Therefore, in everyday life the magnitude value is called Richter scale.

Earthquake magnitude and earthquake intensity rating scale

The Richter scale contains conventional units (from 1 to 9.5) - magnitudes, which are calculated from vibrations recorded by a seismograph. This scale is often confused with earthquake intensity scale in points(according to a 7 or 12-point system), which is based on the external manifestations of an earthquake (impact on people, objects, buildings, natural objects). When an earthquake occurs, it is its magnitude that first becomes known, which is determined from seismograms, and not its intensity, which becomes clear only after some time, after receiving information about the consequences.

Correct usage: « magnitude 6.0 earthquake».

Previous use: « earthquake measuring 6.0 on the Richter scale».

Misuse: « magnitude 6 earthquake», « earthquake measuring 6 magnitudes on the Richter scale» .

Richter scale

$M\_s\; =\; \backslash lg\; (A/T)\; +\; 1.66\; \backslash lg\; D\; +\; 3.30.$

These scales do not work well for the largest earthquakes - when M~8 comes saturation.

Seismic moment and Kanamori scale

The seismic energy released by a 1 megaton nuclear explosion (1 megaton = 4.184 10 15 J) is equivalent to an earthquake with a magnitude of about 7. It is worth noting that only a small part of the explosion energy is converted into seismic vibrations.

Frequency of earthquakes of different magnitudes

In a year on Earth, approximately:

• 1 earthquake with a magnitude of 8.0 or higher;
• 10 - with a magnitude of 7.0-7.9;
• 100 - with a magnitude of 6.0-6.9;
• 1000 - with a magnitude of 5.0-5.9.

The strongest recorded earthquake occurred in Chile in 1960—later estimates put Kanamori's magnitude at 9.5.

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An excerpt characterizing the magnitude of the earthquake

The historical sea, not as before, was directed by gusts from one shore to another: it seethed in the depths. Historical figures, not as before, rushed in waves from one shore to another; now they seemed to be spinning in one place. Historical figures, who previously at the head of the troops reflected the movement of the masses with orders of wars, campaigns, battles, now reflected the seething movement with political and diplomatic considerations, laws, treatises...
Historians call this activity of historical figures reaction.
Describing the activities of these historical figures, who, in their opinion, were the cause of what they call the reaction, historians strictly condemn them. All famous people of that time, from Alexander and Napoleon to m me Stael, Photius, Schelling, Fichte, Chateaubriand, etc., are subject to their strict judgment and are acquitted or condemned, depending on whether they contributed to progress or reaction.
In Russia, according to their description, a reaction also took place during this period of time, and the main culprit of this reaction was Alexander I - the same Alexander I who, according to their descriptions, was the main culprit of the liberal initiatives of his reign and the salvation of Russia.
In real Russian literature, from a high school student to a learned historian, there is not a person who would not throw his own pebble at Alexander I for his wrong actions during this period of his reign.
“He should have done this and that. In this case he acted well, in this case he acted badly. He behaved well at the beginning of his reign and during the 12th year; but he acted badly by giving a constitution to Poland, making the Holy Alliance, giving power to Arakcheev, encouraging Golitsyn and mysticism, then encouraging Shishkov and Photius. He did something wrong by being involved in the front part of the army; he acted badly by distributing the Semyonovsky regiment, etc.”
It would be necessary to fill ten pages in order to list all the reproaches that historians make to him on the basis of the knowledge of the good of humanity that they possess.
What do these reproaches mean?
The very actions for which historians approve of Alexander I, such as: the liberal initiatives of his reign, the fight against Napoleon, the firmness he showed in the 12th year, and the campaign of the 13th year, do not stem from the same sources - the conditions of blood , education, life, which made Alexander’s personality what it was - from which flow those actions for which historians blame him, such as: the Holy Alliance, the restoration of Poland, the reaction of the 20s?
What is the essence of these reproaches?
The fact that such a historical person as Alexander I, a person who stood at the highest possible level of human power, is, as it were, in the focus of the blinding light of all the historical rays concentrated on him; a person subject to those strongest influences in the world of intrigue, deception, flattery, self-delusion, which are inseparable from power; a face that felt, every minute of its life, responsibility for everything that happened in Europe, and a face that is not fictitious, but living, like every person, with its own personal habits, passions, aspirations for goodness, beauty, truth - that this face , fifty years ago, not only was he not virtuous (historians do not blame him for this), but he did not have those views for the good of humanity that a professor now has, who has been engaged in science from a young age, that is, reading books, lectures and copying these books and lectures in one notebook.
But even if we assume that Alexander I fifty years ago was mistaken in his view of what is the good of peoples, we must involuntarily assume that the historian judging Alexander, in the same way, after some time will turn out to be unjust in his view of that , which is the good of humanity. This assumption is all the more natural and necessary because, following the development of history, we see that every year, with every new writer, the view of what is the good of humanity changes; so that what seemed good appears after ten years as evil; and vice versa. Moreover, at the same time we find in history completely opposite views on what was evil and what was good: some take credit for the constitution given to Poland and the Holy Alliance, others as a reproach to Alexander.
It cannot be said about the activities of Alexander and Napoleon that they were useful or harmful, because we cannot say for what they are useful and for what they are harmful. If someone does not like this activity, then he does not like it only because it does not coincide with his limited understanding of what is good. Does it seem good to me to preserve my father’s house in Moscow in 12, or the glory of the Russian troops, or the prosperity of St. Petersburg and other universities, or the freedom of Poland, or the power of Russia, or the balance of Europe, or a certain kind of European enlightenment - progress, I must admit that the activity of every historical figure had, in addition to these goals, other, more general goals that were inaccessible to me.
But let us assume that so-called science has the ability to reconcile all contradictions and has an unchanging measure of good and bad for historical persons and events.
Let's assume that Alexander could have done everything differently. Let us assume that he could, according to the instructions of those who accuse him, those who profess knowledge of the ultimate goal of the movement of mankind, order according to the program of nationality, freedom, equality and progress (there seems to be no other) that his current accusers would have given him. Let us assume that this program was possible and drawn up and that Alexander would act according to it. What would then happen to the activities of all those people who opposed the then direction of the government - with activities that, according to historians, were good and useful? This activity would not exist; there would be no life; nothing would have happened.

