Causes of earthquakes and consequences. The nature of earthquakes. Earth disasters - earthquakes What causes earthquakes

seismic earthquake lithosphere

Based on the reason that caused them, earthquakes are divided into four types.

Volcanic. Volcanic earthquakes occur quite often. Seismic activity occurs in areas of active volcanic activity. Because zones of seismic and volcanic activity often coincide, it can be very difficult to determine the type of earthquake. One can definitely speak of a volcanic type of earthquake only in situations where seismic activity is observed simultaneously with increased volcanic activity.

The consequences of volcanic earthquakes extend no more than 30-50 km near the volcano, the focus is located at a shallow depth, and the epicenter is located next to the volcanic cone. The energy of volcanic earthquakes is characterized by the energy of explosions that accompanies rapidly occurring chemical reactions.

Tectonic. The area of ​​tremors during tectonic earthquakes can spread over 1500-2000 km, and seismic waves often travel around the entire globe. The consequences include changes in the earth's surface, the appearance of mountains, lakes and depressions, and severe destruction of houses and buildings. The energy of tectonic earthquakes is comparable to the energy of nuclear reactions.

Most scientific researchers believe that the cause of seismic activity during tectonic earthquakes is the release of stress caused by the shear, torsion or sliding of lithospheric plates relative to each other. The proof of the hypothesis is the location of earthquake foci along existing faults. Repeated faults cause earthquakes. This hypothesis is also confirmed by the greater intensity of shear waves. If substances were compressed and stretched without breaking, longitudinal waves would have greater intensity.

The most catastrophic earthquakes are associated with the remaking of the Earth in zones of young tectonic folding. With the onset of critical stress, faults are formed, and at the moment of formation of faults, seismic tremors occur. The shock energy released at the fault point (the source of the earthquake) is transmitted through elastic waves in the earth's crust and reaches the surface, destroying everything around. The hypocenters of many tectonic earthquakes are located at depths of 10 to 50 km, and some are deeper. All the epicenters of the tremors are located on the fault lines of the Earth's crust, and the hypocenters are located on the fault planes, which gradually plunge into the bowels of the Earth.

Denudation. The processes of landslides occurring when rocks are destroyed by temperature fluctuations, water and wind are called denudation. Denudation earthquakes are caused by external factors that are controlled by solar heat flows. Denunation accounts for less than 1% of all earthquakes.

Anthropogenic. Safe earthquakes up to magnitude 4 can be caused by anthropogenic causes. For example, a large number of weak tremors began to be recorded in some states of America after the start of shale gas and heavy oil production. In both cases, water hammer technology is used, followed by pumping liquid into the resulting cavities. Several NG production fields were closed until the reasons were clarified.

The development of oil, gas, coal and diamond deposits can lead to subsidence of areas of varying sizes. Since the formation of stress in the earth's crust occurs slowly, the results of human activity may only be observed after 500 -1000 years.

Anthropogenic causes also include small tremors following underground tests of thermonuclear bombs. In addition, it is known that the fall of a large meteorite to Earth 65 million years ago caused powerful earthquakes, volcanic eruptions, tsunamis and led to the extinction of 80% of all life on our planet. It was also noticed that a rapid drop or sudden increase in atmospheric pressure triggers tremors, especially in seismically active zones.

Scientists are also making observations on the connection between seismic activity and the phases of the Moon and the Sun. The influence of the Sun and Moon is especially noticeable when the Earth is at the closest distance from them. An increased influence of the Moon on the occurrence of earthquakes during the full and new moon has been established; There is a connection between earthquakes and an increase in the density of the solar wind emitted by the Sun.

The element of earthquakes finds its manifestation mainly where there is an active fault. Catastrophic tremors rarely repeat themselves in the same place. For example, before 1948 in Ashgabat, an earthquake of similar magnitude occurred in the 14th century. Seismologists suggest that the time of stress accumulation at one point is measured in millennia.

As a result of seismic activity, the level of cities such as Stavropol and Tula rises, and Samara, Odessa and Baku subsidence. In some large cities, there is a simultaneous subsidence of one part of the city and an uplift of another. The eastern part of Moscow is going down, while the western part is gradually rising.

American geologist C. Liese believes that faults are only a consequence, and seismic activity is caused by other reasons, due to deeper processes (for example, due to sudden changes in the volume of matter, conductivity of matter and atomic structure).

But the most important reason for seismic tremors is the movement of the Earth’s core, which is also called the inner sun and the heart of our planet. We know a lot about our common “heart” theoretically, but practically nothing at all. Without the movement of this troublemaker, life on Earth would be impossible. Stopping the core will cause the disappearance of the magnetic field of our planet. The solar wind will be able to freely penetrate the Earth, evaporating all the water and volatile substances from the surface of the earth's crust and turning our planet into a desert similar to Mars.

The movements of the core continuously transform the surface of the Earth, and the consequences of these transformations bring a lot of suffering to humanity. Scientists have gained enough knowledge about the causes of natural disasters and are carefully studying their consequences.

I would like to believe that soon Earth researchers will create an accurate method for predicting all natural disasters, and earthquakes in the first place.

Earthquakes are a natural phenomenon that even today attracts the attention of scientists not only due to their lack of knowledge, but also due to their unpredictability, which can harm humanity.

What is an earthquake?

An earthquake is an underground tremors that can be felt by a person largely depending on the power of vibration of the earth's surface. Earthquakes are not uncommon and occur every day in different parts of the planet. Often, most earthquakes occur at the bottom of the oceans, which avoids catastrophic destruction within densely populated cities.

The principle of earthquakes

What causes earthquakes? Earthquakes can be caused by both natural causes and man-made ones.

Most often, earthquakes occur due to faults in tectonic plates and their rapid displacement. For a person, a fault is not noticeable until the moment when the energy generated from the rupture of rocks begins to break out to the surface.

How do earthquakes occur due to unnatural causes? Quite often, a person, through his carelessness, provokes the appearance of artificial tremors, which in their power are not at all inferior to natural ones. Among these reasons are the following:

• - explosions;
• - overfilling of reservoirs;
• - above-ground (underground) nuclear explosion;
• - collapses in mines.

