Earthquake is a phenomenon in which the local medium in the earth ruptures rapidly, producing seismic waves, thus causing ground vibration in a certain range. Earthquake is the rapid vibration of the earth's surface, which was also called earthquake in ancient times. Like wind, rain and lightning, it is a natural phenomenon that often happens on the earth. Ground vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes at the bottom of the sea or in coastal areas will cause huge waves, which is called tsunami. Earthquakes are extremely frequent, and there are about 5 million earthquakes every year in the world.
The place where seismic waves are emitted is called the source. The vertical projection of the source on the ground, and the point closest to the source on the ground is called the epicenter. It was the first place to receive vibration. 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. Earthquakes of the same size have different damage degrees to the ground due to different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa.
Destructive earthquakes usually occur in shallow earthquakes. For example, the focal depth of the Tangshan earthquake in 1976 was 12km.
The strongest ground motion of a destructive earthquake is called the extreme earthquake zone, which is often the area where the epicenter is located.
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 jumping up and down, but you can feel horizontal shaking.
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.
Earthquakes have a certain temporal and spatial distribution law.
In terms of time, earthquakes have periodic phenomena of alternating active periods and quiet periods.
From a spatial point of view, the distribution of earthquakes has certain zones, called seismic zones, which are mainly concentrated in the Pacific Rim and Mediterranean-Himalayan seismic zones. The Pacific seismic belt almost concentrates more than 80% of shallow earthquakes (0 km ~ 70 km) in the world, and all moderate earthquakes (70 km ~ 300 km) and deep earthquakes release about 80% of the total energy.
important
Magnitude refers to the magnitude of an earthquake and is an index to measure the intensity of an earthquake. It is determined by the energy released by each seismic activity measured by seismograph. The magnitude is usually represented by the letter M. At present, the standard of magnitude used in China is the international Richter scale, which is divided into 9 levels. Earthquakes with a magnitude less than 2.5 are usually called small earthquakes, earthquakes with a magnitude of 2.5-4.7 are called inductive earthquakes, and earthquakes with a magnitude greater than 4.7 are called destructive earthquakes. The magnitude difference per 1.0 and the energy difference are about 30 times; For every difference of 2.0, the energy difference is about 900 times. For example, an earthquake of magnitude 6 released energy equivalent to the atomic bomb dropped by the United States in Hiroshima, Japan. An earthquake of magnitude 7 is equivalent to 32 earthquakes of magnitude 6, or 1000 earthquakes of magnitude 5.
According to the magnitude, earthquakes can be divided into the following categories:
The magnitude of a weak earthquake is less than 3. The magnitude of the felt earthquake is equal to or greater than 3 and less than or equal to 4.5.
The magnitude of moderate-strong earthquakes is greater than 4.5 and less than 6. The magnitude of a strong earthquake is equal to or greater than 6. Among them, magnitude 8 or above is also called a giant earthquake.
earthquake intensity
The damage caused by earthquakes of the same scale is not necessarily the same; The losses caused by the same earthquake are different in different places. In order to measure the degree of earthquake damage, scientists "made" another "ruler"-earthquake intensity. 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. The factors affecting the intensity are magnitude, focal depth, distance from the focal point, ground conditions and stratigraphic structure.
Generally speaking, as far as the relationship between intensity and source and magnitude is concerned, the greater the magnitude, the shallower the source and the greater the intensity. Generally speaking, after the earthquake, the damage in the epicenter area is the heaviest and the intensity is the highest; This intensity is called epicentral intensity. From the epicenter to the surrounding areas, the earthquake intensity gradually decreased. So the earthquake has only one magnitude, but the damage caused by different regions is different. In other words, an earthquake can be divided into several regions with different intensities. This is the same reason that the degree of damage from far and near is different after the bomb explodes. The amount of explosives in the bomb is like magnitude; Bombs destroy different places as much as they do.
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.
