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CHILDREN'S SCIENTIFIC LABORATORY
Tsunami. Children's Science Lab
Directory / Children's Science Lab "Tsunami" - a big wave in the harbor. Translation from Japanese. The disaster began at three o'clock in the morning with a strong jolt. It lasted only a few seconds ... After 15 minutes, a strong noise was heard from the sea. It seemed that the sea rushed to land. From the side of the spit, where the buildings of the seal area were located, there was a terrible crack and roar ... At dawn, the spit looked completely clean, only in one place some kind of shapeless pile was visible ... From the diary of P. Novograblenov, the first Soviet observer of seismic events in Kamchatka, 1923. Long before the start Far from the Pacific coast, in Leningrad, in the building of the State Hydrological Institute, scientists built a new Ust-Kamchatsk. Of course, it was just a model of the city, but on a large scale. Part of the Kamchatka Bay, the mouth of the Kamchatka River, city buildings are recreated in every detail on it - a whole area of over 4000 km2 is housed in a small laboratory. The coast, the seabed of the model are made of concrete, and the land with all the details of the terrain is made of plasticine. Scientists densely sprinkled the entire coast with sawdust. Electric wires were lowered into the water. To top it all off, a movie camera chirped somewhere under the ceiling. What is this? Isn't it a game? Then why else under the action of compressed air, like the bellows of a huge accordion, does the bottom sink or rise and waves rise in the toy Kamchatka Bay? Scientists decided to repeat the catastrophe that happened in. 1923. Then an earthquake, which happened far in the sea, gave rise to a high wave, and it splashed onto the shore and destroyed the city. Kamchatka, the Kuril and Japanese Islands, Sakhalin, Alaska - even from a simple enumeration it can be seen that tsunamis most often appear in the Pacific Ocean. In the waters of the largest ocean, dozens of volcanoes awaken every year, strong earthquakes occur, and most often under the ocean floor, where the earth's crust is much thinner. If it were possible to expose the bottom of the Pacific Ocean, then one could count nine huge zones in which faults or swelling of the earth's crust constantly occur. Near Japan, the ocean floor is perhaps the most restless. It has many faults hundreds of kilometers long. Along these now healing, now re-opening "wounds" the blocks of the earth's crust are constantly shifting or moving apart. Most of the faults are along the coast. But there are also transverse faults. And where the longitudinal and transverse faults on the earth's crust intersect, especially strong tremors occur. From there, the highest tsunamis should be expected. Here and on the model hundreds of times scientists staged tsunami raids on plasticine shores. Electric sensors determined fluctuations in the "sea" level. The boundary of sawdust not washed off the shore indicated where the wave could rise, and filming recorded the speed of surface currents. All this together helped to restore for certain the picture of the catastrophe described by Novograblenov. And not only to restore, but also to make important conclusions: the industrial and residential buildings of an expanding city should be built in those places where the highest wave cannot rise. The recommendations of hydrological scientists are now exactly being followed. But not every earthquake causes a tsunami. Only when a section of the seabed - a kind of giant piston - raises or lowers the multi-kilometer column of water above it, waves appear on the surface of the ocean. This phenomenon can be compared to what happens if a plug can be abruptly lifted or lowered from the bottom of a bathtub filled with water. For a moment, the bottom section seems to disappear. The column of water resting on it "fails", and a hole is formed on the surface. In the ocean, the height of such a hole can reach several hundred meters, and the height of the water column can be several kilometers. This giant discharge of a liquid column is the future of the tsunami. During an earthquake, a block of the earth's crust can also hit upwards. Then the ocean floor swells. The water column rises above the surrounding surface, which also generates a high wave. The height of such waves directly above the earthquake sources reaches several hundred meters. But already a few hundred kilometers from the epicenter, its gentle crest rarely exceeds a height of 2 m. That is why ships on the high seas are not threatened by a meeting with a high wave. It is quite another matter when the ship gets into a storm. Ten-meter wind waves throw it like a chip. And here's what's remarkable. Wind waves fluctuations in the surface