The location where a tectonic plate breaks is the source of an earthquake. Not only the strength of the potential push, but also its duration will depend on the depth of its location. If the source is located 100 kilometers from the surface, then its strength will be more than noticeable. Most likely, this earthquake will lead to the destruction of houses and buildings. Occurring in the sea, such earthquakes cause tsunamis. However, the source can be located much deeper - 700 and 800 kilometers. Such phenomena are not dangerous and can only be recorded using special instruments - seismographs.

The place where the earthquake is most powerful is called the epicenter. It is this piece of land that is considered the most dangerous for the existence of all living things.

Studying earthquakes

A detailed study of the nature of earthquakes makes it possible to prevent many of them and make the life of the population living in dangerous places more peaceful. To determine the power and measure the strength of an earthquake, two basic concepts are used:

• - magnitude;
• - intensity;

The magnitude of an earthquake is a measure that measures the energy released during release from the source in the form of seismic waves. The magnitude scale allows you to accurately determine the origins of vibrations.

Intensity is measured in points and allows you to determine the ratio of the magnitude of tremors and their seismic activity from 0 to 12 points on the Richter scale.

Features and signs of earthquakes

Regardless of what causes an earthquake and in what area it is localized, its duration will be approximately the same. One push lasts on average 20-30 seconds. But history has recorded cases when a single shock without repetitions could last up to three minutes.

Signs of an approaching earthquake are the anxiety of animals, which, sensing the slightest vibrations on the surface of the earth, try to get away from the ill-fated place. Other signs of an imminent earthquake include:

• - the appearance of characteristic clouds in the form of oblong ribbons;
• - change in water level in wells;
• - malfunctions of electrical equipment and mobile phones.

How to behave during earthquakes?

How to behave during an earthquake to save your life?

• - Maintain reasonableness and calm;
• - When indoors, never hide under fragile furniture, such as a bed. Lie down next to them in the fetal position and cover your head with your hands (or protect your head with something extra). If the roof collapses, it will fall on the furniture and a layer may form, in which you will find yourself. It is important to choose strong furniture whose widest part is on the floor, i.e. this furniture cannot fall;
• - When outside, move away from tall buildings and structures, power lines that may collapse.
• - Cover your mouth and nose with a wet cloth to prevent dust and fumes from entering if any object catches fire.

If you notice an injured person in a building, wait until the tremors end and only then get into the room. Otherwise, both people may be trapped.

Where do earthquakes not occur and why?

Earthquakes occur where tectonic plates break. Therefore, countries and cities located on a solid tectonic plate without faults do not have to worry about their safety.

Australia is the only continent in the world that is not at the junction of lithospheric plates. There are no active volcanoes and high mountains on it and, accordingly, there are no earthquakes. There are also no earthquakes in Antarctica and Greenland. The presence of the enormous weight of the ice shell prevents the spread of tremors across the surface of the earth.

The probability of earthquakes occurring on the territory of the Russian Federation is quite high in rocky areas, where the displacement and movement of rocks is most actively observed. Thus, high seismicity is observed in the North Caucasus, Altai, Siberia and the Far East.

In this article we will look at causes of earthquakes. The very concept of an earthquake is known to all people, and even children, but what are the reasons that suddenly the ground under your feet begins to move and everything around collapses?

First of all, it must be said that earthquakes are conventionally divided into several types: tectonic, volcanic, landslide, artificial and man-made. We will briefly look at all of them right now. If you want to know, be sure to read to the end.

1. Tectonic causes of earthquakes

Most often, earthquakes occur due to the fact that they are in constant motion. The top layer of lithospheric plates is called tectonic plates. The platforms themselves move unevenly and constantly press on each other. However, they remain alone for a long time.

Gradually, the pressure increases, as a result of which the tectonic plate makes a sudden push. It is this that produces vibrations in the surrounding rock, which is why an earthquake occurs.

San Andreas Fault

Transform faults are huge cracks in the Earth where plates rub against each other. Many readers will be aware that the San Andreas Fault is one of the most famous and longest transform faults in the world. It is located in the state of California in .

Photo of the San Andreas fault

The platforms moving along it cause devastating earthquakes in the cities of San Francisco and Los Angeles. Interesting fact: in 2015, Hollywood released a movie called “San Andreas Fault.” He talks about the corresponding disaster.

1. Volcanic causes of earthquakes

One of the causes of earthquakes is volcanoes. Although they do not produce strong vibrations of the earth, they last quite a long time. The reasons for the tremors are related to the fact that deep in the depths of the volcano the tension formed by lava and volcanic gases increases. As a rule, volcanic earthquakes last for weeks or even months.

However, history knows cases of tragic earthquakes of this type. An example is the Krakatoa volcano, located in Indonesia, which erupted in 1883.

Krakatoa still gets excited sometimes. Real photo.

The force of its explosion was at least 10 thousand times greater than the force of . The mountain itself was almost completely destroyed, and the island split into three small parts. Two-thirds of the land disappeared under water, and the rising tsunami destroyed everyone who still had a chance to escape. More than 36,000 people died.

1. Landslide causes of earthquakes

Earthquakes caused by giant landslides are called landslides. They are local in nature, and their strength is usually small. But there are exceptions here too. For example, in Peru, in 1970, a landslide with a volume of 13 million cubic meters descended from Mount Huascaran at a speed of over 400 km/h. About 20,000 people died.

1. Technogenic causes of earthquakes

Earthquakes of this type are caused by human activity. For example, artificial reservoirs in places not intended for this by nature provoke pressure on the plates with their weight, which serves to increase the number and strength of earthquakes.

The same applies to the oil and gas industry, when large quantities of natural materials are extracted. In a word, man-made earthquakes occur when a person took something from nature from one place and transferred it to another without asking.

1. Artificial causes of earthquakes

From the name of this type of earthquake, it is easy to guess that the blame for it lies entirely with man.

