Indirect signs of the onset of a strong earthquake. Harbinger of an earthquake. Preventive safety measures

Harbingers of earthquakes

By monitoring changes in various properties of the Earth, seismologists hope to establish a correlation between these changes and the occurrence of earthquakes. Those characteristics of the Earth, the values \u200b\u200bof which regularly change before earthquakes, are called precursors, and the deviations from normal values \u200b\u200bthemselves are called anomalies.

Below we will describe the main (it is believed that there are more than 200) earthquake precursors that are currently being studied.

Seismicity. The position and number of earthquakes of various magnitudes can serve as an important indicator of an impending strong earthquake. For example, a strong earthquake is often preceded by a swarm of weak aftershocks. Detecting and counting earthquakes requires a large number of seismographs and associated data processing devices.

The movements of the earth's crust. Geophysical networks using a triangulation network on the Earth's surface and satellite observations from space can reveal large-scale deformations (shape changes) of the Earth's surface. Extremely accurate surveys are carried out on the surface of the Earth using laser light sources. Repeated surveys require a lot of time and money, so sometimes several years pass between them and changes on the earth's surface will not be noticed in time and accurately dated. Nevertheless, such changes are an important indicator of deformation in the earth's crust.

Subsidence and uplift of sections of the earth's crust. Vertical movements of the Earth's surface can be measured using precise land leveling or tide gauges at sea. Since the tide gauges are set on the ground and record the position of the sea level, they detect long-term changes in the mean water level, which can be interpreted as the rise and fall of the land itself.

The slopes of the earth's surface. To measure the angle of inclination of the earth's surface, an instrument called a tilt meter was designed. Tiltmeters are usually installed near faults at a depth of 1–2 m below the surface of the earth, and their measurements indicate significant changes in slope shortly before weak earthquakes.

Deformations. To measure the deformations of rocks, wells are drilled and strain gauges are installed in them, fixing the value of the relative displacement of two points. The deformation is then determined by dividing the relative offset of the points by the distance between them. These instruments are so sensitive that they measure deformations in the earth's surface due to the earth's tides caused by the gravitational pull of the moon and sun. Earth tides, which are the movement of the earth's crustal masses, similar to sea tides, cause changes in the height of land with an amplitude of up to 20 cm. Crypometers are similar to strain gauges and are used to measure the creep, or slow relative movement of the faults.

Seismic wave velocities. The speed of seismic waves depends on the stress state of the rocks through which the waves propagate. The change in the velocity of longitudinal waves - first its decrease (up to 10%), and then, before the earthquake, - a return to the normal value, is explained by the change in the properties of rocks with the accumulation of stresses.

Geomagnetism. The earth's magnetic field can experience local changes due to deformation of rocks and movement of the earth's crust. Special magnetometers have been developed to measure small variations in the magnetic field. Such changes were observed before earthquakes in most areas where magnetometers were installed.

Earthly electricity. Changes in the electrical resistance of rocks can be associated with an earthquake. Measurements are carried out using electrodes placed in the soil at a distance of several kilometers from each other. In this case, the electrical resistance of the earth layer between them is measured. Experiments carried out by seismologists from the US Geological Survey found some correlation of this parameter with weak earthquakes.

Radon content in groundwater. Radon is a radioactive gas found in groundwater and well water. It is constantly released from the Earth into the atmosphere. The changes in the radon content before the earthquake were first noticed in the Soviet Union, where the ten-year increase in the amount of radon dissolved in the water of deep wells was replaced by a sharp drop before the Tashkent earthquake of 1966 (magnitude 5.3).

Water level in wells and boreholes. The groundwater table before earthquakes often rises or falls, as was the case in Haicheng, China, apparently due to changes in the stress state of rocks. Earthquakes can also directly affect water levels; borehole water can fluctuate during the passage of seismic waves, even if the well is far from the epicenter. The water level in wells located near the epicenter often experiences stable changes: in some wells it becomes higher, in others it is lower.

Change in the temperature regime of the near-surface earth layers. Infrared imaging from space orbit makes it possible to “examine” a kind of thermal blanket of our planet - an invisible thin layer in centimeters thick, created near the earth's surface by its thermal radiation. Nowadays, many factors have accumulated that indicate a change in the temperature regime of the near-surface earth layers during periods of seismic activation.

Changes in the chemical composition of waters and gases. All geodynamically active zones of the Earth are distinguished by significant tectonic fragmentation of the earth's crust, high heat flux, vertical discharge of waters and gases of the most variegated and unstable chemical and isotopic composition in time. This creates the conditions for entering underground

Animal behavior. Over the centuries, the extraordinary behavior of animals before an earthquake has been repeatedly reported, although until recently, reports of this have always appeared after the earthquake, not before it. It is impossible to say whether the described behavior was actually associated with an earthquake, or whether it was just a common occurrence that happens every day somewhere in the vicinity; in addition, the messages mention both those events that seem to have happened a few minutes before the earthquake, and those that happened a few days.

Migration of earthquake precursors

Significant difficulty in determining the location of the source of a future earthquake from observations of precursors is a large area of \u200b\u200bdistribution of the latter: the distances at which precursors are observed are tens of times larger than the size of the rupture in the source. At the same time, short-term precursors are observed at greater distances than long-term ones, which confirms their weaker connection with the focus.