layer of the ocean. Deeper than 30 m there is a stagnant zone. There, in the words of the famous oceanologist Zh. I. Cousteau, there is a real world of silence. But the tsunami really lives up to its name of a high wave. A two-meter hump is just its top, while the base of the wave rests on the ocean floor. By the way, we note: the weight of such a wave is more than one hundred million tons. And if you consider that it does not stand still, but literally flies across the ocean at the speed of a passenger jet, then its energy is enormous. Calculations have shown that in order to get an artificial tsunami of medium power, you need to blow up a thick bomb weighing a billion tons at the bottom of the ocean! If in the open ocean a big wave is absolutely harmless, then as it approaches the shore, its temper changes. Due to the friction of water particles on the roughness of the bottom, the speed of movement of the wave bottom is significantly reduced. Near the shore, it grows in height, takes on an irregular shape and overturns its crescent-shaped crest far forward. P. Novograblenov measured the height of the tsunami that destroyed Ust-Kamchatsk. The water wall then rose from the sea higher than an eight-story building! The height of a tsunami also largely depends on the configuration of the shore. If we are on the shore of a bay with a narrow entrance, we have nothing to fear. The wave will spend a significant part of its energy to overcome the narrow passage. A completely different matter is an open, wedge-shaped bay. Here, as the wave moves towards the top of the wedge, it shortens in length, but increases in height. For this reason, river mouths, elongated straits are the most dangerous places. Mankind cannot actively fight against the formidable natural phenomenon of nature. So far, we have to think more about defense than about fighting. After all, it is impossible to oppose the strength of a tsunami with one's own strength or count on the strength of coastal protection structures. Even the most perfect and strong dam is unlikely to withstand the onslaught of hundreds of millions of cubic meters of water. That is why, when it comes to building any structures on the shore, a complete large-scale copy is created in the laboratory. With such modeling, a destructive wave is easily imitated and its landfalls are studied. But scientists are interested in the model not only of a separate, albeit an extended section of the coastal zone. Now, if it were possible to create an accurate model of the Pacific Ocean with all the islands, coasts of Asia and America? And such a model is not a fantasy. Of course, it cannot be made of concrete and plasticine. All the geometric dimensions of the continents, the front of the wave, its speed and energy, the depths of the ocean at different points, and much more can be entered into the memory of a high-speed computer. And the computer will decide where to wait for the highest wave, at what time. Such work has already been done for the tsunami that covered the Japanese port of Niigata in 964 at the Leningrad Hydrometeorological Institute and at Stanford University (USA). The results of calculations on mathematical models were compared at the recent tsunami symposium in Honolulu. Soviet and American mathematical models almost coincided. This is just a special case of active cooperation between the two countries. For more than twenty years, an extensive network of interconnected coast stations has been operating on the Pacific coasts of the USSR, Japan and the USA. Scientists are constantly exchanging information, looking for more effective ways to detect a large wave in order to notify the population of coastal areas of impending danger as quickly as possible. For the third year in a row, the Soviet ship "Valeryan Uryvaev" has been making voyages across the Far Eastern seas, from which new Soviet scientific instruments are being installed in the ocean. The study of the formidable natural phenomenon of nature continues, and, as you can see, in several directions. In front of you is a section of the ocean. Sensitive devices are installed on the shore, on the islands, surface and underwater buoy stations. Some conduct observations of the seismic activity of the earth's crust, and determine the epicenter of an earthquake by the speed of propagation of elastic vibrations. Sensors of ocean level fluctuations separate tsunami waves from wind and tidal waves, and establish the appearance of the first large waves. Laser rangefinders on satellites not only fix the epicenter, swelling or dip of the ocean level at the time of the earthquake, but also determine the direction and speed of the tsunami. Such a wide network of recording instruments is supposed to be installed in the most tsunami-prone points of the Pacific Ocean.