For example, North Korea tested a nuclear bomb in 2006, which caused a small earthquake recorded in many countries. That is, any activity of the inhabitants of the earth, which is obviously guaranteed to entail an earthquake, is an artificial cause of this type of disaster.

Is it possible to predict earthquakes?

Indeed it is possible. For example, in 1975, Chinese scientists predicted an earthquake and saved many lives. But this cannot be done with a 100% guarantee, even today. An ultra-sensitive device that records an earthquake is called a seismograph. The vibrations of the earth are recorded on the rotating drum by a recorder.

Seismograph

Animals also feel acutely anxious before earthquakes. Horses begin to rear up for no apparent reason, dogs bark strangely, and snakes crawl out of their holes to the surface.

Earthquake scale

Typically, the strength of earthquakes is measured using the Earthquake Scale. We will present all twelve points so that you have an idea of ​​what it is.

• 1 point (invisible) - the earthquake is recorded exclusively by instruments;
• 2 points (very weak) - can only be noticed by domestic animals;
• 3 points (weak) - noticeable only in some buildings. Feels like driving over bumps in a car;
• 4 points (moderate) - noticed by many people, may cause movement of windows and doors;
• 5 points (quite strong) - glass rattles, hanging objects sway, old whitewash may crumble;
• 6 points (strong) - with this earthquake, slight damage to buildings and cracks in low-quality buildings are noted;
• 7 points (very strong) - at this stage the buildings suffer significant damage;
• 8 points (destructive) - destruction is observed in buildings, chimneys and cornices are falling, cracks of several centimeters can be seen on the mountain slopes;
• 9 points (devastating) - earthquakes cause collapses of some buildings, old walls collapse, and the speed of crack propagation reaches 2 centimeters per second;
• 10 points (destructive) - collapses in many buildings, in most - serious damage. The soil is riddled with cracks up to 1 meter wide, and there are landslides and landslides all around;
• 11 points (catastrophe) - large landslides in mountainous areas, numerous cracks and a picture of general destruction of most buildings;
• 12 points (severe catastrophe) - the relief is changing globally almost before our eyes. Huge collapses and total destruction of all buildings.

In principle, on a twelve-point earthquake scale, any catastrophe caused by tremors of the earth’s surface can be assessed.

These are some of the most terrible natural disasters, claiming tens and hundreds of thousands of human lives and causing devastating destruction over vast areas.

On December 7, 1988, a powerful earthquake occurred in Armenia, named Spitak after the name of the city, which was completely wiped off the face of the Earth. Then, in a few seconds, more than 25 thousand people died, and several hundred thousand were injured. The Ashgabat earthquake on the night of October 5-6, 1948 claimed more than 100 thousand lives.

In China in 1920, 200 thousand people died, in 1923 in Japan - more than 100 thousand. There are many examples of catastrophic earthquakes that resulted in large casualties. For example, in 1755 in Lisbon, in 1906 in San Francisco, in 1908 in Sicily, in 1950 in the Himalayas, in 1957 in Western Mongolia and in 1960 in Chile. In 1976, 250 thousand people became victims of the very strong Tangshan earthquake in China. 3,100 people died in an earthquake in 1980 in Italy, 2,500 in 1981 in Iran.

In 1993, a powerful earthquake struck the Japanese city of Kobe, causing fires that devastated entire neighborhoods and caused loss of life. In 1994, powerful tremors shook San Francisco, collapsing highway overpasses. The earthquake in the north of Sakhalin in 1995 in Neftegorsk turned into a tragedy, when several buildings collapsed, under the rubble of which 2 thousand people died.

In the winter of 1998, a powerful earthquake struck Afghanistan. This list can be continued endlessly, since earthquakes of varying strengths and in different regions of the globe occur constantly, causing enormous material damage and leading to numerous casualties.

That is why scientists from various countries are making great efforts to study the nature of earthquakes and their forecast. Unfortunately, it is still not possible to predict the location and time of an earthquake, with the exception of a few cases.

CAUSES OF EARTHQUAKES AND THEIR PARAMETERS

Any earthquake is an instant release of energy due to the formation of a rock rupture that occurs in a certain volume called the earthquake focus, the boundaries of which cannot be defined strictly enough and depend on the structure and stress-strain state of the rocks in a given location. Deformation that occurs abruptly emits elastic waves. The volume of deformed rocks plays an important role in determining the strength of the seismic shock and the energy released.

Large spaces of the Earth's crust or upper mantle, in which ruptures occur and inelastic tectonic deformations occur, give rise to strong earthquakes: the smaller the volume of the source, the weaker the seismic tremors. The hypocenter, or focus, of an earthquake is the conditional center of the source at depth, and the epicenter is the projection of the hypocenter onto the Earth's surface. The zone of strong vibrations and significant destruction on the surface during an earthquake is called the pleistoseist region.

Based on the depth of the hypocenters, earthquakes are divided into three types: 1) shallow-focus (0-70 km), 2) medium-focus (70-300 km), 3) deep-focus (300-700 km). Most often, earthquake foci are concentrated in the earth's crust at a depth of 10-30 km. As a rule, the main underground seismic shock is preceded by local tremors - foreshocks. Seismic tremors that occur after the main shock are called aftershocks. Aftershocks that occur over a significant period of time contribute to the release of stress in the source and the emergence of new ruptures in the thickness of the rocks surrounding the source.