Dilatancy theory

The theory that can explain some of the precursors is based on laboratory experiments with rock samples at very high pressures. Known as the “dilatancy theory,” it was first put forward in the 1960s by W. Brace of the Massachusetts Institute of Technology and developed in 1972 by A.M. Noor from Stanford University. In this theory, dilatancy refers to the increase in rock volume upon deformation. When the earth's crust moves, stresses increase in the rocks and microscopic cracks form. These cracks change the physical properties of rocks, for example, the speed of seismic waves decreases, the volume of the rock increases, the electrical resistance changes (it increases in dry rocks and decreases in wet rocks). Further, as water penetrates into the cracks, they can no longer collapse; consequently, the rocks increase in volume and the Earth's surface can rise. As a result, water spreads throughout the expanding bed, increasing pore pressure in fractures and reducing rock strength. These changes can lead to earthquakes. An earthquake releases accumulated stresses, water is squeezed out of the pores, and many of the previous rock properties are restored.

T. ZIMINA

Earthquake in the city of Kobe (Japan). 1995 year. Building in the downtown area.

Earthquake in the city of Kobe (Japan). 1995 year. A crack in the ground at the ship pier.

Earthquake in San Francisco (USA). 1906 year.

Every year, several hundred thousand earthquakes occur on the globe, and about a hundred of them are destructive, bringing death to people and entire cities. Among the worst earthquakes of the outgoing twentieth century - the earthquake in China in 1920, which killed more than 200 thousand people, and in Japan in 1923, during which more than 100 thousand people died. Scientific and technical progress proved to be powerless in the face of the terrible elements. And more than fifty years later, hundreds of thousands of people continue to die during earthquakes: in 1976, during the Tien Shan earthquake, 250 thousand people died. Then there were terrible earthquakes in Italy, Japan, Iran, the United States (California) and in our territory the former USSR: in 1989 in Spitak and in 1995 in Neftegorsk. More recently, in 1999, the disaster overtook and buried about 100 thousand people under the rubble of their own homes during three terrible earthquakes in Turkey.

Although Russia is not the most earthquake-prone place on Earth, earthquakes in our country can bring a lot of troubles: over the past quarter of a century, 27 significant earthquakes have occurred in Russia, that is, with a force of more than seven on the Richter scale, earthquakes. The situation is partly saved by the low population density of many seismically dangerous regions - Sakhalin, the Kuril Islands, Kamchatka, Altai Territory, Yakutia, the Baikal region, which, however, cannot be said about the Caucasus. Nevertheless, in the zones of possible devastating earthquakes in Russia, a total of 20 million people live.

There is evidence that in the past centuries in the North Caucasus there have been destructive earthquakes with an intensity of seven to eight points. The region of the Kuban Lowland and the lower reaches of the Kuban River is especially seismically active, where in the period from 1799 to 1954 there were eight strong earthquakes of magnitude six to seven. The Sochi zone in the Krasnodar Territory is also active, since it is located at the intersection of two tectonic faults.

The last fifteen years have turned out to be seismically turbulent for our planet. The territory of Russia was no exception: the main seismically dangerous zones - the Far Eastern, Caucasian, Baikal - became more active.

Most of the sources of strong shocks are located near the largest geological structure that crosses the Caucasus region from north to south, in the Transcaucasian transverse uplift. This rise separates the basins of rivers flowing westward into the Black Sea and eastward into the Caspian Sea. Strong earthquakes in this area - Chaldyranskoe 1976, Paravan 1986, Spitak 1988, Racha-Dzhavskoe 1991, Barisakhskoe 1992 - gradually spread from south to north, from the Lesser Caucasus to the Bolshoi and finally reached the southern borders of the Russian Federation.

The northern end of the Transcaucasian transverse uplift is located on the territory of Russia - Stavropol and Krasnodar territories, that is, in the area of \u200b\u200bMineralnye Vody and on the Stavropol arch. Weak earthquakes of magnitude two or three in the Mineralnye Vody region are common. Stronger earthquakes occur here on average once every five years. In the early 90s, fairly strong earthquakes with an intensity of three to four points were recorded in the western part of the Krasnodar Territory - in the Lazarevsky region and in the Black Sea depression. And in November 1991, an earthquake of a similar strength was felt in the city of Tuapse.

Most often, earthquakes occur in areas of rapidly changing topography: in the area of \u200b\u200bthe transition of the island arc to the oceanological trench or in the mountains. However, there are also many earthquakes on the plain. So, for example, on the seismically calm Russian platform, about a thousand weak earthquakes were recorded over the entire observation period, most of of which originated in the areas of oil production in Tatarstan.

Is earthquake forecast possible? Scientists have been looking for the answer to this question for many years. Thousands of stations, densely enveloping the Earth, are watching the breath of our planet, and whole armies of seismologists and geophysicists, armed with instruments and theories, are trying to predict these terrible natural disasters.

The bowels of the earth are never calm. The processes taking place in them cause movements of the earth's crust. Under their influence, the planet's surface is deformed: it rises and falls, stretches and contracts, giant cracks form on it. A dense network of cracks (faults) covers the entire Earth, breaking it into large and small areas - blocks. Along faults, individual blocks can move relative to each other. So, the earth's crust is a heterogeneous material. Deformations in it accumulate gradually, leading to local development of cracks.

