. According to this, it can be divided into tectonic earthquakes, volcanic earthquakes, and impact earthquakes. Human activities can also cause earthquakes, called induced earthquakes, such as reservoir earthquakes.
1. Tectonic earthquake
Tectonic earthquake is an earthquake caused by tectonic changes, especially fault activities. The vast majority of earthquakes around the world are tectonic earthquakes, accounting for about 90% of the total earthquakes. Most of them are shallow-source earthquakes, with a wide range of impact, and are very severely damaged to the ground and buildings, which often cause major losses to life and property.
Most of the strong earthquakes in my country are shallow-source tectonic earthquakes, and more than 80% of them are related to fault activities. For example, the Yunnan Tonghai earthquake on January 5, 1970 was caused by the re-acting of the Qujiang fault. The Ganzi and Luhuo earthquake in Sichuan in February 1973 (magnitude 7.9) was caused by the re-acting of the Xianshui River fault. After the earthquake, a ground crack that was heading towards NW310° and was more than 100 km long was formed on the ground.
Many famous earthquakes in the world also belong to tectonic earthquakes. The 1906 San Francisco earthquake (magnitude 8.3) in the United States was related to the San Andres fault activity. The 1923 Kanto earthquake in Japan (magnitude 8.3) was related to the NW-SE direction fault activity passing through Sagami Bay . A series of strong earthquakes occurred in Chile from May 21 to June 22, 1960 (three earthquakes with a magnitude 8 or above, and more than 10 earthquakes with a magnitude 7 or above), all of which occurred in the Peru trench fault zone that is 1,400 km from north to south.
(I) The cause and source mechanism of tectonic earthquakes
This issue is the most core issue in earthquake forecasting theory, and it is also a problem that is still being discussed and needs to be solved.
In the earth's crust and upper mantle, due to the continuous movement of matter, a huge force that squeezes and pushes rocks often occurs, namely ground stress . Under the action of ground stress, rocks accumulate a large amount of strain energy; once this energy exceeds the limit value that rocks can withstand, it will cause the rock to suddenly break in an instant, releasing a large amount of energy, and some of them propagate in the form of elastic waves ( seismic wave ). When the local seismic waves reach the ground, the ground will vibrate, which is an earthquake.
Judging from the earthquakes that have occurred, its occurrence is closely related to the existing active structures (especially live faults), and the epicenters of many strong earthquakes are distributed in the active fault zone. If we look at it globally, the distribution of seismic zones is closely related to plate boundaries. These boundaries are actually some fracture structures that are tensile, extruded or horizontally staggered.
How does rupture activity produce earthquakes with great energy and how it is active? There are currently several related hypotheses.
1. Elastic jump theory
is the earliest and most widely used hypothesis about the cause of earthquakes. It is a hypothesis proposed based on the discovery of the San Andres fault in the 1906 San Francisco earthquake in the United States. The hypothesis believes that the occurrence of earthquakes is due to the fault of rocks in the earth's crust, and the rock itself is elastic. When the fault occurs, the rocks that have already undergone elastic deformation have jumped back in the opposite direction after the force disappears, and return to the state before deformation. This bounce can generate amazing speed and power, and releases the energy accumulated over a long time in an instant, causing an earthquake. In short, seismic waves are caused by the overall elastic jump of the rocks on both sides of the fault surface, and come from the fault surface. As shown in Figure 8-3, the rock formation undergoes elastic deformation (B), the force exceeds the elastic strength of the rock, and a fault occurs (C). Then the two plates of rocks in the fault bounce back as a whole and return to their original state, so an earthquake occurs. This hypothesis can better explain the causes of shallow-source earthquakes, but it is difficult to explain medium- and deep-source earthquakes. Because in a very deep underground, the rock is already plastic and it is impossible to have elastic rebound.
2. Peristalticism is also called submersible movement and submersible movement. The surface soil and rock layer can move slowly downward under the action of gravity. There is no obvious interface between its moving body and the base, and the deformation and movement are both transitional relationships. This deformation and movement are called peristalsis. The peristalsis rate is only a few millimeters to several centimeters per year.
