Causes of tsunamis: signs of occurrence and danger of tsunamis. The largest tsunamis in the world: wave height, causes and consequences


A is the depth of the ocean (the so-called shallow water approximation, when the wavelength is significantly greater than the depth). With an average depth of 4 km, the propagation speed is 200 m/s or 720 km/h. In the open ocean, the wave height rarely exceeds one meter, and the wave length (the distance between crests) reaches hundreds of kilometers, and therefore the wave is not dangerous for shipping. When waves enter shallow water, near the coastline, their speed and length decrease, and their height increases. Near the coast, the height of a tsunami can reach several tens of meters. The highest waves, up to 30-40 meters, form along steep shores, in wedge-shaped bays and in all places where focusing can occur. Coastal areas with closed bays are less dangerous. A tsunami usually appears as a series of waves; since the waves are long, more than an hour can pass between wave arrivals. That is why you should not return to the shore after the next wave leaves, but wait a few hours.

The wave height in coastal shallow water (H shallow), which does not have protective structures, can be calculated using the following empirical formula:

H fine = 1.3 · H deep. · (B deep / B shallow) 1/4, m

where: H deep. - initial wave height in a deep place;

B depth - depth of water in a deep place; B chalk - water depth in the coastal shallows;

Causes of tsunami formation

Most common reasons

Other Possible Causes

  • Human activity. In our age of atomic energy, man has in his hands a means of causing shocks that were previously only available to nature. In 1946, the United States carried out an underwater atomic explosion with a TNT equivalent of 20 thousand tons in a sea lagoon 60 m deep. The wave that arose at a distance of 300 m from the explosion rose to a height of 28.6 m, and 6.5 km from the epicenter still reached 1.8 m. But for long-distance propagation of the wave, it is necessary to displace or absorb a certain volume of water, and a tsunami from underwater landslides and explosions are always local in nature. If several hydrogen bombs are simultaneously detonated on the ocean floor, along any line, then there will be no theoretical obstacles to the occurrence of a tsunami; such experiments have been carried out, but have not led to any significant results compared to more accessible types of weapons. Currently, any underwater testing of atomic weapons is prohibited by a series of international treaties.
  • The fall of a large celestial body can cause a huge tsunami, since, having a huge falling speed (tens of kilometers per second), these bodies have colossal kinetic energy, and their mass can be billions of tons or more. This energy is transferred to the water, resulting in a wave.
  • Wind can cause large waves (up to about 20 m), but such waves are not tsunamis, since they are short-period and cannot cause flooding on the coast. However, the formation of a meteo-tsunami is possible with a sharp change in pressure or with a rapid movement of the anomaly atmospheric pressure. This phenomenon is observed in the Balearic Islands and is called Rissaga.

Signs of a tsunami

  • A sudden rapid withdrawal of water from the shore over a considerable distance and drying out of the bottom. The further the sea recedes, the higher the tsunami waves can be. People on the shore who are unaware of the danger may remain out of curiosity or to collect fish and shells. In this case, it is necessary to leave the shore as soon as possible and move as far away from it as possible - this rule should be followed when, for example, in Japan, on the Indian Ocean coast of Indonesia, or Kamchatka. In the case of a teletsunami, the wave usually approaches without the water receding.
  • Earthquake. The epicenter of an earthquake is usually in the ocean. On the coast, the earthquake is usually much weaker, and often there is no earthquake at all. In tsunami-prone regions, there is a rule that if an earthquake is felt, it is better to move further from the coast and at the same time climb a hill, thus preparing in advance for the arrival of the wave.
  • Unusual drift of ice and other floating objects, formation of cracks in fast ice.
  • Huge reverse faults at the edges of stationary ice and reefs, the formation of crowds and currents.

