Is it possible to predict earthquakes? Seismology: how earthquakes are predicted
Today, science is moving forward with great strides, and people can forecast and predict many natural phenomena, including natural disasters, in advance. An earthquake is one of the most dangerous manifestations of the nature of our planet; it can cause enormous damage. Is it possible to predict such geological disturbances today? How do scientists do this? The answers to these questions are of interest to many people, primarily those who live in seismically hazardous areas.
Science has provided humanity with certain capabilities in predicting geological disasters, although predictions are not always 100% accurate. It's worth talking about how they are made.
What causes earthquakes?
Earthquakes are a consequence of geological processes occurring in the mantle and crust. Lithospheric plates move, and in normal situations this movement is barely noticeable. However, stress accumulates on crustal faults due to uneven movements, which ultimately causes earthquakes. These phenomena are not observed everywhere; they are characteristic of geologically turbulent places at the junctions of the earth's crust. The most unstable place is the so-called “ring of fire”, stretching along the outskirts of the Pacific Ocean. It frames the largest lithospheric plate on the planet, on which this ocean is located.
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How has the Earth's surface changed?
Any, even the slightest, movement of such a mass of the earth’s crust cannot occur painlessly, so earthquakes occur constantly along its periphery. There is also massive volcanic activity there.
Predictions of earthquakes in the past
People have long sought to predict natural disasters. The first successful steps in this direction were made thousands of years ago in geologically turbulent regions. In China, ancient scientists were able to create an unusual vase, which was found by modern archaeologists during excavations. Ceramic dragons sit on the edge of the vase, each holding a ball in its mouth. At the slightest vibrations of the earth, harbingers of an impending earthquake, balls fell out of the mouths of the dragons - first of all, from the direction of the source of the future earthquake. This way people could find out in time about an imminent disaster, and even about which side the source of the cataclysm would be located.
Japan also had its own developments - this country has always been a turbulent place. Here people relied more on observations of nature. Before an earthquake, bottom fish rise to the upper layers of water; catfish show particular concern. This was noticed by fishermen, who each time in such cases hurried home to warn their loved ones about the impending disaster.
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Why do earthquakes happen?
Interesting fact : Catfish in Japanese legends is seen as a fish symbolizing earth and stability. Perhaps this is due precisely to the fact that in a calm geological situation the fish swims peacefully and slowly at the bottom, and before earthquakes it begins to rush around and look for shelter.
It was also noted that the fire burning on a candle or splinter sharply goes down before earthquakes, and the candle burns out very quickly. This is due to geomagnetic changes that occur before a cataclysm. Also everywhere, people noted the anxiety of pets and their desire to leave the house before a disaster. Based on these and other signs, people of the past often managed to save themselves, their loved ones or property by leaving their homes and cities in time.
Modern methods of earthquake prediction
Today, seismographs are used to prevent earthquakes. These devices are particularly sensitive sensors that record any vibrations on the surface of the earth. Since microshocks are first observed before any earthquake, the device gives fairly accurate predictions. He records these warning signs and transmits the information to scientists, who warn people through the media. Today, every individual can have their own small seismograph at their disposal - there are individual seismic monitors on sale that record changes and transmit them within a network, which allows you to receive and send warnings.
20% of the territory of Russia belongs to seismically active areas (including 5% of the territory is subject to extremely dangerous 8-10 magnitude earthquakes).
Over the past quarter century, about 30 significant earthquakes, that is, with a magnitude of more than seven on the Richter scale, have occurred in Russia. 20 million people live in zones of possible destructive earthquakes in Russia.