Richter scale designed to determine the strength of earth vibrations. In other words, the ruler helps to determine the power of earthquakes. The system is international. The Italian Mercalli began to develop it. Who is Richter and why did he take the laurels from his predecessor? We'll find out.

History of the Richter scale

Richter earthquake scale adopted in the 1930s. The Mercalli system was not just renamed, but modified. The Italian was weak for the 12-point basis. Minimum tremors – one.

Earthquakes from 6 points were considered strong. This did not suit all states. In Russia, for example, they focused on 10-point limits, and in Japan on 7-point limits. Meanwhile, the age of globalization has arrived.

A single standard was required so that the data from all seismographs could be understood anywhere on Earth. This is where Charles Richter got involved. The American suggested using the decimal logarithm.

The calculation of the vibration amplitude is directly proportional to the needle deflection on the seismograph. At the same time, Richter introduced a correction in accordance with the distance of the area from the epicenter of the earthquake.

Richter magnitude scale was officially adopted in 1935. The world began to focus not just on 10 points, but also on the 10-point difference between adjacent ruler marks.

A magnitude 2 earthquake is 10 times stronger than a magnitude 1 earthquake. 3-point pushes are 10 times more powerful than 2-point ones, and so on. But how to determine the strength of shaking? How to understand that the movements of the earth’s crust are exactly 3, 7, 9-point?

Richter scale - scores in visual and physical manifestations

The scores help measure the intensity of surface tremors. Their strength is greater in the bowels of the Earth, where the rift occurs. Some of the energy is lost on the way to the solid crust of the planet. It turns out that the closer the source is to the surface, the higher the intensity. 1 point is not noticeable by people.

2 points are recognized only on the upper floors of high-rise buildings; weak vibrations are felt. At 3 points, the chandeliers sway. Noticeable shaking inside buildings, even low-rise ones, is 4 points.

Magnitude 5 earthquakes are detected not only in houses, but also on the street. At 6 points, glass may break, furniture and dishes may move. It becomes difficult to stay on your feet during a magnitude 7 earthquake. Cracks are spreading along brick walls, flights of stairs are collapsing, and landslides are forming on roads.

At 8 points, buildings are already collapsing, as well as communications laid underground are torn. 9-point tremors lead to disturbances in water bodies and can cause, for example, a tsunami. The soil is cracking.

It crumples and breaks during magnitude 10 earthquakes. 11 points... Stop. After all, the Richter scale ends at ten. In fact of the matter. Gaps in the knowledge of ordinary people led to a mixture of the Mercalli and Richter systems.

The surface intensity of the tremors was measured in points using the Italian scale. She, apparently, did not sink into oblivion, but unofficially joined the American one. Mercalli has both 11 and 12 points.

At 11, brick buildings will collapse to the ground, and only a reminder will remain of the roads. 12 points is a catastrophic earthquake that changes the topography of the earth. The cracks in it reach a width of 10-15 meters.