The source of an earthquake is characterized by the intensity of the seismic effect, expressed in points and magnitude. In Russia, the 12-point Medvedev-Sponheuer-Karnik intensity scale (MSK-64) is used. According to this scale, the following gradation of earthquake intensity is adopted: I-III points - weak, IV-V - noticeable, VI-VII - strong (dilapidated buildings are destroyed), VIII - destructive (strong buildings are partially destroyed, factory chimneys fall), IX - devastating (most buildings are destroyed), X - destructive (bridges are destroyed, landslides and collapses occur), XI - catastrophic (all structures are destroyed, the landscape changes), XII - disastrous disasters (cause changes in the terrain over a vast territory). The magnitude of an earthquake according to Charles F. Richter is defined as the decimal logarithm of the ratio of the maximum amplitudes of seismic waves of a given earthquake (A) to the amplitude of the same waves of some standard earthquake (Ax). The larger the wave span, the correspondingly greater the ground displacement:

Magnitude 0 means an earthquake with a maximum amplitude of 1 μm at an epicentral distance of 100 km. At a magnitude of 5, minor damage to buildings is observed. The devastating tremor has a magnitude of 7. The strongest recorded earthquakes reach magnitudes of 8.5-8.9 on the Richter scale. Currently, earthquake assessment in magnitude is used more often than in points.

There is a relationship between the intensity (I0) of an earthquake at the epicenter, which is expressed in points, and the magnitude (M)

I0 = 1.7 "M - 2.2; M = 0.6" I0 + 1.2.

A more complex equation characterizes the relationship between the intensity of the oscillation I0, the magnitude M and the source depth H:

I0 = аМ - b log Н + с,

where a, b, c are coefficients determined empirically for a specific earthquake region.

Lines connecting points with the same intensity of vibrations are called isoseists. At the epicenter of an earthquake, the Earth's surface experiences mainly vertical vibrations. As you move away from the epicenter, the role of the horizontal component of the oscillations increases.

The energy released during earthquakes is E = p2rV (a / T), where V is the speed of propagation of seismic waves, r is the density of the upper layers of the Earth, a is the displacement amplitude, T is the oscillation period. The source material for energy calculations is seismogram data. B. Gutenberg, like Charles Richter, who worked at the California Institute of Technology, proposed a connection between the energy of an earthquake and its magnitude on the Richter scale:

log E = 9.9 + 1.9M - 0.024M 2.

This formula shows a colossal increase in energy with increasing earthquake magnitude.

The energy of earthquakes is several million times higher than the energy of a standard atomic bomb of 100 kt (1000 "1018 erg). For example, during the Ashgabat earthquake in 1948, 1023 erg was released, during the Khait earthquake in Tajikistan in 1949 - 5 "1024 erg, in 1960 Chile - 1025 erg. Around the globe, on average, about 0.5" 1026 ergs of energy are released each year due to earthquakes.

An important concept in seismology is specific seismic power, that is, the amount of energy released per unit volume, for example 1 m3, per unit time 1 s. Seismic waves, formed during instantaneous deformation in the foci of earthquakes, produce the main destructive work on the Earth's surface. There are three main types of elastic waves that create seismic vibrations that are felt by people and cause destruction: volumetric longitudinal (P-waves) and transverse (S-waves), as well as surface waves (Fig. 3).

Longitudinal waves represent alternating zones of compression and tension in rocks, and they travel through solid, liquid and gaseous substances. During their propagation, longitudinal waves seem to alternately compress rocks or stretch them. Part of the energy of P-waves, emerging from the bowels of the Earth to its surface, is transmitted to the atmosphere in the form of sound waves, which are perceived by people at a frequency of more than 15 Hz. P waves are the fastest of body waves. The speed of propagation of P-waves, where m is the shear modulus, r is the density of the medium in which the wave propagates, and l is the coefficient associated with the bulk modulus K,

Transverse waves, when propagating, shift particles of matter at right angles to the direction of their path. They do not propagate in a liquid medium, since the shear modulus in a liquid is zero. The speed of transverse waves is less than longitudinal waves. These seismic waves rock and shift the soil surface both vertically and horizontally:

The second type includes surface seismic waves, the propagation of which is limited to a zone close to the Earth's surface. They are like ripples spreading across the surface of a lake. There are surface Love waves and Rayleigh waves.

Love waves (L) cause soil particles to oscillate from side to side in a horizontal plane parallel to the earth's surface at right angles to the direction of their propagation. Rayleigh waves (R) arise at the interface between two media and affect the particles of the medium, causing them to move vertically and horizontally in a vertical plane oriented in the direction of wave propagation. The speed of Rayleigh waves is less than that of Love waves, and both of them propagate more slowly than longitudinal and transverse seismic waves and attenuate quite quickly with depth, as well as with distance from the epicenter of the earthquake.

REGISTRATION OF EARTHQUAKES

Seismic waves propagating from the source of an earthquake in all directions, reaching the surface of the Earth, can be recorded by special devices - seismographs, which record insignificant ground vibrations from earthquakes that occurred even on the opposite side of the globe.

The first seismographs appeared only about 100 years ago, and the recordings of seismic waves - seismograms made by them - make it possible to determine the magnitudes of earthquakes and the location (epicenters) of the latter. The part of the seismograph that directly records the seismogram is called a seismometer and consists of a pendulum suspended on a thin spring that responds to the slightest vibrations of the ground.

The actual recording of these vibrations is carried out either on a rotating drum with a pen and ink, or on magnetic tape using an electromagnetic system that converts the vibrations into current, or with a light beam on moving photographic paper. Seismograms must reflect the movement of soil in two mutually perpendicular horizontal directions and one vertical, which requires three seismometers.

Decoding seismograms involves interpreting and recording the exact time of arrival of various waves P, S, L and R, which not only propagate at different speeds, but also arrive at the seismograph from different directions. By determining the time of entry of different waves and knowing the speed of their propagation, it is possible to determine the distance to the source of the earthquake - the hypocenter. The existing worldwide network of seismic stations with many hundreds of seismographs makes it possible to immediately record earthquakes occurring anywhere in the world. More than several hundred thousand earthquakes felt by people are recorded every year, but only about 100 earthquakes can be classified as destructive. This continuous seismic activity is a consequence of modern tectonic movements in the most superficial shell of the Earth - the lithosphere.

PROPAGATION OF EARTHQUAKES

AND THEIR GEOLOGICAL POSITION

The distribution of earthquakes on the globe is quite natural and is generally well explained by the theory of lithospheric plate tectonics. The largest number of earthquakes is associated with convergent and divergent plate boundaries, that is, with zones where plates either collide with each other or diverge and grow due to the formation of new oceanic crust.

