\u003e Earthquake like geophysical process... Basic concepts of seismology. Assessment of intensity in points

The science of earthquakes, seismology, although young, has made serious progress in understanding the object of its research. The names of A.P. Orlova, I. V. Mushketova, K.I. Bogdanovich, V.N. Weber, B.B. Golitsina, G.A. Gamburtseva, S.V. Medvedeva, Yu.V. Riznichenko are bright reference points on the growth curve of domestic and world seismology.

At the cost of the efforts of several generations of researchers, specialists now have a good idea of \u200b\u200bwhat happens during an earthquake and how it manifests itself on the Earth's surface. But superficial phenomena are the result of what is happening in the depths. And the main attention of specialists is now focused on the knowledge of deep-seated processes in the bowels of the Earth, processes leading to an earthquake, accompanying it and following it.

The theory of earthquakes as a geophysical process is still being developed. Although physical and mathematical modeling is now widely used in research of this kind, knowledge of various natural phenomena associated with earthquakes is largely based on observations on the earth's surface.

Scientific geology (its formation dates back to the 18th century) made the correct conclusions that it was mainly young areas of the earth's crust that were shaken. In the second half of the nineteenth century. has already been chosen general theory, according to which the earth's crust was subdivided into ancient stable shields and young, mobile mountain structures. It turned out that young mountain systems - the Alps, Pyrenees, Carpathians, Himalayas, Andes - are subject to strong earthquakes, while the ancient shields (the Czech Massif belongs to them) are areas where strong earthquakes are absent.

The most commonly used seismological terms associated with the concept of "earthquake" include the following: focus, hypocenter, epicenter, magnitude, point.

The focus of a tectonic earthquake is understood as a closed volume of the earth's substance, in which destruction occurred in a fairly short time, up to 1-3 minutes. As a rule, in the area of \u200b\u200bthe focus there is a displacement (displacement) of one part of the volume relative to another. The place where the movement begins is called the hypocenter.

It is from this point that the process of generating seismic waves begins, which can lead to destruction outside the source. The vertical projection of the hypocenter onto the earth's surface is called the epicenter.

The point concept characterizes the intensity of the shaking at the observation point. In our country, since 1964, a 12-point scale MSK - 64 has been used. It should be noted that non-seismologists in points often characterize the very strength of an earthquake in the source. This is not true, but it appears regularly in newspaper reports. As a rule, this applies to the Richter scale, which uses the dimensionless magnitude of the earthquake magnitude M, which is proportional to the logarithm of the energy released in the source. The confusion arose in connection with two circumstances: 1) the magnitude of the earthquakes known so far does not exceed 9 units (in the catalogs there is only M (max.) Equal to 8.9), that is, the magnitude is numerically close to the values \u200b\u200bof the shaking points; 2) we are accustomed to the fact that any parameter has a dimension (meters, kilograms, degrees), but the logarithms of any parameters are always dimensionless. Therefore, if there are messages in the press like “the earthquake had 7 points on the Richter scale”, then in reality this means that the magnitude of the earthquake is M \u003d 7. And it can be felt at different points with a force of 10 points, 8 points, 5 points - it depends on the distance to the hearth. Thus, if the score depends on the distance to the source, then the magnitude does not.

The MSK-64 scale is compiled in relation to buildings and structures that do not have earthquake-resistant structural reinforcement. I will give here a description of the first four points of this scale without changes, and starting from the fifth, when damage to buildings is possible, I will describe the main distinguishing features of earthquakes and their likely impact on buildings of modern development in Kamchatka. When describing each point, the frequency of recurrence of earthquakes of a given strength for Petropavlovsk-Kamchatsky is indicated in brackets.

1 b a l l. Imperceptible earthquake. The vibration intensity lies below the sensitivity limit, ground shaking is detected and recorded only by seismographs.

2 b a l l a. Weak earthquake. The vibrations are only felt by individualslocated indoors, especially on the upper floors.