Residents of the Far Eastern region of Russia suffer the most from earthquakes and tsunamis. The Pacific coast of Russia is located in one of the “hottest” zones of the “Ring of Fire”. Here, in the area of transition from the Asian continent to the Pacific Ocean and the junction of the Kuril-Kamchatka and Aleutian island volcanic arcs, more than a third of Russia’s earthquakes occur; there are 30 active volcanoes, including such giants as Klyuchevskaya Sopka and Shiveluch. It has the highest density of distribution of active volcanoes on Earth: for every 20 km of coastline there is one volcano. Earthquakes occur here no less often than in Japan or Chile. Seismologists usually count at least 300 significant earthquakes per year. On the seismic zoning map of Russia, the areas of Kamchatka, Sakhalin and the Kuril Islands belong to the so-called eight- and nine-point zone. This means that in these areas the intensity of shaking can reach 8 and even 9 points. Destruction may also result. The most destructive earthquake measuring 9.0 on the Richter scale occurred on Sakhalin Island on May 27, 1995. About 3 thousand people died, the city of Neftegorsk, located 30 kilometers from the epicenter of the earthquake, was almost completely destroyed.
Seismically active regions of Russia also include Eastern Siberia, where 7-9 point zones are distinguished in the Baikal region, Irkutsk region and the Buryat Republic.
Yakutia, through which the boundary of the Euro-Asian and North American plates passes, is not only considered a seismically active region, but is also a record holder: earthquakes with epicenters north of 70° N often occur here. As seismologists know, the bulk of earthquakes on Earth occur near the equator and in mid-latitudes, and in high latitudes such events are recorded extremely rarely. For example, on the Kola Peninsula, many different traces of high-power earthquakes have been discovered - mostly quite old. The forms of seismogenic relief discovered on the Kola Peninsula are similar to those observed in earthquake zones with an intensity of 9-10 points.
Other seismically active regions of Russia include the Caucasus, spurs of the Carpathians, and the coasts of the Black and Caspian Seas. These areas are characterized by earthquakes with a magnitude of 4-5. However, during the historical period, catastrophic earthquakes with a magnitude of more than 8.0 were also recorded here. Traces of a tsunami were also found on the Black Sea coast.
However, earthquakes can also occur in areas that cannot be called seismically active. On September 21, 2004, two series of tremors with a force of 4-5 points were recorded in Kaliningrad. The epicenter of the earthquake was 40 kilometers southeast of Kaliningrad near the Russian-Polish border. According to maps of general seismic zoning of the territory of Russia, the Kaliningrad region belongs to a seismically safe area. Here the probability of exceeding the intensity of such tremors is about 1% within 50 years.
Even residents of Moscow, St. Petersburg and other cities located on the Russian Platform have reason to worry. On the territory of Moscow and the Moscow region, the last of these seismic events with a force of 3-4 points took place on March 4, 1977, on the nights of August 30-31, 1986 and May 5, 1990. The strongest known seismic tremors in Moscow, with an intensity of over 4 points, were observed on October 4, 1802 and November 10, 1940. These were “echoes” of larger earthquakes in the Eastern Carpathians.
In the last days of June 1981, the capital of Peru, the golden-columned Lima, was in turmoil: American scientist Brian Bradley predicted that on Sunday, June 28, the city would be destroyed by an unusually powerful earthquake. Dozens of powerful tremors will turn crowded city blocks into dust, after which tsunami waves will fall on the smoking ruins, sweeping away with a terrible onslaught everything that, by some miracle, manages to survive. The coastal areas of the city around Callao Bay will fall below the ocean level and become the seabed. Blooming “sun-faced” Lima will disappear from the face of the Earth in a few moments.
As the “day of judgment” approached, the situation in the capital became tense. Thousands of distraught people stormed airports, train stations and ship piers, trying to leave the city condemned to death. Lines of cars, carts, pack mules and pedestrians with handcarts and knapsacks on their backs clogged the highways and country roads from the doomed city in search of salvation. Prices for gasoline and food soared, crime increased alarmingly, houses and land were urgently sold for next to nothing, hospitals were suffocating from the influx of people crippled in the growing panic.
But the hour indicated by the soothsayer approached, passed... and nothing happened. Torn to pieces, but unharmed and still beautiful, Lima continued to serenely bathe in the rays of the tropical sun. Nothing happened the next day or in the next few days. Gradually, the wounds inflicted on the city by the panicked flight of the population healed, the incident began to be forgotten and turned into a historical anecdote. The unlucky predictor of the failed catastrophe was recognized as a false scientist and declared a charlatan.