Highly seismic region - active margins of the Pacific Ocean, where oceanic plates subduct, that is, sink under the continental ones and the stresses arising in the cold and heavy plate are discharged in the form of numerous earthquakes, the hypocenters of which form an inclined seismofocal zone, extending into the upper mantle to depths of 600- 700 km.

Such inclined ultra-deep seismofocal zones were established and described by the Dutch geophysicist S.V. Visser in 1936, Japanese geophysicist K. Wadachi in 1938 and Russian scientist A.N. Zavaritsky in 1946. However, thanks to later research by the American seismologist H. Benioff in 1949, they were called Benioff seismofocal zones.

Earthquakes also accompany the formation of rifts in mid-ocean ridges and on continents, but there, unlike compression conditions in subduction zones, they occur under geodynamic conditions of extension or shear.

Another region of strong and frequent earthquakes is the Alpine folded mountain belt, stretching from Gibraltar through the Alps, Balkans, Anatolia, Caucasus, Iran, the Himalayas to Burma and arose only 15-10 million years ago as a result of the collision of enormous lithospheric plates: African -Arabian and Hindustan, on the one hand, and Eurasian, on the other. The compression process continues to this day, so the constantly accumulating stresses are continuously discharged in the form of earthquakes. The largest number of earthquake hypocenters in this belt are confined to the earth's crust, that is, to depths of up to 50 km, although there are also deep ones (up to 300 km), but inclined seismofocal zones are poorly expressed and are rare. It is interesting that the distribution of epicenters in the plan outlines, for example, in Iran and Afghanistan almost aseismic large blocks that turned out to be “welded” together in the process of collision; their junction zones are still active. Within the CIS, the most seismically active regions include the Eastern Carpathians, the Crimean Mountains, the Caucasus, the Kopet Dag, Tien Shan and Pamir, Altai, and the lake region. Baikal and the Far East, especially Kamchatka, the Kuril Islands and Sakhalin Island, where the devastating Neftegorsk earthquake with a magnitude of 7.5 occurred on May 28, 1995, and the death toll was 2 thousand people.

All of these regions have mountainous, often high-mountain topography, indicating that they are currently experiencing active tectonic movements, and the vertical rate of rise of the earth's surface exceeds the rate of erosion. In many regions, for example in Transcarpathia, the Caucasus, and Lake Baikal, the last volcanic eruptions occurred geologically recently, and in Kamchatka and the Kuril Islands they are still occurring today. It is precisely these areas that are characterized by high seismic activity, which is directly correlated with tectonic activity. It should be noted that earthquakes also occur in stable areas of the earth’s crust, on platforms, including ancient ones. True, these earthquakes are quite rare and generally relatively weak. However, there are also strong ones, as, for example, on the Epipaleozoic young Turan plate in the Kyzylkum desert in the Gazli region in 1976 and 1984, and the Gazli village was completely destroyed twice.

The overwhelming majority of earthquakes (more than 85%) occur in compression conditions, and only 15% - in tension conditions, which is consistent with the modern geodynamics of geological structures and the nature of the movements of lithospheric plates.

EARTHQUAKE MECHANISM

The mechanism of earthquakes is a very complex process, which seismologists are only beginning to understand. The source of a strong earthquake is some sudden displacement in a certain volume of rocks along a relatively large rupture plane, so the earthquake mechanism is the kinematics of movement in the source. There are several most common models of the earthquake source mechanism.

The earliest model, developed by H. Reid in 1911, is based on elastic recoil during shear deformation of rocks in which the tensile strength is exceeded. Model N.V. Shebalina (1984) suggests that the main role in the occurrence of short-period oscillations with high accelerations is played by complications, roughness or “hooks” along the main discontinuity along which the displacement occurs. “Hooks” prevent free sliding - creep, and they are responsible for the accumulation of stress in the source. The model of avalanche-unstable crack formation (ALC), developed in Russia by V.I. Myachkin, lies in the rapid increase in the number of cracks, their interaction with each other and ultimately the appearance of a main or main rupture, the displacement along which instantly releases the accumulated stress with the formation of elastic waves. Another model of American geophysicists W. Brace and A.M. The Nura, formed in the late 60s, suggests an important role for dilatancy, that is, an increase in the volume of rock during deformation. The microscopic cracks that arise when water gets into them are not able to close again, the volume of the rock increases, and the stresses increase, at the same time the pore pressure increases and the strength of the rock decreases. All this leads to a release of tension - to an earthquake.

There is a model of unstable sliding, most fully developed by the American geophysicist K. Scholz in 1990, which consists of “sticking” contacts of mutually moving rock blocks with a relatively smooth surface structure of the displacement surface. Sticking leads to the accumulation of shear stresses, the release of which is transformed into an earthquake.

TSUNAMI

If an earthquake occurs in the ocean, above its epicenter, with a sudden vertical displacement of the bottom throughout the entire body of water, peculiar underwater waves arise, moving at speeds of up to 800 km/h in all directions from the epicenter. In the open ocean, these long waves are practically imperceptible, but as you approach the flat shore, in bays and bays, the height of the waves increases many times over, forming a steep wall of water up to 10-15 m high, and often more, crashing onto the shore with colossal force and roar, sweeping away everything in its path. For example, the city of Hilo on the Hawaiian Islands was hit by a devastating tsunami in 1946 and 1960, killing more than 200 people. Interestingly, the tsunami of May 22, 1960 was generated by an earthquake near Chile and the waves reached Hilo harbor only 15 hours later, having traveled a distance of 10,500 km at a speed of about 700 km/h. The 1996 tsunami on the Japanese coast led to the death of 26 thousand people. In Russia, the danger of a tsunami threatens the eastern coast of Kamchatka and the Kuril Islands, where a warning service has been created, and villages are built on high places inaccessible to waves.

