Cause of earthquake

To put it simply, the main reason for the earthquake is that the major plates of the earth are squeezing each other, and volcanic eruption is also one of the reasons.

Earthquakes are divided into natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes, which are caused by the fracture and dislocation of rocks deep underground, so that the long-term accumulated energy is suddenly released and spread in all directions in the form of seismic waves, making houses shake and move to the ground. Tectonic earthquakes account for more than 90% of the total number of earthquakes. Followed by earthquakes caused by volcanic eruptions, known as volcanic earthquakes, accounting for about 7% of the total number of earthquakes. In addition, earthquakes will also occur in some special circumstances, such as cave collapse (collapse earthquake) and large meteorites hitting the ground (meteorite impact earthquake).

Artificial earthquakes are earthquakes caused by human activities. Such as vibration caused by industrial blasting and underground nuclear explosion; High-pressure water injection in deep wells and water storage in large reservoirs increase the pressure on the earth's crust and sometimes induce earthquakes.

The place where seismic waves are generated is called the source. The vertical projection of the source on the ground is called the epicenter. The depth from the epicenter to the source is called the focal depth. Generally, the focal depth less than 70km is called shallow earthquakes, the depth of 70-300km is called Zhongyuan earthquake, and the depth greater than 300km is called deep earthquake. Destructive earthquakes usually occur in shallow earthquakes. For example, the focal depth of the Tangshan earthquake in 1976 was 12km.

Thermal convection of mantle materials. It is driven by the energy generated by the decay of radioactive elements in the earth. It is the external expression of energy release inside the earth. The internal energy release mainly takes the following forms: earthquake, volcano, plate movement and geological structure. The earthquake is one of them.

There is a seismic source in the earth, which releases energy (seismic wave) and causes vibration in a certain range.

Other geological disasters or natural disasters can also indirectly induce earthquakes.

Thermal convection of mantle materials. It is driven by the energy generated by the decay of radioactive elements in the earth. It is the external expression of energy release inside the earth. The internal energy release mainly takes the following forms: earthquake, volcano, plate movement and geological structure. The earthquake is one of them.

Surface processes such as precipitation, wind, ocean currents and rivers are all driven by the external energy of the earth, namely the sun.

Causes of earthquake disasters and preventive countermeasures

What caused the earthquake?

Earthquakes can be divided into natural earthquakes and man-made earthquakes (such as nuclear explosions). Earthquakes are generally called natural earthquakes, and can be divided into (1) tectonic earthquakes (2) volcanic earthquakes (3) impact earthquakes (such as meteorite impacts) according to their causes. Among them, the crustal movement (tectonic earthquake) caused by plate movement is the main one.

Because there is a kind of stress in the earth that pushes the rock strata, when the stress is greater than the strength that the rock strata can bear, the rock strata will be dislocated, and this dislocation will suddenly release huge energy and produce an elastic wave, which we call seismic wave. When it reaches the surface, it will cause the earth to shake. This is an earthquake.

What kind of faults can be classified?

Compared with the formation morphology before and after the fault, it can be divided into four types:

(1) A normal fault that arches upward at an obtuse angle.

(2) Normal fault with acute angle arched upwards.

(3) Right-shifted faults.

(4) The left-lateral fault moves to the left.

What is the source and epicenter?

(1) source: the starting point of earthquake dislocation.

(2) epicenter: the projection point of the source on the surface.

What are shallow earthquakes and deep earthquakes?

Earthquakes with a focal depth of 0~30 kilometers are called extremely shallow earthquakes. 31~ 70km is called shallow earthquakes. 7 1~300 km is called a moderate earthquake. 30 1 ~ 700 km is called a deep earthquake.

What is an earthquake sequence?

Successive arrangement, that is, earthquake sequence. The so-called earthquakes in the same series refer to all earthquakes that occur in similar time and are related in time, including foreshocks, main earthquakes and aftershocks; They are defined as follows:

(1) Foreshock: In the same series of earthquakes, the earthquake that occurred before the main earthquake is called. Only the foreshock is short and of little significance.

(2) Main earthquake: The largest earthquake in the same series is called the main earthquake. If there are two biggest earthquakes, the first one is called the main one.