Now let's figure out what the marks on the true Richter scale indicate. It is “tied” to a magnitude that Mercalli did not take into account. Magnitude determines the energy released during movements in the earth's interior. It is not the external manifestations of the earthquake that are considered, but its internal essence.

Richter scale - magnitude table

While the scores can be determined by observing changes on the surface of the planet, the magnitude is calculated only from seismograph readings. The calculations are based on the type of waves of a typical, average earthquake.

The indicator is put into logarithm with the maximum amplitudes of specific shocks. Magnitude is proportional to this logarithm.

The strength of the energy released during an earthquake depends on the size of its source, that is, the length and width of the fault in the rocks. Typical Richter shocks can be measured not only in whole numbers, but also in fractional numbers.

Thus, a magnitude of 4.5 leads to minor damage. The fault parameters are only a few meters vertically and in length. A source of several kilometers usually produces earthquakes with a magnitude of 6.

The fault is hundreds of kilometers long - magnitude 8.5. There is also a 10 on the Richter scale. But this is, so to speak, an unrealistic limit. There have been no earthquakes with a magnitude greater than 9 on Earth. Apparently it won't happen.

For magnitude 10, a fault depth of more than 100 kilometers is needed. But, at such a depth, the earth is no longer solid, the substance turns into liquid - the mantle of the planet. The length of a ten-fold outbreak should exceed 1000 kilometers. But such faults are not known to scientists.

Earthquakes with a magnitude of 1 do not occur, or rather are not recorded by instruments. The weakest tremors, felt by both seismographs and people, are 2 points. Yes, magnitude indicators are sometimes also called points. But, it is more correct to pronounce only the number, so that there is no confusion with the Mercalli scale.

There is an approximate relationship between the severity of an earthquake and its magnitude. At the same time, it is important to take into account the depth of the shock source. The easiest way to compare the indicators is by looking at the table.

Kilometers	Magnitude
5	5	6	7	8
10	7	8-9	10	11-12
20	6	7-8	9	10-11
40	5	6-7	8	9-10

It can be seen that the same magnitude can lead to different destruction depending on the depth of the source. There are other reasons to judge what it will be like earthquake in points? Richter scale points They also depend on the seismic resistance of buildings in the area of ​​tremors and the nature of the soil.

In good-quality buildings, the force of an earthquake is perceived differently than in houses built without taking into account possible movements of the earth's crust. Charles Richter talked about this back in the 1930s.

The scientist not only created an international scale, but spent his entire life fighting for reasonable construction, taking into account all the risks of a particular area. It was thanks to Richter that many countries tightened building construction standards.