To predict an earthquake is possible, you need to know how it occurs. The basis of modern concepts of the origin of an earthquake source are the provisions of fracture mechanics. According to the approach of the founder of this science, Griffiths, at some point the crack loses its stability and begins to avalanche
spread. In a heterogeneous material, before the formation of a large crack, various precursors of this process necessarily appear. At this stage, an increase for some reason in the stresses in the region of the rupture and its length does not lead to a violation of the stability of the system. The intensity of the precursors decreases over time. Instability stage - an avalanche-like crack propagation occurs after a decrease or even complete disappearance of precursors.

If we apply the provisions of fracture mechanics to the process of occurrence of earthquakes, then we can say that an earthquake is an avalanche propagation of a crack in a heterogeneous material - the earth's crust. Therefore, as in the case of material, this process is preceded by its precursors, and immediately before a strong earthquake, they should completely or almost completely disappear. It is this feature that is most often used in predicting an earthquake.

The forecast of earthquakes is also facilitated by the fact that the avalanche-like formation of cracks occurs exclusively on seismogenic faults, where they have repeatedly occurred earlier. So, observations and measurements for the purpose of forecasting are carried out in certain zones according to the developed seismic zoning maps. Such maps contain information on earthquake sources, their intensity, recurrence periods, etc.

Earthquake prediction is usually done in three stages. First, possible seismically hazardous zones are identified for the next 10-15 years, then a medium-term forecast is made - for 1-5 years, and if the probability of an earthquake in a given place is high, then a short-term forecast is carried out.

The long-term forecast is intended to identify seismically hazardous areas for the coming decades. It is based on the study of long-term cyclicity of the seismotectonic process, identification of periods of activation, analysis of seismic calm, migration processes, etc. Today, on the map of the globe, all areas and zones are outlined where, in principle, earthquakes can occur, which means that it is known where it is impossible to build, for example, nuclear power plants and where to build earthquake-resistant houses.

The mid-term forecast is based on the identification of earthquake precursors. More than a hundred types of medium-term precursors have been recorded in the scientific literature, of which about 20 are mentioned most often. As noted above, anomalous phenomena appear before earthquakes: constant weak earthquakes disappear; the deformation of the earth's crust, electrical and magnetic properties of rocks change; the level of groundwater is falling, their temperature is decreasing, and their chemical and gas composition is changing, etc. The difficulty of medium-term forecasting is that these anomalies can manifest themselves not only in the focus zone, and therefore none of the known medium-term precursors can be attributed to universal ...

But it is important for a person to know when and where exactly he is in danger, that is, you need to predict an event in a few days. It is these short-term forecasts that are still the main difficulty for seismologists.

The main sign of an impending earthquake is the disappearance or reduction of medium-term precursors. There are also short-term precursors - changes occurring as a result of the already begun, but still latent development of a large crack. The nature of many types of precursors has not yet been studied, so you just have to analyze the current seismic situation. The analysis includes measuring the spectral composition of oscillations, the typical or abnormal nature of the first arrivals of shear and longitudinal waves, identifying a tendency towards clustering (this is called a swarm of earthquakes), assessing the probability of activation of certain tectonically active structures, etc. Sometimes, preliminary shocks are used as natural indicators of an earthquake - foreshocks. All this data can help predict the time and place of a future earthquake.

According to UNESCO, this strategy has already predicted seven earthquakes in Japan, the United States and China. The most impressive forecast was made in the winter of 1975 in the city of Haicheng in northeastern China. The area was observed for several years, an increase in the number of weak earthquakes made it possible to announce a general alarm on February 4 at 14:00. And at 1936 hours an earthquake of more than seven points occurred, the city was destroyed, but there were practically no victims. This success greatly encouraged scientists, but a number of disappointments followed: the predicted strong earthquakes did not occur. And reproaches fell on seismologists: the announcement of a seismic alarm presupposes the shutdown of many industrial enterprises, including continuous operation, power outages, gas supply interruptions, and evacuation of the population. It is obvious that an incorrect forecast in this case results in serious economic losses.

In Russia, until recently, earthquake forecasting did not find its practical implementation. The first step in organizing seismic monitoring in our country was the establishment at the end of 1996 of the Federal Center for Earthquake Prediction of the Geophysical Service of the Russian Academy of Sciences (FTP RAS). Now the Federal Forecasting Center is included in the global network of similar centers, and its data are used by seismologists around the world. It collects information from seismic stations or integrated observation points located throughout the country in earthquake-prone areas. This information is processed, analyzed and, on its basis, a current earthquake forecast is drawn up, which is weekly transmitted to the Ministry. emergencies, and it, in turn, makes decisions on the conduct of the relevant activities.

The RAS Urgent Reporting Service uses reports from 44 seismic stations in Russia and the CIS. The incoming forecasts were quite accurate. Last year, scientists predicted in advance and correctly the December earthquake in Kamchatka with a magnitude of up to eight within a radius of 150-200 km.

