Introduction
Earthquakes are tremors and vibrations of the Earth's surface caused by natural causes (mainly tectonic processes), or (sometimes) artificial processes (explosions, filling of reservoirs, collapse of underground cavities in mine workings). Small tremors can also be caused by the rise of lava during volcanic eruptions. In other words, vibrations of the Earth caused by sudden changes in the condition of the planet's interior. These vibrations are elastic waves propagating at high speed through the rock mass. The most powerful earthquakes are sometimes felt at distances of more than 1,500 km from the source and can be recorded by seismographs (special highly sensitive instruments). The area where vibrations originate is called the earthquake source, and its projection onto the Earth's surface is called the earthquake epicenter. The sources of most earthquakes lie in the earth's crust at depths of no more than 16 km, but in some areas the depths of the sources reach 700 km.
About a million earthquakes occur throughout the Earth each year, but most are so small that they go unnoticed. Really strong earthquakes, capable of causing widespread destruction, occur on the planet about once every two weeks. Most of them fall on the bottom of the oceans, and therefore are not accompanied by catastrophic consequences (if an earthquake under the ocean does not occur without a tsunami).
Types of earthquakes
Tectonic earthquakes occur as a result of a sudden release of stress, for example, during movement along a fault in the earth's crust (research in recent years shows that deep earthquakes can also be caused by phase transitions in the Earth's mantle that occur at certain temperatures and pressures). Sometimes deep faults come to the surface. During the catastrophic earthquake in San Francisco on April 18, 1906, the total length of surface ruptures in the San Andreas fault zone was more than 430 km, the maximum horizontal displacement was 6 m. The maximum recorded value of seismogenic displacements along the fault was 15 m.
Volcanic earthquakes occur as a result of sudden movements of magmatic melt in the bowels of the Earth or as a result of the occurrence of ruptures under the influence of these movements.
Man-made earthquakes can be caused by underground nuclear tests, filling reservoirs, oil and gas production by injecting liquid into wells, blasting during mining, etc. Less powerful earthquakes occur when cave vaults or mine workings collapse.
Causes of earthquakes
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.
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.
Energy released during earthquakes
E = p2rV (a / T),
where V is the speed of propagation of seismic waves,
r - density of the upper layers of the Earth,
a is the displacement amplitude,
T - period of oscillation. 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. For example, during the Ashgabat earthquake in 1948, 1023 ergs of energy were released, during the Khait earthquake in Tajikistan in 1949 - 5 "1024 ergs, in 1960 in Chile - 1025 ergs. Around the globe, on average, about 0.5 ergs are released per year due to earthquakes "1026 ergs of energy.
An important concept in seismology is specific seismic power, that is, the amount of energy released per unit volume, for example 1 m 3, 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.
The content of the article
EARTHQUAKES, vibrations of the Earth caused by sudden changes in the condition of the planet's interior. These vibrations are elastic waves propagating at high speed through the rock mass. The most powerful earthquakes are sometimes felt at distances of more than 1,500 km from the source and can be recorded by seismographs (special highly sensitive instruments) even in the opposite hemisphere. The area where vibrations originate is called the earthquake source, and its projection onto the Earth's surface is called the earthquake epicenter. The sources of most earthquakes lie in the earth's crust at depths of no more than 16 km, but in some areas the depths of the sources reach 700 km. Thousands of earthquakes occur every day, but only a few of them are felt by humans.
Mentions of earthquakes are found in the Bible, in the treatises of ancient scientists - Herodotus, Pliny and Livy, as well as in ancient Chinese and Japanese written sources. Until the 19th century Most reports of earthquakes contained descriptions heavily flavored with superstition and theories based on scanty and unreliable observations. A. Perry (France) began a series of systematic descriptions (catalogues) of earthquakes in 1840. In the 1850s, R. Malle (Ireland) compiled a large catalog of earthquakes, and his detailed report on the Naples earthquake in 1857 became one of the first strictly scientific descriptions of large earthquakes.
Causes of earthquakes.
Although numerous studies have been carried out since ancient times, it cannot be said that the causes of earthquakes have been fully studied. Based on the nature of the processes at their sources, several types of earthquakes are distinguished, the main ones being tectonic, volcanic and man-made.
Tectonic earthquakes
arise as a result of a sudden release of stress, for example, during movement along a fault in the earth’s crust (research in recent years shows that deep earthquakes can also be caused by phase transitions in the Earth’s mantle, occurring at certain temperatures and pressures). Sometimes deep faults come to the surface. During the catastrophic earthquake in San Francisco on April 18, 1906, the total length of surface ruptures in the San Andreas fault zone was more than 430 km, the maximum horizontal displacement was 6 m. The maximum recorded value of seismogenic displacements along the fault was 15 m.