Well, it’s easy to understand the impressionable residents of the Peruvian capital, who chose to flee the city over certain death under the ruins of their houses. Their country is located in a very seismically dangerous area of the globe. Over the five centuries that have passed since the discovery of the New World, 35 destructive earthquakes have occurred in Peru, and scientific observations over the past 100 years have recorded several thousand tremors of varying strength. There are probably few families in the country who would not mourn their loved ones who lost their lives in seismic disasters. The beautiful Lima also suffered repeatedly from strong earthquakes; in other tragic years, the underground elements destroyed most of the city.
Thus, the panic alarm of the residents of Lima had the most serious reasons. But back to the ill-fated Brian Bradley. On what and on what grounds he based his assumptions is still unknown. Therefore, it is not right now to condemn him in absentia, call him a pseudoscientist and accuse him of quackery, as the temperamental Latin American newspapers did. It’s better to first try to understand the essence of the question: is it possible, using the methods of modern science, to predict the onset of earthquakes, that is, to determine the place where they will occur, their intensity and time? After all, such forecasts (if they are issued in advance), like weather forecasts, will allow the population of threatened areas to prepare for expected natural disasters, take preventive measures and, if not prevent, then at least significantly reduce heavy losses and losses.
The possibility of seismic forecasting was suggested by the experience of observing natural phenomena, which, preceding seismic shocks, serve as harbingers of approaching catastrophes. It has long been noted that before some earthquakes a faint diffuse glow spreads over the ground; sometimes it is accompanied by flashing flashes or similar lightning, reflections on the clouds (this happened in 1966 in Tashkent). In other places, a foggy haze appears, which spreads over the surface of the earth and disappears after shaking. It happens that before the tremors, a light rising breeze flows from the ground (in Japan it is called “chiki”) or a muffled underground rumble is heard; in this case, random oscillations of the magnetic needle occur and the lifting force of the permanent magnets changes.
All these physical processes that precede seismic vibrations influence the behavior of animals, allowing them to anticipate impending misfortune. Chronicles, historical documents and oral traditions of the peoples of Asia, America and Southern Europe tell about this. In the palaces of Chinese emperors, special freshwater fish were kept in special aquariums, which, with their restlessness, warned of the approach of a natural disaster. Before the earthquake, the population of Japan observed the sudden appearance of large schools of eels, tuna and salmon in the sea, unknown deep-sea species floated to the surface, and the usual widespread species suddenly disappeared. Many octopuses swam to the shores, usually nesting in the crevices of underwater rocks.
Frogs, snakes, worms and centipedes crawl out of their shelters before an earthquake. Rats leave their holes in advance. Birds fly towards quieter areas inland. Horses, donkeys, sheep and pigs show increased nervousness. Cats and dogs have a special premonition; There are known cases when dogs forced their owners to leave buildings that were subsequently destroyed by underground shocks.
There are also people endowed with the ability to anticipate seismic vibrations; Most often these are neurotic patients with increased mental excitability, but there are also healthy people who are characterized by heightened susceptibility. For example, in 1855, a servant of a Japanese samurai predicted a strong earthquake in the city of Iedo (the ancient name of Tokyo).
Based on all these observations, scientists came up with the idea of the possibility of scientific prediction of earthquakes. This idea arose in the 50s of our century almost simultaneously in different countries that were subjected to the crushing onslaught of seismic disasters. To implement it, it was necessary to learn to use instruments to detect physical harbingers of tremors and use the data obtained for forecasting.
By this time, it had already been clearly established that earthquakes occur during rapid movements of blocks of the earth’s crust along the faults separating these blocks. It would seem that it is worth making observations of the behavior of geological faults - and the forecast problem will be solved: an increase in the activity of the fault will indicate the approaching threat of seismic tremors.