Several different intensity scales are used all over the world. The improved mcquarrie intensity scale, called M.M. intensity scale for short, is widely used in western countries, and there are 12 intensity grades from I to I. In Japan, there is no feeling at 0 degrees, and the feeling is divided into 8 grades: I to VII degrees and * * * *. The former Soviet Union and China divided the intensity scale according to 12 intensity grade. China revised the earthquake intensity table (see table) in 1980.
China earthquake intensity scale
1 degree; No sense-only instruments can record;
2 degrees; Micro-feeling-a particularly sensitive person feels in complete stillness;
3 degrees; Feeling less-several people in the room feel resting and the clothes rack swings slightly;
4 degrees; A lot of feelings-most people are indoors, a few people are outdoors, hanging objects are swaying, and unstable utensils are creaking;
5 degrees; Wake up-most people can feel it outdoors, animals are restless, doors and windows are rustling, and cracks appear on the wall.
6 degrees; Panic-people stand unsteadily, livestock flee, utensils fall, simple sheds are damaged, and steep slopes slide;
7 degrees; Damage to the house-slight damage to the house, damage to the archway and chimney, cracks on the surface, sand blasting and bubbling;
8 degrees; Building damage-many houses were damaged, a few damaged roadbed collapsed, and underground pipelines were broken;
9 degrees; Buildings are generally destroyed-most houses are destroyed, a few collapse, arches and chimneys collapse, and railway tracks bend;
10 degree; Buildings were generally destroyed-houses were pushed down, roads were destroyed, a lot of rocks collapsed, and water waves rushed to the shore;
1 1 degree; Destruction-a large number of houses collapsed, a large section of levee collapsed, and the surface changed greatly;
12 degrees; Landscape changes-all buildings are generally destroyed, the terrain changes dramatically, and animals and plants are destroyed;
For example, the 1976 Tangshan earthquake, with a magnitude of 7.8 and an epicenter intensity of 11 degrees. Affected by the Tangshan earthquake, the earthquake intensity in Tianjin is eight degrees, and that in Beijing is six degrees, only four to five degrees away from Shijiazhuang and Taiyuan.
seismism
When an earthquake occurs, the most basic phenomenon is the continuous vibration of the ground, mainly the obvious shaking.
People in the earthquake zone sometimes feel jumping up and down before they feel a big earthquake. This is because seismic waves travel from underground to the ground, and longitudinal waves arrive first. Shear waves then produce a large horizontal vibration, which is the main cause of earthquake disasters. 1960 during the Chile earthquake, the biggest shaking lasted for 3 minutes. The first disaster caused by the earthquake was the destruction of houses and buildings. For example, in the 1976 Tangshan earthquake in China, 70% ~ 80% of buildings collapsed, causing heavy casualties.
Earthquakes also have a great impact on the natural landscape. The main consequence is that there are faults and ground fissures on the ground. The surface faults of large earthquakes often extend from tens to hundreds of kilometers, and often have obvious vertical and horizontal offsets, which can reflect the characteristics of structural changes at the source (see the Houwei earthquake and the San Francisco earthquake). However, not all surface faults are directly related to the motion of the source, and may also be caused by the secondary influence of seismic waves. Especially in areas with thick surface sediments, ground fissures often appear at the edge of hillsides, banks of rivers and both sides of roads. This is often due to topographical factors. Without support on one side, the topsoil is loose and cracked due to shaking. The shaking of the earthquake makes the topsoil sink, and the shallow groundwater will rise to the surface along the ground fissure, forming the phenomenon of sand blasting and water inrush. A big earthquake will change the local topography, or uplift or sink. Urban and rural roads are cracked, rails are twisted and bridges are broken. In modern cities, water, electricity and communication are blocked due to the rupture of underground pipelines and the cutting of cables. The leakage of gases, toxic gases and radioactive substances will lead to secondary disasters such as fire, poisoning and radioactive pollution. In mountainous areas, earthquakes can also cause landslides and landslides, which often lead to the tragedy of burying villages and towns. The collapsed rocks blocked the river and formed an earthquake lake upstream. 1923 During the Great Kanto Earthquake in Japan, a mudslide occurred in Kanagawa Prefecture, which went down the valley as far as 5 kilometers.