15.08.2016

The previously discussed concept of “intensity” of an earthquake characterizes the extent of its consequences for a certain area, without indicating its (earthquake) strength (power) as a whole as a physical phenomenon. Therefore, at the end of the 19th century there were proposals (scales) to estimate the intensity of an earthquake only in the epicentral zone. Subsequently, there were proposals to judge the strength of an earthquake by the size of the areas affected by it. An earthquake that caused damage in areas with a larger diameter was considered to be in the stronger class. As can be seen from table. 1.5, on the one hand, the characteristics of the intensity of an earthquake in many cases are determined by the level of susceptibility of people (which cannot be expressed in quantitative terms), and on the other hand, the degree of damage to buildings and structures is significantly determined by the quality of construction and ground conditions. When establishing the strength of an earthquake based on the areas of damaged areas, the question arises about the depth of the source. Thus, an urgent need arose to evaluate the strength of an earthquake, regardless of its consequences, by some numerical parameter obtained using an instrument (seismograph) during an earthquake, regardless of the location of recording. Since the cause of all macroseismic effects included in any intensity scale and observed during earthquakes is ground movement, it is natural to vary the value of ground movement when estimating the strength of an earthquake. This is how the idea of ​​earthquake magnitude arose. The magnitude of an earthquake is a measure of assessing its strength by the magnitude of the movements of soil particles and the time of this earthquake. The Latin word “magnitude” and translated into Russian means “magnitude”. In fact, when talking about the magnitude of an earthquake, it is necessary to mean its magnitude. The greater the level of movement of soil particles during an earthquake, the greater its magnitude, i.e. the stronger the earthquake itself.
Many experts in the field of seismology took part in formulating the concept of magnitude. In particular, workers at seismic stations often wondered about the discrepancy between the degree of anxiety or fear of people caused by an earthquake and the nature of its actual seismogram recorded at the station. A weak local shock has always had a large response, while a strong distant earthquake in a sparsely populated desert, mountains or in the ocean often goes unnoticed except by the employees of seismic stations themselves, who have seismograms of the earthquake. Seismologists themselves also found it more difficult to correctly classify earthquakes by their strength, regardless of their consequences. A major contribution to detailing the concept of magnitude was made by California Institute of Technology (Pasadena) professor Charles Richter, who developed a plan for separating strong and weak earthquakes on an objective instrumental basis rather than subjective judgments about their consequences. The main axiomatic principle of the assessment is that of two earthquakes having the same hypocenter, the large (strong) one should be recorded with a large amplitude of ground vibrations at any station. For the same earthquake strength, a seismograph installed close to the epicenter will record greater ground movements than at a far distance. Consequently, to determine the magnitude, the first question arose about choosing the location where the earthquake was recorded.
As noted above, Richter raised the question of dividing earthquakes into strong and weak. Therefore, there was a need to establish a “standard” earthquake as a standard. For a standard earthquake, Richter chose the registration location at a distance of 100 km from the epicenter. On the other hand, even at the same distance from the epicenter, the magnitude of movements of soil particles in areas with different engineering-geological characteristics differ significantly. Therefore, it was agreed that the recording device should be installed in areas with rocky soils. Richter chose the Wood-Anderson short-period torsional seismograph, which was widely used in the 30s of the last century, as an instrument. The main parameters of this seismograph: the period of free oscillations of the pendulum - 0.8 sec, the attenuation coefficient - h = 0.8, the magnification factor - 2800 (the real movement of the soil on the recording tape increases 2800 times). This is how Richter himself formulated the concept of magnitude: “The magnitude of any shock is determined” as the decimal logarithm of the maximum amplitude of the record of this shock, expressed in microns, recorded by a standard short-period Wood-Anderson torsional seismograph at a distance of 100 km from the epicenter.” Let us note in advance that it is not necessary to have the Wood-Anderson seismograph exactly at a distance of 100 km from the epicenter every time (this can happen completely by accident), simply, as will be indicated below, it is necessary to introduce corrections to bring the measurement results obtained at other distances and other seismographs, to those that would be received at a distance of 100 km by a Wood-Anderson seismograph.
Therefore, the magnitude of the earthquake, which is denoted by the letter M, will be

where Ac is the amount of movement of rocky soil on the seismogram in microns, recorded by a Wood-Anderson seismograph at a distance of 100 km. If on an earthquake seismogram recorded by a Wood-Anderson seismograph, at a distance of 100 km, the maximum ground movement is equal to 1 micron (1 micron = 0.001 millimeters), then the magnitude of this earthquake is taken to be equal to M = Ig1 = 0. But this does not mean that there was no earthquake, it was just very weak. Similarly, if the maximum ground movement is 10 microns, then the magnitude of such an earthquake will be Igl0 = 1. In reality, the magnitude M=1 will correspond to the earthquake during which, at a distance of 100 km from the epicenter, the actual movement of the rocky ground will be equal to:

Based on the above definition of magnitude, it is surprising to note that it can also have negative values. So, if on an earthquake seismogram recorded by a Wood-Anderson seismograph, at a distance of 100 km from the epicenter, the ground movement is equal to 0.1 microns, then the magnitude of such an earthquake will be

In this case, the actual ground movement will be

Recording such ground movement is, of course, not an easy task. It involves the creation of a seismograph with high magnification factors. Fortunately, we note that to date, ultra-sensitive seismographs have been created that are capable of recording earthquakes with magnitudes up to M=3. Thus, with an increase in magnitude by one unit, the amplitude of ground vibrations increases 10 times. For greater clarity, in Table. Table 1.7 shows the actual values ​​of displacements at a distance of 100 km from the epicenter for earthquakes from the weakest with magnitude M=1 to the strongest with magnitude M=9.0.

The weakest earthquake felt by humans has a magnitude of M=1.5. Earthquakes with a magnitude of M=4.5 or more already cause damage to buildings and structures. Earthquakes from 1< M < 3 называются микроземлетрясениями, а с M < 1 - ульграмикроземлетрясениями.
The Richter magnitude scale (if it can even be called a scale) has no upper limit. Therefore, it is often called the “open” scale, since no one can predict when and with what strength the strongest earthquake will occur, although the upper limit of magnitude is determined (limited) by the limiting strength of the earth's rocks. Apparently, this can also be said about the lower limit of the scale, since over time, by improving seismographs, opportunities are created for recording the weakest earthquakes.
In the Armenian version of this book, published in 2002, we noted two earthquakes as the strongest since the beginning of instrumental recordings, having a magnitude of M-8.9. Both of these earthquakes occurred under the ocean in subduction zones. The first earthquake occurred in 1905 off the coast of Ecuador, the second in 1933 on the coast of Japan. In 2002, we posed a rhetorical question: maybe our planet is not capable of generating earthquakes with a magnitude greater than 8.9 and believed that only time could answer this question. A little time passed and we received the answer to this question: earthquakes with a magnitude greater than 8.9 are possible on our planet Earth. This happened on December 26, 2004. The most catastrophic earthquake on Earth occurred on the coast of the island of Sumatra with a magnitude of more than 9.0, causing a huge tsunami and causing the death of more than 300,000 people.
Obviously, if an earthquake is recorded not by a Wood-Anderson seismograph, but by any other seismograph, then the magnitude of the earthquake will be