Volcanic earthquakes
occur as a result of sudden movements of magmatic melt in the bowels of the Earth or as a result of the occurrence of ruptures under the influence of these movements.
Man-made earthquakes
can be caused by underground nuclear tests, filling reservoirs, oil and gas production by injection of liquid into wells, blasting during mining, etc. Less strong earthquakes occur when cave vaults or mine workings collapse.
Seismic waves.
Oscillations propagating from the source of an earthquake are elastic waves, the nature and speed of propagation of which depend on the elastic properties and density of rocks. Elastic properties include the bulk modulus, which characterizes the resistance to compression without changing shape, and the shear modulus, which determines the resistance to shear forces. The speed of propagation of elastic waves increases in direct proportion to the square root of the values of the parameters of elasticity and density of the medium.
Longitudinal and transverse waves.
These waves appear first on seismograms. The first to be recorded are longitudinal waves, during the passage of which each particle of the medium is first compressed and then expanded again, experiencing reciprocating motion in the longitudinal direction (i.e. in the direction of wave propagation). These waves are also called R- waves, or primary waves. Their speed depends on the elastic modulus and rigidity of the rock. Near the earth's surface speed R-wave is 6 km/s, and at very great depths - approx. 13 km/s. The next to be recorded are transverse seismic waves, also called S-waves, or secondary waves. As they pass, each rock particle oscillates perpendicular to the direction of wave propagation. Their speed depends on the shear resistance of the rock and is approximately 7/12 of the speed of propagation R- waves
Surface waves
spread along the earth's surface or parallel to it and do not penetrate deeper than 80-160 km. This group includes Rayleigh waves and Love waves (named after the scientists who developed the mathematical theory of the propagation of such waves). When Rayleigh waves pass through, rock particles describe vertical ellipses lying in the focal plane. In Love waves, rock particles oscillate perpendicular to the direction of wave propagation. Surface waves are often abbreviated as L-waves. Their propagation speed is 3.2-4.4 km/s. During deep-focus earthquakes, surface waves are very weak.
Amplitude and period
characterize the oscillatory movements of seismic waves. Amplitude is the amount by which the position of a soil particle changes during the passage of a wave compared to the previous state of rest. The period of oscillation is the period of time during which one complete oscillation of a particle occurs. Near the source of the earthquake, vibrations with different periods are observed - from fractions of a second to several seconds. However, at large distances from the center (hundreds of kilometers), short-period oscillations are less pronounced: for R-waves are characterized by periods from 1 to 10 s, and for S-waves – a little more. The periods of surface waves range from a few seconds to several hundred seconds. The amplitudes of oscillations can be significant near the source, but at distances of 1500 km or more they are very small - less than a few microns for waves R And S and less than 1 cm – for surface waves.
Reflection and refraction.
Encountering layers of rocks with different properties along their path, seismic waves are reflected or refracted, just as a ray of light is reflected from a mirror surface or refracted when passing from air to water. Any changes in the elastic characteristics or density of the material along the path of propagation of seismic waves cause them to be refracted, and with sudden changes in the properties of the medium, part of the wave energy is reflected ( cm. rice.).
Paths of seismic waves.
Longitudinal and transverse waves propagate throughout the Earth, while the volume of the medium involved in the oscillatory process continuously increases. The surface corresponding to the maximum movement of waves of a certain type at a given moment is called the front of these waves. Since the elastic modulus of a medium increases with depth faster than its density (up to a depth of 2900 km), the speed of wave propagation at depth is higher than near the surface, and the wave front appears to be more advanced inland than in the lateral (lateral) direction. The path of a wave is a line connecting a point at the wave front to the source of the wave. Directions of wave propagation R And S are curves convex downwards (due to the fact that the speed of waves is greater at depth). Wave trajectories R And S coincide, although the former spread faster.
Seismic stations located far from the epicenter of an earthquake record not only direct waves R And S, but also waves of these types, already reflected once from the Earth’s surface - RR And SS(or PR 1 And S.R. 1), and sometimes - reflected twice - RRR And SSS(or PR 2 and S.R. 2). There are also reflected waves that travel one section of the path as R-wave, and the second, after reflection, - like S-wave. The resulting converted waves are designated as PS or SP. In seismograms of deep-focus earthquakes, other types of reflected waves are also observed, for example, waves that were reflected from the Earth's surface before reaching the recording station. They are usually denoted by a small letter followed by a capital letter (for example, pR). These waves are very convenient to use to determine the depth of the earthquake source.