(3) Aftershock: In the same series of earthquakes, the earthquake after the main shock is called it.

What are the main seismic waves?

Seismic waves can be divided into two categories according to their propagation paths:

1. Body wave: It can spread inside the earth and can be divided into the following categories according to the fluctuation of trade:

(1)P wave (longitudinal wave or compression wave): It is similar to sound wave in nature, and the particle motion is consistent with the wave propagation direction, with the fastest speed.

(2)S-wave (shear wave or shear wave): The particle motion is perpendicular to the propagation direction of Apollo, resulting in up-and-down or left-and-right vibration, followed by speed.

2. Surface waves: propagating along the earth's surface or internal interface, which can be mainly divided into:

(1) Love wave: The particle moves along the horizontal plane perpendicular to the Apollo propagation direction.

(2) Rayleigh wave: the particle vibrates along an elliptical trajectory on a vertical plane parallel to the propagation of seismic waves.

such as

What is plate movement?

Plate tectonics mainly explains the structures that currently occur in the upper layer of the earth and the causes of earthquakes. The outermost layer of the earth is cold and hard movable rock, called lithosphere, with an average thickness of about 100 km. Below the lithosphere, the asthenosphere is composed of liquid substances with high viscosity, which becomes plastic under the action of high temperature and high pressure, so that the lithosphere floats on it.

The basic concept of plate tectonics is to divide the lithosphere into several rigid plates, including the larger Eurasian plate, American plate, African plate, Indian Ocean plate, Pacific plate, Antarctic plate and several smaller plates (see attached figure). Plates are subjected to tension, pressure, gravity and mantle convection. Different plates move slowly at a relative speed of several centimeters per year. Most earthquakes, volcanoes and orogeny are caused by the interaction of adjacent plates.

There are three main plate boundaries,

(1) divergent boundary represents crustal extension and fracture. On the mid-ocean ridge, two adjacent plates are separated from each other, resulting in a new lithosphere, whose material comes from the upper part of the mantle and is produced by melting. Here, the earth's crust expands and extends to both sides due to the tension, and earthquakes often occur along the divergent boundary, and the focal depth is mostly within 100 km.

(2) Convergence boundary, where two plates collide with each other, and the heavier plate is inserted under the lighter plate (dip angle is about 30 ~ 45), so that the messenger lithosphere disappears and returns to the mantle. This inserted part is called subduction zone. Because of the friction between the two plates, earthquakes can occur continuously along the subduction zone, resulting in an earthquake zone with a focal depth from very shallow to about 700 kilometers. Earthquakes are very frequent near Hualien, Taiwan, because it is the junction of Eurasia plate and Philippine Sea plate.

(3) The conservative boundary did not create a new lithosphere or make it disappear. Two adjacent plates moved laterally and rubbed against each other, resulting in an earthquake with a shallow focal depth. Taitung longitudinal valley fault is the junction of Eurasia plate and Philippine Sea plate.

What are the methods of earthquake prediction?

Although the mechanism of earthquake occurrence has not been clarified and the theory of earthquake prediction has not been fully established, there are still many tentative research methods for earthquake prediction, and the common ones are: (1) geodesy (2) tidal observation (3) continuous observation of crustal changes (4) seismic activity (5) seismic wave velocity (6).

We choose to introduce some important methods as follows:

Geodesic method:

According to many records in the past, the crust will change when a big earthquake occurs, sometimes before the earthquake. Therefore, it is possible to predict whether a major earthquake will occur by measuring crustal changes and analyzing earthquake precursors. For example, before the Niigata earthquake in Japan in 1864, the site subsided. Because local surveys are often carried out, it is found that there are indeed precursor phenomena before the earthquake.

In addition, the area where the crust changes will increase with the increase of earthquake scale, that is to say, the wider the range of abnormal crustal changes, the greater the scale of possible earthquakes.

Is the earthquake controllable?

In areas threatened by earthquake disasters all over the world, it is urgent to strengthen the study of earthquakes because of the serious losses caused by earthquake disasters. First of all, I hope to develop accurate earthquake prediction, just like the weather forecast now. Before the earthquake, we can inform the people in the area where the earthquake will occur, so that we can leave the earthquake area calmly and seek good luck and avoid disaster.