For this purpose, systematic instrumental observations were organized on many seismically active faults that experienced destructive earthquakes. It was expected that before the seismic tremors there would be an increase in the deformation of the tensile layers of rocks, the rise and fall of the contacting blocks of the earth's crust, sharp changes in the inclination of the layers (the so-called "tilt storms"), weak small tremors preceding the main shock ("microearthquakes") caused by the piezoelectric effect is an increase in the strength of telluric currents emanating from the seismic source, anomalous changes in the geomagnetic field (“local magnetic storms”) and a number of other phenomena that foreshadow the release of tectonic stress in the depths.
In fact, the situation was much more complicated. Indeed, in many cases the expected phenomena were observed; but often they contradicted the theoretical model of the process or revealed a completely unexpected, inexplicable course. Thus, in earthquake-prone areas of Alaska, a very slow (several centimeters per year) subsidence of the earth’s surface usually occurred. Three times - in 1923, 1924 and 1952 - abrupt “dips” were observed, during which the dives accelerated 5-6 times; however, no seismic phenomena were observed.
The destructive Anchorage earthquake in Alaska occurred in 1964 without any prerequisites in the form of a sharp subsidence or rise of layers. In the Japanese province of Niigata, where, on the contrary, gradual soil uplift prevailed, in 1959 the rate of uplift suddenly increased 10 times. A strong earthquake did not follow this jump, but broke out without visible precursors only five years later. The same inconsistencies were noted in the observed changes in the inclination of layers, the behavior of geomagnetic and electric fields, etc., although in some cases seismic tremors, as theoretically expected, were preceded by sharp outbreaks of anomalies.
Over three decades of research and search, it has not been possible to identify indisputable patterns that can be relied upon when predicting seismic shocks. Therefore, now none of the experts dares to assert that certain phenomena in the earth’s crust can be regarded as unambiguous harbingers of earthquakes and provide reliable grounds for predictions.
Currently, the circle of scientists working on the problem of earthquake forecasting is divided into two camps - skeptics and optimists. Skeptics believe that given the current state of our knowledge, which is completely insufficient, this problem is insoluble. At one time, the President of the USSR Academy of Sciences M.V. Keldysh called it fantastic. The most prominent American seismologist, Charles Richter, writes: “This is a tempting will-o’-the-wisp... At present, no one can say with certainty that an earthquake will occur at a given time in a given place. It is unknown whether such a prediction will be possible in the future.” The famous Soviet researcher of seismicity in Eastern Siberia V.P. Solonenko ironically cites a saying attributed to the Chinese sage Confucius: “It is difficult to catch a black cat in the dark, especially if it is not there.”
Optimists both in our country and abroad believe that the science of earthquake forecasting is on the right track and is already making significant progress. As a reliable precursor of tremors, they cite, for example, the flow of helium, argon, radon, chlorine, fluorine and other elements originating from the deep zones of the Earth into groundwater before seismic shocks, identified by Soviet scientists in some areas of the Caucasus and Central Asia; They also pin their hopes on studying the processes of dilatancy, the development of which also precedes the discharge of seismic elements. However, it has not yet been clarified how universal these phenomena are for territories with different geological structures. Some experts attach great importance to determining the periodicity of seismic processes. Thus, Japanese scientists, who have established a period of seismic activity of 69 years for the Tokyo area, are waiting with trepidation for 1992, when, in their opinion, a “great catastrophe” similar to the earthquake with a magnitude of 8.2 that devastated the capital of the Land of the Rising in 1923 could happen again. sun. But recurrence phenomena are still very poorly studied, since systematic observations of earthquakes in the earth’s crust have been carried out for only about 100 years.
Under these conditions, it is clear what risks earthquake forecasters are exposed to and what responsibility they take on. There's nothing surprising about Brian Bradley's prediction, unless of course he is. was made on the basis of genuine scientific data, but was not confirmed. On the contrary, it would be surprising if everything that was predicted happened.
