The word “tsunami” came from the Japanese language and was fixed in the dictionary of concepts known to almost every inhabitant of the Earth. Translated, it means “port or coastal wave.” I must admit that the name is quite appropriate, given the mortal danger and the numerous destruction that huge waves are causing the Japanese port areas and coastal settlements.
Tsunami, wave formation, signs of approaching the shore, preparation and behavior during and after the tsunami.
First of all, tsunamis should not be compared with tidal waves. Tides are merely periodic sea level fluctuations caused by the forces of attraction of the moon and the sun. Sometimes the strongest storms can lift waves up to 25 meters. However, this phenomenon is far from the tsunami. If you dive deeper under the wind wave, it will become obvious that its strength decreases with depth, and there will come a moment when the excitement completely stops.
Classical tsunamis extend from the very bottom of the ocean (sea), and their strength remains the same at any depth. Even if it is more than one kilometer. This is a natural consequence of the fact that tsunamis are usually caused by significant seismic disturbances of the seabed. That is why scientists sometimes extend the concept of seismic waves to tsunamis..
Tsunami Wave Formation.
Physically, everything looks quite simple. On a certain section of the bottom there is a rise. As a result, a column of water rises, resting on this layer. Accordingly, on the surface of the water this also manifests itself as a certain elevation, the area of which sometimes reaches 25,000 km2. In the event that the bottom sinks, the surface of the water goes down and a cavity forms.
. In the latter case, a very typical example was the powerful tsunami near the coast of Indonesia, which occurred in August 1883 and was caused by the eruption of the volcano Krakatau. The waves, reaching a height of 41 meters, destroyed about 300 settlements and claimed the lives of more than 40,000 people.
The speed of wind waves never exceeds 100 km / h. As a rule, it is even smaller. As for the tsunami, its speeds are comparable to those of jets. In an hour, a deadly wave overcomes up to 1000 km. However, in the open sea, the tsunami is not dangerous, because here the waves are low and gentle. Sometimes the distance between the ridges is hundreds of kilometers.
For this reason, tsunami waves in the open sea are extremely difficult to notice. Ships located at sea at depths of 180 meters or more may not be afraid of the tsunami. The crews will notice it only when it crashes ashore.
Tsunami approaching the shore.
When approaching the coast and reaching shallow water, the waves transform. Friction on the bottom reduces the speed of movement, but unevenly. The depth at which the rear of the wave is located is usually greater than the depth below its front. Therefore, the rear part moves a little faster. As a result, the wave is braked, simultaneously contracting and increasing in height. The next waves immediately roll over and, colliding with the 1st wave, merge into it.
Near the coastline, the tsunami is able to turn into a breaker falling on the coast in the form of a water shaft, or boron. But usually it looks like a fast-appearing and strong tidal wave that far exceeds the usual tidal level.
It may seem strange, but the first sign of a tsunami is usually not a water ramp moving towards the coast, but a significant deviation of water from the coast with exposure of vast sections of the bottom. In shallow bays and bays, fish are beating, seashells are lying around, etc. The fact that the first approaches the shore — the crest or bottom of the wave, depends on what will happen (a water shaft or a retreat of water).
Seismic waves that can cause tsunamis appear during earthquakes. The bulk of the mechanical energy of the earthquake is spent on rock destruction in the area of the hypocenter. Only an insignificant part of the mechanical energy reaches the surface, turning into seismic waves. In the Pacific Ocean, tsunamis are very often observed due to the high seismicity of its basin. On average, one powerful tsunami necessarily occurs on the coasts of the countries of the Pacific region for a year.
It should be remembered that the tsunami moves with great speed. Therefore, if the wave is visible, then it will not be possible to escape. In each wave there is a movement of water particles in a circle or ellipse. In a tsunami situation, this factor manifests itself especially strongly and plays a role in lowering the water level. This partially explains why the water recedes far from the coast before the arrival of the 1st wave. Swimmers, before the tsunami arrives directly, feel water dragging them from the shore.
Tsunami formation due to melting icebergs and glaciers.
In addition to the traditional causes of the formation of huge waves (a kind of tsunami), scientists have revealed yet another, rather unusual. Based on the calculations, the wave is able to form when an unstable iceberg capsizes or breaks into pieces.
Chunks of ice, broken off from floating blocks in the polar regions, are only the beginning of the chain, which leads to the formation of a tsunami. When the iceberg gradually melts, separate pieces separate from it. Because of what, its mass is redistributed. It acquires a vertically elongated shape, becomes unstable, falls on its side and forms a wave quite powerful in some areas of the Earth. For example, in the region of Greenland. Such waves can cause significant damage and casualties..
In the polar regions near Greenland and Antarctica, a lot of potentially dangerous icebergs float. Calculations show that the fall of the iceberg can lead to the formation of a tsunami of the same strength as in an earthquake up to 6 magnitudes. Moreover, the wave height depends on the height of the iceberg and is 1% of it. Thus, if the average height of icebergs in Antarctica is 400 meters, and the largest is 1 km, then the height of the tsunami in the 1st case will be 4 meters, and in the 2nd – 10 meters.