where A is the maximum value of the actual soil movement in microns, recorded by any seismograph (not on a seismogram).
For example, during the 1988 Spitak earthquake at the engineering seismometric station N5 of the city of Yerevan, the SM-5 seismometer recorded a maximum soil movement of 3.5 mm or 3500 microns (Fig. 3.19). The Yerevan-Spitak distance is approximately 100 km, so the magnitude of the Spitak earthquake will be approximately

M = lg 2800*3500 = lg10v7 = 7.0,

which was confirmed by many seismic stations around the world.
A natural question arises - how to determine the magnitude if the seismograph is installed not at a distance of 100 km from the epicenter, but at an arbitrary distance. To do this, Richter himself constructed a calibration curve for the California earthquakes to go from the amplitudes observed at an arbitrary epicentral distance to the amplitudes expected at a distance of 100 km. This type of magnitude is currently called local magnitude - ML, and is determined by Richter's formula

where A is the maximum value of the actual movement of the soil along volumetric shear waves S and microns, recorded by any seismograph, Δ is the epicentral distance in kilometers.
Formula (1.92a) is applicable only for shallow local earthquakes of the type studied by Richter with Δ ≤ 600 km.
For earthquakes with a central distance Δ ≥ 600 km, surface waves with long periods predominate in the seismograms. For shallow-focus remote earthquakes (teleseismic), Gutenberg derived the following formula for the magnitude Ms:

where A is the horizontal component of the actual ground movement (in microns) caused by surface waves with a period of about 20 seconds.
The International Association of Seismology and Physics of the Earth (IASPEI) recommends the following expression for Ms:

where (A/T)max is the maximum of all A/T values ​​(amplitude/period) for various wave groups on the seismogram. For T=20sec, equation (1.92c) almost coincides with equation (1.92b).
The peculiarity of the listed three formulas (1.92) is that as the epicentral distance Δ increases, the maximum ground movement A decreases and vice versa, therefore, as a result, the same earthquake recorded at different distances from the epicenter will have almost the same magnitude. Equations (1.92) are considered applicable only for shallow-focus earthquakes with a focal depth h of no more than 60 km. For deeper earthquakes, the magnitude scale is based on the amplitude of teleseismic body waves mв and is determined by the formula:

where T is the period of the measured wave, and A is the amplitude of the soil, C(h, Δ) is an empirical coefficient depending on the depth of the source and the epicentral distance, determined from special tables.
The following relationship between mв and Ms has been empirically established

Note that the values ​​of mn and M coincide at mn = M=6.75, above this M=mn, below M=mn.

All of the above reasoning and formulas, despite their apparent simplicity, in their practical application encounter certain difficulties associated with the translation of the values ​​of ground movements recorded by a modern seismograph to the records of the Wood-Anderson seismograph, with the establishment of the angle of incidence of the seismic wave front, the depth of the source and fixation on the seismogram of the positions of the first arrivals of body and surface waves P, S, L and their periods, as well as those related to the ground conditions of the earthquake registration site. Therefore, all seismic stations have their own correction factors for determining magnitude. All calculations are made using computer programs or special nomograms. One of these nomograms, borrowed from, is shown in Fig. 1.43. But, despite all this, due to the complexity of the essence of the earthquake itself, the heterogeneity of the propagation paths of seismic waves and the non-identity of seismographs, the magnitude values ​​of the same earthquake calculated at different seismic stations always differ from each other, and the difference can reach a value of 0.5 .
We consider it necessary to note once again that the development of the concept of assessing the strength of an earthquake using a magnitude scale is a fundamental step in the development of quantitative seismology. No other measure describes the overall size of an earthquake as completely and accurately. The magnitude scale makes it possible, having at least one instrumental record (seismogram) of an earthquake on the Earth's surface, regardless of the location of the incident and the degree of the consequences caused, to quantify the scale and power of the earthquake.

