Earthquakes are divided into two categories: natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes, it is due to the deep underground rock rupture, the wrong move to the long-term accumulation of energy dramatically released, in the form of seismic waves spread out in all directions, to the ground caused by the house shaking the ground. Tectonic earthquakes account for more than 90% of all earthquakes. The next type of earthquakes are those caused by volcanic eruptions, called volcanic earthquakes, which account for about 7% of the total number of earthquakes. In addition, some special circumstances can also produce earthquakes, such as the collapse of the cavern (fall earthquake), a large meteorite impact on the ground (meteorite impact earthquake) and so on.
There are many causes of earthquakes, which can be categorized as tectonic earthquakes, volcanic earthquakes and shock earthquakes, and human activities can also lead to earthquakes, which are called induced earthquakes, such as reservoir earthquakes.
I. Tectonic earthquakes
Tectonic earthquakes are earthquakes caused by tectonic changes, especially rupture activities. The vast majority of earthquakes in the world are tectonic earthquakes, accounting for about 90% of the total number of earthquakes. Most of them are shallow-source earthquakes, which have a wide range of impacts and cause very strong damage to the ground and buildings, often resulting in major losses of life and property.
China's strongest earthquakes are mostly shallow-source tectonic earthquakes, of which more than 80% are related to rupture activities. For example, on January 5, 1970, the Yunnan Tonghai earthquake (magnitude 7.7), was caused by the re-activation of the Qujiang rupture. 1973 February Sichuan Ganzi, Fuhuo earthquake (magnitude 7.9), was caused by the re-activation of the Freshwater River rupture, and after the earthquake in the ground to form a direction of the NW310 °, the length of more than 100km of the earth's fissure.
Many famous earthquakes in the world are also tectonic earthquakes, such as the 1906 San Francisco earthquake (magnitude 8.3), which was related to the San Andreas rupture, and the 1923 Kanto earthquake (magnitude 8.3), which was related to the NW-SE oriented rupture that crossed Sagami Bay, as well as a series of earthquakes that occurred from May 21 to June 22, 1960 in Chile (3 earthquakes of magnitude 8 or higher, and more than 10 earthquakes of magnitude 7 or higher). magnitude or higher, and more than 10 earthquakes of magnitude 7 or higher) in Chile from May 21 to June 22, 1960, all occurred on the 1400-km-long north-south Peruvian Trench Fracture Zone.
(I) Causes and Source Mechanisms of Tectonic Earthquakes
This issue is the most central issue in the theory of earthquake forecasting, and it is also an issue that continues to be explored and needs to be solved.
In the earth's crust and upper mantle, due to the constant movement of materials, a huge force that squeezes and pushes the rocks against each other, i.e., geostress, is often generated. Rocks under the action of geostress, the accumulation of a large amount of strain energy; when this energy once more than the rock can withstand the limit value, it will make the rock in a split second sudden fracture, releasing a large amount of energy, part of which in the form of elastic waves (seismic waves) propagated, when the seismic waves to the ground, the ground shook, which is the earthquake.
From the point of view of the earthquakes that have occurred, its occurrence has a close relationship with the already existing active structure (especially active faults), and the epicenters of many powerful earthquakes are distributed in the active fracture zones. On a global scale, the distribution of seismic zones is closely related to plate boundaries. These boundaries are actually also tensile, extrusive or horizontally staggered by a number of fracture tectonics.
There are several hypotheses about how rupture activity can produce earthquakes of high energy and how it does so.
1. The elastic rebound hypothesis is the earliest and most widely used hypothesis on the cause of earthquakes, based on the discovery that the San Andreas Fault moved horizontally during the 1906 San Francisco earthquake. The hypothesis suggests that earthquakes occur because rocks in the earth's crust have fractured and moved incorrectly, and the rocks themselves are elastic; rocks that have deformed elastically at the time of the fracture will bounce back in the opposite direction as a whole after the force has disappeared, returning to the state they were in before they were deformed. This kind of bouncing can produce amazing speed and power, the long-term accumulation of energy in a split second release, resulting in earthquakes. In short, seismic waves are generated by the overall elastic rebound of the rocks on both sides of the fault plane, originating from the fault plane. As in Figure 8-3, the elastic deformation of the rock layer (B), the force exceeds the elastic strength of the rock, rupture (C), then the two plates of rock on the fault as a whole bounced back to the original state, so the earthquake occurred. This hypothesis can better explain the causes of shallow-source earthquakes, but for medium and deep-source earthquakes is not a good explanation. Because in the underground quite deep place, the rock has plasticity, it is impossible to occur elastic rebound phenomenon.