EARTHQUAKE FORECASTING

Earthquake forecasting is the most important problem addressed by scientists in many countries around the world. However, despite all efforts, this issue is still far from being resolved. Forecasting earthquakes includes both identifying their precursors and seismic zoning, that is, identifying areas in which an earthquake of a certain magnitude or intensity can be expected. Earthquake prediction consists of a long-term forecast for tens of years, a medium-term forecast for several years, a short-term forecast for several weeks or the first months, and the announcement of an immediate seismic alarm. The most impressive reliable earthquake prediction was made in the winter of 1975 in the city of Haichen in northeast China. After observing this area for several years using different methods, it was concluded that a strong earthquake was possible in the near future. The increase in the number of weak earthquakes made it possible to declare a general alarm on February 4 at 14:00. People were taken out into the streets, shops and businesses were closed, and rescue teams were prepared. At 19:36 a strong earthquake with a magnitude of 7.3 occurred, the city of Haichen was destroyed, and there were few casualties. But even along with other successful earthquake predictions, they are the exception rather than the rule.

Seismic zoning of different scales and levels is carried out on the basis of taking into account many features: geological, in particular tectonic, seismological, physical, etc. Compiled and approved maps are required to take into account all construction organizations, despite the fact that an increase in the estimated strength of an earthquake by at least 1 point entails represents a multiple increase in construction costs, as it is associated with the need for additional strengthening of buildings.

Seismic zoning of the territory involves several levels from small to large scale. For example, for cities or large industrial enterprises, detailed maps of microseismic zoning are drawn up, on which it is necessary to take into account the features of the geological structure of small areas, the composition of soils, the nature of their water content, the presence of rocky outcrops and their types. The least favorable are waterlogged soils (hydraulic hammer), loose loams, and loess with high subsidence. Alluvial plains are more dangerous during earthquakes than rock outcrops. All this must be taken into account when constructing and designing buildings, hydroelectric power stations, and factories.

Earthquake-resistant construction is given great attention in all countries, especially for such critical facilities as nuclear power plants, hydroelectric power plants, chemical and oil refineries. The design and construction of buildings in seismic zones requires making them resistant to earthquakes. As aptly noted in the book by J. Geer and H. Shah (1988), the most important thing in the design of earthquake-resistant buildings is to “bind” the building, that is, to connect all the elements of the building: beams, columns, walls and slabs into a single strong, but together along with a flexible structure capable of withstanding ground vibrations. Thanks to such measures, buildings of 35-45 floors are being built in Mexico City, and even 60 floors in Tokyo, a highly seismic area. Such buildings have flexibility, that is, the ability to sway and bend, like trees in a strong wind, but not collapse. Fragile materials, such as brick or raw brick, are destroyed immediately. Let's also not forget that Japan has many nuclear power plants, but their buildings are designed to withstand very strong earthquakes. Old buildings are tied together with steel hoops or cables, strengthened from the outside with a reinforced concrete frame, and secured with reinforcement passing through all the walls. Existing norms and rules are not, of course, able to fully ensure the safety of objects during an earthquake, but they significantly reduce the consequences of natural disasters and therefore require strict implementation.

There are a large number of different earthquake precursors, ranging from seismic and geophysical to hydrodynamic and geochemical. These can be illustrated with several examples. Thus, strong earthquakes, as opposed to weak ones, in a particular area occur over significant periods of time, measured in tens and hundreds of years, since after the stress is released, time is needed for it to increase to a new critical value, and the rate of stress accumulation according to G.A. Sobolev does not exceed 1 kg/cm2 per year. K. Kasahara showed in 1985 that for the destruction of rock it is necessary to accumulate elastic energy of 103 erg/cm3 and the volume of rock that releases energy during an earthquake is directly related to the amount of this energy. Consequently, the greater the magnitude of the earthquake, and therefore the energy, the greater the time interval between strong earthquakes. Data on the seismically active Kuril-Kamchatka island arc allowed S.A. Fedotov established the repeatability of earthquakes with magnitude M = 7.75 every 140? 60 years. In other words, a certain periodicity or seismic cycle is revealed, which makes it possible to give, although a very approximate, long-term forecast.

Seismic precursors include consideration of clustering of earthquake swarms; reduction of earthquakes near the epicenter of a future strong earthquake; migration of earthquake sources along a large seismically active rupture; aseismic slips along the rupture plane at depth, occurring before a future sudden shift; acceleration of viscous flow in the focal region; formation of cracks and movements along them in the area of ​​stress concentration; heterogeneity of the structure of the earth's crust in the zone of seismic ruptures. Of particular interest as precursors are foreshocks, which, as a rule, precede the main seismic shock. However, the main unresolved difficulty lies in the difficulty of recognizing real foreshocks against the background of routine seismic events.

As geophysical precursors, precise measurements of deformations and slopes of the earth's surface are used using special devices - deformers. Before earthquakes, the rate of deformation increases sharply, as it did before the Niigata (Japan) earthquake in 1964. Precursors also include changes in the travel speeds of longitudinal and transverse seismic waves in the focal area immediately before the earthquake. Any change in the stress-strain state of the earth's crust affects the electrical resistance of rocks, which can be measured at high current strength to a depth of 20 km. The same applies to variations in the magnetic field, since the stressed state of rocks affects fluctuations in the magnitude of the piezomagnetic effect in magnetic minerals.

Measurements of fluctuations in groundwater levels are quite reliable as precursors, since any compression in rocks leads to an increase in this level in boreholes and wells. Using the hydrogeodeformation method, successful short-term predictions were made: for example, in Japan in Izu-Oshima on January 14, 1978, in Ashgabat before the strong earthquake on September 16, 1978 with M = 7.7. Changes in the content of radon in groundwater and wells are also used as precursors.

The entire variety of earthquake precursors has been repeatedly analyzed in order to identify general patterns and eliminate errors. Geophysicist T. Rikitaki conducted a statistical analysis of the relationships between the duration of anomalies T and its amplitude A and the expected magnitude M, identifying three classes of precursors. For medium-term precursors, he obtained the equation

log DT = aM - b,

where a = 0.76; b = -1.83, and T is a day. At M = 5-7, the time for the manifestation of precursors is the first months - the first years.