Caution - danger! The Far Eastern Hydrometeorological Institute has a tsunami department. Its task is to create a new automated service for alerting the population of coastal zones about impending danger. Off the coast of Kamchatka, the Kuril Ridge and Sakhalin, as well as far in the ocean, directly in the zone of possible earthquakes, scientists are installing many instruments and sensors. First of all, sensitive instruments - seismographs - monitor the seismic activity of the Earth. They capture elastic waves, which determine the coordinates of the epicenter in the energy of an underwater earthquake. If the energy is high, and the epicenter is located in an area where high waves most often appear, then a warning signal is transmitted via wire and radio lines to hydrometeorological stations that monitor sea level. Having received a signal, observers monitor the readings of self-recording level gauges and try to register the first, usually small, tsunami waves. But finding them is not so easy. Wind waves roll onto the shore every half a minute. Twice a day, the ocean level rises during high tides. But tsunami waves hit the coast with an interval of 10-150 minutes. How, then, to distinguish a wind wave, a tidal wave from a tsunami? A float floats in a vertically installed pipe that communicates with the sea. It rises or falls and sets the pen in motion, recording level fluctuations on the tape. A column of liquid at a depth of, say, 10 m creates a pressure equal to one atmosphere. But the sea is rarely calm. Therefore, if a pressure gauge is installed at a certain depth, one can judge the height of the wave by its readings. Wind and tidal waves, overlapping one another, seem to obscure the first, still low tsunami waves. It is very difficult to distinguish them with the help of float and hydrostatic instruments. In addition to them, another device was installed. It was called the tsunami wave detector.
Let's get acquainted with its device (see. Fig.). Metal corrugated cup 1 is compressed under the action of hydrostatic pressure. Two capillaries of different diameters 2 connect the cavity of the cup with two identical chambers 3, inside which there are also corrugated cups, but of a smaller size. Their internal cavities communicate with the measuring chamber 4, divided by a membrane into two parts. The internal cavities of the three cups are filled with an incompressible fluid. The sensor is installed on the membrane. How does the detector react to sea level fluctuations? Tidal waves only come ashore twice a day. The sea level is slowly changing, therefore, the hydrostatic pressure gradually increases in the place where the device is installed. The metal cup is gradually compressed, displacing part of the liquid almost without resistance through the capillaries into the internal cavity of the measuring chamber. The pressure on both sides of the membrane is the same, the device is silent. The device is silent even when there are ordinary wind waves on the sea. Encountering significant resistance in the capillaries, the liquid does not have time to flow at a sufficient speed. In this case, a constant pressure acts on the membrane. Only when the tsunami waves approach does the effect of different capillary resistance begin to show. A capillary with a larger diameter creates less resistance to fluid flow, and the pressure on one side of the membrane becomes greater than on the other. The membrane bends, the sensor automatically turns on the light and sound alarms at the station. This is how the coastal warning service works. However, the institute's scientists are striving to improve the efficiency of the warning system and gain some time from the tsunami. Sensitive devices are taken out as far as possible from the coast and are connected by cable or radio to coast stations. A whole network of stations is already equipped on the islands, on moored floats - buoys. In seismically active zones at a depth of 5-6 km, automatic seismographs and sensitive tsunami wave detectors with string transducers are installed. The detectors act like tuning forks, like piano strings stretched on a rigid frame. One has only to turn the peg with the key in any direction, as the pitch of the string changes. The converter is based on the same principle. Between the center of the membrane, which is affected by the measured hydrostatic pressure, and the body of the device, a thin steel wire is stretched - a string. If the ocean is calm, the string sounds at the same frequency. But as soon as the waves appear, the membrane sags, the tension of the string decreases. The electronic device picks up the change in pitch and sends a signal through the wire up to the buoy. Coastal, island and buoy stations are not all that the automated service will have. To detect tsunami waves, experiments are now underway using a laser. It is known that thanks