At a depth of 2900 km the speed P-waves decrease sharply from >13 km/s to ~ 8 km/s; A S-waves do not propagate below this level, corresponding to the boundary of the earth's core and mantle . Both types of waves are partially reflected from this surface, and some of their energy returns to the surface in the form of waves, denoted as R with R And S with S. R-waves pass through the core, but their trajectory is sharply deviated and a shadow zone appears on the Earth’s surface, within which only very weak waves are recorded R-waves. This zone starts at a distance of approx. 11 thousand km from the seismic source, and already at a distance of 16 thousand km R-waves appear again, and their amplitude increases significantly due to the focusing influence of the core, where wave velocities are low. R-waves passing through the earth's core are designated RKR or Rў . The seismograms also clearly distinguish waves that travel like waves along the path from the source to the core S, then pass through the core as waves R, and upon output the waves are again converted to the type S. In the very center of the Earth, at a depth of more than 5,100 km, there is an inner core that is presumably in a solid state, but its nature is not yet entirely clear. Waves penetrating through this inner core are denoted as RKIKR or SKIKS(cm. rice. 1).
Registration of earthquakes.
The device that records seismic vibrations is called a seismograph, and the recording itself is called a seismogram. A seismograph consists of a pendulum suspended inside a housing by a spring and a recording device.
One of the first recording devices was a rotating drum with paper tape. As the drum rotates, it gradually moves to one side, so that the zero line of the recording on the paper looks like a spiral. Every minute, vertical lines are drawn on the graph - time stamps; For this purpose, very precise watches are used, which are periodically checked against the exact time standard. To study nearby earthquakes, marking accuracy is required - down to a second or less.
In many seismographs, induction devices are used to convert a mechanical signal into an electrical one, in which, when the inert mass of the pendulum moves relative to the body, the magnitude of the magnetic flux passing through the turns of the induction coil changes. The resulting weak electric current drives a galvanometer connected to a mirror, which casts a beam of light onto the photosensitive paper of the recording device. In modern seismographs, vibrations are recorded digitally using computers.
Earthquake Magnitude
usually determined on a scale based on seismograph recordings. This scale is known as the magnitude scale, or Richter scale (named after the American seismologist C. F. Richter, who proposed it in 1935). The magnitude of an earthquake is a dimensionless quantity proportional to the logarithm of the ratio of the maximum amplitudes of a certain type of waves of a given earthquake and some standard earthquake. There are differences in methods for determining the magnitudes of nearby, distant, shallow (shallow) and deep earthquakes. Magnitudes determined from different types of waves differ in magnitude. Earthquakes of different magnitudes (on the Richter scale) manifest themselves as follows:
2 - the weakest felt shocks;
4 1/2 - the weakest shocks, leading to minor damage;
6 - moderate destruction;
8 1/2 - the strongest known earthquakes.
Earthquake intensity
are assessed in points during a survey of the area based on the magnitude of the destruction of ground structures or deformations of the earth's surface caused by them. To retrospectively assess the intensity of historical or more ancient earthquakes, some empirically obtained relationships are used. In the United States, intensity ratings are usually made using a modified 12-point Mercalli scale.
1 point. It is felt by a few particularly sensitive people in especially favorable circumstances.
3 points. People feel it like vibration from a passing truck.
4 points. Dishes and window glass rattle, doors and walls creak.
5 points. Felt by almost everyone; many sleepers wake up. Loose objects fall.
6 points. It is felt by everyone. Minor damage.
8 points. Chimneys and monuments fall, walls collapse. The water level in wells changes. Capital buildings are severely damaged.
10 points. Brick buildings and frame structures are destroyed. Rails become deformed and landslides occur.
12 points. Complete destruction. Waves are visible on the earth's surface.
In Russia and some neighboring countries, it is customary to evaluate the intensity of fluctuations in MSK points (12-point Medvedev-Sponheuer-Karnik scale), in Japan - in JMA points (9-point scale of the Japan Meteorological Agency).
Intensity in points (expressed in whole numbers without fractions) is determined by examining the area in which the earthquake occurred, or by interviewing residents about their feelings in the absence of destruction, or by calculations using empirically obtained and accepted formulas for a given area. Among the first information about an earthquake that has occurred, it is its magnitude that becomes known, not its intensity. Magnitude is determined from seismograms even at large distances from the epicenter.