However, whether the earthquake prediction is 100% accurate or not, the occurrence of earthquakes is inevitable. Therefore, it is not impossible to make the upcoming earthquake disappear further, or to reduce the power of the upcoming major earthquake to a moderate earthquake or a small earthquake.

After years of research, scientists have established a new theory called "plate crustal structure". After a long period of pushing, the pressure on plates with the same structure in the earth's crust is increasing day by day. When they can't bear the load at a certain moment, earth-shattering vibrations will occur.

Therefore, scientists think about how to eliminate the crustal stress in some way when the crustal stress is gradually increasing in places where earthquakes may occur, or artificially create some small earthquakes to guide the crustal stress to dissipate in the form of small earthquakes.

I. Tectonic earthquake

Tectonic earthquake is an earthquake caused by structural changes, especially fault activity. The vast majority of earthquakes in the world are tectonic earthquakes, accounting for about 90% of the total number of earthquakes. Most of them belong to shallow earthquakes, which has a wide range of influence, and the damage to the ground and buildings is very strong, often causing heavy losses of life and property.

Most of the strong earthquakes in China are shallow tectonic earthquakes, and more than 80% of them are related to fault activity. For example, 19701the Yunnan Tonghai earthquake (M = 7.7) on October 5th was caused by the reactivation of Qujiang fault. 1973 The Ganzi and Luhuo M7.9 earthquakes in February, Sichuan Province were caused by the reactivation of the Xianshuihe fault. After the earthquake, a ground fissure with the strike of NW3 10 and the length of 100 km was formed on the ground.

Many famous earthquakes in the world also belong to tectonic earthquakes. The San Francisco earthquake 1906 (M = 8.3) is related to the activity of the San Andreas fault. 1923 The Great Kanto Earthquake in Japan (M = 8.3) is related to the NW-SE fault activity passing through Sagami Bay. A series of strong earthquakes occurred in Chile from May 2 1 year to June 22/960 (3 earthquakes with magnitude above 8, 10 earthquakes with magnitude above 7), all of which occurred in the Peru trench fault zone with a length of 1400km from north to south.

(a) Causes and focal mechanisms of tectonic earthquakes

This problem is the core problem in the theory of earthquake prediction, and it is also a problem that is still being discussed and needs to be solved at present.

In the crust and upper mantle, due to the continuous movement of materials, there is often a huge force that squeezes and pushes rocks, that is, geostress. Rock has accumulated a lot of strain energy under the action of in-situ stress. When this energy exceeds the limit that the rock can bear, it will make the rock break suddenly in an instant, releasing a lot of energy, some of which will spread in the form of elastic waves (seismic waves). When local seismic waves reach the ground, the ground will vibrate. This is an earthquake.

Judging from the earthquakes that have occurred, its occurrence is closely related to the existing active structures (especially active faults), and the epicentres of many strong earthquakes are distributed in active fault zones. Globally, the distribution of seismic zones is closely related to plate boundaries. These boundaries are actually extensional, compressive or horizontally staggered fault structures.

At present, there are several hypotheses about why fault activity produces earthquakes with great energy and how it moves.

1. Elastic rebound theory is the earliest and most widely used hypothesis about the causes of earthquakes, which is based on the discovery of the horizontal movement of the San Andreas fault during the 1906 San Francisco earthquake. Suppose that the earthquake occurred because the rocks in the crust broke and dislocated, and the rocks themselves were elastic. After the force disappears, the rock that has been elastically deformed rebounds in the opposite direction and returns to its original state. This kind of bounce can produce amazing speed and strength, release the energy accumulated for a long time in an instant, and trigger an earthquake. In a word, seismic waves are caused by the elastic rebound of rocks on both sides of the fault plane, and come from the fault plane. As shown in Figure 8-3, the stress on the rock stratum causes elastic deformation (b), and the force exceeds the elastic strength of the rock, resulting in fracture (c). Then, the two rocks of the fault bounced back and returned to their original state, and the earthquake happened. This hypothesis can better explain the cause of shallow earthquakes, but it is not easy to explain the moderate and deep earthquakes. Because in quite deep underground, the rock is already plastic, and it is impossible to rebound elastically.

