and length or the propagation velocity depend not only on the two basic parameters depend only on the depth of the sea.

Speed

The speed of a tsunami depends on the depth of the sea from the deeper the sea, the faster and the more shallow, the slower the tsunami. The speed c of a tsunami wave (more precisely, their phase velocity ) is derived from the root of the product of the gravitational acceleration g ( 9.81 m / s ² ) and water depth h; Thus, the following applies:

The propagation velocity is thus in oceans (water depth 5000 m) 800 km / h Which is comparable to the speed of travel of an aircraft. Tsunamis can so within a few hours to cross entire oceans and up to 20,000 km spread, without being noticed immediately. In contrast, the waves generated by wind speeds between 8 and 100 km / h At low water depth, so near the coast, the tsunami, as can be seen on the adjacent animation slows down. This reduces the wavelength, making it eventually results in an increase of wave height, and to break the shaft.

Gravity waves come about through the gleichtaktige movement of large masses of water. Every single part volume of water moved, this is only tiny amounts. For a shallow-water gravity wave with amplitude A in a water depth h one can even specify quantitatively: The speed with which the one involved in the wave matter moves circularly, is by a factor of A / h is smaller than the phase velocity c of the wave. For a large tsunami, this factor is of the order 10-5: When a wave in the open sea with c = 200 m / s (= 720 km / h ) spreads, the water elements move only at 2 mm / s, which is opposite currents and wind waves is completely negligible and not directly observable. At the same time it explains the only low energy loss of the gravity wave during their migration.

Wavelength

Tsunamis because their wavelength is much larger than the depth of the sea, so-called shallow water waves. Typical wavelengths at tsunamis are between 100 and 500 km. The wavelengths of wind- generated waves only achieve between 100 and 200 meters. In general, for waves the relationship

Between speed, wavelength and wave period.

With the tsunami speed from above and the specification of the wavelength, typical wave periods over

Be calculated as:

Is the time that elapses before the arrival of the second shaft.

The longer the wavelength, the lower the energy loss during the propagation. In circular propagation is the energy with which a wave incident on a coastal strip, to a first approximation inversely proportional to the distance from the origin of the tsunami.

Speed ​​and wavelength of a tsunami as a function of water depth

Amplitude

The wave height (amplitude) of the tsunami depends on the energy and the water depth. When tsunamis long wavelength applies:

This means that the amplitude increases at a lower depth. In the open ocean it decreases with increasing distance only by a factor from ( spherical waves that propagate into the deep, take by a factor of from ). This can be illustrated when a stone is thrown into a shallow puddle. The amplitude of the water waves only decreases significantly because the energy is distributed over a larger circular wave crest. The energy loss due to the internal friction of the water molecules is extremely small, and the pulse is passed almost without attenuation to the adjacent water molecules. The energy of a tsunami wave attenuates in the open sea only by their geometric spreading from. Tsunami waves can therefore circumnavigate the globe several times. When tsunamis smaller wavelength - usually not caused by earthquakes - the amplitude decrease much faster with distance.

In the open ocean, the amplitude is rarely more than a few feet. The water level is thus increased only slowly and only by a small amount and lowered again, which makes the presence of a tsunami is usually not noticed on the open sea.

The destructive power of a tsunami is not determined basically by its amplitude, but by the wave period and by the transported amount of water.

Impinging on the coast

The energy of the waves, which was widely distributed on the open ocean, is focused by non-linear mechanisms when the tsunamis occur near the coasts. Then the waves are slowed, compressed and put it on yourself.

Increasing the amplitude

Near the coast, the water is flat. As a result, wavelength and phase velocity decrease ( see table). Due to the conservation of total energy ( see conservation of energy ), the available energy is converted into potential energy, so that the amplitude of the wave and the velocity of matter involved increase. The energy of the tsunami wave becomes more and more concentrated until it hits with full force on the coast. The energy content of a wave train is proportional to h cross section times the wavelength times the square of the particle velocity and is independent of the wave crest height in the above-mentioned approximation

Typical amplitudes of the impact of a tsunami on the coast are in the order of ten meters; was on 24 April 1771 in the vicinity of the Japanese island of Ishigaki reported in flat terrain, from a peak of 85 meters. Near the shore of a deep lake shore part, the amplitude increase to about 50 meters. Runs a tsunami in a fjord, so the shaft can accumulate to well over 100 meters.

In the Lituya Bay in Alaska waves were detected, although they did not exceed 100 meters in height, but a 520 meter high hill rolled over (Mega Tsunami ). However, these gigantic waves did not occur as a remote effect of an earthquake, but by water displacement in the fjord itself: Violent earthquake could slopes slide into the fjord and brought this suddenly overflowing.