2. Peristalsis said peristalsis, also known as subduction, submergence. Surface soil and rock layer under the action of gravity can move slowly downward for a long time, its mobile body and the base of the interface between the absence of obvious, and the amount of deformation and movement are transitional relationship, this deformation and movement is known as peristalsis. Peristalsis rate of several millimeters to several centimeters per year.
It has been found that buildings constructed on active faults and the active faults themselves have this creep phenomenon, which is relatively slow and steady sliding, in the absence of earthquakes. For example, there is an Anatolian active fault zone 110km north of Ankara, Turkey, and the walls of the buildings located on this fault zone have been found to have the phenomenon of fault breakage, and the creep is about 2cm per year.
The faults in the Middle East after earthquakes have been observed, and it is found that some of them are accompanied by no-earthquake creep, and the amount of creep is about 1cm per year.
What is the situation under which creep is likely to occur is not very clear. Some experiments show that in the high-pressure, low-temperature, high rock porosity (water), containing soft minerals such as dolomite, calcite, serpentine and other rocks, easy to produce stable creep. It is also believed that creep is easily produced at higher peripheral pressures or higher temperatures.
There is a phenomenon gradually proved for the facts, that is, the rock layer in the long-term creep of the section or in the active fault creep accounted for a higher percentage of long-term activity in the section, due to the energy through the slow creep and the gradual release of the earthquake, but rarely occur strong. In the Arjinshan area of China, there are large-scale shear faults, which are active faults, and through satellite image analysis, it is found that there is a creep phenomenon, and the modern water system is cut through, the displacement is obvious, and the misalignment is also very large, but there are few earthquakes ever recorded, and it is presumed that the activity mode of this fault is dominated by the no-earthquake creep.
According to the data on the relationship between creep and the size of earthquakes, it is shown that the largest earthquakes can only be of magnitude 5 in the sections where creep accounts for more than 50% of the long-term activity, while large earthquakes of magnitude 8 or higher may occur in the sections where creep accounts for less than 10% of the long-term activity.
3. Stick-slip said in the deeper parts of the ground, the rock on both sides of the fault if you want to slide must overcome the strong friction, so in the usual case of the two plates of rock as if they are stuck together, who can not move. However, when the stress accumulates to be equal to or greater than the friction, the two plates of rock will suddenly slide. Through the sudden sliding, the energy is released, and the two disks are bonded together again until the energy accumulates to a certain level leading to the next sudden sliding. Experiments have proved that the form of destruction of objects under high pressure, is bonded along the fracture surface and sliding alternately, the fracture occurred intermittent sharp sliding phenomenon, after many stress landing, the accumulated strain energy release, this statement is called viscous sliding said.
There are many factors affecting the way the fault activity: one is the temperature, the temperature is lower than 500 ℃, the rock on both sides of the fault surface is easy to produce stick-slip; the temperature is higher than 500 ℃, it is easy to produce creep and creep. The second is the rock composition, the rock is brittle and hard (such as quartzite, quartz sandstone, etc.), the rock on both sides of the fault tends to stick-slip; the rock is soft, then creep is dominant. Third, the porosity and water content of the rock, rock pore size, high porosity, containing more water, of course, easy to creep; on the contrary, the rock pore is small, low porosity, containing less water, it is more in the form of stick-slip. In addition, the size of the surrounding pressure will also affect the fault activity. If the two plates of the fault continuous viscous slip, is a period of frequent earthquakes.
In fact, the same active fault can be active in different ways at different depths, and the same fault can be active in different ways at different times. For example, the San Andreas Fault, at a depth of more than 4km, is characterized by stable creep without earthquakes; 4-12km is characterized by stick-slip movement accompanied by earthquakes; and below 12km (due to high temperatures), stable creep is again dominant. Therefore, the depth of the seismic source on the San Andreas Fault does not exceed 20km.