People found that buildings and active faults themselves on active faults also had this kind of peristalsis in the absence of earthquakes, that is, relatively slow and stable sliding. For example, there is an Anatoria active fault zone 110km north of Ankara, Turkey. The walls of buildings located on this fault zone were found to have faults, and the peristalsis amount was about 2cm per year. Some people also observed faults after earthquakes in , the Middle East, area, and found that some areas were accompanied by earthquake-free peristalsis, and the peristalsis amount was about 1cm per year. Under what circumstances
is prone to creeping, it is not clear. Some experiments show that under high pressure and low temperature, high porosity (water-containing), and containing weak minerals such as dolomite , calcite , serpentine , serpentine and other rocks, stable peristalsis is prone to occur. Some people also believe that peristalsis is prone to occur at higher confining pressures or higher temperatures.
There is a phenomenon gradually proved by facts that in the rock formations, areas with long-term peristaltic movement or areas with relatively high percentage of long-term activity in active faults, due to the gradual release of energy through slow peristaltic movement, severe earthquakes rarely occur. There is a large-scale shear fault in the Alvin Mountains region of my country, which is an active fault. Through satellite image analysis, it is found that there is a peristalsis phenomenon. Modern water systems are cut through, with obvious displacement and large distances. However, there are few earthquake records in history. It is speculated that the activity mode of this fault is mainly based on non-seismic peristalsis.
According to data on the relationship between peristalsis and earthquake size, when peristalsis accounts for more than 50% of long-term activity, the maximum earthquake can only be magnitude 5, while when peristalsis accounts for less than 10% of long-term activity, major earthquakes of magnitude 8 or above may occur.
3. Slimming
is in deeper underground areas. If the rocks on both sides of the fault slide, they must overcome strong friction. Therefore, under normal circumstances, the two plates of rocks seem to stick to each other and no one can move. However, when the stress accumulates to equal to or greater than the friction force, the two plates of rocks will suddenly slide. Through sudden sliding, energy is released and the two disks are stuck until the energy accumulates to a certain extent, causing the next sudden sliding. Experiments have shown that the destruction form of an object under high pressure is alternately bonded and sliding along the fracture surface, and the cross-section undergoes intermittent rapid jumping and sliding. After multiple stress drops, the accumulated strain energy is released. This statement is called sticky and slip.
has many factors that affect the movement of faults: First, the temperature is when the temperature is lower than 500℃, and the rock mass on both sides of the fault surface is prone to stick and slippage; the temperature is higher than 500℃, it is prone to peristalsis and creep. The second is the rock composition, which is brittle and hard in lithology (such as quartzite , quartz sandstone , etc.), and the rocks on both sides of the fault are often mainly sticky and slippery; the lithology is soft, and it is mainly creeping. Third, the porosity and moisture content of rocks. If the rock has large pores, high porosity, and contains a lot of moisture, it is of course easy to peristalsis; on the contrary, rocks have small porosity, low porosity, and less moisture, so they are mostly sticky and slippery. In addition, the magnitude of the confining pressure will also affect the way the fault moves. If the faults are continuously sticky and slipped, it is a period of frequent earthquakes.
In fact, the same active fault can have different ways of activity at different depths, and the same fault can also have different ways of activity at different periods. For example, in the San Andres fault, the depth of more than 4km is a stable peristalsis without shock; 4-12km is a stick-slip movement accompanied by earthquakes; below 12km (due to high temperatures) is mainly stable peristalsis. Therefore, the depth of the seismic focal level in the San Andres fault zone shall not exceed 20km.
4. Phase Transformation says
Some people believe that deep earthquakes are caused by the phase transition process of deep matter. Under high temperature and high pressure conditions, underground substances cause the structure of the rock to suddenly change, causing the rock volume to suddenly shrink or expand, forming an explosive vibration source, and thus an earthquake occurs. This statement failed to provide specific arguments from multiple aspects, and therefore failed to gain widespread popularity. In recent years, according to the analysis of the propagation of longitudinal earthquake waves in deep underground, faults and staggering occurred in the deep earthquake, proving that earthquakes occur and fault activities are related.At the same time, plate tectonic theory points out that when lithosphere plates submerge underground, middle and deep earthquakes occur inside plates that subside to the mantle, rather than in mantle asthroid material, so the phase change theory naturally loses its basis for existence.
(II) Characteristics of tectonic earthquakes
The characteristics of tectonic earthquakes are frequent activities, long duration, wide range of impacts, and strong destructiveness.
1. Earthquake sequence Any earthquake occurs through a long-term birth process, that is, stress accumulation process, which can last for more than ten years, decades or even hundreds of years.