The piling up of the water masses happens only by the gradual flattening of the water, the consequent reduction of the propagation velocity and thus the wavelengths, which must lead to the increase of the amplitude of the water masses. Is also the coast or bay -shaped, it is additionally a lateral superposition or focusing of the masses of water, which can increase the induced by the vertical water profile amplitude increase still significantly, particularly for the occurring resonance ( wavelengths in the order of the linear bay dimensions). At high cliffs of the mainland of the tsunami may indeed accrue to considerable surf heights, but then penetrates usually not far into the hinterland before. Furthermore, rising atolls with linear dimensions much smaller than the wavelength of tsunamis in the open ocean are hardly noticed and only washed over flat steeply from the deep sea.

Refraction effects

The variation of the wave propagation velocity on approach to the coast of the tsunami will depend on the depth profile of the sea bottom. Depending on local conditions can cause refraction effects: just as light in the transition from air into water or glass changes its direction, as well as a tsunami changes direction as it passes obliquely through a zone in which changes in the ocean depths. Depending on the origin of tsunamis and underwater topography this may lead to individual coastal areas to focus the tsunami. This effect is not sharply separated from the funnel effect of a fjord and may overlap with this one.

Retreat of the sea

As an acoustic signal, there is also a tsunami is not of a single wavelength, but a whole set of waves with different frequencies and amplitudes. Waves of different frequencies propagate at slightly different speeds. Therefore, the individual waves add a package in from place to place and from minute to minute different ways. A tsunami can be observed at a point on the coast first wave crest or trough as first. If the cause of the tsunami a Hangabrutsch or breaking down of a continental plate, so water is accelerated to the sole way. Water is displaced, and at first it creates a trough. Then the water moves back, and the wave crest is formed. Upon arrival of the wave at the coast, first withdraw the coastline, possibly by several 100 meters. When the tsunami hits an unprepared population, it can happen that people are attracted by the unusual spectacle of the receding sea, rather than that they use the remaining minutes until the arrival of the tsunami in order to save on higher ground.

Stokes flow

If the amplitude of a tsunami near the coast is no longer negligible small compared to the water depth, then a part of the vibration of the water converts to a general horizontal movement called Stokes flow. In the immediate coast this rapid horizontal movement is more responsible than the rise of the water level for destruction.

Near the coast, the Stokes flow has a theoretical speed of

With the phase velocity of the tsunami and the gravitational acceleration, so:

The Stokes flow thus reaches several tens of km / h

Hazards and protective

Tsunamis are among the most devastating natural disasters, with which man can be faced, since a powerful tsunami can its destructive power over thousands of kilometers carry far or even wear around the globe. Without protective coastal rocks already three meter high waves can penetrate several hundred meters deep into the country. The damage caused by a tsunami in penetrating, be increased when the water mass flow off again. The peak height of a tsunami has only limited explanatory power of his destructive power. Especially at low altitudes country can also be a low wave height of only a few meters cause similar destruction like a big tsunami, with dozens meters.

On 26 December 2004 were killed by the big tsunami in Southeast Asia at least 231,000 people. The wave was triggered by one of the strongest earthquake since records began. The devastating effect here was based mainly on the large volume of water that hit per kilometer coastline on the land, while the wave height was relatively low, usually with only a few meters.

Danger zones

Most commonly tsunamis occur on the western and northern edges of the Pacific plate, the Pacific Ring of Fire.

Japan had the highest number of deaths complain due to its geographical location in the last thousand years by tsunami; During this time about 160,000 people died. In the last 100 years, however, focused only 15 percent of the 150 registered tsunamis, damage or cost lives. Today, Japan has an effective early warning system, and for the population are regular training programs instead. Many Japanese coastal towns protect themselves by building huge dams, such as a ten -meter high and 25 meter wide walls on the island of Okushiri.

In Indonesia, however, now looks even half of the tsunami disaster, because most coastal residents are not informed about the signs that announce a tsunami. In most cases also the land is very flat and the water mass flow to the interior. See also: earthquake in the Indian Ocean tsunami in 2004 and prior to Java July 2006.

Also occur along the European coasts to tsunamis, although much less frequently. As the African plate pushes northward under the Eurasian plate, can be caused by earthquakes in the Mediterranean and in the Atlantic also tsunamis.

A meteorite impact could trigger a tsunami. The probability that the heavenly bodies collides with the sea, is greater than that it hits the ground, because oceans constitute the largest part of the earth's surface. In order to trigger a tsunami, but very large meteorites are needed.

Effects

Compared to the direct damage caused by earthquakes, volcanic eruptions, landslides or avalanches, which mostly occur only locally or in geographically relatively narrow areas, tsunamis can cause even to thousands of kilometers distant shores devastation and loss of human life.

