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

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

When the movement of the earth's crust happens closer to the coast, the wave speed drops, and its height, on the contrary, rises and can sometimes grow up to 30 or 40 meters. It is these massive layers of water that crash onto the shore, and it is they who are called tsunamis.

Reasons for the birth of a 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. So, 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 the appearance of a tsunami with a wave of more than 500 meters. Of course, such cases are very rare, because landslides occur more often under water in river deltas, 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 the tsunami. Such an eruption under water is similar to an ordinary earthquake. However, the mechanism and consequences of crustal movement are fundamentally different. If a strong volcanic eruption occurs, not only tsunamis are formed from it, during the eruption the rock cavity cleaned with lava is filled with water, after the eruption an underwater depression or the so-called underwater lake is formed. As a result of the eruption, a very long wave is born. An example of a relatively recent birth of this type of wave is the volcanic eruption of Krakatoa.

The cause of the formation 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, the formation of a tsunami occurs in almost the same way: water moves vertically, and then returns to its original position.

Tsunamis are among the most dangerous hydrological phenomena of natural origin. Tsunami is a type of sea waves. The word "tsunami" comes from Japanese and means "big wave".

Sea waves are oscillatory movements of the aquatic environment of the seas and oceans, caused by the force of the winds, tides, underwater earthquakes and volcanic eruptions.

Tsunamis are not associated with winds, storms and hurricanes. Tsunamis also occur in calm weather, because they originate at the bottom of the ocean (sea) as a result of geological changes in the lithosphere. The causes of tsunamis can be sharp shifts of the bottom during strong earthquakes, large underwater landslides and volcanic eruptions.

Tsunami- These are marine gravity waves of great length, resulting from the vertical shift of significant sections of the seabed.

In most cases, tsunamis are caused by underwater earthquakes occurring under the ocean floor or near its coast. Tsunamis can also be generated by eruptions of underwater volcanoes. However, tsunamis occur only after those earthquakes that are associated with the rapid formation of faults, collapses and landslides at the bottom of the ocean. The fault is a rapid displacement of blocks of bottom rocks of the earth's crust and gives an impetus that sets in motion huge masses of water. This displacement pushes the water and causes a tsunami to form.

Big wave - Tsunami. Japanese artist of the 19th century K. Hokusai.

A tsunami, like any sea wave, is characterized by the height, length and speed of the waveform.

The height of a sea wave is the vertical distance between the crest of a wave and its bottom. Wavelength is the horizontal distance between two peaks (crests) of adjacent waves. The speed of movement of a waveform is the linear speed of horizontal movement of some element of the wave, such as a crest.

The height of a tsunami wave above its source in the ocean is 1–5 m. The wavelength can be 150–300 km. Tsunami propagation speed ranges from 50 to 1000 km/h.

The wavelength of a tsunami, its height and propagation speed depend on the depth of the ocean. The greater the depth of the ocean, the greater the wavelength and the speed of its propagation, the lower the wave height. Thus, the speed of tsunami propagation when crossing the waters of the Pacific Ocean, where the average depth is about 4 km, is 650-800 km/h, and when propagating along deep-sea places in the ocean, it can reach 1000 km/h. When a tsunami approaches the coast, where the depth decreases to 100 m, the tsunami propagation speed drops to 100 km/h. With a decrease in depth, the wavelength decreases, but the height of a tsunami wave when it enters shallow water increases sharply and can reach from 10 to 50 m.

Thus, when approaching the shore and, especially, when entering narrowing bays, tsunamis slow down their speed, but their height increases sharply. As a result, giant water shafts 10-15 m high, and sometimes up to 30-50 m high, can collapse on the coast. The damage caused by a tsunami can many times exceed the consequences of the earthquakes that caused them.

Where do tsunamis most often occur on Earth?

Most often, tsunamis hit the shores of the Pacific Ocean (75%), which is associated with the high volcanic activity of this basin. Over the past millennium, the Pacific coast has been hit by tsunamis about 1000 times, while tsunamis have been observed on the coasts of the Atlantic and Indian Oceans only a few dozen times.

In Russia, the eastern coast of Kamchatka and the Kuril Islands, Sakhalin Island and the Pacific coast are most prone to tsunamis.