2. peristalsis is also called peristalsis and peristalsis. Under the action of gravity, the earth-rock layer on the surface can move down slowly for a long time. There is no obvious interface between the moving body and the basement, and the deformation and movement are transitional, which is called creep. The speed of peristalsis is only a few millimeters to a few centimeters per year.

It is found that buildings built on active faults also have this creeping phenomenon, that is, they slide relatively slowly and steadily without earthquakes. For example, there is an Anatolian active fault zone in the north of Ankara 1 10km, and it is found that the walls of buildings located on this fault zone are staggered and creep about 2cm every year. Some people have also observed the faults after the Middle East earthquake, and found that some areas are accompanied by earthquake-free creep, and the creep amount is about 1cm per year.

It is not clear under what circumstances it is easy to produce peristalsis. Experiments show that stable creep is easy to occur under the conditions of high pressure and low temperature, high porosity (water content) and soft minerals such as dolomite, calcite and serpentine. Others think that peristalsis is easy to occur at higher confining pressure or higher temperature.

One phenomenon has been gradually proved by facts, that is, long-term creep of rock strata or high percentage creep of active faults, because energy is gradually released through slow creep, so strong earthquakes rarely occur. There is a large-scale shear fault in Altun Mountain area of China, which is an active fault. Through the analysis of satellite images, it is found that there is a creeping phenomenon, and the modern water system is cut through, with obvious displacement and large offset, but there are few earthquake records in history. It is speculated that the active mode of the fault is mainly aseismic peristalsis.

According to the data of the relationship between creep and earthquake size, the largest earthquake can only be of magnitude 5 in areas where creep accounts for more than 50% of long-term activity, while in areas where creep accounts for less than 10% of long-term activity, large earthquakes of magnitude 8 or above may occur.

3. Stick-slip theory says that in the deep underground, the rocks on both sides of the fault must overcome strong friction if they want to slide, so under normal circumstances, the two sets of rocks seem to stick together and no one can move. However, when the stress accumulated to be equal to or greater than the friction force, the two rocks suddenly slipped. By suddenly sliding, energy is released, and the two disks are stuck together until the energy accumulates to a certain extent, leading to the next sudden sliding. Experiments show that the failure mode of an object under high pressure is alternately bonding and sliding along the fracture surface, and the cross section jumps and slides intermittently, and the accumulated strain energy is released after multiple stress drops. This statement is called stick-slip theory.

There are many factors that affect the fault activity mode: first, when the temperature is lower than 500℃, the rocks on both sides of the fault plane are easy to stick and slip; When the temperature is higher than 500℃, it is easy to creep and creep. Second, the rock composition is brittle and hard (such as quartzite and timely sandstone). ), and the rocks on both sides of the fault are often stick-slip; If the lithology is soft, it is mainly peristalsis. Third, the porosity and water content of rocks, rocks with large porosity, high porosity and high water content are naturally prone to creep; On the contrary, rocks with small pores, low porosity and low water content are mostly sticky and slippery. In addition, the magnitude of confining pressure will also affect the active mode of faults. If the two plates of the fault continue to stick and slip, it is a period of frequent earthquakes.

In fact, the same active fault can have different activities at different depths, and the same fault can also have different activities at different times. For example, the San Andreas fault, with a depth of more than 4km, is a stable peristalsis without earthquakes; 4- 12 km is the stick-slip movement accompanying the earthquake; Below 12km (due to high temperature) is mainly stable peristalsis. Therefore, the focal depth of the earthquake on the San Andreas fault zone is less than 20 kilometers.

4. Phase transition theory Some people think that deep earthquakes are caused by the phase transition process of deep materials. Under the condition of high temperature and high pressure, the underground material causes the sudden change of the crystal structure of rock minerals, which leads to the sudden contraction or expansion of rock volume, forming an explosive vibration source, and then an earthquake occurs. This theory has not been proved in many ways, so it has not been widely popularized. In recent years, according to the analysis of the propagation of P-wave earthquakes in the deep underground, faults and dislocations have also occurred in the places where deep earthquakes occurred, which proves that earthquakes are related to fault activities. At the same time, the theory of plate tectonics points out that when the lithosphere plate subducts underground, the moderate-deep earthquake occurs inside the subducted mantle plate, not inside the mantle asthenosphere material, so the theory of phase transition naturally loses its foundation.