4. Some people believe that deep-source earthquakes are caused by the process of phase transition of deep materials. Underground material in the high temperature and high pressure conditions, causing sudden changes in the mineral crystal structure of the rock, resulting in sudden contraction or expansion of the rock volume, the formation of an explosive vibration source, so the earthquake. This theory fails to give specific arguments from various aspects, and thus fails to gain wide popularity. In recent years, according to the analysis of the propagation of seismic longitudinal waves in the deep underground, the deep-source earthquakes are located in the same part of the fracture and fault movement, proving that the occurrence of earthquakes is related to fracture activities. At the same time, the theory of plate tectonics points out that when the lithospheric plate subducts into the ground, the medium- and deep-source earthquakes occur inside the plate that is subdued to the mantle, but not in the mantle soft rheosphere material, so the theory of phase transition naturally loses its basis for existence.
(2) Characteristics of Tectonic Earthquakes
Tectonic earthquakes are characterized by frequent activities, long duration, wide coverage and strong destructiveness.
1. Sequence of earthquakes Any earthquake occurs after a long period of gestation process, that is, the stress accumulation process, this process can be as long as ten years, decades or even hundreds of years.
But within a certain period of time (a few days, weeks, years), in the same geologic tectonic zone or the same source of the body, but a series of large and small earthquakes with the cause of the connection, such a series of earthquakes is called the earthquake sequence. In an earthquake sequence, if there is a particularly large earthquake, known as the main shock; in the main shock often occurs before a series of weak or smaller earthquakes, known as foreshocks; in the main shock after the main shock often occurs a series of earthquakes smaller than the main shock, known as aftershocks.
One of the important features of tectonic earthquakes is that they often occur in such a sequence. This feature may be related to the process of generating tectonic earthquakes. Generally speaking, when the ground stress is about to increase to more than the strength of the rock, the rock layer first produces a series of small misalignment (or along the fault zone stick-slip to start the alternation process), resulting in the formation of a number of small earthquakes, i.e., foreshock. Then the ground stress continues to increase, to the point where the rock layer can not withstand the time, will cause the rock layer of the whole sliding or new fracture sliding, the formation of a large earthquake, i.e., the main quake. After the mainshock, the equilibrium between the rock strata still needs to go through a period of activity and adjustment to release the remaining energy in the rock strata, thus causing some small aftershocks. At the earthquake site, it is often seen that on the ruptured ground, many secondary fissures appear again, staggered in between, indicating that the movement has not completely stopped until the complete destruction of many locations that have not yet been destroyed, and all of the remaining strain energy has been released. This situation is similar to the compression of the spring process, when the force disappears, the stored potential energy is converted into kinetic energy to jump back and restore the original state, but it is difficult to recover at once, but also after a period of time to slowly churning to adjust to restore the original equilibrium position. This phenomenon is called the spring effect. Rock is also elastic, so there should be this elastic effect. 1920 Ningxia (formerly Gansu) Haiyuan earthquake, aftershocks for three years. Its intensity and frequency are sometimes high and sometimes low, but the general trend is to gradually decay until the calm.
2. Types of earthquake sequences Although tectonic earthquakes are often in a certain sequence, their energy release pattern, the activity time and proportion of large and small earthquakes are often different. According to the analysis of strong earthquakes in China since October 1949, earthquake sequences can be summarized into three types:
(1) Single-earthquake Also known as isolated earthquakes. The foreshocks and aftershocks of such earthquakes are few and weak, and the magnitude difference with the mainshock is so great that the seismic energy of the whole sequence is almost entirely released through the mainshock. These earthquakes are relatively rare; the Dingyuan earthquake in Anhui province in the fall of 1966 and the Linyi earthquake in Shandong province in March 1967 had no observed foreshocks or aftershocks, and the magnitude was very small, only 4-4.5 magnitude.
(2) the main shock type earthquake is one of the most common types, the main shock magnitude is particularly prominent, the release of energy accounted for more than 90% of the whole series; foreshock may or may not be, but there are many aftershocks. 1975 February 4, Liaoning Haicheng earthquake (magnitude 7.3), the 24 hours before the quake **** more than 500 foreshock, the main shock and aftershock after the occurrence of a number of aftershocks. 1976 July 28 Tangshan earthquake (magnitude 7.3), the main shock after the aftershock of the aftershock. The Tangshan earthquake of July 28, 1976 (magnitude 7.8), on the other hand, had essentially no foreshocks, but aftershocks continued for several years.