Nevertheless, scientists are forced to admit that the main task of seismology has not yet been solved. One can only talk about the trends in the development of the seismic situation, but rare accurate forecasts give hope that in the near future people will learn to adequately meet one of the most formidable manifestations of the power of nature.

Photo by O. Belokoneva.

Professor of the Tomsk Polytechnic Institute A.A.Vorobyov believes that flares are caused by mechano-electrical processes in rocks during compression and tension.

Every year, several hundred thousand earthquakes occur on the globe, some of which become destructive. But even modern seismologists are practically able to predict exactly when, where and how strong the tremors will be. It is known that animals can anticipate an earthquake and behave very tensely, nervously and try to get out of an unfavorable place as soon as possible. Sometimes, before an earthquake, a rumble is heard from the ground. Scientists believe it is caused by tectonic plate movement. And sometimes mysterious flashes of light can be observed in the sky.

Everyone knows that Japan has suffered and is suffering the most from the elements. It was the Japanese who were the first to begin to analyze various natural phenomena, the precursors of earthquakes. And perhaps they are the first to write in their historical chronicles about the unusual light phenomena that arose just before the movement of the earth under their feet. 373 BC. - one of the first documented evidence of such a strange phenomenon in the Land of the Rising Sun.

For a long time, the phenomenon of light flashes associated with earthquakes was ignored by geophysicists and seismologists, believing that the ruptures of high-voltage lines and flashes of gas bursting in pipes are to blame. Only in recent decades, scientists have become seriously interested in him, since the evidence recorded on video has become much more.

Professor of the Tomsk Polytechnic Institute A.A.Vorobyov believes that the flares are caused by mechano-electrical processes in rocks during compression and tension. If millions of tons of natural minerals are compressed and unclenched, a powerful electric machine will work under the earth's surface, generating high-voltage fields and radio waves. When rocks collapse, we can see intense electrical discharges similar to lightning flashes.

All these phenomena precede an earthquake. And they can be observed a day before it, for hours, but most often minutes before the shock itself. It should be noted that an electric discharge occurs when any rock and even coal seams are destroyed. Perhaps, sometimes the flashes of light captured by the camera are nothing more than explosions in coal mines, when the air-methane mixture is set on fire in the latter by natural electrical processes.

Scientists have also found that a few hours before the onset of the earthquake in the atmosphere at an altitude of about 100 km above the future epicenter, the intensity of the glow of the green line of atomic oxygen increases. In their opinion, the excitation of the upper layers of the atmosphere occurs under the influence of infra-sound waves from the source of the impending earthquake. If the earthquake is large, then infrasonic waves, when propagating upward, can transfer part of their energy to oxygen atoms, making them glow with a wavelength characteristic of this element. Usually the glow is weak and hardly noticeable. But with a sharp increase in the concentration of such particles, flashes of light can be observed with the naked eye at night. Light can pulsate, have a different shade and move across the sky.

Each strong earthquake leads to a partial unloading of the stresses accumulated in this place of the seismically active region. In this case, the stresses in absolute value decrease in the area of \u200b\u200bthe earthquake source by only 50–100 kg / cm 2, which is only the first percent of those existing in the earth's crust. However, this is enough for the next strong earthquake in a given place to occur after a fairly significant period of time, calculated in tens and hundreds of years, since the rate of stress accumulation does not exceed 1 kg / cm 2 per year. The energy of the earthquake is drawn from the volume of rocks surrounding the source. Since the maximum elastic energy that a rock can accumulate before destruction is defined as 10 3 erg / cm 3, there is a directly proportional relationship between the energy of an earthquake and the volume of rocks that give up their elastic energy during an earthquake. Naturally, the time interval between successive strong earthquakes will increase with increasing energy (magnitude) of the earthquake. We thus arrive at the concept seismic cycle.

Based on the analysis of the seismicity of the Kuril-Kamchatka arc, it is substantiated that earthquakes of magnitude M\u003d 7.75 are repeated on average after 140 ± 60 years. Seismic cycle duration Tdepends on the energy of the earthquake E:

It is essential for the prediction of earthquakes that the seismic cycle breaks down into 4 main stages. The earthquake itself lasts for several minutes and constitutes stage I. Then comes stage II of aftershocks that gradually decrease in frequency and energy. For strong earthquakes, it lasts for several years and takes about 10% of the seismic cycle. During the stage of aftershocks, gradual unloading of the focal area continues. Then comes a long stage of seismic dormancy, which takes up to 80% of the entire time of the seismic cycle. During this stage, there is a gradual restoration of stress. After they again approach the critical level, seismicity revives and increases until the next earthquake. Stage IV of seismicity activation takes about 10% of the seismic cycle. Most earthquake precursors occur at stage IV.

Seismological precursors... The concept seismic gaps presented in its modern form by S.A. Fedotov. He found that the aftershock areas of earthquakes do not overlap. At the same time, the next strong earthquakes tend to be located between the sources of the already occurred. On this basis, a method was constructed for the long-term forecast of the locations of the next earthquakes, taking into account the stage of the seismic cycle and the rate of energy accumulation in the seismically active zone.