Consequences of earthquakes.
Strong earthquakes leave many traces, especially in the area of the epicenter: the most common are landslides and slides of loose soil and cracks on the earth's surface. The nature of such disturbances is largely determined by the geological structure of the area. In loose and water-saturated soil on steep slopes, landslides and collapses often occur, and the thick layer of water-saturated alluvium in valleys is more easily deformed than hard rocks. On the surface of alluvium, subsidence basins are formed and filled with water. And even not very strong earthquakes are reflected in the terrain.
Displacements along faults or the occurrence of surface ruptures can change the plan and altitude position of individual points of the earth's surface along a fault line, as happened during the 1906 San Francisco earthquake. During the October 1915 earthquake in the Pleasant Valley in Nevada, a ledge 35 km long and up to 4.5 m high formed on the fault. During the May 1940 earthquake in the Imperial Valley in California, movements occurred along a 55-kilometer section of the fault, and horizontal displacements of up to 4 were observed .5 m. As a result of the Assam earthquake (India) in June 1897 in the epicentral region, the height of the area changed by no less than 3 m.
Significant surface deformations can be traced not only near faults and lead to a change in the direction of river flow, damming or rupture of watercourses, disruption of the regime of water sources, and some of them temporarily or permanently cease to function, but at the same time new ones may appear. Wells and boreholes are filled with mud, and the water level in them changes noticeably. During strong earthquakes, water, liquid mud or sand can be ejected from the ground in fountains.
When moving along faults, damage occurs to roads and railways, buildings, bridges and other engineering structures. However, well-built buildings rarely collapse completely. Typically, the degree of destruction is directly dependent on the type of structure and the geological structure of the area. During earthquakes of moderate strength, partial damage to buildings can occur, and if they are poorly designed or poorly constructed, then their complete destruction is possible.
During very strong shocks, structures built without taking into account seismic hazards can collapse and suffer severe damage. Typically, one- and two-story buildings do not collapse unless they have very heavy roofs. However, it happens that they move from the foundations and often their plaster cracks and falls off.
Differential movements can cause bridges to move from their supports and cause utilities and water pipes to break. During intense vibrations, pipes laid in the ground can “fold”, sticking into one another, or bend, coming to the surface, and railway rails become deformed. In earthquake-prone areas, structures must be designed and built in compliance with the building codes adopted for the given area in accordance with the seismic zoning map.
In densely populated areas, almost more damage than the earthquakes themselves are caused by fires that occur as a result of ruptures of gas pipelines and power lines, overturning of stoves, stoves and various heating devices. Fighting fires is complicated by the fact that the water supply is damaged and the streets are impassable due to the resulting rubble.
Related phenomena.
Sometimes tremors are accompanied by a clearly audible low hum when the frequency of seismic vibrations lies in the range perceived by the human ear; sometimes such sounds are heard in the absence of tremors. They are quite common in some areas, although significant earthquakes are very rare. There are also numerous reports of the appearance of a glow during strong earthquakes. There is no generally accepted explanation for such phenomena yet. Tsunamis (large sea waves) occur when rapid vertical deformations of the seabed occur during underwater earthquakes. Tsunamis propagate in the deep oceans at speeds of 400–800 km/h and can cause destruction on coastlines thousands of kilometers from the epicenter. On the shores close to the epicenter, these waves sometimes reach a height of 30 m.
During many strong earthquakes, in addition to the main shocks, foreshocks (preceding earthquakes) and numerous aftershocks (earthquakes following the main shock) are recorded. Aftershocks are usually weaker than the mainshock and can recur over weeks or even years, becoming less and less frequent.
Geographical distribution of earthquakes.
Most earthquakes are concentrated in two long, narrow zones. One of them frames the Pacific Ocean, and the second stretches from the Azores east to Southeast Asia.
The Pacific seismic zone runs along the west coast of South America. In Central America it splits into two branches, one following the island arc of the West Indies, and the other continuing north, expanding within the United States, to the western ranges of the Rocky Mountains. Further, this zone passes through the Aleutian Islands to Kamchatka and then through the Japanese Islands, the Philippines, New Guinea and the islands of the southwestern Pacific Ocean to New Zealand and Antarctica.
The second zone from the Azores extends east through the Alps and Turkey. In southern Asia, it expands and then narrows and changes direction to the meridional, passes through the territory of Myanmar, the islands of Sumatra and Java and connects with the circum-Pacific zone in the region of New Guinea.