A Coast offshore reefs, sandbars or shallow water areas can reduce the destructive power of tsunami waves, sometimes special Breakwater buildings like they were built to some particularly vulnerable coastal areas of Japan. But there are also examples where necessary passbands in such shelters, the flow speed and wave height of the tsunami locally hazardous and therefore increased the damage in the area to be protected actually increased.

Experience of Japan indicate that tsunami amplitudes below 1.5 m pose no threat to people and structures in general. But there are cases such as the nightly intrusion of the tsunami of 1992 in Nicaragua, where especially children, who slept on the floor in fishermen's huts on the beach, drowned in the some places only 1 to 1.5 m rising water. When wave heights of 2 m are lightweight structures made ​​of wood, sheet metal, clay, and concrete block buildings usually totally destroyed in waves over 3 m in height. When wave heights of 4 m and the number of deaths increases dramatically. Solid reinforced concrete buildings can withstand tsunami waves of up to 5 m high contrast. Therefore, the upper floors of reinforced concrete high-rise buildings or hotel in case of very short warning times and less chances of escaping outdoors can also be used as places of refuge.

Tsunamis often penetrate hundreds of meters, especially high waves even several kilometers in shallow coastal areas before and there not only devastate human settlements, but also make agricultural land and wells by salinity and silting unusable. Since the mass of water to penetrate and flow back several times, the flooding areas with mud and sand, smashed objects and building parts are covered. Ships in ports are thrown into the country, blocked roads, washed out railroad tracks and thus unusable. Low-lying areas of the harbor and fishing villages are often far under water and have become uninhabitable. These threats come from leaky drums with fuels or chemicals, flooding of sewage treatment plants or sewage pits and bodies of people and animals. Especially in tropical regions increases the acute risk of drinking water poisoning, outbreak of epidemics and the like. The direct tsunami damage are often exacerbated by the outbreak of fire due to broken gas lines and electrical short circuit, often in conjunction with spilled fuel from wrecked ships and vehicles or leaking tanks in ports. Consequential damage resulting from the complete disaster of coastal industrial plants, as in 2011 at the Japanese nuclear power plant in Fukushima, where there was a partial meltdown of uncontrolled release of radioactive substances. Also coastal habitats ( mangroves, coral reefs, and others) can be severely damaged by the tsunami and permanently disturbed.

Early warning systems

Tsunami early warning systems take advantage of that certain information on the possible occurrence of a tsunami can be obtained before the tsunami itself may exert its destructive power. Seismic waves propagate much faster than the tsunami itself, for example, is a sufficiently dense network of seismic stations available, so leave after just a few minutes accurate conclusions about the place and the strength of pull of an earthquake, and thus a potentially emanating therefrom tsunami hazard forecasting. GPS stations measure centimeter displacement of the earth's surface, which can be extrapolated to the sea floor and is an accurate predictor of the tsunami hazard. Buoys measure the tsunami wave directly or on the high seas, so that a pre-warning remains.

Many states have set up technical early warning systems in recent decades that can detect by recording seismographic plate movements tsunamis at its very source, so that the exposed coastal areas can be evacuated by the time gained advantage. This is especially true for the Pacific Ocean. There, a network of sensors has been set up on the sea floor and other important places 1950-1965, which continuously measures all relevant data and reports to Hawaii via satellite to the Pacific Tsunami Warning Center ( PTWC ) in Honolulu. This evaluates the data ongoing and can spread a tsunami warning within 20 to 30 minutes. As the affected States have an effective communication system and regional emergency plans in case of disaster is a good chance that timely rescue measures can be initiated.

Some coastal cities in Japan protect themselves by up to ten meters high and 25 meters wide dikes, whose gates can be closed within a few minutes. In addition people watching from the coastal protection with cameras to sea level changes. An early warning system for earthquakes of magnitude 4 are automatically tsunami alarm, so that the residents can be evacuated.

Unfortunately, some States concerned by the danger these systems have not yet, and its information network is so poorly developed that a warning be restricted or not possible. This concerns in particular the Indian Ocean. It also appears that authorities do not forward out of fear of the loss of tourism revenue source tsunami warnings.

The states of the Indian Ocean have decided after the tsunami disaster in South Asia in 2004, set up a tsunami early warning system.

Indonesia has ordered a German early warning system - the German Indonesian Tsunami Early Warning System ( GITEWS ) - which, on behalf of the German Federal Government, Research Centre for Geosciences (GFZ ) Potsdam and seven other institutions was developed, the November 2008 went into trial operation, and since March 2011 in operating mode is. By seismic sensors and GPS technology allows this complex system even more accurate predictions than the PTWC. At first buoys were also in use, which floated on the sea surface. This proved, however, as unreliable.

Malaysia, the Malaysian National Tsunami Early Warning System ( MNTEWS ) built, which currently allows for alerting the population within twelve minutes after the event. For 2012, the reduction of the alarm time was announced at ten minutes.