However, within a certain period of time (several days, weeks, and years), a series of earthquakes with large and small causal connections can occur on the same geological tectonic belt or in the same source. Such a series of earthquakes is called earthquake sequences. In an earthquake sequence, if an earthquake is particularly large, it is called main shock ; a series of weak or smaller earthquakes often occur before the main shock, which is called front shock ; a series of earthquakes smaller than the main shock often occur after the main shock, which is called aftershocks. One of the important characteristics of
structural earthquakes is that they often occur in sequence. This feature may be related to the process of tectonic earthquakes. Generally speaking, when the local stress is about to strengthen beyond the strength that the rock bears, the rock formation first produces a series of small staggering movements (or alternating processes starting along the fault zone), thus forming many small shocks, namely foreshifts. Then the ground stress continues to increase. When the rock formation cannot bear it, it will cause the overall sliding of the rock formation or the new fault sliding, forming a large earthquake, that is, the main earthquake. After the main earthquake occurs, the equilibrium state between the rock formations still needs to be carried out for a period of time to release the remaining energy in the rock formations, causing some small aftershocks. At earthquake sites, many magnitude 1 cracks appear on the broken ground, mixed in between, indicating that the movement has not stopped completely until many undamaged sites are completely destroyed, and all the remaining strain energy is released. This situation is similar to the process of pressing the spring. When the force disappears, the stored energy is converted into kinetic energy and rebounds back and restores to its original state, but it is difficult to recover at once. It still needs to be slowly trembling and adjusting for a period of time to restore the original balance position. This phenomenon is called the spring effect. Rocks are also elastic, so they should also have this elastic effect. The Haiyuan earthquake in Ningxia (formerly Gansu) in 1920, and the aftershock did not disappear for three years. Its intensity and frequency are sometimes high and sometimes low, but the overall trend is to gradually decay until it calms down.
2. Earthquake sequence type
Although tectonic earthquakes often have a certain sequence, their energy release patterns, activity time and proportion of earthquakes of size and size are often different. According to the analysis and research on strong earthquakes that have occurred in my country since October 1949, the seismic sequence can be summarized into three types:
(1) Single-hair earthquake
is also called isolated earthquake. The pre-shocks and aftershocks of this kind of earthquake are few and weak, and are very different from the main shock magnitude . Almost all the seismic energy of the entire sequence is released through the main shock. There are few such earthquakes. No foreshocks and aftershocks were observed in the Dingyuan earthquake in Anhui in the autumn of 1966 and the Linyi earthquake in Shandong in March 1967. The magnitude is very small, with only magnitude 4-4.5.
(2) Main earthquake type earthquake
is the most common type. The main earthquake magnitude is particularly prominent and the energy released accounts for about 90% of the entire series; the foreshocks are either there or not, but there are many aftershocks. On February 4, 1975, the Liaoning Haicheng earthquake (magnitude 7.3), more than 500 foreshocks occurred within 24 hours before the earthquake, and many aftershocks occurred after the main earthquake. On July 28, 1976, the Tangshan earthquake (magnitude 7.8) was basically no foreshock, but aftershocks continued for several consecutive years.
(3) seismic group earthquake
consists of many earthquakes with similar magnitudes, without prominent main earthquakes. This type of earthquake has many foremost and aftershocks and is large, often appear in groups, has a long activity time, is slow attenuation speed, and has a large range of activity. For example, from February 28 to March 22, the magnitude of the Xingtai earthquake gradually rose from 3.6, 4.6, 5.3, 6.8, and 6.8 to 7.2, and a major earthquake occurred. Sometimes this type of earthquake is formed by combining or confusing two main seismic earthquakes.
sometimes has a complex earthquake sequence, as if it is composed of several single hair, main shock type, and group shock type. For example, the Mabian earthquake in Sichuan Province from August to September 1971.
seismic sequence type may be related to the uniformity and complexity of rocks and structures. According to experiments, when the medium is uniform and the stress in the medium is not concentrated, there is no small rupture before the main rupture, and there is very little rupture after the main rupture; when the medium is uneven and the stress is partially concentrated or highly concentrated, certain or many small ruptures will occur before and after the main rupture.
studies seismic sequence types and can help predict and predict trends in seismic activity. For example, when the main earthquake occurred in 1967, it was judged as a main earthquake type earthquake based on its small forward earthquake and small magnitude (2.3 magnitude), and there will be no major aftershocks after the main earthquake. Facts show that the inference is correct.