Generation and types of earthquakes
Earthquakes are divided into natural earthquakes and artificial earthquakes. In addition, earthquakes will also occur under some special circumstances, such as large meteorites hitting the ground (meteorite impact earthquake). There are many reasons for the vibration of the earth's surface. According to the causes of earthquakes, earthquakes can be divided into the following categories:
1, tectonic earthquake
Long-term accumulated energy is released sharply due to the fracture and dislocation of rocks in the deep underground, and spreads in all directions in the form of seismic waves. The ground motion caused by the ground is called tectonic earthquake. This kind of earthquake has the highest frequency and the greatest destructive power, accounting for more than 90% of the global earthquakes.
2. Volcanic earthquake
Earthquakes caused by volcanism, such as magmatism and gas explosion, are called volcanic earthquakes. Volcanic earthquakes can only occur in volcanic active areas, and earthquakes in volcanic active areas only account for about 7% of global earthquakes.
3. Collapse earthquake
An earthquake caused by the collapse of an underground cave or mine top is called a collapse earthquake. Such earthquakes are relatively small in scale and few in frequency. Even if it exists, it often occurs in limestone areas with dense caves or large underground mining areas.
4. Induced earthquake
Earthquakes caused by reservoir impoundment and oil field water injection are called induced earthquakes. This kind of earthquake only occurs in some specific reservoir areas or oil fields.
5. Artificial earthquake
The ground vibration caused by underground nuclear explosion and explosive blasting is called artificial 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.
Earthquake professional knowledge
The most familiar fluctuation is the observation of water waves. When a stone is thrown into a pond, the water surface is disturbed, and ripples extend outward around the place where the stone enters the water. This wave train is caused by the particle motion of water near the water wave. However, water does not flow along the direction of water wave propagation; If a cork floats on the water, it will jump up and down, but it will not move from its original position. This disturbance is continuously transmitted through the simple back and forth motion of water particles, and the motion is transmitted from one particle to the previous particle. In this way, the water waves brought the energy of the water surface broken by stones to the edge of the pool and stirred up waves on the shore. The earthquake motion is very similar to this. The vibration we feel is the vibration of elastic rock caused by the energy of seismic wave.
The physical properties of the first wave are like sound waves. Sound waves, even ultrasonic waves, propagate in the air by alternately squeezing (pushing) and expanding (pulling). Because liquids, gases and solid rocks can be compressed, the same type of waves can pass through water bodies such as oceans and lakes and the solid earth. In an earthquake, this type of wave propagates from the fault in all directions at the same speed, alternately squeezing and stretching the rock they pass through, and its particles move forward and backward along the propagation direction of these waves, in other words, the movement of these particles is perpendicular to Apollo. Forward and backward displacement is called amplitude. In seismology, this type of wave is called P wave, that is, longitudinal wave, which is the first wave to arrive.
Unlike air, elastic rocks can be compressed but not sheared, while elastic substances can allow the second wave to propagate by shearing and twisting objects. The second arrival wave produced by earthquake is called S wave. When shear waves pass through, the behavior of rocks is completely different from that of longitudinal waves. Because shear waves involve shear rather than compression, the motion of rock particles is transverse to the offset direction. These rocks can move vertically or horizontally, which is similar to the lateral movement of light waves. The coexistence of P wave and S wave makes the seismic wave sequence have a unique combination of properties, which makes it different from the physical expression of light wave or sound wave. Because shear motion cannot occur in liquid or gas, S waves cannot propagate in it. The unique properties of P wave and S wave can be used to detect the existence of deep fluid zone in the earth.
S-waves are polarized, only those light waves that vibrate laterally (up and down, horizontal, etc. ) can pass through a polarizing lens on a certain plane. The passing light wave is called plane polarized light. When sunlight passes through the atmosphere, there is no polarization, that is, there is no preferred lateral vibration of light waves. The refraction of special plastic such as crystal or polarizer can change unpolarized light into plane polarized light.