A seismic gap should be understood as the long-term absence of strong earthquakes in the area of \u200b\u200ba seismically active fault between the foci of already occurring earthquakes. The term "long-term" means tens and even hundreds of years. There are increased stresses between the ends of the ruptures from the sources of earlier earthquakes, which increase the likelihood of the next seismic event in this place. The difficulty of using this precursor is that, given the very short history of earthquake registration, firstly, it is difficult to identify places where earthquakes have already occurred in the distant past, and secondly, in practice, it turns out that a significant number of gaps are found in seismically active regions. and not in all it is possible to establish the stage of the seismic cycle. Some may turn out to be non-seismic areas as a result of tectonic features or due to unfavorably oriented stress state.

In contrast to the seismic gap, which has existed in the seismically active area for many years, sometimes in the III stage of the seismic cycle, against the background of the increasing intensification of seismicity, a relatively short-term seismic lull... A detailed analysis of this situation allows us to propose the following basic rules for detecting seismic calm:

evaluation of the homogeneity of the seismic catalog;

determination of the minimum magnitude recorded without gaps;

elimination of groups and aftershocks;

quantitative assessment of the magnitude and significance of the anomaly;

quantifying the onset of the anomaly;

estimation of the size of the anomalous area.

In the case of an extended and fairly uniform in strength seismically active fault, the transfer of stresses to the edge of the fault from an earthquake that has occurred can contribute to the formation of a sequence of subsequent earthquakes in a chain along the fault. An analogy with a gradual abrupt lengthening of a crack is appropriate here. A more general reason seismic migrationthere may be deformation waves propagating along seismogenic belts. A possible source of the deformation wave is the strongest earthquake of the past. A change in the deformation field can contribute to the initiation of earthquakes in those places where significant tectonic stresses have accumulated. Deformation waves can cause migration effects of strong earthquakes found in Central Asia and the Caucasus. Consider a sequence of earthquakes with M \u003e 6 on a 700-kilometer section of the Caucasian branch of the North Anatolian fault. The beginning of the migration of earthquakes, apparently, was the Erzurum earthquake of 1939, M\u003d 8. The migration process spread in a northeastern direction at an average speed of 12 km / year. In 1988 and 1991. in accordance with this trend, destructive earthquakes occurred in Armenia (Spitak) and Georgia (Rachinsky). The phenomenon of migration is successfully used for long-term forecasting. It was in this way that the Alai earthquake in Kyrgyzstan on November 1, 1978 was predicted.

The occurrence of earthquakes is quite common. Royrefers to a group of earthquakes that slightly differ in magnitude, the probability of occurrence of which in a certain spatial cell for a fixed time interval significantly exceeds the probability following from the law of random distribution. Poisson's law is adopted as the latter. To distinguish a swarm from a sequence of aftershocks of a strong earthquake, the following rule is adopted: if in a group of earthquakes the magnitude of the main shock M r exceeds the magnitude of the next strongest M r –1 by a small amount ( M r - M r –1 = 0.3), then this group can be identified as a swarm and one should expect a main earthquake with a magnitude twice M r .

The distance between adjacent seismic events in a group is determined by the interaction of the stress fields of their sources. Group of Nor more earthquakes computed in a space-time window T–R, the boundaries of which (in time and distance) are set as follows:

T(K) = a·ten bK ; (2.12)

R(K) = c· L . (2.13)

where K – the energy class of the earthquake, relative to which the parameters of the space-time window are determined when the grouping events are found; L- the length of the rupture in the source of an earthquake of a given energy class, which is found by relation (2.7); a, b- empirical parameters of the model, value from\u003d 3, which corresponds to the zone of influence of stresses of each rupture on neighboring ones and to the value of the concentration criterion of fracture of solids considered below.

Predictive parameter of seismogenic fracture density,which is an analogue of the concentration criterion of destruction during the transition to the scale of a seismically active region, is based on the application of the kinetic theory of the strength of solids to rocks. It is believed that an earthquake occurs after a critical concentration of smaller ruptures has accumulated in its source area. To construct maps of the seismogenic fracture density parameter K cf the seismically active zone is divided into overlapping elementary volumes V,in each of which the values \u200b\u200bare calculated K sr for the time interval Δ T j increasing with some step Δ t, according to the formula:

, (2.14)

where N- the number of earthquakes per unit volume; LIs the average length of ruptures of these earthquakes, calculated as

. (2.15)

The length of the rupture in the focus i-th earthquake is calculated by the formula (2.7).

It follows from (2.14) that K avg after the start of counting has high values, gradually decreasing as a strong earthquake approaches. For different seismically active regions of the world, before strong earthquakes, so many breaks of previous sizes accumulate in their foci that the average distance between adjacent breaks is equal to three times their average length. In these cases, an avalanche-like combination of accumulated ruptures occurs, leading to the formation of a main (main) rupture, causing a strong earthquake. The model of avalanche-unstable cracking (LNT) is based on two phenomena: the interaction of stress fields of cracks and the localization of the cracking process. It is natural to expect the manifestation localization of the seismic processbefore strong earthquakes. It can be found by calculating accumulation maps of the number of seismic events, energy, or fracture surfaces over successive time intervals.