However, there are examples of successful forecasts. The first such forecast was made on February 4, 1975 in the Chinese province of Liaoning. By order of the authorities, the population of the cities of Haichen and Yingkou left their homes on this day, and measures were taken to prevent the destruction of factories, food warehouses, children's institutions and hospitals. At 19:36 a strong earthquake occurred (with a magnitude of 7.3), which destroyed almost all residential premises, many factories, dams and other engineering and industrial structures. Thanks to the security measures taken, there were very few casualties. After this, two more small earthquakes were predicted. However, Chinese scientists failed to foresee the tragic Tien Shan disaster on July 27, 1976, in which 680 thousand were killed and over 700 thousand were injured, and the total number of victims exceeded 1.4 million people.
Our country has experience in predicting one of the minor (5 magnitude) tremors in the Tashkent region, a small earthquake in the uninhabited area of the Alai Valley near Andijan, and several other similar seismic phenomena in other areas of Central Asia.
It must be said that in all the examples given there is no guarantee that the accuracy of the prediction is due to the accuracy of the forecast, and not to a random coincidence. There are a number of counter-examples, when forecasts of supposedly future earthquakes were not confirmed.
From time to time, mass sources of information suddenly begin to beat the kettledrums and widely announce extraordinary successes in the field of seismic forecasting, and it seems as if most of the problems of this important scientific area have already been solved. However, in fact, the situation is not at all so encouraging and the false pathos of this information remains on the conscience of its authors and distributors.
Indeed, except for a single case in Liaoning Province (Haicheng), during the 30-year period of work on the problem of seismic forecasting, not a single catastrophic earthquake was predicted in any region of the globe. In particular, as the famous Soviet researcher B.A. Petrushevsky points out, in the USSR no warning forecasts were made either for the Tashkent region in 1966, or for the Gazli region in 1976 and 1984, which is why the destruction there was so unexpected and severe. On the one hand, modern forecasting cannot yet identify the main harbingers of the upcoming release of seismic stresses and determine the location of the earthquake: during the dramatic disaster in the Chinese Tien Shan in 1976, observations delineated a vast seismic zone, but they could not determine the source of the seismic release; In this respect, the forecast of volcanic eruptions is in a better position because it deals with specific points on the ground.
On the other hand, the lack of ability to recognize and control the “trigger mechanism” of earthquakes does not allow us to determine the exact time of the event: after the 1964 Anchorage earthquake, many scientists came to the conclusion that it was provoked by a high sea tide, which acted as a “trigger mechanism” , increasing the load on the earth's crust. Before the earthquake this was not clear to anyone; at the same time, according to other experts, the initiator of the shock was a strong disturbance in the magnetic field, recorded 1 hour before the disaster. In addition, scientists do not yet have any direct methods for calculating the strength of possible vibrations.
Apparently, the most fair assessment of the problem of predicting earthquakes was made by C. Richter, who believes that at the current level of science, predicting the discharge of seismic energy is possible - without an exact date - only on certain tectonic faults that have been studied systematically and for a long time. It is likely that in the future, with the improvement of space survey methods and the deployment of a network of stationary ground observations, it will be possible to predict seismic phenomena over vast regions of the earth's surface.
It should be noted that seismic forecasting, while helping to solve the problem of reducing the number of human casualties, does nothing to prevent material losses and destruction during earthquakes. Therefore, work to clarify seismic zoning with differentiation of the territory according to the degree of danger, the development of earthquake-resistant construction in hazardous areas and the reduction of economic activities in highly hazardous areas are of much greater importance; these activities are aimed at solving both problems. Without setting themselves the goal of knowing exactly when an earthquake will occur, they allow themselves to be prepared for it at any time.
Recently, ideas have been expressed in engineering seismology about the possibility of controlling earthquakes. It has been noticed that underground nuclear explosions cause a series of subsequent, weaker earthquakes; similar phenomena occur after water is pumped into the subsoil through deep wells under high pressure. It is assumed that with such technical means it is possible to release energy accumulated in the depths and discharge it in small portions, preventing destructive tremors. Sensible experts note: there is no guarantee that the process will develop the way we want.