Tsunami danger

It may not be clear why a tsunami several meters high turned out to be catastrophic, while waves of the same (and even much greater) height that arose during the storm did not lead to casualties or destruction. There are several factors that lead to catastrophic consequences:

  • The height of the wave near the coast in the event of a tsunami, generally speaking, is not a determining factor. Depending on the configuration of the bottom near the coast, the tsunami phenomenon can occur without a wave at all, in the usual sense, but as a series of rapid ebbs and flows, which can also lead to casualties and destruction.
  • During a storm, only the surface layer of water moves. During a tsunami - the entire thickness of water, from the bottom to the surface. At the same time, during a tsunami, a volume of water splashes onto the shore that is thousands of times greater than storm waves. It is also worth considering the fact that the length of the crest of storm waves does not exceed 100-200 meters, while the length of the tsunami crest extends along the entire coast, and this is more than one thousand kilometers.
  • The speed of tsunami waves, even near the shore, exceeds the speed of wind waves. The kinetic energy of tsunami waves is also thousands of times greater.
  • A tsunami, as a rule, generates not one, but several waves. The first wave, not necessarily the largest, wets the surface, reducing resistance for subsequent waves.
  • During a storm, excitement increases gradually; people usually manage to move to a safe distance before large waves arrive. The tsunami comes suddenly.
  • Destruction from a tsunami can increase in the harbor - where wind waves are weakened, and therefore residential buildings can be located close to the shore.
  • Lack of basic knowledge among the population about possible danger. So, during the 2004 tsunami, when the sea retreated from the coast, many local residents remained on the shore - out of curiosity or out of a desire to collect fish that had not managed to escape. In addition, after the first wave, many returned to their homes to assess the damage or try to find loved ones, unaware of subsequent waves.
  • The tsunami warning system is not available everywhere and does not always work.
  • The destruction of coastal infrastructure aggravates the disaster, adding catastrophic man-made and social factors. Flooding of lowlands and river valleys leads to soil salinization.

Tsunami warning systems

Tsunami warning systems are based mainly on the processing of seismic information. If an earthquake has a magnitude of more than 7.0 (in the press this is called points on the Richter scale, although this is a mistake, since magnitude is not measured in points. The magnitude is measured in points, characterizing the intensity of ground shaking during an earthquake) and the center is located under water, then A tsunami warning is issued. Depending on the region and the population of the shores, the conditions for generating an alarm signal may be different.

The second possibility of warning about a tsunami is a warning “after the fact” - a more reliable method, since there are practically no false alarms, but often such a warning can be generated too late. The warning after the fact is useful for teletsunamis - global tsunamis that affect the entire ocean and arrive at other ocean boundaries a few hours later. Thus, the Indonesian tsunami in December 2004 is a teletsunami for Africa. A classic case is the Aleutian tsunami - after a strong splash in the Aleutians, you can expect a significant splash in the Hawaiian Islands. Bottom hydrostatic pressure sensors are used to detect tsunami waves in the open ocean. A warning system based on such sensors with satellite communication from a near-surface buoy, developed in the USA, is called DART (en:Deep-ocean Assessment and Reporting of Tsunamis). Having detected a wave in one way or another, one can quite accurately determine the time of its arrival in various populated areas.

An essential aspect of the warning system is the timely dissemination of information among the population. It is very important that the population understands the threat a tsunami poses. Japan has many educational programs on natural disasters, and in Indonesia the population is mostly unfamiliar with tsunamis, which was the main reason large quantity victims in 2004. It is also important the legislative framework on the development of the coastal zone.

The largest tsunamis

XX century

  • 5.11.1952 Severo-Kurilsk (USSR).

see also

Sources

  • Pelinovsky E. N. Hydrodynamics of tsunami waves / IAP RAS. Nizhny Novgorod, 1996. 277 p.
  • Local tsunamis: warning and risk reduction, collection of articles. / Edited by Levin B.V., Nosov M.A. - M.: Janus-K, 2002
  • Levin B.V., Nosov M.A. Physics of tsunamis and related phenomena in the ocean. M.: Janus-K, 2005
  • Earthquakes and tsunamis - study guide - (contents)
  • Kulikov E. A. “Physical foundations of tsunami modeling” (training course)

Tsunami in art

  • "Attention, tsunami!" - Feature Film(Odessa Film Studio, 1969)
  • “Tsunami” - song by V. S. Vysotsky, 1969
  • “Tsunami” is the name of the album of the group “Night Snipers” ().
  • “Tsunami” - a novel by Gleb Shulpyakov
  • "Tsunami" - Korean film, 2009
  • “2012 (film)”, 2009
  • Film "Deep Impact", 1998
  • Tsunami 3D - thriller 2012
  • Catastrophic natural phenomena. Electronic version of the rescuer's textbook by a team of authors (Shoigu S.K., Kudinov S.M., Nezhivoy A.F., Nozhevoy S.A., under the general editorship of Vorobyov Yu.L.), published by the Ministry of Emergency Situations of Russia in 1997.