2. Volcanic earthquake
refers to earthquakes caused by volcanic activities. This kind of earthquake can be caused by an earthquake directly by a volcanic eruption; it may also be caused by tectonic changes, resulting in earthquakes; or it may be caused by tectonic changes, resulting in earthquakes. Therefore, volcanic earthquakes are often closely related to tectonic earthquakes.
is one of the few volcanic earthquakes, accounting for about 7% of the total. The depth of the focal focal is not large, generally not more than 10km. Some earthquakes occur near volcanoes with a depth of 1-10km. Their occurrence has no direct or clear relationship with volcanic eruption activities, but it is related to changes in the geostress distribution caused by changes in underground magma or gas state. This earthquake is called A-type volcanic earthquake. Some earthquakes are concentrated in a narrow range near the active crater. The depth of the focal focal depth is shallower than 1 km and the impact range is very small. It is called a B-type volcanic earthquake. Sometimes underground magma crashes to the ground but does not spew out of the surface, and an earthquake can also occur, called a latent volcanic earthquake.
Modern volcanic belts such as Italy, Japan, Philippines , Indonesia , Kamchatka Peninsula , etc. are most likely to occur volcanic earthquakes.
3. Impact earthquakes
This kind of earthquake is caused by landslides, landslides, etc., or due to long-term dissolving of groundwater in carbonate rock areas, many underground caves formed, and the top of the cave collapses. The latter is also called a collapse earthquake. The number of earthquakes of this type is very small, accounting for about 3% of the total earthquake. The source of the earthquake is very shallow, the impact range is small, and the magnitude is not large. In 1935, a collapse earthquake occurred in Baishou County, Guangxi. The collapse area was about 40,000 m2. The ground collapsed into a deep pool. The slogan heard dozens of miles away, and the nearby roof tiles were shaken. For example, in March 1972, in the coal mine goaf in the western Datong coal mine in Shanxi, a large-scale roof collapse caused an earthquake, with a maximum magnitude of 3.4, and the buildings in the epicenter area were slightly damaged.
4. Reservoir earthquakes
There were no or very few earthquakes in some places. Later, due to the construction of the reservoir, earthquakes often occurred, which was called reservoir earthquakes. This shows that this earthquake is related to the action of water, and of course it is also related to certain structural and stratigraphic conditions, and the action of water is only an inducing factor. For example, the Xinfengjiang Reservoir in Heyuan, Guangdong Province, has gradually increased its earthquake frequency around the reservoir area since it stored water in 1959. On March 19, 1962, a magnitude 6.4 earthquake occurred, with an epicenter intensity of 8 degrees, making it one of the largest known reservoir earthquakes. As of 1972, the district had recorded nearly 260,000 earthquakes (Figure 8-4). For example, the famous Aswan Reservoir in Egypt has a dam height of 110m and a storage capacity of reaches 16.5 billion m3. It was officially started in 1960, intercepted and stored water in 1964, and officially put into operation in 1968. There were no earthquakes in history before the construction of the reservoir in this area. There were small and micro earthquakes since 1980. In November 1981, a magnitude 5.6 earthquake occurred in the 60km southwest of the dam site. In 1982, a magnitude 5 and magnitude 4.6 earthquake occurred at the same location.
In addition, deep well water injection and underground water pumping can also trigger earthquakes. For example, there is a Rocky Mountain military factory in Colorado, USA. It dug a 3,614m deep well to treat wastewater and poured water into the ground with high pressure. Frequent earthquakes occurred in 1962. After the water injection is stopped, the seismic activity will weaken; when the water injection is resumed, the earthquake will increase again.
The causes of the above earthquakes, especially the reservoir earthquake, have attracted great attention. It is generally believed that under certain geological tectonic conditions that are conducive to earthquakes (such as active faults, dense or cross-sectional faults, or transitional areas where differential movements of lifting and lowering are present, reservoir water storage can induce earthquakes.In addition to human factors induced earthquakes, certain natural factors such as sunspot activity period, the new year and hope of the lunar calendar, etc. are also likely to induce earthquakes. Various trigger mechanisms are waiting for in-depth research. Modern science explains the causes of earthquakes as follows:
As the earth is constantly moving and changing, it gradually accumulates huge energy. In some fragile areas of the earth's crust, the rock formations suddenly break or cause the original fault to move. This is an earthquake. Most of the
earthquakes occur in the earth's crust.
earthquakes are divided into four types: tectonic earthquakes, volcanic earthquakes, sinking earthquakes and induced earthquakes.