to the laser, it was possible to measure the distance from the Earth to the Moon with an accuracy of several tens of centimeters. And why not install a laser rangefinder on the satellite to measure ocean level fluctuations? Perhaps soon there will be satellites that will monitor tsunami waves. In addition to the ocean itself, the ionosphere can tell about the appearance of high waves. When a section of the earth's crust falls or rises sharply under water, a column of atmospheric air rises or falls along with the water column. In the upper layers, acoustic waves arise, which distort the radio waves reflected from the ionosphere. Since acoustic waves outpace the tsunami by several hours, scientists believe that the ionospheric method will also be used in the warning service. Information from all instruments and sensors installed on the ocean floor, buoy stations and the shore will be sent to a single center of the institute and sent to a computer. The machine will calculate and make a recommendation: in which area should the highest wave be expected and how soon. An alarm will go off in this area - people will have time to move to a safe place. Do you know that... ... Tsunamis can be caused not only by displacements of huge earth blocks of the ocean floor. During the eruption of Krakatau in the summer of 1883, an explosion of unprecedented force shook the earth. The island-volcano (its dimensions were approximately 5 by 10) exploded into the air, and fragments of rock with a volume of 20 km3 fell into the waters of the Sunda Strait. It was they who caused a giant wave, which, although already weakened, was recorded on the shores of France and England, that is, it passed the Indian Ocean, circled Africa and entered the Atlantic. ... The atmosphere can also generate tsunamis. As soon as the atmospheric pressure somewhere above the ocean drops by only 1 mm, the water level in this area will rise by 13 mm. And atmospheric pressure sometimes drops by many tens of millimeters, as happens during typhoons. Something resembling a hill is created on the water surface, which, with a sharp shift of the cyclone, instantly settles and generates waves. ... In July 1958, on the coast of Alaska, a large avalanche descended from the slopes of Mount Fairweather, containing a mass of ice, snow, and soil. The wave rising after it reached more than 500 m in height. It is not surprising that she "with her head" covered the nearby island. ...Recently, tsunami waves have been detected... on the Moon. According to astronomers, the numerous ring-shaped mountain structures surrounding most of the lunar craters with a diameter of 200 km may be preserved tsunami wolves. Meteorites falling on the still not cooled down surface of the Moon pierced its thin hardened shell. Molten rock rose from the bowels into the hole formed. Like an ordinary liquid, it formed waves, which froze forever. ...Thirteen years ago, on the island of Urup, which is part of the Kuril chain, there lived a large herd of sea otters. After two devastating tsunami raids, the shallow coastal waters were covered with stones. The food balance of animals was disturbed, and their numbers were sharply reduced. But here's an interesting pattern. Shortly after the tsunami, an environmental explosion was noted on the same island. The urupsia herd not only quickly recovered, but also increased. According to the Sakhalin zoologist Viktor Voronov, tsunamis both destroy and create. A giant plow raises a huge amount of nutrients from the depths. Waves plow and fertilize the coastal shelf. In such a nutritious "broth" phyto- and zooplankton rapidly develop, schools of fish grow. Therefore, the sea otter chose the island as its place of residence, which is annually subjected to tsunami attacks. ... Calculated and experimentally, scientists came to the conclusion that tsunami waves decay with distance from the epicenter in proportion to the distance, taken approximately to the power of 5/6. Fluctuations in the earth's crust under the ocean floor can cause not one but several waves. Which of them is the most dangerous - the first, second, third? It turns out that the tsunami alternates in its relative growth as it moves away from the place where it originated. For example, near the epicenter, the second wave is higher than the first. But the farther from the source, the greater the serial number is the maximum wave. ...The energy characteristic of an earthquake is the magnitude measured by a seismograph. The magnitude scale was proposed by Charles Richter. The most powerful earthquake has a magnitude slightly less than 9. Seismologists believe that if the magnitude on the Richter scale is 7 or more, then the occurrence of a tsunami is almost completely inevitable. If less, then the probability of a tsunami is close to zero. Author: V.Rotov
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