Notes

Links

Tsunami in Wiktionary
Tsunami on Wikimedia Commons
Tsunami in Wikinews

As a result of the earthquake, shifts begin to occur, as one part of the bottom begins to rise, and the rest sinks. This all leads to the movement of water reaching the surface, but when this entire mass tries to return to its original state, huge waves form.

If tremors occur in the open ocean, the height of the waves generated there very rarely exceeds 1 meter; it is believed that deep ocean earthquakes are not dangerous for shipping, since the waves have a large width between the crests.

When will the movement earth's crust happens closer to the shore, the speed of the wave decreases, and its height, on the contrary, increases and can sometimes grow to 30 or 40 meters. It is these massive layers of water that hit the shore, and they are called tsunamis.

Reasons for the birth of the wave

As mentioned above, an underwater earthquake is one of the most common causes of the formation of huge waves. It accounts for up to 85% of all tsunamis, but scientists say that not all tremors in the ocean provoke the birth of high waves. Thus, about 7% of huge waves are formed due to landslides. For example, we can cite a case that took place in Alaska: there was a landslide that fell into the water from a height of 1100 meters and thereby provoked a tsunami with a wave of more than 500 meters. Of course, such cases are very rare, because landslides under water in river deltas occur more often, and they do not pose a danger.

Another reason for the formation of a tsunami is a volcanic eruption, which accounts for up to 4.99% of tsunamis. Such an underwater eruption is similar to a regular earthquake. However, the mechanism and consequences of cortical movement are fundamentally different. If a strong volcanic eruption occurs, not only tsunamis are formed from it, during the eruption process the rock cavity cleared by lava is filled with water, after the eruption an underwater depression or a so-called underwater lake is formed. The eruption produces a very long wave. An example of a relatively recent birth of this type of wave is the Krakatoa volcanic eruption.

The cause of a tsunami can be meteorites, or rather their fall into the ocean, but such cases are very rare. In each of the above cases, a tsunami is formed in almost the same way: water moves vertically and then returns to its original position.

The word "tsunami" is a Japanese term and means "wave in the bay." IN modern use, a tsunami is an ocean wave that is caused by displacement of water and is fundamentally different from an ordinary wave. As a rule, an ordinary wave is formed by winds or the natural gravitational influence of the Sun and Moon. Underground earthquakes, volcanic eruptions, landslides or even underwater explosions can displace masses of water, creating a large wave or series of waves - a phenomenon known as a tsunami.

Tsunamis are often called tidal waves, but this is not an accurate description since tides have little effect on giant waves. Scientists often use the term "seismic sea waves" as a more precise name for what we usually call a tsunami or tidal wave. In most cases, a tsunami is not a single wave, but a series of large waves.

How does a tsunami start?

The strength and behavior of a tsunami are difficult to predict. Any earthquake or underwater event can be a precursor to a tsunami, although most underwater earthquakes or other seismic events do not create giant waves, which is why they are so difficult to predict. A fairly large earthquake may not cause a tsunami at all, but a small one may provoke very large, destructive waves. Scientists believe that it is not so much the strength of the earthquake as its origin. An earthquake in which tectonic plates shift abruptly vertically is more likely to cause a tsunami than horizontal movement.

Far out in the ocean, tsunami waves are not very high, but they move very quickly. The National Oceanic and Atmospheric Administration (NOAA) reports that some tsunami waves can travel at speeds of more than 100 km/h. Far from the sea, where the water depth is deep, the wave may be almost imperceptible, but as the tsunami approaches land and the ocean depth decreases, the speed of the tsunami wave slows and the height increases sharply - along with its destructive potential.

How does a tsunami approach the coast?

A strong earthquake in the coastal area is a signal that a tsunami may occur and therefore immediate evacuation is necessary. In regions where the threat of tsunamis is persistent, authorities should have a siren system or other means of transmitting information, as well as established plans for evacuating low-lying areas. Once a tsunami reaches the shore, the waves can last anywhere from 5 to 15 minutes and do not follow any particular pattern. NOAA warns that the first wave is usually not the largest.