Seismic scale

Earthquakes- tremors and vibrations of the Earth's surface caused by natural causes (mainly tectonic processes) or artificial processes (explosions, filling of reservoirs, collapse of underground cavities in mine workings). Small tremors can also cause lava to rise during volcanic eruptions.

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. Fortunately, most of them occur 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).

Earthquakes are best known for the devastation they can cause. Destructions of buildings and structures are caused by soil vibrations or giant tidal waves (tsunamis) that occur during seismic displacements on the seabed.

Introduction

The cause of an earthquake is the rapid displacement of a section of the earth's crust as a whole at the moment of plastic (brittle) deformation of elastically stressed rocks at the source of the earthquake. Most earthquakes occur near the Earth's surface. The displacement itself occurs under the action of elastic forces during the discharge process - a decrease in elastic deformations in the volume of the entire section of the slab and a displacement to the equilibrium position. An earthquake is a rapid (on a geological scale) transition of potential energy accumulated in elastically deformed (compressed, sheared or stretched) rocks of the earth's interior into the energy of vibrations of these rocks (seismic waves), into the energy of changes in the structure of rocks at the source of the earthquake. This transition occurs when the tensile strength of the rocks at the source of the earthquake is exceeded.

The tensile strength of crustal rocks is exceeded as a result of an increase in the sum of forces acting on it:

1. Forces of viscous friction of mantle convection flows on the earth's crust;
2. Archimedean force acting on the light crust from the heavier plastic mantle;
3. Lunar-solar tides;
4. Changing atmospheric pressure.

These same forces also lead to an increase in the potential energy of elastic deformation of rocks as a result of displacement of plates under their action. The potential energy density of elastic deformations under the influence of the listed forces increases in almost the entire volume of the slab (in different ways at different points). At the moment of an earthquake, the potential energy of elastic deformation in the earthquake source quickly (almost instantly) decreases to the minimum residual energy (almost to zero). Whereas in the vicinity of the source, due to the displacement of the plate as a whole during an earthquake, elastic deformations increase somewhat. That is why repeated earthquakes - aftershocks - often occur in the vicinity of the main earthquake. In the same way, small “preliminary” earthquakes - foreshocks - can provoke a large one in the vicinity of the initial small earthquake. A large earthquake (with large plate displacement) can cause subsequent induced earthquakes even at distant plate edges.

Of the listed forces, the first two are much larger than the 3rd and 4th, but their rate of change is much less than the rate of change of tidal and atmospheric forces. Therefore, the exact time of arrival of an earthquake (year, day, minute) is determined by changes in atmospheric pressure and tidal forces. Whereas much larger, but slowly changing forces of viscous friction and Archimedean force set the time of arrival of an earthquake (with a focus at a given point) with an accuracy of centuries and millennia.

Deep-focus earthquakes, the foci of which are located at depths of up to 700 km from the surface, occur at convergent boundaries of lithospheric plates and are associated with subduction.

Seismic waves and their measurement

Types of seismic waves

Seismic waves are divided into compression waves And shear waves.

• Compression waves, or longitudinal seismic waves, cause vibrations of the rock particles through which they pass along the direction of wave propagation, causing alternating areas of compression and rarefaction in the rocks. The speed of propagation of compression waves is 1.7 times greater than the speed of shear waves, so seismic stations are the first to record them. Compression waves are also called primary(P-waves). The speed of the P-wave is equal to the speed of sound in the corresponding rock. At frequencies of P-waves greater than 15 Hz, these waves can be perceived by ear as an underground hum and rumble.
• Shear waves, or seismic transverse waves, cause rock particles to vibrate perpendicular to the direction of propagation of the wave. Shear waves are also called secondary(S-waves).

There is a third type of elastic waves - long or superficial waves (L-waves). They are the ones who cause the most destruction.

Measuring the strength and impacts of earthquakes

A magnitude scale and an intensity scale are used to evaluate and compare earthquakes.

Magnitude scale

The magnitude scale distinguishes earthquakes by magnitude, which is the relative energy characteristic of the earthquake. There are several magnitudes and, accordingly, magnitude scales: local magnitude (ML); magnitude determined from surface waves (Ms); body wave magnitude (mb); moment magnitude (Mw).

The most popular scale for estimating earthquake energy is the local Richter magnitude scale. On this scale, an increase in magnitude by one corresponds to a 32-fold increase in the released seismic energy. An earthquake with a magnitude of 2 is barely noticeable, while a magnitude of 7 corresponds to the lower limit of destructive earthquakes covering large areas. The intensity of earthquakes (cannot be assessed by magnitude) is assessed by the damage they cause in populated areas.