3 b a l l a. Weak earthquake. It is felt by not many people indoors, in the open air - only in favorable conditions. The vibrations are similar to the tremors generated by a passing light truck. Attentive observers notice a slight swaying of hanging objects, somewhat more violent on the upper floors.

4 b a l l a. Noticeable concussion. The earthquake is felt inside the building by many people, in the open air by a few. In some places they wake up, but no one is frightened. The vibrations are similar to the jolt generated by a passing heavy truck. Rattling near doors, dishes. The creak of walls, floors. Trembling furniture. Hanging objects swing slightly. The liquid in open vessels fluctuates slightly. In cars standing still, the shock is noticeable.

5 b a l about (15-25 times in 100 years). Almost all sleeping people wake up, water fluctuates and partially splashes in vessels, light objects can tip over, dishes break. Buildings are not damaged.

6 points (10-15 times in 100 years). Many people get scared, hesitation interferes with walking. Buildings stagger, pendant lights swing violently. Crockery falls and breaks, objects fall from the shelves. Furniture can move. Scattering of whitewash, fine cracks in the plaster.

7 points (4-6 times in 100 years). Severe fright, hesitation interferes with standing. Furniture moves and may fall. In any building, there are cracks in the partitions. Cracks in the plaster, thin cracks in the walls, cracks in the seams between blocks and in partitions, loss of seams, often thin cracks in blocks.

8 b a l about in (1-3 times in 100 years). Knocks down. Cracks in the ground on slopes. In any buildings - damage, sometimes partial destruction of partitions. Cracks in load-bearing walls, plaster collapses, block displacement, block cracks.

9 points (approximately once every 300 years). Cracks in the ground everywhere. Landslides on the slopes. In any building - the collapse of partitions. Destruction of part of the load-bearing walls, damage and displacement of some panels.

Rubleny houses made of logs and beams, as a rule, tolerate 9-point tremors without destruction.

The causes of earthquakes will immediately become clear as soon as we imagine the dynamic nature of the Earth and the slow movements that occur in its crust - the lithosphere. The thickness of the bark is highly variable. Under continents it is equal to 30-35 km, and large mountains, significantly higher than the average level of the earth's surface, are almost always accompanied by deep "roots". So, in Tibet, the thickness of the crust was more than 70 km. The base of the crust under the oceans is about 10 km below sea level. Its small thickness is well illustrated by the following example: if the Earth is reduced to the size of an egg, then the hard crust will be as thick as the shell. This hard layer, however, is not solid: it is broken into several large pieces called slabs.

Forces operate beneath the lithosphere, forcing the plates to move at a speed, usually of several centimeters per year. The reason for these underlying forces is not entirely clear. They can be caused, for example, by slow currents of hot plastic substance in the bowels. Currents arise from thermal convection combined with the dynamic effects of the Earth's rotation. In some areas, new substance rises up from the bowels of the earthpushing the plates aside (this happens, for example, in the Mid-Atlantic Ridge); in other places they slide along one another (like along the San Andreas Fault in California); There are areas called subduction (underthrust) zones where one plate is pushed under another when it meets (for example, in the ocean off the western coast of South and Central America, off the coast of Alaska and Japan). The inconsistency in the movement of the plates in any direction causes the rock mass to crack, thus creating earthquakes.

Not surprisingly, the majority of earthquakes (almost 95%) occur at the edges of the plates. Earthquakes caused by plate movement are called tectonic. Although they usually occur at plate boundaries, a small proportion of them occur within the plates. Some other earthquakes, such as in the Hawaiian Islands, have volcanic origin and already very rarely they are caused by human activity (filling reservoirs, pumping water into wells, mining, big explosions).

The earthquake zone surrounding the Pacific Ocean is called the Pacific Belt: about 90% of all earthquakes in the world occur here. Another area of \u200b\u200bhigh seismicity, which includes 5-6% of all earthquakes, is the Alpine belt, which stretches from the Mediterranean to the east through Turkey, Iran and North India. The remaining 4-5% of earthquakes occur along mid-ocean ridges or within plates.