There is also a smaller zone in the central part of the Atlantic Ocean, following along the Mid-Atlantic Ridge.
There are a number of areas where earthquakes occur quite frequently. These include East Africa, the Indian Ocean and in North America the valley of the St. River. Lawrence and northeastern USA.
Compared to shallow-focus earthquakes, deep-focus earthquakes have a more limited distribution. They have not been recorded within the Pacific zone from southern Mexico to the Aleutian Islands, and in the Mediterranean zone - west of the Carpathians. Deep-focus earthquakes are characteristic of the western edge of the Pacific Ocean, Southeast Asia and the western coast of South America. The zone with deep-focus sources is usually located along the zone of shallow-focus earthquakes on the continental side.
Earthquake forecast.
To improve the accuracy of earthquake forecasts, it is necessary to better understand the mechanisms of stress accumulation in the earth's crust, creep and deformations on faults, to identify the relationships between heat flow from the Earth's interior and the spatial distribution of earthquakes, and also to establish patterns of earthquake recurrence depending on their magnitude.
In many areas of the globe where there is a possibility of strong earthquakes, geodynamic observations are carried out in order to detect earthquake precursors, among which changes in seismic activity, deformation of the earth's crust, anomalies in geomagnetic fields and heat flow, sharp changes in the properties of rocks (electrical, seismic, etc.), geochemical anomalies, water regime disturbances, atmospheric phenomena, as well as abnormal behavior of insects and other animals (biological precursors). This kind of research is carried out at special geodynamic testing sites (for example, Parkfield in California, Garm in Tajikistan, etc.). Since 1960, many seismic stations have been operating, equipped with highly sensitive recording equipment and powerful computers that allow them to quickly process data and determine the location of earthquake sources.
Ancient Greek philosophers more than two thousand years ago expressed correct, but, of course, far from complete judgments about what are the causes of earthquakes. They explained them by failures in the roofs of caves, the formation of which was also correctly combined with. It has been suggested that the earthquake is
an underground thunderstorm that finds no way out.Many centuries have passed since then, and only careful observation of nature and a thorough study of the phenomena occurring in it made it possible in the second half of the 19th century to correctly explain the causes of earthquakes.
Types of earthquakes
Currently, there are three types of earthquakes:- landslides,
- volcanic,
- tectonic.
Volcanic earthquakes. Earthquakes tectonic nature are closely related to mountain-building processes (translated from ancient Greek “tectonics” - the art of construction), associated with the structure of the earth’s crust and the movement of tectonic plates. They are characterized by frequent repetition, large scope and duration. Earthquakes of this type account for the greatest number of human casualties and material losses. Relationship between earthquakes and other natural phenomena
Not content with studying the earthquake process itself, researchers seek to establish connection between earthquakes and other natural phenomena.- The fact that the largest number of earthquakes occurs in autumn and winter gives some scientists reason to see this as more than a coincidence. Indeed, making its annual journey, it moves not in a circle, but in an ellipse. The sun is not placed at the center of this curve. In winter the Earth is closer to the Sun, in summer it is further away. This naturally suggests the conclusion about the possibility of the Sun influencing earthquakes.
- Considerations are also expressed about the influence of the Moon, (more details:), which is the closest of all the luminaries to the Earth and whose influence explains the correct alternations along the ocean coast every 6 hours 12.5 minutes.
- Of even greater interest is the attempt to explain the cause of earthquakes by the influence of air. Long-term observations of the state of the atmosphere in Italy have shown a close connection between air pressure and fluctuations in the earth’s crust: a decrease in pressure increases them, an increase, on the contrary, reduces them.
- A sharp drop in pressure, indeed, in some cases preceded earthquakes. Therefore, it was suggested that a decrease in pressure could serve as an impetus for the displacement of layers of the earth's crust that are in unstable equilibrium, and thereby cause an earthquake.
- A connection is also established between fluctuations of the magnetic needle and earthquakes. In some cases, the needle deviation was observed even two days before the earthquake.
- The behavior of some domestic animals is also very indicative: even the day before they experience a certain amount of anxiety - they run away from the yard, do not take food; donkeys bray, cows moo, dogs howl and huddle close to people; pigeons and sparrows fly away from their homes, birds leave gardens and forests.