The earth has one unfortunate property: it sometimes slips away from under your feet, and this is not always associated with the results of a cheerful party in a friendly circle. Ground shaking causes asphalt to stand on end and houses collapse. What's there at home?! — catastrophic earthquakes can uplift or destroy mountains, dry up lakes, and turn rivers around. In such situations, residents of houses, mountains and coasts have only one thing left to do: try to survive as best as possible.
People have been confronted with the violence of the earth's firmament approximately since the time when they descended onto this firmament from the trees. Apparently, the first attempts to explain the nature of earthquakes date back to the beginning of the human era, in which underground gods, demons and other pseudonyms of tectonic movements appear abundantly. As our ancestors acquired permanent housing with accompanying fortresses and chicken coops, the damage from shaking the ground beneath them became greater, and the desire to appease Vulcan, or at least predict his disfavor, became stronger.
However, different countries in ancient times were shaken by different entities. The Japanese version gives the leading role to giant catfish living underground, which sometimes move. In March 2011, another fish riot led to a powerful earthquake and tsunami.
Scheme of tsunami propagation in the Pacific Ocean. The painting shows in color the height of the waves diverging in different directions, generated by an earthquake near Japan. Let us recall that the earthquake on March 11 brought down a tsunami wave on the coast of Japan, leading to the death of at least 20 thousand people, widespread destruction and the transformation of the word “Fukushima” into a synonym for Chernobyl. Responding to a tsunami requires great speed. The speed of ocean waves is measured in kilometers per hour, and seismic waves are measured in kilometers per second. Due to this, there is a time reserve of 10-15 minutes, during which it is necessary to notify the residents of the threatened area.
Unstable Firmament
The earth's crust is in very slow but continuous motion. Huge blocks press against each other and become deformed. When the stresses exceed the tensile strength, the deformation becomes inelastic - the earth's solids break, and the layers shift along the fault with elastic recoil. This theory was first proposed almost a hundred years ago by American geophysicist Harry Reid, who studied the 1906 earthquake that almost completely destroyed San Francisco. Since then, scientists have proposed many theories, detailing the course of events in different ways, but the fundamental principle has remained broadly the same.
The depth of the sea is variable. The arrival of a tsunami is often preceded by a retreat of water from the shore. Elastic deformations of the earth's crust preceding an earthquake leave water in place, but the depth of the bottom relative to sea level often changes. Sea depth monitoring is carried out by a network of special instruments - tide gauges, installed both on the shore and at a distance from the shore.
The variety of versions, alas, does not increase the volume of knowledge. It is known that the source (in scientific terms, the hypocenter) of an earthquake is an extended area in which the destruction of rocks occurs with the release of energy. Its volumes are directly related to the size of the hypocenter - the larger it is, the stronger the shaking. The foci of destructive earthquakes extend over tens and hundreds of kilometers. Thus, the source of the 1952 Kamchatka earthquake was about 500 km long, and the Sumatran earthquake, which caused the worst tsunami in modern history in December 2004, was at least 1,300 km long.
The dimensions of the hypocenter depend not only on the stresses accumulated in it, but also on the physical strength of the rocks. Each individual layer that finds itself in the destruction zone can either crack, increasing the scale of the event, or survive. The final result ultimately turns out to depend on many factors invisible from the surface.
Tectonics in pictures. The collision of lithospheric plates leads to their deformation and stress accumulation.
Seismic climate
Seismic zoning of a territory makes it possible to predict the strength of possible tremors in a given location, even without indicating the exact location and time. The resulting map can be compared with a climate map, but instead of the atmospheric climate, it displays a seismic climate - an assessment of the possible strength of an earthquake in a given place.
The initial information is data on seismic activity in the past. Unfortunately, the history of instrumental observations of seismic processes goes back a little over a hundred years, and in many regions even less. Some help can be provided by collecting data from historical sources: descriptions even by ancient authors are usually enough to determine the severity of an earthquake, since the corresponding scales are built on the basis of everyday consequences - the destruction of buildings, people's reactions, etc. But this, of course, is not enough - humanity still too young. Just because there hasn't been a magnitude 10 earthquake in a certain region over the past couple of thousand years, that doesn't mean it won't happen there next year. As long as we are talking about ordinary low-rise construction, a risk of this level can be tolerated, but the placement of nuclear power plants, oil pipelines and other potentially dangerous objects clearly requires greater precision.