Earthquakes are one of the most terrible natural disasters, causing not only devastating destruction, but also claiming tens and hundreds of thousands of human lives. Earthquakes have always caused horror with their strength, unpredictability, and consequences. A person in such cases feels himself surrendered to the power of "God's wrath." The earthly firmament, the most unshakable in human imagination, suddenly turns out to be mobile, it rises in waves and splits into deep gorges.

Known big number catastrophic earthquakes, during which the number of victims amounted to many thousands (Fig. 18.0). In 1556 in China terrible earthquake led to the death of 830 thousand people, and many hundreds of thousands were injured. The Lisbon earthquake in Portugal in 1755 claimed more than 60 thousand lives. Messina earthquake in 1923 - 150 thousand; Tangshan in China in 1976 - 650 thousand. The list goes on and on and on and on. In Armenia on December 7, 1888, as a result of the Spitak earthquake, more than 25 thousand people died and 250 thousand were injured. On May 28, 1995, in the north of Sakhalin, a powerful earthquake razed the town of neftegorsk to the ground, where more than 2,000 people died.

Earthquakes of various strengths and in different parts of the globe occur constantly, leading to enormous material damage and casualties among the population. Therefore, scientists different countries do not abandon attempts to determine the nature of the earthquake, to identify its causes and, most importantly, to learn how to predict it, which, unfortunately, with the exception of isolated cases has not yet been possible.

The mechanism of the earthquake and its parameters. An earthquake of a tectonic type, i.e. associated with the internal endogenous forces of the earth, is a process of cracking, going at a certain finite speed, and not instantaneously. It presupposes the formation and renewal of many different-scale breaks, with the ripping of each of them not only with the release, but also with the redistribution of energy in a certain volume. When we talk about the fact that the force of external influence on rocks has exceeded their strength, it should be borne in mind that in geomechanics, the strength of rocks as a material, which is relatively high, and the strength of a rock mass, including, in addition to the material of rocks, also structural weakened zones. Thanks to the latter, the strength of the rock mass is significantly lower than the strength of the rocks themselves.

The rate of propagation of ruptures is several km / sec and this process of destruction covers a certain volume of rocks, which is called the focus of an earthquake.

The center of the focus is called a hypocenter, a conditionally point source of short-period oscillations (Fig. 18.1.1).

Figure: 18.1.1. The source of the earthquake and the propagation of shaking in the volume of the rock: 1 - the source area or hypocenter, 2 - the projection of the hypocenter on the earth's surface - the epicenter

Isoseist lines on the surface - lines of equal shaking in points. In most cases, although not always, ruptures are of a shear nature and the earthquake source covers a certain volume around it. Seismology studies elastic waves dynamically propagating in the frequency range of 10-3 –102 Hz at speeds of 2-5 km / sec. The projection of the hypocenter onto the earth's surface is called the epicenter of an earthquake. The intensity of the epicenter earthquake is depicted by lines of equal earthquake intensity - isoseist. The area of \u200b\u200bmaximum scores around the epicenter is called the pleistoseist area.

The main underground seismic shock, an earthquake, is usually preceded by earthquakes or foreshocks, which indicate a critical increase in stresses in rocks. After the main seismic shock, seismic shocks are usually observed, but weaker than the main shock. They are called aftershocks and indicate the process of stress relief during the formation of new ruptures in the rock mass.

According to the depth of hypocenters (foci), earthquakes are divided into 3 groups:

shallow focus 0-60 km;

medium focus - 60-150 km;

deep focus 150-700 km.

But most often, the hypocenters of earthquakes are concentrated in the upper part of the earth's crust at depths of 10-30 km, where the crust is characterized by the greatest rigidity and fragility.

Fast, albeit uneven, displacements of rock masses along the fracture plane cause deformation waves - elastic vibrations in the rock mass, which, propagating in all directions and reaching the surface of the earth, perform the main destructive work on it. The chapter has already discussed the main types of body and surface seismic waves. The first includes longitudinal - p (more high-speed) and transverse - s (less high-speed) waves (see Fig. 2.2.2). The second - the waves of Lava - l and Rayleigh - r. Waves p represent an alternation of compression and expansion and are able to pass through solid, liquid and gaseous substances, while s waves, during their propagation, shift particles of the substance at right angles to the direction of their path.