Seismically resistant building
If science still cannot say the decisive word for preventing earthquakes (more details:), then the art of engineering already has experience in constructing aseismic, i.e., indestructible buildings. Seismically resistant buildings must meet special requirements. So, when studying catastrophic earthquakes, for example in San Francisco (1906), (more details:), the best preserved are giant twenty-story skyscrapers built of reinforced concrete, as well as monumental buildings on a solid foundation. The Ashgabat earthquake (1948) gave similar conclusions: amid the general destruction in the city, buildings connected by a metal frame, such as, for example, the huge buildings and the tower of a textile factory, were well preserved. The main basis for preserving a building is the strong connection of all its parts, which is achieved by an iron frame (frame), (more details:) and a reliable foundation, resting not on a thin layer of surface sediment, but on bedrock. A building of this type vibrates during an earthquake as one whole, the connection of the individual parts is not broken, and they perfectly withstand the shocks from which everything around is destroyed.
Preserved buildings. During the earthquake in San Francisco, tremors were felt only in the lower floors of skyscrapers, and in the upper ones they were so weakened that people playing billiards on the 17th floor (at an altitude of 90 meters from the surface of the earth) calmly played balls. Good results are also obtained by the circular type of buildings, which represent a combination of individual oval-shaped rooms in the building. The absence of corners makes each such round room like a tower or minaret, which usually withstands earthquake shocks well. In Ashgabat, the concrete elevator towers were almost undamaged, while the first floor of the building adjacent to them was completely crushed, as a result of which the entire structure sank and leaned. The experience of wooden construction of well-built houses on a solid foundation also deserves great attention. Indeed, taking into account the damage of the Vernensky earthquake showed the advantage of wooden buildings: while in the city there was not a single stone house without damage, all wooden houses survived. Despite its relatively young age, seismology already provides many valuable practical instructions that help not only in questions about the causes of earthquakes, but also in the structure of the Earth and in subsoil exploration. Instead of expensive exploration methods, explosions of dynamite cartridges are now carried out at a certain depth, in the thickness of the rocks under study, then, based on seismograph records, after mathematical processing, a conclusion is made about the presence of the desired deposit, deeply hidden underground. 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.
Acid rain is a serious environmental problem caused by environmental pollution. Their frequent appearance frightens not only scientists, but also ordinary people, because such precipitation can have a negative impact on human health. Acid rain is characterized by a low pH level. For normal precipitation, this figure is 5.6, and even a slight violation of the norm is fraught with serious consequences for living organisms caught in the affected area.
With a significant shift, the reduced level of acidity causes the death of fish, amphibians, and insects. Also, in the area where such precipitation is observed, you can notice acid burns on the leaves of trees and the death of some plants.
Negative consequences of acid rain also exist for humans. After a rainstorm, toxic gases accumulate in the atmosphere, and inhaling them is highly discouraged. A short walk in acid rain can cause asthma, heart and lung diseases.
Acid rain: causes and consequences
The problem of acid rain has long been global in nature, and every inhabitant of the planet should think about their contribution to this natural phenomenon. All harmful substances that enter the air during human activity do not disappear anywhere, but remain in the atmosphere and sooner or later return to the earth in the form of precipitation. Moreover, the consequences of acid rain are so serious that it sometimes takes hundreds of years to eliminate them.
In order to find out what the consequences of acid rain can be, you need to understand the very concept of the natural phenomenon in question. So scientists agree that this definition is too narrow to describe the global problem. Only rain cannot be taken into account - acid hail, fog and snow are also carriers of harmful substances, since the processes of their formation are largely identical. In addition, toxic gases or dust clouds may appear during dry weather. They are also a type of acid precipitation.
Causes of acid rain formation
The cause of acid rain lies largely in the human factor. Constant air pollution with acid-forming compounds (sulfur oxides, hydrogen chloride, nitrogen) leads to imbalance. The main “suppliers” of these substances into the atmosphere are large enterprises, in particular those working in the field of metallurgy, processing of oil-containing products, burning coal or fuel oil. Despite the availability of filters and cleaning systems, the level of modern technology still does not allow us to completely eliminate the negative impact of industrial waste.
Acid rain is also associated with an increase in vehicles on the planet. Exhaust gases, although in small proportions, also contain harmful acidic compounds, and in terms of the number of cars, the level of pollution becomes critical. Thermal power plants also contribute, as well as many household items, such as aerosols, cleaning products, etc.
In addition to human influence, acid rain can also occur due to some natural processes. Thus, their appearance is caused by volcanic activity, during which large amounts of sulfur are released. In addition, it produces gaseous compounds during the breakdown of certain organic substances, which also leads to air pollution.
How is acid rain formed?