(2) Characteristics of tectonic earthquakes

Tectonic earthquakes are characterized by frequent activities, long duration, wide spread and strong destructiveness.

1. The occurrence of any earthquake in the earthquake sequence has to go through a long-term gestation process, that is, the stress accumulation process, which can last for more than ten years, decades or even hundreds of years.

However, in a certain period (days, weeks, years), a series of large and small earthquakes with genetic connections can occur on the same geological structure belt or on the same focal body. Such a series of earthquakes is called an earthquake sequence. In an earthquake sequence, if an earthquake is particularly large, it is called a main earthquake; A series of weak or small earthquakes often occur before the main earthquake, which is called foreshock; After the main earthquake, a series of earthquakes smaller than the main earthquake often occur, which are called aftershocks.

An important feature of tectonic earthquakes is that they often occur in this order. This feature may be related to the tectonic earthquake process. Generally speaking, when the local stress is about to strengthen beyond the strength of the rock, the rock stratum first produces a series of tiny dislocations (or begins an alternating process along the fault zone), thus forming many small earthquakes, that is, foreshocks. Then the in-situ stress continues to increase, and when the rock stratum can't bear it, it will cause the whole rock stratum to slide or a new fault to slide, forming a major earthquake, which is the main earthquake. After the main earthquake, the equilibrium state between rock layers needs to be adjusted for a period of time to release the residual energy in the rock layers, thus causing some small aftershocks. At the earthquake site, many secondary cracks can often be seen on the broken ground, which shows that the movement has not stopped completely until many undamaged places are completely destroyed and the remaining strain energy is completely released. This situation is similar to the process of pressing the spring. When the force disappears, the stored potential energy is converted into kinetic energy and bounces back to its original state, but it is difficult to recover. It takes a period of slow vibration adjustment to restore the original equilibrium position. This phenomenon is called spring effect. Rock is also elastic, so it should also have this elastic effect. 1920 Haiyuan earthquake in Ningxia (formerly Gansu), the aftershocks have not disappeared for three years. Its intensity and frequency are high and low, but the general trend is to gradually decay until it subsides.

2. Types of earthquake sequences Although tectonic earthquakes are often in a certain sequence, their energy release laws, active time and the proportion of large and small earthquakes are often different. According to the analysis of the strong earthquakes in China since June 1949+00, the earthquake sequence can be divided into three types:

(1) A single earthquake is also called an isolated earthquake. The foreshocks and aftershocks of this kind of earthquake are few and weak, and the magnitude is quite different from that of the main earthquake. Almost all the seismic energy of the whole sequence is released by the main shock. Such earthquakes are rare. No foreshocks and aftershocks were observed in the Dingyuan earthquake in Anhui Province in the autumn of 1966 and the Linyi earthquake in Shandong Province in March of 1967, and the magnitude was only 4-4.5.

(2) The main earthquake is the most common type, and its magnitude is particularly prominent, and the energy released accounts for more than 90% of the whole series; Foreshocks may or may not exist, but there are many aftershocks. 1On February 4th, 975, an earthquake of magnitude 7.3 occurred in Haicheng, Liaoning Province. More than 500 foreshocks occurred in the 24 hours before the earthquake, and many aftershocks occurred after the main earthquake. 1The Tangshan earthquake (M = 7.8) on July 28th, 976 basically had no foreshock, but the aftershocks lasted for several years.

(3) Group earthquakes are composed of many earthquakes with similar magnitude, and there is no prominent main earthquake. There are many foreshocks and aftershocks of this kind of earthquake, which often appear in groups, with long activity time, slow attenuation speed and large activity range. For example, the Xingtai earthquake of 1966 gradually rose from 3.6, 4.6, 5.3, 6.8 and 6.8 to 7.2 from February 28th to March 22nd, which triggered a major earthquake. Sometimes, this type of earthquake is a combination or confusion of two major earthquakes.

Sometimes the earthquake sequence is more complicated, and it seems to be composed of several haplotypes, main types and earthquake swarms. For example, the Mabian earthquake in Sichuan in August and September, 197 1.