The appearance of foreshocks marks the end of stage III of the seismic cycle and indicates the completion of the process of seismicity localization. In this sense, foreshocks are of great interest, since they can be considered as a short-term precursor of an earthquake, indicating the exact location of the hypocenter. However, no reliable criteria have yet been found for detecting foreshocks against the background of seismic events. Therefore, foreshocks are identified, as a rule, after an earthquake has occurred, when the position of the source is known. In rare cases, before the main shock, there are such powerful series of foreshocks that they are highly likely to indicate a possible strong earthquake and are used for forecasting. The most significant case of this kind took place before the Haicheng earthquake. M \u003d7.3 (China) February 4, 1975

In seismological practice, foreshocks include events that occurred in a few seconds, minutes, hours, and, in extreme cases, days in the source area of \u200b\u200ba strong earthquake. However, foreshocks can also be called events that happened in the source area earlier, but with a high degree of probability indicate the preparation process in this place of a strong earthquake. Such foreshocks may include phenomena that have been studied in detail and called distant aftershocks. This kind of seismic events was given the following definition.

Let A- strong earthquake with a magnitude M> M a , after which aftershocks take place;

IN- earthquake in a smaller range of magnitudes ( M b <M<M c), what happened for some time T a b after the earthquake Aat a distance no more D a b from him;

FROM- impending strong earthquake ( M> M c). Earthquakes INand FROMare located outside the area of \u200b\u200bordinary earthquake aftershocks A.The hypothesis about distant aftershocks is that the earthquake INoccurs in the vicinity of the impending earthquake FROMnot by chance.

To identify non-accidental occurrence of an event INin a seismically active area, it is important to set a short period of time T a b and moderate distance D a b , unlikely to occur INin a given space-time window in comparison with the law of random distribution. Relatively weak earthquakes, indicating the place of a stronger future, occur not only immediately after the previous strong earthquake, but also in a short time interval before it. They are called induced foreshocks and can occur at distances of several hundred kilometers from the initiating strong earthquake. This fact suggests that during the preparation of a strong earthquake, a significant volume of the earth's crust in the seismically active region is activated. The phenomena of distant aftershocks and induced foreshocks are explained by the high sensitivity to external influences of the rock under conditions close to loss of stability.

Geophysical, hydrogeodynamic and geochemical precursors... From consideration of the models of earthquake preparation (dilant-diffusion model (DD), avalanche-unstable cracking (LNT), unstable sliding model, consolidation model) it follows that the stages of origin and development of the source should be accompanied by inelastic deformations of rocks. At the same time, the greatest changes in the deformation field of the earth's crust should be expected in the softest areas represented by fault zones. In this regard, we consider the hypothesis of the occurrence deformation anomalies... In the seismically active region of the Kopetdag and the seismically calm Pripyat trough, which are characterized by thick sedimentary mantles, local anomalies of vertical movements with a width of about 1–2 km were revealed, which form in 10 –1 –10 years with a high-gradient character of movements (10–20 mm / km year ).

Generalization of the observation results led to the conclusion about three main types of local anomalies:

1. The most pronounced γ-type anomalies are represented by the sinking of benchmarks in the zones of tectonic faults under conditions of subhorizontal extension.

2. During sub-horizontal compression, β-type anomalies are recorded, representing surface uplift on a larger base compared to γ-type anomalies (regional bend).

3. The anomaly has S-shaped (step-like) shape. All of them develop against the background of a slower quasi-static slope of the surface with a change in regional stresses.

Let us consider an example of γ-type anomalies in Kamchatka along a 2.6 km leveling profile that crosses the fault zone. The profile includes 28 pickets. In the interval 1989-1992 it was used for repeated observations with a frequency of 1 time per week. Vertical displacements of the earth's surface with an amplitude of several centimeters were detected with a measurement accuracy of 0.1 mm. The width of the anomalies ranged from 200 to 500 m. They were not found in the part of the profile that was outside the fault zone. The results of measurements in successive time intervals showed that they reflect the pulsating nature of the magnitude of the anomalies. An increase in the amplitude of anomalies was revealed before earthquakes occurring at a distance of up to 200 km from the observation profile. However, local anomalies do not occur over all faults. In addition, in separate time intervals, they stop developing, turning from kinematic to static. Hence, it follows that for the appearance of local anomalies, it is necessary to fulfill certain conditions for changing the regional stress field and the properties of the material (parameters) of the fault zones within which they arise. In this regard, it is appropriate to call such anomalies parametric. A γ-type anomaly can arise, for example, due to a change in the regional stress field and subsidence of rocks in the fault zone. However, subsidence can also take place at a constant regional stress due to changes in the properties of the fault, for example, due to variations in pore pressure. The relative deformation of rocks in the zone of the γ-type anomaly can reach values \u200b\u200bof 10 –5 1 / year, which is consistent with field observations.