All harmful substances released into the air react with solar energy, carbon dioxide or water, resulting in acidic compounds. Together with drops of moisture, they rise into the atmosphere and form clouds. As a result, acid rain occurs, snowflakes or hailstones form, which return all absorbed elements to the earth.
In some regions, deviations from the norm of 2-3 units were noticed: the permissible acidity level is 5.6 pH, but in China and the Moscow region there was precipitation with values of 2.15 pH. At the same time, it is quite difficult to predict where exactly acid rain will appear, because the wind can carry the formed clouds quite far from the place of pollution.
Composition of acid rain
The main elements in acid rain are sulfuric and sulfurous acids, as well as ozone, which is formed during thunderstorms. There is also a nitrogen variety of sediments, in which the main core is nitric and nitrous acids. Less commonly, acid rain can be caused by high levels of chlorine and methane in the atmosphere. Also, other harmful substances can get into precipitation, depending on the composition of industrial and household waste that enters the air in a particular region.
Consequences: acid rain
Acid rain and its effects are a constant subject of observation for scientists around the world. Unfortunately, their forecasts are very disappointing. Precipitation with a low acidity level is dangerous for flora, fauna, and humans. In addition, they can lead to more serious environmental problems.
Once in the soil, acid rain destroys many nutrients that are necessary for plant growth. At the same time, they also draw toxic metals to the surface. Among them are lead, aluminum, etc. With a sufficiently concentrated acid content, precipitation leads to the death of trees, the soil becomes unsuitable for growing crops, and it takes years to restore it!
An earthquake is one of the most terrible natural phenomena. Earthquakes are recorded every day around the world. But most of them are so insignificant that they can only be detected with the help of sensors and instruments. However, a couple of times a month, scientists manage to record a strong vibration of the earth’s crust, which is capable of serious destruction.
Description of the earthquake
Earthquakes are vibrations of the earth's crust and tremors that are caused by natural or artificially created causes. What can cause an earthquake? Any earthquake is an instant release of energy that occurs due to the rupture of rocks. The volume of the rupture is called the focus of the earthquake. It plays an important role, since the amount of energy released and the force of the push depend on its size.
The source of an earthquake is a rupture, after which there is a displacement of the earth's surface. This break does not happen immediately. First, the plates collide with each other. As a result, friction occurs and energy is generated. It gradually grows and accumulates.
At some point, the stress becomes maximum and exceeds the friction force. This is when the rock breaks. The energy released in this way generates seismic waves. They have a speed of about 8 km/s and cause vibrations in the earth.
It should be noted that the deformation of rocks occurs spasmodically, that is, an earthquake consists of several stages. The strongest shock is preceded by oscillations (foreshocks), followed by aftershocks. Such fluctuations can occur for several years before the main shock occurs.
It is very difficult to calculate which shock will be the strongest. This is why many earthquakes come as a complete surprise and lead to serious disasters. In addition, there are cases when strong tremors of the earth at one end of the planet lead to earthquakes on the opposite side.
Causes of earthquakes
There are several reasons why earthquakes occur.
Among them:
- volcanic;
- tectonic;
- landslide;
- artificial;
- technogenic.
There is also such a thing as a seaquake.
Tectonic
This is the most common cause of earthquakes. It is as a result of the displacement of tectonic plates that the largest number of disasters occur. Usually this shift is small and amounts to only a few centimeters. However, it sets in motion the mountains that are located above, it is they who release enormous energy. As a result of this, cracks appear on the surface of the earth, along the edges of which all objects located on it are displaced.
Volcanic
Earthquakes can be caused by volcanic activity. Volcanic fluctuations rarely lead to serious consequences; they are usually recorded over a fairly long period of time. The contents of a volcano exert pressure on the earth's surface, which is called volcanic tremor. As the volcano prepares to erupt, periodic explosions of steam and gas can be observed. They are the ones that generate seismic waves.
Earthquakes can be caused by either an active or an extinct volcano. In the latter case, hesitations indicate that he may still wake up. It is studies of seismological activity that help predict eruptions. Scientists often find it difficult to determine the cause of tremors. In this case, an earthquake caused by a volcano is characterized by a close location of the epicenter to the volcano and a small magnitude.
Landslide
Rock falls can also cause earthquakes. They can occur either naturally or as a result of human activity. In this case, tectonic earthquakes can also cause a collapse. But even the collapse of a significant mass of rock causes minor seismic activity.
Earthquakes caused by rock falls have low intensity. More often than not, even a large volume of rock is not enough to cause strong vibrations. Most often, a disaster occurs precisely because of a landslide, and not because of the earthquake itself.