The problem turns out to be solvable if we move from individual earthquakes to consideration of the flow of seismic events, characterized by certain patterns, including density and recurrence. In this case, it is possible to establish the dependence of the frequency of earthquakes on their strength. The weaker the earthquakes, the greater their number. This dependence can be analyzed using mathematical methods, and having established it for a certain period of time, albeit small, but supported by instrumental observations, it is possible to extrapolate with sufficient reliability the course of events after hundreds and even thousands of years. The probabilistic approach makes it possible to impose acceptable accuracy restrictions on the scale of future disasters.
Seismic zoning map OSR-97D. The colors indicate the maximum destructive power of earthquakes with a repetition period of about 10,000 years. This map is used in the construction of nuclear power plants and other critical facilities. One of the manifestations of earthly activity are volcanoes. Their eruptions are colorful and sometimes destructive, but the seismic shocks they generate are, as a rule, weak and do not pose an independent threat.
As an example of how this is done, we can cite the OSR-97 set of seismic zoning maps currently in use in Russia. When compiling it, faults were identified based on geological data - potential sources of earthquakes. Their seismic activity was modeled using very complex mathematics. The virtual streams of seismic events were then checked against reality. The resulting dependencies could be relatively confidently extrapolated into the future. The result was a series of maps showing the maximum score of events that can be repeated in a given territory with a periodicity of 100 to 10,000 years.
Harbingers of trouble
Seismic zoning makes it possible to understand where to “place the straw”. But in order to minimize the damage, it would be good to know the exact time and place of the event - in addition to assessing the “climate,” also have a “weather” forecast.
The most impressive short-term earthquake forecast was made in 1975 in the Chinese city of Haichen. Scientists who had been monitoring seismic activity for several years sounded the alarm on February 4 at around 2 p.m. Residents were taken to the streets, and shops and industrial enterprises were closed. An earthquake with a magnitude of 7.3 occurred at 19:36, causing significant damage to the city, but there were few casualties. Alas, this example remains one of very few so far.
Stresses accumulating in the earth's thickness lead to changes in its properties, and in most cases they can be “caught” by instruments. Several hundred such changes—seismologists call them harbingers—are known today, and their list is growing year after year. Increasing earth stresses change the speed of elastic waves in them, electrical conductivity, groundwater level, etc.
One of the typical consequences of a devastating earthquake. Experts would rate the intensity of the shaking at about 10 points (on a 12-point scale).
The problem is that harbingers are capricious. They behave differently in different regions, appearing to researchers in different, sometimes bizarre combinations. In order to confidently put together a “mosaic”, you need to know the rules for its composition, but we do not have complete information and it is not a fact that one day there will be one.
Studies from the 1950s to the 1970s showed a correlation of radon content in groundwater in the Tashkent area with seismic activity. The radon content before earthquakes within a radius of up to 100 km changed 7–9 days before the shock, first increasing to a maximum (five days), and then decreasing. But similar studies in Kyrgyzstan and the Tien Shan did not show a stable correlation.
Elastic deformations of the earth's crust lead to a relatively rapid (months and years) change in the altitude of the area. These changes have been “caught” for a long time and reliably. In the early 1970s, American experts identified a surface uplift near the town of Palmdale in California, standing directly on the San Andreas Fault, to which the state owes its reputation as a seismically troubled place. Considerable effort, money and equipment were spent on trying to track the development of events and warn in time. By the mid-1970s, the rise of the surface increased to 35 cm. A decrease in the speed of elastic waves in the earth's thickness was also noted. Observations of the harbingers continued for many years, costing a lot of dollars, but... no catastrophe occurred, the condition of the area gradually returned to normal.
In recent years, new approaches to forecasting have emerged related to the consideration of seismic activity at the global level. In particular, Kamchatka seismologists, traditionally at the “cutting edge” of science, reported predictive successes. But the attitude towards prognostication of the scientific world as a whole would still be more correctly characterized as cautious skepticism.