Geomagnetic harbingers Since ancient times, earthquakes have received great attention, since due to the existence of the piezomagnetic effect and the presence of magnetic minerals in rocks, changes in the stress state should be reflected in variations in the geomagnetic field. There are two points of view on the nature of geomagnetic precursors. One connects them with electrokinetic phenomena, the second - with piezomagnetism. Similar geomagnetic observations were carried out in the area of \u200b\u200bAshgabat with a certain layout of benchmarks. The estimated rms measurement error did not exceed 0.5 nT. Variations of changes in the total vector of the geomagnetic field are determined T along three profiles before the earthquake on September 7, 1978 with a magnitude of 4.4. It was determined that anomalous changes in the bay-like shape of up to 6 nT appeared 6–8 months before the seismic shock on all benchmarks along the profiles along the fault zones. At the same time, the amplitude of the anomalies decreased as the picket moved away from the fault. Time of development of anomalies Tcoincided with the variation in the slope of the earth's surface recorded by a tilt meter installed in a pit near one of the benchmarks. This gives great confidence to attribute geomagnetic variations to tectonic origin. Calculations and comparison with measurements of telluric currents led to the conclusion that the anomalies are caused by the electrokinetic effect of the filtration flow of groundwater varying in power. The greatest changes in the latter occurred in the fault zones.

Geomagnetic precursors of piezomagnetic nature were identified in the Baikal region, and their physical nature was confirmed by quantitative calculations. It was also found that variations in mechanical stresses in rocks of 0.01 MPa due to seasonal fluctuations in the level of Lake Baikal lead to changes in the magnetic field recorded in the coastal zone. T1 nT.

After the first work on the use of DC dipole sounding at the Garm test site and precursors of electrical resistance, work in this direction was actively carried out at the Garm test site, as well as in Kyrgyzstan and Turkmenistan. Deep electrical studies are carried out by the methods of frequency sensing (FS) and sounding by formation (SZ).

The first systematic work to detect electrotelluric precursors (ETP) were carried out in the early 60s. in Kamchatka. Their peculiarity was synchronous registration at several stations, and at each station a number of measuring lines and non-polarizable electrodes were used to exclude near-electrode processes. It was found that anomalous changes in the potential difference are recorded before the earthquakes in Kamchatka, which are not correlated with variations in the geomagnetic field and meteorological factors. Work in the Garm region and in the Caucasus confirmed the main features of this type of anomaly: a bay-like change Ein the first tens of millivolts, regardless of the length of the measuring line and a large "long-range" (up to several hundred kilometers from the epicenter of the earthquake). In addition, it is shown that ETP anomalies are associated with faults in the earth's crust and are "parametric", ie, associated with changes in the electrokinetic and electrochemical properties of rocks in the fault zone under the influence of a slowly varying stress field.

When searching electromagnetic precursorsin the radio wave range, the counting rate of electromagnetic pulses (EMP) was recorded. During the work, a set of frequencies was used, but the most interesting results were obtained in the 81 kHz range. There are known count rate anomalies before three earthquakes in Japan. The epicentral distances were the first hundreds of kilometers, which ensured the registration of EMP by the reflected beam, if we assume that the signal appeared in the epicentral region. The envelope level of the count rate began to increase 0.5–1.5 h before the seismic shock and dropped sharply to the initial level immediately after the earthquake. It turned out that in the epicentral area of \u200b\u200ban earthquake, both an increase and a decrease in the EMP activity before the earthquake can be noted. So, for example, when 2 days before the earthquake in the Carpathians on March 4, 1977 from M\u003d 7 and a focal depth of 120 km, a gradual increase in the number of signals to the receiving station in the azimuth, which indicated the epicenter, was noted. The presence of the remote station made it possible to conclude that this increase is caused by the better transmission of signals from distant thunderstorms over the epicentral region. Note that in addition to the general increase in the number of signals, there is an increase in the swing in the diurnal cycle. Further studies showed that before the Alai earthquake on November 1, 1978, from M\u003d 7 and the Spitak earthquake on December 7, 1988 with M\u003d 6.9, on the contrary, fading of the signal transmission over the epicentral regions was noted. All this led to the conclusion that precursors in electromagnetic pulses can be a reflection of the changed geoelectric conditions over the epicenter of the impending earthquake, for example, due to anomalous ionization of the atmosphere.

The largest number of recorded reliable precursors of earthquakes, with the exception of seismic ones, are related to measurements of the groundwater level. This is due to two reasons. First, a well and even a well are sensitive volumetric strain gauges and directly reflect changes in the stress-strain state in the ground. Second, only hydrogeology has accumulated long observation series on an extensive network of wells and wells. Despite the variety of forms of manifestation hydrogeodynamic precursor, in the epicentral area of \u200b\u200bthe impending earthquake, the following sequence is more often observed: several years before a strong earthquake, a gradually accelerating drop in the level is observed, followed by a sharp rise in the last days or hours before the shock. This type is also manifested in the flow rate of sources or self-flowing wells. Usually, the magnitude of anomalous changes in the level of groundwater in wells before an earthquake is several centimeters, but unique cases of high-amplitude anomalies were also noted.

During the two Gazli earthquakes of 1976 with magnitudes 7 and 7.3, an anomaly of 15.6 m was recorded, and the well was located at a distance of 530 km from the earthquake sources. One of the possible explanations for this phenomenon was given. Let the observation well penetrate two or more aquifers or fracture systems. If they are separated by weakly permeable layers of rocks, then the piezometric levels Hand water conductivity Tsuch horizons will differ among themselves. For a system of two horizons, the water level in the well will be determined by the ratio

. (2.16)

If, in the process of tectonic deformation, the contact of the well with one of the horizons is broken, or, conversely, a previously isolated horizon opens, this can lead to an abrupt change in the water level in the well. This mechanism is a specific manifestation of a more general law that describes the nonlinearity of the system when the percolation threshold is reached.