CONCLUSION

In ancient times, earthquakes were considered a punishment sent to people by angry gods. Now we know how and where earthquakes occur, we know all the parameters of this natural disaster, we know how to protect ourselves from it and reduce the catastrophic consequences, at least partially. The globe is marked with areas and zones in which an earthquake of one or another magnitude may occur. Thousands of seismographs, strainmeters, and accelerographs listen to the pulse of the Earth around the clock. But just like thousands of years ago, we are not able to foresee where, what strength and, most importantly, when the next strike of the underground elements will occur. Currently, the degree of predictability of long- and medium-term forecasts has a probability of 0.7-0.8. The situation is worse with short-term forecasts, for which significant connections with precursors have not yet been established. Any earthquake forecast is probabilistic, and the main goal of seismology has not yet been achieved.

1. Yunga S.L. Methods and results of studying seismotectonic deformations. M.: Nauka, 1990. 191 p.

2. Myachkin V.I. Earthquake preparation processes. M.: Nauka, 1978. 232 p.

3. Bolt B.A. Earthquakes. M.: Mir, 1981. 256 p.

4. Earthquakes in the USSR. M.: Nauka, 1990. 323 p.

5. Sobolev G.A. Fundamentals of earthquake forecasting. M.: Nauka, 1993. 312 p.

6. Mogi K. Prediction of earthquakes. M.: Mir, 1988. 382 p.

Nikolai Vladimirovich Koronovsky, professor, head. Department of Dynamic Geology, Faculty of Geology, Moscow State University. M.V. Lomonosov, Honored Scientist of the Russian Federation; specialist in the field of volcanism, tectonics and regional geology of the Alpine belt. Author of textbooks "A Short Course in Regional Geology of the USSR" (1976, 1984), "Fundamentals of Geology" (co-author A.F. Yakushova), a number of monographs and 235 articles on various issues of geology.

Valery Aleksandrovich Abramov, Doctor of Geological and Mineralogical Sciences, Professor of the Far Eastern State Technical University, Researcher of the Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences. Area of ​​scientific interests - seismology.

Hundreds of thousands of earthquakes occur on our planet every year. Most of them are so small and insignificant that only special sensors can detect them. But there are also more serious fluctuations: twice a month the earth’s crust shakes violently enough to destroy everything around it.

Since most tremors of such force occur at the bottom of the World Ocean, unless they are accompanied by a tsunami, people are not even aware of them. But when the land shudders, the element is so destructive that the number of victims goes into the thousands, as happened in the 16th century in China (during earthquakes of magnitude 8.1, more than 830 thousand people died).

Earthquakes are underground tremors and vibrations of the earth's crust caused by natural or artificially created causes (movement of lithospheric plates, volcanic eruptions, explosions). The consequences of high-intensity tremors are often catastrophic, second only to typhoons in terms of the number of victims.

Unfortunately, at the moment, scientists have not studied the processes that occur in the depths of our planet so well, and therefore the forecast of earthquakes is rather approximate and inaccurate. Among the causes of earthquakes, experts identify tectonic, volcanic, landslide, artificial and man-made vibrations of the earth's crust.

Tectonic

Most of the earthquakes recorded in the world arose as a result of movements of tectonic plates, when a sharp displacement of rocks occurs. This can be either a collision with each other, or a thinner plate being lowered under another.

Although this shift is usually small, amounting to only a few centimeters, the mountains located above the epicenter begin to move, releasing enormous energy. As a result, cracks form on the earth's surface, along the edges of which huge areas of the earth begin to shift, along with everything that is on it - fields, houses, people.

Volcanic

But volcanic vibrations, although weak, continue for a long time. Usually they do not pose any particular danger, but catastrophic consequences have still been recorded. As a result of the powerful eruption of the Krakatoa volcano at the end of the 19th century. the explosion destroyed half the mountain, and the subsequent tremors were so powerful that they split the island into three parts, plunging two-thirds into the abyss. The tsunami that arose after this destroyed absolutely everyone who had managed to survive before and did not have time to leave the dangerous territory.

Landslide

It is impossible not to mention landslides and large landslides. Usually these tremors are not severe, but in some cases their consequences can be catastrophic. So, it happened once in Peru, when a huge avalanche, causing an earthquake, came down from Mount Ascaran at a speed of 400 km/h, and, having leveled more than one settlement, killed more than eighteen thousand people.

Technogenic

In some cases, the causes and consequences of earthquakes are often related to human activities. Scientists have recorded an increase in the number of tremors in areas of large reservoirs. This is due to the fact that the collected mass of water begins to put pressure on the underlying earth’s crust, and water penetrating through the soil begins to destroy it. In addition, an increase in seismic activity has been noticed in oil and gas production areas, as well as in the area of ​​mines and quarries.

Artificial

Earthquakes can also be caused artificially. For example, after the DPRK tested new nuclear weapons, sensors recorded moderate earthquakes in many places on the planet.

An undersea earthquake occurs when tectonic plates collide on the ocean floor or near the coast. If the source is shallow and the magnitude is 7, an underwater earthquake is extremely dangerous because it causes a tsunami. During the shaking of the sea crust, one part of the bottom falls, the other rises, as a result of which the water, in an attempt to return to its original position, begins to move vertically, generating a series of huge waves moving towards the coast.

Such an earthquake together with a tsunami can often have catastrophic consequences. For example, one of the most powerful seaquakes occurred several years ago in the Indian Ocean: as a result of underwater tremors, a large tsunami arose and, hitting the nearby coasts, led to the death of more than two hundred thousand people.

The tremors begin

The source of an earthquake is a rupture, after the formation of which the earth's surface instantly shifts. It should be noted that this gap does not occur immediately. First, the plates collide with each other, resulting in friction and energy that gradually begins to accumulate.

When the stress reaches its maximum and begins to exceed the frictional force, the rocks rupture, after which the released energy is converted into seismic waves moving at a speed of 8 km/s and causing vibrations in the earth.