One of the signals that a tsunami is approaching is that the water moves away from the shore very quickly (but in such a situation you will have very little time to evacuate). Unlike the portrayal of tsunamis in films, the most dangerous tsunamis are not those that hit the shore as high waves, but those that have long waves containing a huge volume of water. In scientific terms, the most destructive waves are those that come ashore with a significant wavelength and not necessarily a large amplitude. On average, a tsunami lasts about 12 minutes - six minutes of "takeoff" during which water can flow inland for considerable distances, and then it recedes for about six minutes. However, sometimes tsunamis can last for several hours.

Tsunami in history

  • The first historically recorded tsunami occurred in 426 BC, and was described by the ancient Greek historian Thucydides in the book “History of the Peloponnesian War”, in which he wrote that the cause of such waves are earthquakes in the ocean.
  • An earthquake, the cause of a tsunami in 365 AD, devastated Alexandria in North Africa.
  • The 1908 Messina earthquake and tsunami killed more than 123,000 people in Sicily and Calabria.
  • happened strong earthquake, off the coast of Indonesia. The energy released by the earthquake caused a tsunami that hit the coasts of Indonesia, Sri Lanka, India and Thailand. Over 200,000 people died.
  • In March 2011, a magnitude 9.0 earthquake rocked Japan, sending massive waves across its coastline. More than 18,000 people became victims; buildings, roads, seaports and railways; A serious accident occurred at a nuclear power plant.

Environmental consequences of the tsunami

A tsunami is one of the most dangerous disasters that can occur on our planet. Only earthquakes and eruptions of underground magma can compare with them in their consequences.

It is quite natural that, like most others terrible events, the tsunami is of considerable interest to humanity. Some are looking for information about what destruction giant waves have caused throughout the history of mankind or, others are interested in how a tsunami occurs, and still others want to know what parameters it is characterized by. And they do it right - forewarned is forearmed.

The process of tsunami formation

An earthquake occurring underwater or near the coast (less commonly, an earthquake) leads to vibrations of the ocean surface. Masses of water moving vertically cause the appearance of not too high, but rather long waves that can travel several thousand kilometers at a speed of up to 1000 km/h and reach the shore. Approaching the coastal zone, it slows down and decreases in length, while simultaneously increasing in height. The narrower the bay or bay that the waves reach, the higher they will be - sometimes their size exceeds 50 m. However, even a ten-meter tsunami is already enough to create significant destruction.

Main signs of a tsunami

You can find out about the occurrence of a tsunami thanks to warning systems, which are certainly available in tsunami-prone areas. Although there are signs of a tsunami that are visible even to the naked eye ordinary people who found themselves in the coastal zone shortly before the occurrence of this natural phenomenon. Among them are:

– rapid retreat of the sea from the coast;

– tremors indicating an earthquake occurring nearby, which, as is known, can lead to a tsunami;

– numerous debris of various origins floating on the surface of the water, and sharp surges of water at the edge of the ice or reefs.

Factors accompanying a tsunami

As a rule, the occurrence of a tsunami is accompanied by destructive consequences that are caused by factors such as the action of first a sea wave and then an air wave, as well as hydrodynamic pressure.

Secondary factors in the occurrence of the phenomenon are:

– flooded area;

– destroyed buildings;

– dead people and animals;

– contaminated water and soil (which can, in turn, lead to the emergence and spread of infections);

– destroyed forests and agricultural lands.

Tsunamis and their characteristics

The main characteristics for this natural phenomenon are:

– the height of the tsunami wave, that is, the vertical distance from its crest to its base. When it occurs, this value is 0.5–5 m, but near the shore it can reach 70 m;

– wavelength equal to the distance between the crests of adjacent waves. Usually this value ranges from one to several tens (less often, up to two to three hundred) kilometers and depends on the depth of the ocean;

– the speed at which the tsunami moves. Typically between 50 and 100 km/h for the main part of the route, although 1,000 km/h can sometimes be reached near the epicenter.