Let us dwell on the spatial features of hydrogeodynamic (GHD) precursors. Based on measurements of the water level, a number of coefficients are calculated, the most important of which is the change in volumetric deformation of rocks. An analysis of the maps of the GHD - the fields of the Caucasus during the Spitak earthquake showed that, starting from August 1988, there was a tendency for the development of the extension structure in the area of \u200b\u200bthe future earthquake. The development of the Spitak structure proceeded towards an increase in its size with a simultaneous increase in the intensity of deformations. By December 1, 1988, the structure had grown in such a way that its elongated axis reached 400 km, and its width was about 150 km. The center of the structure, characterized by a drop in the water level in the wells, was located in the epicentral zone of the future earthquake. The maximum intensity of the anomaly and the dimensions of the extension structure was observed 11 h before the earthquake. The anomaly began to decrease 40 min before the shock.

Geochemical precursors indicate an abnormal increase in radon content in thermomineral water of deep origin (before the Tashkent earthquake on April 25, 1966, M \u003d 5.1). The high probability of the anomaly's connection with the earthquake was evidenced by the rapid return of the radon content to the normal level after the shock. The longest time series of observations on the well system were obtained at the Tashkent prognostic range. This made it possible to identify prognostic levels for a number of parameters and, in combination with geophysical methods, contributed to the issuance of a short-term forecast for the Alai earthquake of November 1, 1978 with a magnitude of 7. One of the obstacles to the use of geochemical methods for predicting earthquakes is the unidentified effective sensitivity to the deformation field and the size of the area, responsible for the observed variations. Geochemical forecasting methods can be applied as complementary to others, primarily hydrogeodynamic and deformation methods.

Many earthquakes, especially large ones, were preceded by some phenomena not typical for this area. As a result of the systematization of data on large earthquakes of the 17th - 21st centuries, as well as on the annals in which events related to earthquakes are mentioned, a number of typical phenomena were established that can serve as operational precursors of earthquakes. Since earthquakes have different mechanisms of occurrence, occur in different geological conditions, at different times of the day and year, the accompanying phenomena that serve as precursors can also be different.

As of the beginning of the 2010s, almost all forerunner phenomena have a scientific explanation. Nevertheless, it is extremely rare to use them for prompt warning, since the precursor phenomena are not specific for earthquakes. For example, atmospheric light phenomena in the atmosphere can occur during periods of geomagnetic storms or have a man-made nature, and disturbance of animals can be caused by an impending cyclone.

Currently, the following phenomena are distinguished that can serve as harbingers of earthquakes: foreshocks, abnormal atmospheric phenomena, changes in the level of groundwater, restless behavior of animals.

Main article: Foreshock

Foreshocks are moderate earthquakes that precede a strong one. High foreshock activity in combination with other phenomena can serve as an operational precursor. So, for example, the Chinese Seismological Bureau, on this basis, began the evacuation of a million people the day before the strong earthquake in 1975.

Although half of the major earthquakes are preceded by foreshocks, only 5-10% of the total number of earthquakes are foreshocks. This often generates false alerts.

Optical phenomena in the atmosphere

It has been noticed for a long time that many large earthquakes are preceded by optical phenomena in the atmosphere unusual for a given area: flashes similar to auroras, light columns, clouds of strange shape. They appear as if immediately before the aftershocks, but sometimes they can occur in several days. Since these phenomena are usually noticed by chance by people who do not have special training, who cannot give an objective description before the mass appearance of mobile photo and video devices, the analysis of such information is very difficult. Only in the last decade, with the development of satellite monitoring of the atmosphere, mobile photography and car DVRs, unusual optical phenomena before the earthquake were reliably recorded, in particular before the Sichuan earthquake.

According to modern concepts, unusual optical phenomena in the atmosphere are associated with such processes in the zone of a future earthquake as:

The release of gases into the atmosphere from vapors from stressed rocks. The type and nature of the phenomena depend on the outgoing gases: combustible methane and hydrogen sulfide can give flame torches, which was observed, for example, before the Crimean earthquakes, radon, under the influence of its own radioactivity, fluoresces with blue light and causes fluorescence of other atmospheric gases, sulfur compounds can cause chemiluminescence.

Electrification of strained rocks, which causes electrical discharges on the earth's surface and in the atmosphere in the area of \u200b\u200bthe future focus.

Change in groundwater level

It was established after the fact that many large earthquakes were preceded by anomalous changes in the level of groundwater, both in wells and wells, as well as in springs and springs. In particular, before the Chuya earthquake, in some places on the soil surface, springs suddenly appeared from which water began to flow quickly enough. However, a significant proportion of earthquakes did not cause prior changes in aquifers.

Restless animal behavior

It has been reliably evidenced that the main shocks of many strong earthquakes are preceded by inexplicable disturbance of animals over a large area. This was observed, for example, during the Crimean earthquakes of 1927, before the Ashgabat earthquake. But, for example, before the Spitak earthquake and the earthquake in Neftegorsk, no mass anomalous animal behavior was observed.