Taiwan adopted on 14 November 2011 an undersea seismic observation system in operation. The attached in approximately 300 m water depth in a submarine cable components of the early warning system are distributed over a distance of 45 km and to shorten the warning time for tsunamis and earthquakes continue.

The coordination of existing systems into a global system is driven since mid-2005. For the detection of the earthquake, the seismic evaluations of the UN are used, which are typically used for monitoring of the complete nuclear test ban treaty CTBT. For this purpose only the alarm systems in the national alarm systems need to be integrated because the detection capabilities are already available. The messages of these artificial or natural caused by nuclear explosions earthquake converge in Vienna with the Comprehensive 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 ) is built up in the Atlantic and in the Mediterranean.

In all early warning systems, the problem is that false alarms while an unnecessary evacuation may be costly and undermine people's confidence in the forecasts.

Behaviors in acute tsunami hazard and tsunami warning

When staying in tsunami prone coastal regions, inter alia, the following are recommended:

• Keep up near the coast and you feel a strong earthquake, then immediately rush to a high or offshore haven, since strong offshore earthquake can cause a tsunami. However, following such a quake in only about 10 to 20 % of cases also a dangerous tsunami. Nevertheless, you should not be hasty return to deeper offshore areas, but best to wait an official all-clear, unless you can safely estimate from your position that the tremors within an hour no tsunami followed.
• Are you on the coast and take a true unexpected rapid rise or fall of the water table within minutes, then you rush also now at a high or offshore haven. In any case, you should run out into sudden dry bays. The first high wave follows with certainty within a few minutes.
• Take strong earthquake tremors within a building true, then they behave as earthquake- specific instructions. Is the building in a potential tsunami inundation area, then leave the building immediately after the decay of the shock and rush you to a higher or more distant coastal retreat (except for sound and not damaged by the quake reinforced concrete high-rise buildings ).
• Find people in your area about your perceptions and warn them accordingly.
• Are you still swept by the wave, then you should, wherever possible, try to immediately rise to a high, stable rock or a stable home or hold on to a tree or pole, because to climb as high as possible and to remain. Only in this way you may be able to avoid being detected by subsequent waves or washed out by the flooding water masses then back into the open sea. Even if the surrounding water no longer flows and has calmed down like a lake, you should not climb down from your elevated, secure location, because the water could either flow back shortly from the sea or the next wave could come immediately. Then, when finally all the water has flowed back into the sea, the flooding will repeat every 30 to 60 minutes with decreasing intensity, for example.
• Safe havens should already be by no means leave after withdrawal of the first wave (s ) again. They may have to endure more than five hours on the higher places of refuge and should return to low-lying coastal areas until they are officially clear.

Typical phenomena of tsunamis

• Tsunamis consist of a series of consecutive, very long period sea waves. These are usually caused by strong submarine earthquake, but also by volcanic eruptions or landslides.
• Most tsunamis occur in the Pacific Ocean, it exists but also in all other oceans and marine areas. Although tsunamis are rare, they pose a great danger represents a reliable protection against tsunamis can not be reached, except to avoid potentially tsunami -prone areas in the settlement and development in low-lying (less than 30 m above sea level. NN ) fields.
• Tsunamis can cause great destruction within a few minutes on the coasts near its origin and cost many lives. Strong tsunamis take effect but also in distant shores because they can spread across entire ocean basins of hours of time in the course.
• The speed at which tsunamis spread depends on the water depth. In deep oceans it reaches 800 km / h and in shallow water, it is about 30 to 50 km / h
• A tsunami usually consists of several wave crests, which up to follow each other at a distance of tens of minutes over an hour and often accumulate in subsequent wave mountains to maximum heights on the coast.
• The distances between the wave crests in deep, open sea some 100 km and reduce in shallow water areas up to about ten kilometers.
• The wave heights are low in deep, open sea, usually smaller than 80 cm and can be fixed on the basis of long wavelengths for ships safe and only by special buoys or satellite altimetry. When approaching the coast, especially in shallow bays, the water masses more than 30 up to 50 m can but about ten meters, in extreme cases, high pile, flat land flooding inland behind the coast up to several kilometers and can cause horrific devastation.
• Persons on shore take an approaching tsunami is not necessarily true as a wave, but as a sudden, in comparison to the ebb and flow much faster drop or rise in sea levels. You notice, for example, that suddenly water running over the short time before the dry ground for future reference already waist-deep are some moments in the water and cars are swept away like matchboxes. Sea levels are rising more quickly if necessary by several meters and flooded deep -lying coastal areas. Then the water is running in the reverse direction back from the sea and shipped destroyed at the expiration of buildings and debris for miles on the open sea.

List of Tsunamis

See: List of Tsunamis