I must say that the model described by scientists is quite feasible in practice.
Such processes actually occur in nature. The tsunamis resulting from the split of icebergs already occurred near the coast of Greenland, where several ports were destroyed. There are difficulties in answering another interesting question: can such waves reach populated areas?
Apparently, the answer is yes. Since it is no secret that the power of the tsunami as it moves across the ocean almost does not change. So the danger can even come from where it is not expected. After all, nobody monitors icebergs in terms of their tsunami formation.
With increasing temperature on Earth, the threat of melting glaciers and icebergs also increases. Therefore, the risk of tsunami formation is becoming more likely. In addition, there is another, global danger: regardless of their size, the waves formed by the part breaking away from the iceberg cause a chain reaction. Ice shelves also begin to crack and, in such a fragmented state, melt faster. Accordingly, the increase in sea level is accelerating.
How to survive the tsunami?
In order for people, when receiving a danger signal, to manage their remaining time correctly, they must have an idea of the basics of survival in the conditions of the approaching tsunami.
Signs of the approach of the tsunami.
An earthquake acts as a natural signal about a possible tsunami. Before the tragedy, the water in the ocean usually departs from the coast for a considerable distance. Hundreds of meters or even several kilometers. The duration of this “low tide” ranges from several minutes to half an hour. Along with the movement of the waves, sounds similar to thunder are heard, which are heard even before the tsunami approaches..
In addition, before the appearance of huge waves, the coast is able to become covered with a kind of thin water “carpet”. If the coast has ice formations, then cracks may appear in them. An unexpected and strange change in animal behavior also indicates an impending disaster. They will try to go to high places.
The threat of a tsunami can either be reported by walkie-talkie, or notify the population by a siren.
Therefore, it is necessary to follow the messages, while keeping in mind other signs. In any case, residents of a particular region must know the signals about the danger of the tsunami and explain them to family members. It is very important to plan ahead in the event of a natural disaster. Moreover, each family member or employee of the enterprise must know how to behave in the event of a tsunami.
It is necessary to determine whether the house or place of work is in the risk zone of being hit by a wave, and establish the boundaries of these zones, as well as the optimal escape routes. The most dangerous are straits, bays and estuaries.
It is worth taking care in advance of the list of documents, things and medicines that must be taken out as a matter of priority during evacuation. It is better to put things and medicines in a suitcase or backpack. You should also plan ahead in what sequence the evacuation should take place. In what place will family members meet. In the house or on the street.
And in the course of daily activities at home and at work, do not block the corridors and exits with any pieces of furniture and other things. That is, the walkways should be free so that you can quickly leave the room if necessary.
It is important to know the rules of conduct during a direct tsunami strike. To think over in advance the procedure for actions indoors, outdoors, in water. If you prepare a place where important documents, spare clothes, personal hygiene items, a stock of non-perishable products will be stored for at least 2 days in case of evacuation, this will save time and possibly save your life.
When a tsunami hazard signal is received, you must not hesitate and should respond immediately. Using every minute to ensure an adequate level of personal security for yourself and your loved ones. And also, if possible, to help people around.
From a signal to the arrival of a time wave, it can remain from a minute to half an hour, or even more. However, in any case, you need to act calmly and thoughtfully. Then the chances of salvation will increase dramatically. If an alarm catches you in the car, you can immediately drive off to a safe distance, picking up walking people along the way.
It is required to leave the premises quickly, without forgetting to turn off the electricity and shut off the gas. The shortest way is to get to a safe place, located 2-3 km from the water or at least 40 meters high. If there is no time to move, you need to go up to the upper floor and close the windows and doors. If the building is not too reliable, it is better to move to another, more thorough.
And if you had to stay indoors, it will be most safe near the capital internal walls (in the corners formed by such walls), columns. Next, remove objects that are nearby that could fall, especially those made of glass..
If it is obvious that the tsunami will catch on the street, it is more expedient to find a tree or a strong fence. Grasp them or hide in a place least affected by the elements. Once in the water, you need to take off your clothes and shoes. Then try to grab hold of objects floating on the surface. It is important to be careful not to collide with any large object..
You can’t relax after the 1st wave, because others will come after it. At the slightest opportunity you should get to a safe place. If victims need urgent medical attention, do not pass by. Because next time you may need help.
Actions after the tsunami.
When you receive the end-alarm signal, you can return back only by making sure that there are no high waves in the ocean for 2-3 hours. Before entering the house it is necessary to check its strength. Have the windows and doors remained intact, are there no cracks in the walls and ceilings, has the foundation been washed out? Is there a gas leak, is the mains functioning. Then, the state of the house must be reported to the emergency commission.
After making sure that your loved ones are safe, you should take an active part in rescue activities and other urgent work in destroyed buildings, in search of victims and first aid.