When the shear wave passes through the earth, it will be refracted or reflected when it meets the discontinuous interface, and its vibration direction will be polarized. When polarized S-wave rock particles only move in the horizontal plane, it is called SH wave. When rock particles move in a vertical plane containing the wave propagation direction, this S wave is called SV wave.
Most rocks have linear elasticity if they are not forced to vibrate too much, that is, the deformation caused by the acting force changes linearly with the acting force. This linear elastic property is called obeying Hooke's law, which is based on the contemporary British mathematician robert Newton. Hook (1635~ 1703). Similarly, in an earthquake, the deformation of rocks will increase with the increase of force. In most cases, the deformation will remain in the linear elastic range, and the rock will return to its original position after shaking. However, important exceptions sometimes occur in earthquake events. For example, when the soft soil shakes strongly, it will remain permanently deformed, and the soil will not always return to its original position after fluctuating deformation. In this case, the earthquake intensity is difficult to predict.
Elastic motion provides an excellent inspiration for how the energy changes when local seismic waves pass through rocks. The energy related to the compression or extension of the spring is elastic potential energy, and the energy related to the movement of the spring assembly is kinetic energy. The total energy at any time is the sum of elastic energy and motion energy. For an ideal elastic medium, the total energy is a constant. At the position with the largest amplitude, the energy is all elastic potential energy; When the spring oscillates to the middle equilibrium position, the energy is all kinetic energy. We have assumed that there is no friction or dissipation force, so the reciprocating elastic vibration will continue with the same amplitude once it starts. This is of course an ideal situation. During an earthquake, the friction between moving rocks gradually generates heat and dissipates some wave energy. Unless new energy is added, like a vibrating spring, the vibration of the earth will gradually stop. The measurement of seismic wave energy dissipation provides important information about the inelastic characteristics of the earth. However, in addition to friction dissipation, there are other factors that cause seismic vibration to gradually weaken with the increase of propagation distance.
Because the wavefront of sound wave is an expanding sphere, the sound it carries decreases with the increase of distance. Similar to the diffusion of water waves outside the pond, we observed that the height or amplitude of water waves gradually decreased outward. The amplitude decreases because the initial energy spreads more and more widely, which is called geometric diffusion. This diffusion will also weaken the seismic waves passing through the earth's rocks. Unless there are special circumstances, the farther away the seismic wave is from the source, the more its energy is attenuated.
Famous earthquake
The 11th earthquake in China
1556 M8 earthquake in hua county, China, with the death toll as high as 830,000.
1668 At about 8: 00 pm on July 25th, Tancheng earthquake, with magnitude of 8.5, affected 8 provinces 16 1 county, which was one of the biggest earthquakes in China's history, with the destruction area reaching more than 500,000 square kilometers, which was called "the miracle of the ages" in history.
1920 65438+February 16 at 20: 05: 53, a strong earthquake of magnitude 8.5 occurred in Haiyuan county, Ningxia. 240,000 people died, four cities were destroyed and dozens of county towns were damaged.
1927 At 6: 32: 47 on May 23rd, a strong earthquake of magnitude 8 occurred in Gulang, Gansu, China. More than 40,000 people died. When the earthquake happened, the earth cracked, green and black water came out, and sulfur gas was brilliant, killing countless hungry people.
1932 65438+February 25th 10: 04: 27, a magnitude 7.6 earthquake occurred in Changmabao, Gansu, China. 70,000 people died. When the earthquake happened, there was yellow wind and white light "Pentium" on the loess wall; Dust emerged from the rocks, and Jiayuguan Tower, a famous historical site in China, collapsed. Xuefeng on the south bank of Shule River collapsed; The falling rocks in the Thousand Buddha Cave are rolling ... aftershocks are frequent and last for half a year.
On August 25th 1933, 15: 50: 30, a magnitude 7.5 earthquake occurred in Diexi Town, Maoxian County, Sichuan Province, China. When the earthquake happened, the ground was yellow and foggy, and the walls were gone. A shepherd boy flew over two mountains. The huge landslide cut off the Minjiang River and dammed it into a lake.