Tectonic earthquake refers to the fact that when the local stress reaches and exceeds the intensity limit of the rock formation under the action of tectonic motion, the rock formation will suddenly deform and even break, releasing energy at once, causing a large earthquake. This type of earthquake is called tectonic earthquake, accounting for more than 90% of the total earthquake.
Volcanic earthquakes refer to the earthquake that occurs after a volcanic eruption, due to a large amount of magma loss, the underground pressure is reduced or the magma deep underground is not available for replenishment, resulting in the fracture or collapse of the overlying rock formation. There are not many such earthquakes, accounting for only about 7% of the total earthquake.
Falling earthquake is a local earthquake caused by the fall of underground caves or mining goafs. Falling earthquakes are all the result of gravity, with a small scale and fewer times, accounting for only about 3% of the total earthquake.
Artificial earthquake and induced earthquakes are earthquakes caused by artificial blasting, mining, military construction and underground nuclear tests. Since human production activities trigger certain fault activities, earthquakes caused by earthquakes are called induced earthquakes, mainly reservoir earthquakes, deep well pumping and water injection induced earthquakes, nuclear tests trigger earthquakes, mining activities, irrigation, etc. can also induce earthquakes.
earthquake (earthquake) is the rapid vibration of the surface of the earth, also known as earth motion in ancient times. It is like wind, rain, lightning, landslides, and volcanic eruptions. It is a natural phenomenon that often occurs on the earth. It originates from a point underground, which is called the focus. Vibrations come out of the source and spread through the earth. The closest point on the ground to the source of the earthquake is called the epicenter, which is the earliest part to receive vibration. Earth vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes occurring in the seabed or coastal areas can cause huge waves, which are called tsunamis. Earthquakes are extremely frequent, with about 5 million earthquakes occurring every year around the world, which has a great impact on the entire society.
[Earthquake phenomenon] When an
earthquake occurs, the most basic phenomenon is continuous vibration of the ground, mainly obvious sway. The people in the extreme shock zone sometimes feel the up and down first before they feel the big shaking. This is because seismic waves come from the ground to the ground, and longitudinal wave arrives first. horizontal wave then produces a large amplitude horizontal shaking, which is the main reason for earthquake disaster . During the 1960 Chile earthquake, the biggest shaking lasted for 3 minutes. The disaster caused by earthquakes is first of all the damage to houses and structures, causing casualties between people and animals. For example, in the Tangshan earthquake in Hebei, China in 1976, 70% to 80% of the buildings collapsed, causing heavy casualties. Earthquakes also have a great impact on natural landscapes. The main consequence is faults and ground fractures on the ground. The surface faults of major earthquakes often extend for dozens to hundreds of kilometers, and often have obvious vertical and horizontal distances, which can reflect the structural changes at the source (see the Danwei Earthquake, the San Francisco Earthquake). But not all surface faults are directly related to the motion of the source, and they may also be secondary effects caused by seismic waves. Especially in areas with thicker surface sedimentary layers, ground cracks often appear on the edges of slopes, river banks and on both sides of roads. This is often due to terrain factors, and shaking on one side without support causes the topsoil to collapse and crack. The shaking of the earthquake causes the topsoil to sink, and the shallow groundwater will rise along the ground cracks to the surface, forming a sand spray and water spray. A large earthquake can change the local terrain, either uplift or settle. This will cause urban and rural roads to crack, rails to twist, and bridges to break. In modern cities, water outages, power outages and communications are blocked due to cracks in underground pipelines and cut-offs.Leakage of gas , toxic gases and radioactive substances can lead to secondary disasters such as fires and toxic substances, radioactive pollution . In mountainous areas, earthquakes can also cause landslides and landslides, often causing tragedies that bury villages and towns. The collapsed rocks block the rivers and form an earthquake lake upstream. During the Great Kanto earthquake in Japan in 1923, a mudslide occurred in Kanagawa Prefecture, slid down the valley, reaching 5 kilometers.
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