Intensity scales

Medvedev-Sponheuer-Karnik scale (MSK-64)

The 12-point Medvedev-Sponheuer-Karnik scale was developed in 1964 and became widespread in Europe and the USSR. Since 1996, the European Union has used the more modern European Macroseismic Scale (EMS). MSK-64 is the basis of SNiP-11-7-81 “Construction in seismic areas” and continues to be used in Russia and the CIS countries.

Point	Earthquake strength	a brief description of
1	Not felt.	Marked only by seismic instruments.
2	Very weak tremors	Marked by seismic instruments. It is felt only by certain people who are in a state of complete peace in the upper floors of buildings, and by very sensitive pets.
3	Weak	It is felt only inside some buildings, like a shock from a truck.
4	Moderate	Recognized by slight rattling and vibration of objects, dishes and window glass, creaking of doors and walls. Inside the building, most people feel the shaking.
5	Quite strong	In the open air it is felt by many, inside houses - by everyone. General shaking of the building, vibration of furniture. The clock pendulums stop. Cracks in window glass and plaster. Awakening the Sleepers. It can be felt by people outside buildings; thin tree branches are swaying. Doors slam.
6	Strong	It is felt by everyone. Many people run out into the street in fear. Pictures fall from the walls. Individual pieces of plaster are breaking off.
7	Very strong	Damage (cracks) in the walls of stone houses. Anti-seismic, as well as wooden and wattle fence buildings remain unharmed.
8	Destructive	Cracks on steep slopes and wet soil. Monuments move out of place or topple over. Houses are heavily damaged.
9	Devastating	Severe damage and destruction of stone houses. Old wooden houses are crooked.
10	Destructive	Cracks in the soil are sometimes up to a meter wide. Landslides and collapses from slopes. Destruction of stone buildings. Curvature of railway rails.
11	Catastrophe	Wide cracks in the surface layers of the earth. Numerous landslides and collapses. Stone houses are almost completely destroyed. Severe bending and bulging of railway rails.
12	Major disaster	Changes in the soil reach enormous proportions. Numerous cracks, collapses, landslides. The appearance of waterfalls, dams on lakes, deviation of river flows. Not a single structure can withstand.

What happens during strong earthquakes

An earthquake begins with the rupture and movement of rocks at some place deep in the Earth. This location is called the earthquake focus or hypocenter. Its depth is usually no more than 100 km, but sometimes it reaches 700 km. Sometimes the source of an earthquake can be near the surface of the Earth. In such cases, if the earthquake is strong, bridges, roads, houses and other structures are torn and destroyed.

The area of ​​land within which on the surface, above the source, the force of tremors reaches its greatest magnitude is called the epicenter.

In some cases, layers of earth located on the sides of a fault move toward each other. In others, the ground on one side of the fault sinks, forming faults. In places where they cross river channels, waterfalls appear. The vaults of underground caves are cracking and collapsing. It happens that after an earthquake large areas of the earth sink and are filled with water. Earth tremors displace the upper, loose layers of soil from the slopes, forming landslides and landslides. During the earthquake in California last year, a deep crack appeared on the surface. It stretches for 450 kilometers.

It is clear that the sudden movement of large masses of earth in the source must be accompanied by a blow of colossal force. Over the year people [ Who?] can feel about 10,000 earthquakes. Of these, approximately 100 are destructive.

Measuring instruments

To detect and record all types of seismic waves, special instruments are used - seismographs. In most cases, the seismograph has a weight with a spring attachment, which during an earthquake remains motionless, while the rest of the device (body, support) begins to move and shifts relative to the load. Some seismographs are sensitive to horizontal movements, others to vertical ones. The waves are recorded by a vibrating pen on a moving paper tape. There are also electronic seismographs (without paper tape).

Other types of earthquakes

Volcanic earthquakes

Volcanic earthquakes are a type of earthquake in which an earthquake occurs as a result of high tension in the interior of a volcano. The cause of such earthquakes is lava, volcanic gas. Earthquakes of this type are weak, but continue for a long time, many times - weeks and months. However, an earthquake does not pose a danger to people of this type.

Man-made earthquakes

Recently, information has emerged that earthquakes can be caused by human activity. For example, in areas of flooding during the construction of large reservoirs, tectonic activity increases - the frequency of earthquakes and their magnitude increases. This is due to the fact that the mass of water accumulated in reservoirs increases the pressure in rocks with its weight, and seeping water reduces the tensile strength of rocks. Similar phenomena occur when large quantities of rock are removed from mines, quarries, and during the construction of large cities from imported materials.