Having a high speed of movement and a huge mass (in 1 m 3 of water, 1 ton of mass), a tsunami has an enormous destructive power. Running into oncoming coastal obstacles, the wave brings down all its energy on them, rising above them as a huge water wall, crushes, destroys and destroys everything that comes in its way. The destructive power of a tsunami is directly proportional to the speed at which the wave comes ashore.

The intensity of a tsunami based on the results of impact on the coast is estimated on a conditional six-point scale.

• 1 point- the tsunami is very weak, the wave is recorded only by special instruments (seaographs).
• 2 points- weak tsunami, can flood the flat coast. It is noticed only by experts.
• 3 points- average tsunami, noted by everyone. The flat coast is flooded, light ships can be washed ashore, port facilities are subject to minor damage.
• 4 points- strong tsunami. The coast is flooded. Coastal buildings are damaged, have weak and strong damage. Large sailing and small motor boats are washed ashore and then washed back into the sea. The shores are littered with sand, silt, tree fragments, human casualties are possible.
• 5 points- a very strong tsunami. The coastal areas are flooded. Breakwaters and breakwaters are badly damaged. Large ships washed ashore. The damage is also great in the interior parts of the coast. Buildings and structures have strong, medium and weak destruction depending on the distance from the coast. There are high storm surges at river mouths. There are human casualties.
• 6 points- catastrophic tsunami. Complete devastation of the coast and coastal areas. The land is flooded for considerable distances inland.

Varieties of tsunami

The scale of the consequences of a tsunami depends on the destructive power of the wave, the nature and natural features of the shore and coast, the effectiveness and timeliness of the measures taken to reduce damage.

Tsunamis are especially dangerous for settlements, cities and structures located on low-lying shores of the ocean, as well as those located on top of bays and bays that are wide open to the ocean, where tsunamis drive a large mass of water that floods the mouths and valleys of rivers 2-3 km from the sea. Tsunamis can cause large areas to be flooded with seawater.

History knows many examples of catastrophic tsunamis.

In 1703, about 100 thousand people died as a result of a tsunami in Japan.

In October 1994, a 2-3 m high tsunami hit the southern islands of the Kuril chain and the Japanese island of Hokkaido, which led to great destruction and loss of life.

The most powerful earthquake and the tsunami that followed it with a force of 8.9 points were noted in December 2004 in Southeast Asia. The earthquake, which occurred northwest of the island of Sumatra, caused a powerful tsunami that, moving at a speed of 800 km/h, hit the coasts of nine countries. Over 200 thousand people died as a result of the natural cataclysm. The population of Sri Lanka, Thailand, and Indonesia was especially affected.

Test yourself

1. Describe the natural phenomenon of a tsunami.
2. What consequences of a tsunami pose a danger to human life?

After school

In the safety diary, give examples of tsunamis that occurred in the world at the beginning of the 21st century. Specify their consequences and actions for protection of the population. Examples can be collected using the Internet and the media.

Tsunami (Japanese for "big harbor wave") are marine gravitational waves that result from the upward or downward displacement of extended sections of the seabed during underwater and coastal earthquakes. The propagation speed is from 50 to 1000 km / h. The height in the area of ​​occurrence is from 0.1 to 5 m, near the coast - from 10 to 50 m and above.

Tsunamis produce devastating destruction on land. For centuries, this unbridled natural phenomenon has kept people in fear, and therefore there are many inconsistencies about these killer waves.

A tsunami is a huge wave. Firstly, this is not one wave, but a whole series of waves coming ashore one after another. Their number ranges from 3 to 25.
Secondly, not every wave is a tsunami. Storm, ship and other waves are the movement of only the upper layer of water, while the tsunami is the movement of its entire thickness.

A tsunami is generated by an underwater earthquake. A seaquake causes a tsunami in most cases, but not always. Typhoons, tropical cyclones, underwater landslides, or volcanic eruptions can also be causes. The largest waves are formed when a cosmic body - a comet or a meteorite - enters the ocean. The consequences of such a catastrophe can only be imagined and are unlikely to survive. At one time, even dinosaurs died from this.

Any seaquake threatens to cause a tsunami. For a tsunami to occur, the displacement of the bottom surface must be lightning fast and large enough to set the water column in motion. In addition, the earthquake source should not be too deep (up to 20 km.). Therefore, not every change in the relief of the ocean floor generates a giant wave.