(3) Swarm-type earthquakes are sequences of many earthquakes of similar magnitude, with no dominant mainshock. These earthquakes have more and larger foreshocks and aftershocks, often occurring in clusters, with a longer duration of activity, a slower rate of decay, and a larger range of activity. For example, in the 1966 Xingtai earthquake, from February 28 to March 22, the magnitude of the earthquake rose gradually from 3.6, 4.6, 5.3, 6.8, 6.8 to 7.2, and a major earthquake occurred. Sometimes this type of earthquake is formed by combining or confusing two mainshock-type earthquakes.
Sometimes the earthquake sequences are more complex, as if they were formed by a combination of several single-earthquake, mainshock-type, and cluster-type earthquakes. For example, the August-September 1971 Mabian earthquake in Sichuan province.
The type of earthquake sequence may be related to the degree of homogeneity and complexity of rocks and tectonics. According to experiments, when the medium is uniform and the stress within the medium is not concentrated, there is no small rupture before the main rupture, and few small ruptures after the main rupture; when the medium is not uniform and the stress is somewhat locally concentrated or highly concentrated, a certain amount or a lot of small ruptures will be produced both before and after the main rupture.
Studying the types of earthquake sequences can help predict and forecast trends in seismic activity. For example, in the 1967 Hetian earthquake, when the main shock occurred, based on its fewer foreshocks and small magnitude (2.3), it was judged to be a main-shock type of earthquake, which would not be followed by larger aftershocks. The facts show that the inference is correct.
II. Volcanic earthquakes
These are earthquakes caused by volcanic activity. Such earthquakes can be directly caused by volcanic eruptions; may also be caused by volcanic activity caused by tectonic changes, resulting in earthquakes; or tectonic changes caused by volcanic eruptions, resulting in earthquakes. Therefore, volcanic earthquakes and tectonic earthquakes are often closely related.
Volcanic earthquakes are few in number, accounting for about 7% of the total. The depth of the epicenter is not large, generally not more than 10km. some earthquakes occur in the vicinity of the volcano, the depth of the epicenter is 1-10km, the occurrence of which has no direct or clear relationship with the volcanic eruption activities, but with the underground magma or changes in the state of the gas produced by changes in the distribution of geopathic stresses, such earthquakes are known as A-type volcano earthquakes. There are also earthquakes that occur in a narrow area near an active volcanic crater, with a depth of epicenter shallower than 1km and a very small area of influence, which are called B-type volcanic earthquakes. Sometimes the underground magma rushed to near the ground, but not out of the surface, can also produce earthquakes, known as subduction volcanic earthquakes.
Modern volcanic belts such as Italy, Japan, the Philippines, Indonesia, Kamchatka Peninsula, etc. are most prone to volcanic earthquakes.
Three, shock earthquakes
This earthquake, caused by landslides, landslides and other causes, or because of the carbonate rock areas of the rock layer by the long-term dissolution of groundwater to form a lot of underground caves, the roof of the cave collapsed caused. The latter is also known as collapse earthquake. The number of earthquakes in this category is very small, accounting for about 3% of the total number of earthquakes. In 1935, there was a subsidence earthquake in Baishou County, Guangxi, with a collapse area of about 40,000 m2, and the ground collapsed into a deep pool, which was heard for dozens of miles, and the nearby roof tiles were shaken. Another example, in March 1972 in Datong, Shanxi western coal mine mining area, a large area of roof collapse caused by the earthquake, its maximum magnitude of 3.4, the epicenter of the district buildings have minor damage.
Four, reservoir earthquakes
Some places where there was no or very few earthquakes, and then due to the repair of reservoirs, often occurring earthquakes, known as reservoir earthquakes. Explain the role of such earthquakes and water-related, of course, with certain tectonic and stratigraphic conditions, and the role of water is only a triggering factor. For example, Guangdong Heyuan Xinfengjiang Reservoir, since the water storage in 1959, the frequency of earthquakes around the reservoir area gradually increased in March 19, 1962, a 6.4 magnitude earthquake, the epicenter of the intensity of 8 degrees, is one of the largest known reservoir earthquakes. As of 1972, nearly 260,000 earthquakes had been recorded in the area*** (Figure 8-4). Another example is the famous Aswan Reservoir in Egypt, with a dam height of 110m and a capacity of 16.5 billion m3, which was officially opened in 1960, cut off and impounded in 1964, and formally put into operation in 1968. This area in the construction of the reservoir before the history of no earthquakes, from 1980 onwards, small earthquakes, micro-earthquakes, in November 1981 in the 60km southwest of the dam site in the reservoir area of the 5.6 magnitude earthquake; in 1982 the same place and the 5 and 4.6 magnitude earthquakes.