Artificial
Artificial earthquakes and their causes are caused by humans. For example, after the DPRK tested nuclear weapons, moderate tremors were recorded in many places on the planet.
Technogenic
Man-made earthquakes and their causes are also caused by human activity. For example, scientists have recorded an increase in tremors in areas of large reservoirs. The reason for such fluctuations is the pressure of a large volume of water on the earth's crust. In addition, water begins to seep through the soil and destroy it. Also, an increase in seismic activity is recorded in gas and oil production areas.
Seaquake
A seaquake is one of the types of tectonic earthquakes. It occurs as a result of shifting tectonic plates on the ocean floor or near the coast. A dangerous consequence of such a natural phenomenon is a tsunami. This is what causes many disasters.
A tsunami occurs due to the shaking of the sea crust, during which one part of the bottom sinks and the other rises above it. As a result of this, water moves and tries to return to its original position. It begins to move vertically and generates a series of huge waves that go towards the shore.
Earthquake: main characteristics
In order to understand the causes of earthquakes, scientists have developed parameters that determine the strength of the phenomenon.
Among them:
- earthquake intensity;
- epicenter depth;
- energy class;
- magnitude.
Intensity scale
It is based on the external manifestations of the disaster. The impact on people, nature and buildings is taken into account. The closer the earthquake's epicenter is to the ground, the greater its intensity will be. For example, if the epicenter was located at a depth of 10 km and the magnitude was 8, then the intensity of the earthquake would be 11–12 points. With the same magnitude and location of the epicenter at a depth of 50 km, the intensity of the earthquake will be 9–10 points.
The first obvious destruction occurs already during a magnitude 6 earthquake. With such intensity, cracks appear on the walls. But with an earthquake of 11 points, buildings are already destroyed. Earthquakes measuring 12 points are considered the most powerful and catastrophic. They can seriously change not only the appearance of the terrain, but even the direction of water flow in rivers.
Magnitude
Another way to measure the strength of an earthquake is the magnitude scale or Richter scale. This scale measures the amplitude of vibrations and the amount of energy released. If the size of the epicenter in length and width is several meters, then the vibrations are weak and are recorded only by instruments. During catastrophic earthquakes, the length of the epicenter can be up to 1 thousand km. Magnitude is measured in arbitrary units from 1 to 9.5.
Journalists often confuse magnitude and intensity in their reporting. It must be remembered that the description of earthquakes should occur precisely on the intensity scale, which in seismology is synonymous with intensity.
Epicenter depth
There is also a characteristic of an earthquake based on the depth of the epicenter. The deeper the epicenter, the further seismic waves can travel.
- normal - epicenter up to 70 km (this type accounts for approximately 51% of earthquakes);
- intermediate – epicenter up to 300 km (about 36%);
- deep-focus - the epicenter is located deeper than 300 km (about 13% of earthquakes).
Deep-focus earthquakes are typical of the Pacific Ocean. The most significant deep-focus seaquake occurred in Indonesia in 1996 at a depth of 600 km.
Earthquake: causes and consequences
Regardless of the cause, the consequences of earthquakes can be catastrophic. Over the past half a thousand years, they have claimed about 5 million lives. Most of the victims occur in earthquake-prone areas, the main one being China. Such catastrophic consequences can be avoided if earthquake protection is thought through at the state level.
In particular, the possibility of shocks must be taken into account when designing buildings. In addition, it is necessary to teach people living in a seismically active zone what to do in case of an earthquake.
If you feel strong tremors, you need to act as follows.
- If an earthquake finds you in a building, you need to get out of it as quickly as possible. However, you cannot use the elevator.
- On the street, you need to move as far away from tall buildings as possible. Move towards wide streets or parks.
- It is necessary to stay away from electrical wires and move away from industrial enterprises.
- If it is not possible to go outside, then you need to crawl under a strong table or bed. In this case, your head must be covered with a pillow.
- Don't stand in the doorway. If there are strong shocks, it may collapse and part of the wall above the door may fall on you.
- It is safest to stay near the outside walls of the building.
- As soon as the tremors are over, you need to get outside as quickly as possible.
- If an earthquake finds you in a car within the city, you need to get out of it and sit next to it. If you find yourself in a car on the highway, you need to stop and wait out the shocks inside.
If you are covered in debris, do not panic. The human body can survive without food and water for several days. Immediately after earthquakes, rescuers with specially trained dogs work at the disaster site. They easily find living people under the rubble and signal to rescuers.