The characteristics of earthquakes based on the depth of the epicenter are divided into three groups:

1. Normal – epicenter up to 70 km;
2. Intermediate – epicenter up to 300 km;
3. Deep-focus - the epicenter at a depth exceeding 300 km, typical of the Pacific Rim. The deeper the epicenter, the further the seismic waves generated by the energy will reach.

Characteristic

An earthquake consists of several stages. The main, most powerful shock is preceded by warning vibrations (foreshocks), and after it, aftershocks and subsequent tremors begin, and the magnitude of the strongest aftershock is 1.2 less than that of the main shock.

The period from the beginning of foreshocks to the end of aftershocks may well last several years, as, for example, happened at the end of the 19th century on the island of Lissa in the Adriatic Sea: it lasted three years and during this time scientists recorded 86 thousand tremors.

As for the duration of the main shock, it is usually short and rarely lasts more than a minute. For example, the most powerful shock in Haiti, which occurred several years ago, lasted forty seconds - and this was enough to turn the city of Port-au-Prince into ruins. But in Alaska, a series of tremors were recorded that shook the earth for about seven minutes, with three of them leading to significant destruction.

Calculating which shock will be the main one and will have the greatest magnitude is extremely difficult, problematic, and there are no absolute methods. Therefore, strong earthquakes often take the population by surprise. This, for example, happened in 2015 in Nepal, in a country where mild tremors were recorded so often that people simply did not pay much attention to them. Therefore, a ground shaking with a magnitude of 7.9 led to a large number of victims, and the weaker aftershocks with a magnitude of 6.6 that followed it half an hour later and the next day did not improve the situation.

It often happens that the strongest tremors occurring on one side of the planet shake the opposite side. For example, the 2004 magnitude 9.3 earthquake in the Indian Ocean relieved some of the increasing stress on the San Andreas Fault, which is located at the junction of lithospheric plates along the California coast. It turned out to be so strong that it slightly modified the appearance of our planet, smoothing out its bulge in the middle part and making it more rounded.

What is magnitude

One way to measure the amplitude of oscillations and the amount of energy released is the magnitude scale (Richter scale), containing arbitrary units from 1 to 9.5 (it is very often confused with a twelve-point intensity scale, measured in points). An increase in the magnitude of earthquakes by just one unit means an increase in the amplitude of vibrations by ten, and energy by thirty-two times.

The calculations showed that the size of the epicenter during weak vibrations of the surface, both in length and vertically, is measured in several meters, when of average strength - in kilometers. But earthquakes that cause disasters have a length of up to 1 thousand kilometers and extend from the rupture point to a depth of up to fifty kilometers. Thus, the maximum recorded size of the epicenter of earthquakes on our planet was 1000 by 100 km.

The magnitude of earthquakes (Richter scale) looks like this:

• 2 – weak, almost imperceptible vibrations;
• 4 - 5 - even though the shocks are weak, they can lead to minor damage;
• 6 – medium damage;
• 8.5 - one of the strongest recorded earthquakes.
• The largest is considered to be the Great Chilean earthquake with a magnitude of 9.5, which generated a tsunami that, having crossed the Pacific Ocean, reached Japan, covering 17 thousand kilometers.

Focusing on the magnitude of earthquakes, scientists claim that out of the tens of thousands of vibrations that occur on our planet per year, only one has a magnitude of 8, ten - from 7 to 7.9, and a hundred - from 6 to 6.9. It must be taken into account that if the earthquake magnitude is 7, the consequences can be catastrophic.

Intensity scale

To understand why earthquakes occur, scientists have developed an intensity scale based on external manifestations such as the impact on people, animals, buildings, and nature. The closer the epicenter of earthquakes is to the earth's surface, the greater the intensity (this knowledge makes it possible to give at least an approximate forecast of earthquakes).

For example, if the magnitude of the earthquake was eight and the epicenter was at a depth of ten kilometers, the intensity of the earthquake would be between eleven and twelve. But if the epicenter was located at a depth of fifty kilometers, the intensity will be less and will be measured at 9-10 points.

According to the intensity scale, the first destruction can occur already with magnitude six shocks, when thin cracks appear in the plaster. An earthquake of eleven magnitude is considered catastrophic (the surface of the earth's crust becomes covered with cracks, buildings are destroyed). The strongest earthquakes, capable of significantly changing the appearance of the area, are estimated at twelve points.

What to do during earthquakes

According to rough estimates by scientists, the number of people who have died in the world due to earthquakes over the past half-millennium exceeds five million people. Half of them are in China: it is located in a zone of seismic activity, and a large number of people live on its territory (830 thousand people died in the 16th century, 240 thousand in the middle of the last century).

Such catastrophic consequences could have been prevented if earthquake protection had been well thought out at the state level, and the design of buildings had taken into account the possibility of strong tremors: most people died under the rubble. Often, people living or staying in a seismically active zone do not have the slightest idea of ​​how exactly to act in an emergency situation and how to save their lives.

You need to know that if tremors catch you in a building, you need to do everything possible to get out into the open space as quickly as possible, and you absolutely cannot use elevators.

If it is impossible to leave the building, and the earthquake has already begun, leaving it is extremely dangerous, so you need to stand either in the doorway, or in a corner near a load-bearing wall, or crawl under a strong table, protecting your head with a soft pillow from objects that may fall from above. After the tremors are over, the building must be left.

If a person finds himself on the street during the onset of earthquakes, he must move away from the house by at least one third of its height and, avoiding tall buildings, fences and other buildings, move towards wide streets or parks. It is also necessary to stay as far as possible from downed electrical wires of industrial enterprises, since explosive materials or toxic substances may be stored there.

But if the first tremors caught a person while he was in a car or public transport, he urgently needs to leave the vehicle. If the car is in an open area, on the contrary, stop the car and wait out the earthquake.

If it so happens that you are completely covered in debris, the main thing is not to panic: a person can survive without food and water for several days and wait until they find him. After catastrophic earthquakes, rescuers work with specially trained dogs, and they are able to smell life among the rubble and give a sign.