Tsunami intensity scale

Based on their intensity, tsunamis are divided into 6 main types. This classification of tsunamis allows us to characterize the consequences of their action:

– 1 point means that the element was registered only by instruments. Most people are not even aware of such tsunamis;

– a tsunami with an intensity of 2 points can slightly flood the coast, but, again, only specialists can distinguish it from ordinary ocean waves;

– 3 points indicates that the tsunami has become noticeable. Small boats may become beached;

– 4 points. The destruction is quite serious, buildings are damaged, ships are washed ashore and even thrown onto it;

– a magnitude 5 tsunami destroys buildings and can lead to death;

– the element, the intensity of which reaches six points, devastates the coast almost completely.

On the pages of our website we have already talked about one of the most dangerous natural phenomena - earthquakes: .

These vibrations of the earth's crust often give rise to tsunamis, which mercilessly destroy buildings, roads, and piers, leading to the death of people and animals.

Let's take a closer look at what a tsunami is, what are the causes of its occurrence and the consequences it causes.

What is a tsunami

Tsunamis are high, long waves generated by a powerful impact on the entire thickness of ocean or sea water. The term “tsunami” itself is of Japanese origin. Its literal translation is “a big wave in the harbor” and this is not in vain, since in all their power they manifest themselves precisely on the coast.

Tsunamis are generated by sudden vertical displacement lithospheric plates that make up the earth's crust. These gigantic vibrations vibrate the entire thickness of the water, creating a series of alternating ridges and depressions on its surface. Moreover in the open ocean these waves are quite harmless. Their height does not exceed one meter, since the bulk of the oscillating water extends below its surface. The distance between the ridges (wavelength) reaches hundreds of kilometers. The speed of their spread, depending on the depth, ranges from several hundred kilometers to 1000 km/h.

Approaching the shore, the speed and length of the wave begins to decrease. Due to braking in shallow water, each subsequent wave catches up with the previous one, transferring its energy to it and increasing its amplitude.

Sometimes their height reaches 40–50 meters. Such a huge mass of water, hitting the shore, completely devastates the coastal zone in a matter of seconds. The extent of the destruction area deep into the territory in some cases can reach 10 km!

Causes of tsunami

The connection between tsunamis and earthquakes is obvious. But do vibrations in the earth's crust always generate tsunamis? No, tsunami are generated only by underwater earthquakes with a shallow source and magnitude greater than 7. They account for about 85% of all tsunami waves.

Other reasons include:

  • Landslides. Often a whole chain of natural disasters can be traced - a shift of lithospheric plates leads to an earthquake, which generates a landslide that generates a tsunami. This is precisely the picture that can be seen in Indonesia, where landslide tsunamis occur quite often.
  • Volcanic eruptions cause up to 5% of all tsunamis. At the same time, gigantic masses of earth and stone, soaring into the sky, then plunge into the water. A huge mass of water is shifting. Ocean waters rush into the resulting funnel. This dislocation generates a tsunami wave. An example of a disaster of absolutely terrifying proportions is the tsunami from the Karatau volcano in 1883 (also in Indonesia). Then 30-meter waves led to the death of about 300 cities and villages on the neighboring islands, as well as 500 ships.

  • Despite the presence of our planet’s atmosphere, which protects it from meteorites, the largest “guests” from the universe overcome its thickness. When approaching the Earth, their speed can reach tens of kilometers per second. If such meteorite has a large enough mass and falls into the ocean, it will inevitably cause a tsunami.

  • Technological progress has brought not only comfort to our lives, but has also become a source of additional danger. Conducted underground nuclear weapons testing, this is another reason for the occurrence of tsunami waves. Realizing this, the powers possessing such weapons entered into a treaty prohibiting their testing in the atmosphere, space and water.

Who studies this phenomenon and how?

The destructive effect of the tsunami and its consequences are so enormous that humanity has become the problem is to find effective protection against this disaster.

The monstrous masses of water rolling onto the shore cannot be stopped by any artificial protective structures. Most effective protection in such a situation there can only be timely evacuation of people from danger zone. For this necessary enough long term forecast impending disaster. Seismologists do this in collaboration with scientists from other specialties (physicists, mathematicians, etc.). Research methods include:

  • data from seismographs recording tremors;
  • information provided by sensors carried out into the open ocean;
  • remote sensing of tsunamis from outer space using special satellites;

  • development of models for the occurrence and propagation of tsunamis under various conditions.
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