Tsunamis occur only in warm seas. This myth arose because tsunamis occur most of all in the Pacific Ocean, where seaquakes and eruptions of underwater volcanoes occur, and Japan and the Pacific islands most often suffer from their effects. If we talk about landslide tsunamis that occur due to the collapse of rocks of sea cliffs, then they can happen everywhere! In 1964, as a result of an earthquake and the subsequent collapse of the ice, a tsunami occurred in Alaska. It hit the height of its waves: 60 meters!

Before the start of the tsunami, the water recedes from the shore. Canadian mathematician Walter Craig came to the conclusion that only in half of the cases the water really moves away from the coast, foreshadowing a tsunami. It depends, first of all, on the wavelength, and not on the power of the tsunami, as previously thought.

A tsunami is always a high wave! Revealing the secret of the occurrence of this natural phenomenon, it must be said that in fact the height of the tsunami depends on its energy. And the farther from the epicenter, the higher the wave level. Whereas in the open sea a tsunami does not exceed a meter, but moves at a frantic speed, on the shallows the wave speed decreases and gains height. By the way, there may be no waves at all, and the tsunami will pass like a series of rapid ebb and flow. So a tsunami is not just a wall of water falling on the shore, but the movement of the entire water layer, increasing its destructive power when it meets land.

The tsunami comes unnoticed, which is why it is so difficult to escape from it. Indeed, the hallmark of a tsunami is its sudden appearance. But still, it makes itself felt, and if you are attentive, you can notice the approach of a catastrophe. If an earthquake causes a giant wave, everyone on the shore feels the tremors, even if they are not strong. With a strong movement of water, small marine organisms glow. If a tsunami occurs in cold seas, ice breaks and undercurrents arise. In addition, water can move away from the shore, draining the bottom or, on the contrary, slowly flow.

The first wave of a tsunami is always the largest. This is not true. Since tsunami waves move one after another, and the distance between them can reach several tens and even hundreds of kilometers, they reach the coast after a certain time (from a couple of minutes to an hour). After the first wave, the shore gets wet, thereby reducing the resistance for subsequent waves. They are always more destructive.

Animals always feel the approach of a tsunami. Indeed, during the huge tsunami on the coast of Sri Lanka in 2004, not a single animal corpse was found. Eyewitnesses claim that even the fish tried to hide from the impending elements, hiding in the corals. But the truth is that not all animals are catastrophe predictors. For some, the threat will become clear, while the other will not react to it in any way. Therefore, it would be wrong to rely on the intuition of our smaller brothers in everything.

From the tsunami saves only a quick escape deep into the coast. Indeed, this is true, but it is important not only to run away from the coastline, but also to fulfill the simplest requirements: firstly, do not move along the riverbed, where the tsunami wave will quickly overtake you. Secondly, going into the mountains, move up the slope, rising to a height of at least 30 meters from the coastline. Thirdly, if you are on a ship, boat or any other vessel, it is pointless to seek salvation on the shore, and it is better to go farther into the sea. And finally, it should be remembered that the tsunami is back. Only after a certain time you can return to the shore.

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

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

Let us consider in more detail what a tsunami is, what are the causes of their occurrence and the consequences they cause.

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 sounds like this - “a big wave in the harbor” and this is not in vain, since in all its power they appear precisely on the coast.

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

Approaching the shore, the speed and wavelength begin to decrease. Due to braking in shallow water, each subsequent wave overtakes the previous one, transferring its energy to it and increasing the amplitude.

Sometimes their height reaches 40-50 meters. Such a huge mass of water, having fallen on the shore, completely devastates the coastal zone in a matter of seconds. The length of the area of ​​destruction inland in some cases can reach 10 km!

Causes of the tsunami

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

Other reasons include:

• Landslides. Often a whole chain of natural disasters can be traced - the shift of the lithospheric plates leads to an earthquake, it generates a landslide that generates a tsunami. It is this picture that can be traced in Indonesia, where landslide tsunamis occur quite often.
• Volcanic eruptions cause up to 5% of all tsunamis. At the same time, giant masses of earth and stone, shot up into the sky, then plunge into the water. A huge mass of water is shifting. Ocean waters rush into the formed funnel. This dislocation generates a tsunami wave. An example of a catastrophe 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 neighboring islands, as well as 500 ships.