The types of earthquake sequences may be related to the uniformity and complexity of rocks and structures. According to the experiment, when the medium is uniform and the internal stress of the medium is not concentrated, there are no small cracks before the main cracks and there are few small cracks after the main cracks. When the medium is uneven and the stress is locally concentrated or highly concentrated, some or many small cracks will appear before and after the main fracture.

Studying the types of earthquake sequences is helpful to predict and predict the trend of seismic activity. For example, the Hejian earthquake of 1967 was judged to be the main earthquake according to its foreshock and magnitude (2.3), and there would be no major aftershocks after the main earthquake. Facts show that this inference is correct.

Second, volcanic earthquakes.

Refers to earthquakes caused by volcanic activity. This earthquake can be directly caused by volcanic eruption; It may also be structural changes caused by volcanic activity, which may lead to earthquakes; Or volcanic eruption caused by structural changes, which leads to earthquakes. Therefore, volcanic earthquakes are often closely related to tectonic earthquakes.

Volcanic earthquakes are rare, accounting for about 7% of the total. The focal depth is not large, generally not exceeding 10km. Some earthquakes occurred near volcanoes with focal depth of 1- 10 km. Their appearance is not directly or explicitly related to volcanic eruption, but to the change of in-situ stress distribution caused by the change of underground magma or gas state. This kind of earthquake is called type A volcanic earthquake. There are also some earthquakes concentrated in a narrow range near the active crater. The focal depth is shallower than 1km, and the influence range is very small, so it is called B-type volcanic earthquake. Sometimes underground magma rushes to a place close to the ground, but does not gush out of the ground, which can also produce earthquakes, called latent volcanic earthquakes.

Modern volcanic zones such as Italian, Japanese, Philippine, Indonesian and kamchatka peninsula are most prone to volcanic earthquakes.

Third, the impact earthquake.

This kind of earthquake is caused by landslides, landslides, etc. Or many underground caves are formed by the long-term dissolution of groundwater in carbonate areas, and the top of the caves collapses. The latter is also called collapse earthquake. Such earthquakes are rare, accounting for about 3% of the total number of earthquakes. The source is very shallow, the influence range is small, and the magnitude is not large. 1935, a collapse earthquake occurred in Baishou County, Guangxi, covering an area of about 40,000 m2. The ground collapsed into a deep pool, which could be heard dozens of miles away, and the roof tiles nearby were shaking. For another example, in March of 1972, a large area of the roof collapsed in the western goaf of Datong coal mine in Shanxi Province, causing an earthquake with a maximum magnitude of 3.4, and buildings in the epicenter area were slightly damaged.

Fourth, the reservoir earthquake

There are no or few earthquakes in some places, but later, due to the construction of reservoirs, earthquakes often occur, which is called reservoir earthquakes. It shows that this earthquake is related to the action of water, of course, it is also related to certain structural and stratigraphic conditions, and the action of water is only the inducing factor. In addition, deep well water injection and underground pumping can also cause earthquakes. For example, a military factory located in Kishan, Colorado, USA, drilled a deep well of 36 14m to treat wastewater and injected high-pressure water into the ground. 1962 frequently experienced earthquakes. After stopping water injection, seismic activity will be weakened; After water injection was resumed, earthquakes increased again.

The causes of the above earthquakes, especially the reservoir earthquake, have aroused great concern. It is generally believed that reservoir impoundment can induce earthquakes under certain geological structural conditions (such as active faults, dense or intersecting faults, or transitional parts with different elevation movements, etc.). In addition to human factors, some natural factors, such as sunspot activity period, new moon and lunar calendar, are also easy to induce earthquakes. All kinds of trigger mechanisms need further study.

Yingkou people found another reason for the earthquake.

"The cause of the volcano and earthquake.

There is a thick crust on the earth's surface, and magma is usually tightly wrapped inside. The temperature inside the earth is extremely high, and magma flows around there, always trying to find a place to escape to the outside. In some places, the crustal movement is strong and weak. When these places are under pressure, magma gushes out from here. In this way, the volcanic eruption happened. Active volcano, extinct volcano This refers to the situation of volcanic activity. Some volcanoes erupt once and never erupt again, so they become mountains of life and death.