Longitudinal wave speed:

Where μ is the shear modulus; ρ is the density of the medium in which the wave propagates; λ -

Ratio related to modulus of compression to ratio

Shear wave speed:

Because the shear modulus μ in liquid and gas is 0, then transverse waves do not pass through liquids and gases.

Surface waves are like ripples in a lake. Lava waves cause rock particles to vibrate in a horizontal plane parallel to the earth's surface, at right angles to the direction of their propagation. And Rayleigh waves, the speed of which is less than that of Lyav waves, arise at the interface between two media and, acting on the particles, make them move vertically and horizontally in a vertical plane oriented in the direction of wave propagation. Surface waves propagate more slowly than bulk waves and decay rather quickly both at the surface and with depth. Waves p, reaching the Earth's surface, can be transmitted into the atmosphere in the form of sound waves at frequencies above 15 Hz. This explains the "terrible hum" sometimes heard by people during earthquakes. Seismic waves caused by earthquakes can be recorded using the so-called. seismographs are instruments based on pendulums that retain their position when the support on which they are located is oscillating. The first seismographs appeared a hundred years ago. In fig. 18.1.2 shows the schematic diagrams of vertical and horizontal seismographs, as well as an example of a seismogram - records of seismic vibrations, on which the first arrivals of the v and S waves are well observed. the appearance of waves on the seismogram and knowing the speed of their propagation, determine the distance to the epicenter of the earthquake (Fig. 18.1.4). Nowadays, many hundreds of seismographs are installed on the earth's bunk, which immediately register any, even a very weak earthquake and its coordinates.

Now there are more than 140 of these stations in Russia, which is 25 times lower than in Germany, and only 15% of these stations are equipped with modern digital seismographs. There are also 9 data collection and processing centers operating in current and urgent processing modes. Information about the current seismic situation is regularly published in seismological bulletins and catalogs. Now the development and re-equipment of seismic networks in Russia with modern equipment is taking place. Determining the depth of an earthquake focal point is a more difficult task, and existing methods are not very accurate.

Intensity of earthquakes. The intensity or strength of earthquakes is characterized both in points (measure of destruction) and the concept of magnitude (released energy). In Russia, a 12-point scale of earthquake intensity is used.

Figure: 18.1.2. Diagram of a horizontal seismograph with mechanical recording of a seismogram with a tip on a smoked drum of the recorder (a): 1 - device frame, 2, 3 - points of attachment of steel wires to the frame, 4, 5 - points of attachment of threads to the seismograph load rod, 6 - seismograph weight, 7 - a smoked drum. Action of a vertical seismograph (b).

The device reacts very weakly to horizontal shocks.According to this scale, the following gradation of intensity or strength of earthquakes is accepted:

1-3 points - weak

4 - 5 points - tangible

6 - 7 points - strong (dilapidated buildings are destroyed)

8 - destructive (solid buildings, factory pipes are partially destroyed)

9 - devastating (most buildings are destroyed)

10 - destroying (almost all buildings, bridges are destroyed, collapses and landslides occur)

11 - catastrophic (all buildings are destroyed, the landscape changes)

12 - disastrous disasters (complete destruction, changes in the terrain over a vast area).

Figure: 18.1.4. Travel time of seismic waves from the epicenter of an earthquake, used to determine the distance from the epicenter to the point of registration of an earthquake

The degree of shaking on the earth's surface, as well as the area covered by it, depends on many reasons, including the nature of the source, its depth, types of rocks, loose sediments or rocky outcrops, water cut, etc.

In order to quantitatively evaluate the measure of the total energy of seismic waves released during an earthquake, the scale of magnitudes (m) according to C.F. Richter, professor at the California Institute of Technology, is widely used.

М \u003d log (a / t) + blg δ + ε

Where a and t are the amplitude and period of oscillations in the wave, δ is the distance from the observation station to the epicenter of the earthquake, and ε are constants depending on the conditions of the observation station location.