• Despite the fact that our planet has an atmosphere that 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 appearance of tsunami waves. Realizing this, the powers possessing such weapons concluded a treaty prohibiting their testing in the atmosphere, space and in water.

Who and how studies this phenomenon

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

The monstrous masses of water rolling onto the shore cannot be stopped by any artificial protective structures. The most effective protection in such a situation can only be the timely evacuation of people from the danger zone. For this a sufficiently long-term forecast of the coming disaster is needed. This is done by seismologists in collaboration with scientists of other specialties (physicists, mathematicians, etc.). Research methods include:

• data of seismographs registering tremors;
• information supplied by sensors taken out into the open ocean;
• remote measurement of tsunami from outer space using special satellites;

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

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What is a tsunami? How is this natural phenomenon formed? What are the reasons for these giant waves? By what signs can you determine that a tsunami is coming. Let's take a closer look at where they most often occur and give statistics on the most devastating natural disasters that have occurred due to tsunamis over the past 50-60 years.

What is a tsunami?

The definition of the word tsunami when translated from Japanese means "wave in the harbor." i.e. tsunamis are large and long waves that are formed due to the impact on the entire water column. This is the difference between a simple large storm wave and a tsunami, since in a large storm wave the impact occurs only on the surface, while in a tsunami the entire water column is affected. Of course, the larger the body of water, the larger and longer the tsunami. Tsunamis can only form in the seas and oceans. When a tsunami most often forms not one wave, but several, which are thrown onto land with a time interval between them from 2 minutes to 2 hours.

Causes of a tsunami

Scientists share several reasons for the occurrence of such a natural phenomenon as a tsunami. The tsunami mainly comes from the impact on the bottom of the sea or ocean, as a result of which a force is released, which forms the movement of the entire water column - that is, a tsunami.

These are such natural phenomena as:

• - underwater earthquakes;
• - landslides;
• - underwater volcanic eruptions;
• - the fall of a large celestial body into the ocean or sea (for example, the Tunguska meteorite);
• - military tests (for example, tests of nuclear weapons in the ocean or sea).

How does a tsunami occur due to earthquakes?

Large waves are formed due to the displacement of lithospheric plates, while the plates themselves begin to move as a result of underwater earthquakes. The mechanism of wave formation as a result of the displacement of the lithospheric plates is as follows: one plate begins to crawl under the other, as a result, a sufficiently large force is formed that lifts the second lithospheric plate upwards, and this effect also sets the water column in motion.

Other Causes of a Tsunami

Landslides are another cause of such waves as tsunamis. For example, off the coast of Alaska, a large landslide occurred and a large amount of ice and earth rocks collapsed into the water from a great height, resulting in a large and long wave. Off the coast of Alaska, the wave reached a height of more than 500 meters.

Tsunamis as a result of the eruption of an underwater volcano are formed in much the same way as during an earthquake. Since as a result of a volcanic eruption, explosions occur, and when they have great power, they are also ways to cause large and long waves, i.e. tsunamis.

What are tsunamis?

Scientists divide different types of tsunamis depending on the strength and height of the waves, as well as on the catastrophic consequences that these waves cause. Waves from earthquakes can form both large ones from 10 meters in height, and very small ones - waves of 1-2 meters. The farther from the coast, the less destructive effect a tsunami has.

The most destructive tsunamis occur when the epicenter of an earthquake is close to the coast, with an earthquake magnitude of 6.5 on the Richter scale. And with a small earthquake somewhere in the center of the ocean, they can cause waves from 1 meter, which are not dangerous even for ships and liners that are nearby. This is because the tsunami gains its strength and power as it approaches the shore. That is why, being in seismically dangerous coastal zones, you need to know the main signs of a tsunami.

Signs of a tsunami:

• - earthquakes - the more intense the tremors, the stronger the wave will be;
• - a sharp ebb - the farther the sea and ocean coast goes inland, the higher and more powerful the wave will be.

What regions are seismically dangerous zones where a tsunami can form?

Most often, tsunamis form on the coasts of the Pacific Ocean, since more than 80% of the active volcanoes of our planet are located in its waters, and 80% of all earthquakes occur at the bottom of this ocean. Dangerous zones include the western coast of Japan, Sakhalin Island, the coast of Peru, India, Australia, Madagascar.