In addition, earthquakes can also be triggered by deep well injection and underground pumping. Such as the United States, Colorado has a Rocky Mountain Arsenal, for the treatment of wastewater cut a 3614m deep well, with high pressure water injection in the ground, in 1962, frequent earthquakes. When the injection was stopped, the seismic activity decreased; when the injection was resumed, the earthquakes increased again.
The causes of the above earthquakes, especially reservoir earthquakes, have attracted great attention. It is generally believed that under certain favorable geological and tectonic conditions (such as the existence of active faults, dense or intersecting ruptures, or in the transition part of the up-and-down differential movement, etc.), water storage in reservoirs can induce earthquakes. In addition to earthquakes triggered by man-made factors, certain natural factors, such as sunspot activity, the lunar calendar's solstice, hope period, etc., are also prone to triggering earthquakes. Various triggering mechanisms are awaiting in-depth study.
Volcanoes and earthquakes
The Earth's surface has a thick layer of crust, usually magma by the crust is tightly wrapped in the inside. The temperature inside the Earth is particularly high, and magma flows around there, always looking for a place to escape to the outside. There are places where the crustal movement is stronger and the crust is weaker, and when these places are under pressure, the magma rushes out from there. In this way, volcanic eruptions occur. Active volcanoes, extinct volcanoes This refers to volcanic activity. Some volcanoes erupt once and then never erupt, these volcanoes become dead active mountains.
Artificial earthquakes are earthquakes caused by human activity. For example, vibrations caused by industrial blasting and underground nuclear explosions; and earthquakes are sometimes induced by high-pressure water injections in deep wells and by large reservoirs that store water and increase the pressure on the earth's crust.
The place where an earthquake wave originates is called the epicenter. The vertical projection of the epicenter on the ground is called the epicenter. The depth from the epicenter to the source is called the depth of the source. Usually the depth of the epicenter is less than 70 kilometers called shallow earthquakes, the depth of 70-300 kilometers called medium-source earthquakes, the depth of more than 300 kilometers called deep earthquake. Destructive earthquakes are usually shallow earthquakes. For example, the 1976 Tangshan earthquake had a depth of 12 kilometers.
Thermal convection of mantle material. It is driven by the energy generated by the decay of radioactive elements within the Earth. It is an external manifestation of the release of energy within the Earth. Internal energy release takes the form of earthquakes, volcanoes, plate movements, and geologic tectonics. Earthquakes are one of them.
[1] There is a source of vibration within the Earth, which releases energy (seismic waves) and causes vibrations within a certain area.
[2] Other geological or natural disasters can also indirectly induce earthquakes.
Thermal convection of mantle material. Is driven by the energy generated by the decay of radioactive elements within the Earth. Is the external manifestation of the Earth's internal energy release. Internal energy release mainly in the form of: earthquakes, volcanoes, plate movement, geological tectonics. Earthquakes are one of them.
Precipitation, winds, ocean currents, rivers, and other surface processes are driven by energy external to the Earth, i.e., the Sun.
Why do earthquakes occur?
Earthquakes can be categorized into natural earthquakes and man-made earthquakes (e.g. nuclear explosions). Generally known as natural earthquakes, they can be categorized into (1) tectonic earthquakes, (2) volcanic earthquakes, and (3) impact earthquakes (e.g., meteorite impacts) depending on the cause of the earthquakes. Among them, crustal changes caused by plate movements (tectonic earthquakes) are the most important.
Because there is a kind of stress in the earth that pushes the rock layer, when the stress is greater than the strength that the rock layer can withstand, the rock layer will happen to move (dislocation), and this kind of dislocation will suddenly release a huge amount of energy, and produce a kind of elastic waves (elastic waves), which we call seismic waves (seismic waves), and when it arrives at the earth's surface, it will cause the earth to shake. When it reaches the surface, it causes the earth to shake, which is an earthquake.