This is the magnitude calculated from surface waves, although P-wave and S-wave magnitudes are used. Magnitude 0 means an earthquake with a maximum displacement amplitude of 1 Mkm at an epicentral distance of 100 km. At a magnitude of 5, there is little damage to buildings, while a magnitude of 7 marks a devastating earthquake. The strongest recorded earthquakes had a magnitude of 8.9-9.0. It should be emphasized that deep-focus earthquakes usually do not generate surface seismic waves, therefore there are other magnitude scales, for example, teleseismic for distant (more than 2000 km from the epicenter) earthquakes or the unified magnitude of B. Gutenberg, determined by the amplitude of longitudinal body waves. There are many modifications of the scales, which make it possible to estimate the energy of all earthquakes occurring on the globe and, including all nuclear underground and industrial explosions. In particular, the assessment of the seismic moment -

Mo \u003d μs π d φ

Where μ is the shear strength of rocks in the fault zone, s is the surface area of \u200b\u200bthe fault,

- the average displacement along the fault, allows fairly objective assessment of the magnitude of the earthquake.

Magnitude calculated from the seismic moment: there is a definite relationship between the magnitude (m) and the strength of the earthquake, expressed in points (j0).

Jo \u003d 1.7m - 2.2; m \u003d 0.6jo +1.2

The relationship between the magnitude (m), the intensity of earthquakes in points (j0) and the depth of the source. (n) is expressed by the formula:

Jo \u003d am - blgh + c

where a, b and c are the coefficients determined empirically for each specific area where earthquakes have occurred.

The energy released during earthquakes reaches enormous values \u200b\u200band is expressed by the formula:

Е \u003d π 2ρv (a / t)

where ρ is the density of the upper layers of the earth, v is the speed of seismic waves, a is the displacement amplitude, and t is the period of oscillations. The energy can be calculated using the data read from the seismograms.

Well-known geophysicists, B. Gutenberg, who worked, like C.F. Richter, at the California Institute of Technology, proposed b the equation for the relationship between the Energy of an earthquake and its magnitude on the Richter scale

Lge \u003d 9.9 + 1.9m-0.024m2.

This formula demonstrates a tremendous increase in energy as the magnitude of an earthquake increases. Thus, an increase in the magnitude of an earthquake by one unit causes an increase in energy 32 times, while the amplitude of oscillations of the earth's surface increases only 10 times. If the explosion of a standard atomic bomb of 100 kt emits about 1000 ⋅1018 erg, then all strong earthquakes have much greater energy release, and the strongest earthquake ever recorded released energy comparable to that of a hydrogen bomb (Fig. 18.1.3 ). An increase in magnitude by two corresponds to an increase in energy 1000 times.

Figure: 18.1.3. The ratio of the magnitude of earthquakes and released energy

Lge (erg) \u003d am + b

Where a - 1.5, and b - 11.8

The amount of energy released in a unit of rock volume, for example, in 1 m3 per 1 sec is called the specific seismic power.

In Russian seismology, the energy class K is also used to estimate the magnitude of an earthquake. K equals the decimal logarithm of seismic energy, expressed in joules. So with k \u003d 15 e \u003d 1015 J or 1022 erg.

Between the values \u200b\u200bof m and k, there is a relationship k \u003d 1.8 m + 4.6, established for the southern regions of Russia or for The Far East k \u003d 1.5 m + 4.6.

The intensity of the earthquake at the epicenter of the earthquake and in the pleistoseist region is the higher, the closer to the surface the source is. However, with the distance from the epicenter, in this case, the oscillations quickly decay. During earthquakes at great depths, for example, about 100 km in the vrance zone in the Romanian Carpathians, despite M \u003d 5, vibrations were felt even in Moscow in December 1978. With very strong earthquakes with m \u003d 8, seismic vibrations cover a huge area with a radius of about 1000 km. The area engulfed in destruction grows with magnitude. So, at M \u003d 5 and a source depth of 40 km, the area of \u200b\u200bdestruction will be about 100 km2, and at m \u003d 8 - about 20,000 km2.