Earthquakes in the world are mainly concentrated in the following two areas:

(1) Pacific Rim Seismic Belt: It includes the Pacific coast of North and South America, Aleutian Islands and kamchatka peninsula, passes through Thousand Islands and Japanese Islands, passes through Taiwan Province Province of China to the south, and then turns to the Philippines to reach New Zealand to the southeast.

(2) Himalayan-Mediterranean seismic belt: from western Indonesia through Myanmar to Hengduan Mountains and Himalayas in China, across Pamirs, and through Central Asia to the Mediterranean Sea and its coast.

The structure of the ball is like an egg, which can be divided into three layers. The middle layer is the "yolk"-the core; In the middle is "egg white"-mantle; The outer layer is the "eggshell"-the shell. Earthquakes usually occur in the earth's crust. The earth is constantly rotating, and the interior of the crust is constantly changing. The resulting force led to the deformation, fracture and dislocation of the crust and strata, so an earthquake occurred. The place where an earthquake occurs underground is called the source. The place from the source vertically upward to the surface is called the epicenter. The distance from the epicenter to the source is called the focal depth. Earthquakes with a focal concentration of less than 70km are shallow earthquakes, earthquakes with a focal concentration of 70km to 300km are moderate earthquakes, and earthquakes with a focal concentration of more than 300km are deep earthquakes. The earthquake with the deepest focal depth was 1963, which occurred in the northern part of Irian Jaya province, Indonesia, with a focal depth of 786 kilometers. Earthquakes of the same size have different damage to the ground because of different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa.

The distance from a place to the epicenter is called epicentral distance. Earthquakes with epicentral distance less than 1000 km are called near earthquakes, earthquakes with epicentral distance between 100- 1000 km are called near earthquakes, and earthquakes with epicentral distance greater than1000 km are called teleseisms. Among them, the farther the epicentral distance, the smaller the impact and damage.

The ground vibration caused by earthquake is a complex movement, which is the result of the joint action of longitudinal wave and shear wave. In the epicenter, longitudinal waves made the ground jump up and down. Shear waves make the ground shake horizontally. Because longitudinal waves travel faster and decay faster, while shear waves travel slower and decay slower, they are far from the epicenter, so you often can't feel the jump up and down, but you can feel the horizontal shaking.

The magnitude of the earthquake itself is expressed by magnitude, and the magnitude is determined according to the energy of elastic waves released during the earthquake. China generally uses the Richter scale. Earthquakes with magnitude less than 2.5 are usually called small earthquakes, earthquakes with magnitude 2.5-4.7 are called inductive earthquakes, and earthquakes with magnitude greater than 4.7 are called destructive earthquakes. For every magnitude difference of 1, the energy released by the earthquake is about 30 times different. For example, an earthquake of magnitude 7 is equivalent to 30 earthquakes of magnitude 6 or 900 earthquakes with magnitude difference of 0. 1, and the average difference of energy released is 1.4 times.

When a large earthquake occurs in a certain place, a series of earthquakes often occur within a period of time, the largest of which is called the main earthquake, the earthquake before the main earthquake is called the foreshock, and the earthquake after the main earthquake is called the aftershock.

The degree of ground vibration at a certain point during an earthquake is called seismic intensity. China divides the earthquake intensity into 12 degrees.

Although both magnitude and intensity can reflect the strength of an earthquake, their significance is the same. The same earthquake has only one magnitude, but the intensity varies from place to place, and the intensity value varies from place to place. For example, on1February 1990 10, an earthquake of magnitude 5. 1 occurred in Changshu-Taicang. Some people say that Suzhou is level 4 and Wuxi is level 3, which is wrong. No matter where it is, it can only be said that an earthquake of magnitude 5. 1 occurred in Changshu-Taicang, but this time, the earthquake intensities in shaxi town, Taicang, Suzhou and Wuxi were 6 degrees, 4 degrees and 3 degrees respectively.

Seismic intensity is a frequently used term. There are qualitative and quantitative standards for dividing strength. On the China Earthquake Intensity Table, people's feelings and the damage degree of ordinary houses are described, which can be used as the basic basis for determining the intensity.

Earthquake and its cause analysis