Landslide earthquakes

Earthquakes can also be caused by landslides and large landslides. Such earthquakes are called landslides; they are local in nature and have low strength.

Earthquakes of artificial nature

An earthquake can also be caused artificially: for example, by the explosion of a large amount of explosives or during a nuclear explosion. Such earthquakes depend on the amount of material exploded. For example, during the DPRK's testing of a nuclear bomb, a moderate earthquake occurred in 2016, which was recorded in many countries.

The most destructive earthquakes

• January 23 - Gansu and Shaanxi, China - 830,000 deaths
• - Jamaica - Turned into ruins in Port Royal
• - Kolkata, India - 300,000 people died
• - Lisbon - from 60,000 to 100,000 people died, the city was completely destroyed
• - Colabria, Italy - between 30,000 and 60,000 people died
• - New Madrid, Missouri, USA - the city has been reduced to ruins, flooding over an area of ​​500 sq. km
• - Sanriku, Japan - the epicenter was under the sea. A giant wave washed away 27,000 people and 10,600 buildings into the sea
• - Assam, India - Over an area of ​​23,000 sq. km, the relief is changed beyond recognition, probably the largest earthquake in the history of mankind
• - San Francisco, USA 1,500 people died, 10 sq. km destroyed. cities
• - Sicily, Italy 83,000 people died, the city of Messina was reduced to ruins
• - Gansu, China 20,000 people died
• - Great Kanto earthquake - Tokyo and Yokohama, Japan (8.3 Richter) - 143,000 people died, about a million were left homeless as a result of the resulting fires
• - Inner Taurus, Türkiye 32,000 people died
• - Ashgabat, Turkmenistan, Ashgabat earthquake, - 110,000 people died
• - Ecuador 10,000 people died
• - The Himalayas cover an area of ​​20,000 sq. km in the mountains.
• - Agadir, Morocco 12,000 - 15,000 people died
• - Chile, about 10,000 killed, cities of Concepcien, Valdivia, Puerto Mon destroyed
• - Skopje, Yugoslavia about 2,000 killed, most of the city reduced to ruins

“Richter scale” is the common name for a scale showing the magnitude of an earthquake.

The Richter scale characterizes the energy that is released in the form of seismic waves during an earthquake. This system was proposed relatively recently - in 1935.

The Richter scale is sometimes confused with another classification that shows the level of impact of an earthquake on external objects - people, buildings, natural formations. These are actually two different scales.

The Richter system contains arbitrary units from 1 to 9.5, and the intensity scale contains 7 or 12 points. During an earthquake, only its magnitude can be determined immediately, and the intensity can be assessed later, when the consequences of the vibrations become known.

Creation of the Richter scale

The first scale to assess the intensity of an earthquake was proposed back in 1902; its creator was Giuseppe Mercalli, an Italian priest and geologist. This classification can be called scientific with a big stretch: the description of the degree of tremors is made in it on the basis of purely subjective sensations.

For example, an earthquake of magnitude II is described as “felt in a calm environment on the upper floors of buildings”; however, since then construction technologies have changed, there are many more floors, and a “calm environment” is a completely individual concept for each person.

If the house collapses, but people managed to run out, then fewer points are given, and if they died under the rubble, then more. Subsequently, Richter himself improved the Mercalli scale, and in this form it is still sometimes used - mainly in the USA. However, Richter wanted to obtain a truly objective and rigorous system for assessing earthquakes.

He proposed using a standard seismograph that records tremors using needle oscillations. The strength of the earthquake in the proposed system was estimated as the decimal logarithm of the needle movement, despite the fact that the seismograph is located no further than 600 km from the epicenter. The distance from the epicenter affects the accuracy of measurements, so a correction function was introduced into the equation, calculated from the table.

However, this system had its drawbacks: Richter used Southern California earthquakes, the sources of which are shallow, as the basis for calibrating his scale. The first Richter scale ended at 6.8 units, since the equipment of that time did not allow more. The method measured only surface waves, while in deep earthquakes a significant part of the energy is released in the form of body waves.

Apparently, at that time the young scientist lacked knowledge about earthquakes of various types. Long years of observation of this phenomenon made it possible to significantly rework and refine the Richter scale. Currently, several of its varieties are used, used for different cases.

Beno Gutenberg

The honor of creating the Richter scale does not belong to Richter alone. He developed it in collaboration with Beno Gutenberg, a native of Germany. Gutenberg also seriously studied earthquakes, but he was a Jew, so when the Nazis came to power he was forced to flee to the United States. There he founded a seismic laboratory, in which Richter began working with him.