Earthquake foci. It has already been said that the overwhelming majority of earthquakes occur in the upper, relatively more fragile part of the earth's crust at depths of 7-30 km. The mechanism of these earthquakes shows that all of them were formed as a result of displacement along faults with an almost obligatory shear component. Because the earthquake source is located at a depth of earth crust, then it is impossible to conduct direct observations in it and monitor, for example, its activation. Therefore, any description of an earthquake source is based on remote observations, on the use of the laws of fracture mechanics, modeling, etc. Possible rupture planes in the source and its dynamic parameters are determined by theoretical calculations. The latter, as a first approximation, make it possible to understand what the mechanism of destruction was. Whether it was stretching or compression, what was the shear component and its orientation (Fig. 18.1.5).

The size of earthquake foci generally increases with increasing magnitude. If the focus is shallow, then the seismogenic rupture can come to the surface, as happened, for example, during the Spitak earthquake. The source is not a plane, but a certain volumetric block of the lithosphere, within which movements are carried out along a number of separate faults, merging into one large seismogenic rupture.

On May 27, 1995, in the north of Sakhalin Island, powerful earthquake s m \u003d 7.7. The epicenter of the earthquake hit the village of Neftegorsk, completely destroyed. Figure: 18.1.5. Focal area of \u200b\u200bthe Tashkent earthquake of 1966 Under the city. Large circles are the place of the main shock, the smaller ones are aftershocks. Arrows - direction of movement. Near the surface of the arrow - the amount of swelling according to the re-leveling data.

In this case, more than 2,000 residents died. On the ms scale, the intensity of the earthquake was 9 points. The source of the earthquake was located near the surface and emerged on it in the form of a system of ruptures with a total length of 40 km. The main rupture is a right-hand reverse fault, with a horizontal displacement of up to 8 m and a vertical displacement of up to 2 m. The general rupture is supported by a number of smaller ones, forming a complex dynamic system, which can be traced to a depth of 15 km. This main seismic rupture turned out to be inherited from the geologically well-known Upper Piltun right lateral reverse fault, which is steeply dipping towards the northwest. When the near-surface structure of this rupture was studied in detail, the horizons of paleosols were revealed, disturbed, according to E.A. Rogozhin, by seismogenic ruptures 1800, 1400 and 1000 years ago, during even stronger earthquakes than the oilfield one. Earthquake foci in the Kuril-Kamchatka active zone with m \u003d 7.9-8.3 have dimensions already in the first hundreds of km, seismogenic movements in them exceed 10 m and, in general, the sources cover a large volume of the lithosphere within the upper part of the sinking ocean plate.

The mechanism of earthquakes. Paleoseismic dislocations. Traces of earthquakes that occurred in the recent geological past - in the Holocene time, i.e. Over the past 10,000 years, it can be found in relief, thanks to special methods developed here in Russia. Strong earthquakes always leave traces, "wounds" on the surface of the earth. When the regions of the last major earthquakes that occurred in 1988 in Spitak and in 1995 in Neftegorsk were studied in detail, traces of past, equally strong earthquakes in the form of tectonic ledges were revealed; displacements of paleosols horizons; cracks crossing various modern relief elements - valleys, ravines, slopes of mountains and hills, watersheds.

Such seismogenic disturbances are usually superimposed on the relief, completely inconsistent with its elements. As a result of earthquakes, large landslides, wasps, mudslides, landslides occur, which are perfectly deciphered in aerial photographs, and large faults and cracks appear in space images. For example, on the mountain slopes of the central part of the greater Caucasus, shallow ditches and ledges are perfectly visible, intersecting these slopes and located, regardless of the peculiarities of the geological structure of the area. Their relative freshness is evidence, apparently, of recent strong earthquakes. Therefore, the study of paleoseismic dislocations has a great practical meaning, since their presence unambiguously testifies to the active seismicity of the area in the recent geological past and, therefore, the area may again undergo a strong earthquake.