On July 23, the fourth earthquake in a day occurred in Iran, and the number of victims reached 287. A day earlier, tremors with a magnitude of 5.2 were recorded in Chile. In general, over 7 months of 2018, 6881 earthquakes occurred on Earth, taking 227 human lives. But why have scientists never learned to predict these cataclysms? Realist figured it out.
How are seismic zones determined?
Lithospheric plates are in constant motion. Colliding and stretching, they increase stress in the rocks, which leads to their rapid rupture - an earthquake. The source (hypocenter) of an earthquake is located in the bowels of the earth, and the epicenter is its projection on the surface.
The strength of earthquakes is measured on a scale of destruction in points (from 1 to 12), as well as magnitude - a dimensionless quantity that reflects the released energy of elastic vibrations (from 1 to 9.5 on the Richter scale).
The easiest way for science is to identify seismically dangerous zones and long-term forecasting of earthquakes for the next 10-15 years. To do this, researchers analyze the cyclical activation of the seismotectonic process: there is no reason to believe that in the next few hundred years the Earth will begin to behave differently than in a similar period of time in the past.
Is it possible to predict earthquakes
No, at least with sufficient accuracy to allow evacuation programs to be planned. And although most earthquakes occur in predictable locations along well-known geological faults, the reliability of short-term forecasts leaves much to be desired.
“We have models that show that in Southern California the risk of earthquakes of magnitude 7.5 or greater in the next 30 years is 38%. If these models are used to calculate the probability of earthquakes in the coming week, the probability drops to about 0.02%,” comments Thomas Jordan, director of the Southern California Earthquake Center.
This risk is quite small, but still not zero, and since the San Andreas transform fault passes through the state of California, local schools regularly conduct drills to prepare for a large earthquake.
Why are large earthquakes so difficult to predict?
Reliable predictions require identifying signals that would indicate an upcoming large earthquake. Such signals should be characteristic only of large earthquakes: weak and moderate tremors with a magnitude of up to 5 can lead to hanging objects swaying, glass rattling or plaster falling, which does not require evacuation of the population. However, in 5-10% of cases such tremors turn out to be foreshocks, which precede stronger earthquakes. According to statistics, foreshock activity is characteristic of 40% of medium and 70% of large earthquakes.
Seismologists have still not been able to identify specific events that regularly occur only before large earthquakes.
Today, a wide range of potential earthquake predictors have been studied, from increased radon concentrations in the air and unusual animal behavior to deformation of the earth's surface and changes in groundwater levels. But these anomalies are general: each of them can occur even before the weakest shocks.
Why are people not evacuated at the slightest risk of a major earthquake?
The main reason is the high probability of a false alarm. Thus, in 1975, in Haicheng (China), seismologists recorded increasing frequency of weak earthquakes and declared a general alarm on February 4 at 2 pm. After 5 hours and 36 minutes, an earthquake with a magnitude of more than 7 occurred in the city, many buildings were destroyed, but thanks to timely evacuation, the cataclysm occurred with virtually no casualties.
Unfortunately, such successful predictions could not be repeated in the future: seismologists predicted several large earthquakes that did not take place, and the shutdown of enterprises and evacuation of the population only resulted in economic losses.
How earthquake early warning systems work
Japan today has the best early warning system for earthquakes. The country is literally “strewn” with stations that, using sensitive equipment, record seismic waves, identify potential foreshocks and transmit information to the Meteorological Agency, which, in turn, immediately transmits it to TV, the Internet and mobile phones of citizens. Thus, by the time the second seismic wave arrives, the population has already been warned about the epicenter of the earthquake, its magnitude and the time of approach of the second wave.
Despite technological advances, even the Japanese warning system goes off after a natural disaster has occurred. But until researchers thoroughly study the physical processes associated with earthquakes, one cannot count on more. Residents of seismically active zones can only hope that seismometers will become more sensitive, and satellite observation will help speed up forecast times.