1950 At 22: 09: 34 on August 5th, a strong earthquake of magnitude 8.6 occurred in Chayu County, Tibet, China. Hundreds of thousands of square kilometers of Himalayas are instantly unrecognizable: the Yarlung Zangbo River was cut into four sections in the landslide; The whole village was thrown across the river.
The Xingtai earthquake consists of two major earthquakes:15: 29 on March 8, 1966 14 seconds, a 6.8-magnitude earthquake occurred in Longyao County, Xingtai District, Hebei Province,1March 22, 1966 16, 19: 46 seconds, Ningjin County, Xingtai District, Hebei Province.
65438+65438 0970+1:00: 341On 5 October, an earthquake of magnitude 7.7 occurred in Tonghai County, Yunnan Province, China. Death 1562 1 person, disability 3243 1 person. It is the second serious disaster with more than 10,000 deaths in China since the 1949 Yangtze River flood.
On February 4th, 1975, 19: 36: 6, a magnitude 7.3 earthquake occurred in Haicheng county, Liaoning province, China. Because the earthquake was successfully predicted and prevented, it avoided greater and heavier losses, so it was called a miracle in the history of earth science and world science and technology in the 20th century.
1976 At 3: 42: 54.2 on July 28th, a 7.8-magnitude earthquake struck Tangshan, Hebei Province, China. 242,000 people died and 1.6 million people were seriously injured. A heavy industrial city was destroyed, with a direct economic loss of more than 654.38 billion yuan, which was the biggest earthquake in the world in the 20th century.
1988 1 1: 03, 16 10. On October 6th, two major earthquakes of magnitude 7.6 (Lancang) and 7.2 (Gengma) occurred in China. The two earthquakes, which were120km apart, were only 13 minutes apart. Two county towns were razed to the ground, 4 105 people were injured and 743 people died, resulting in economic losses of 25 1 1 100 million yuan.
On May 2, 2008 12 14: 28, Wenchuan County, Sichuan Province (3 1.0 N, 103.4 E), an earthquake of magnitude 8.0 occurred, and the directly severely affected area reached 6,543,800 square kilometers. As of July 4th 12, the Wenchuan earthquake in Sichuan has caused death: 69,225 people were killed, 374,640 people were injured, and18,624 people were missing. Emergency relocation and resettlement 1.500438+0.000 people, with a total affected population of 46.24 million.
The strongest earthquake in the world since the 20th century.
On March 28th, 2005 (09: 00 Beijing time on the 29th), an earthquake measuring 8.5 on the Richter scale occurred off Sumatra Island, which is one of the eleven strongest earthquakes in human history since 1900. The following are the basic information of eleven major earthquakes (in order of magnitude):
1, Chile earthquake (1May 22, 960): 8.9 on the Richter scale (it is reported to be 9.5 again). It happened in the waters of central Chile, and caused tsunami and volcanic eruption. The earthquake killed 5,000 people and left 2 million homeless. This earthquake is the highest in history.
2. Alaska Earthquake (1March 28th, 964): 8.8 on the Richter scale. The tsunami caused 125 deaths and property losses of 3 1 1 billion dollars. Strong earthquakes were felt in most parts of Alaska, Yukon, Canada and Colombia.
3. Alaska Earthquake (1March 9, 957): 8.7 on the Richter scale, which occurred in the waters near Andrea Island and Unac Island in Alaska, USA. The earthquake led to the eruption of Vesivedov volcano, which had been dormant for 200 years, and triggered a tsunami with a height of 15 meters, which spread as far as Hawaii Island.
4. (Parallel) Indonesia Earthquake (65438+February 26th, 2004): 8.7 on the Richter scale, which occurred in Aceh Province, Sumatra Island, Indonesia. The tsunami triggered by the earthquake swept through Sri Lanka, Thailand, Indonesia and India, leaving about 300,000 people missing or dead.
4. (tied) Russian earthquake (1952165438+1October 4): 8.7 on the Richter scale. The tsunami triggered by the earthquake spread to the Hawaiian islands, but no casualties were caused.
5. Ecuador earthquake (190665438+1October 3 1): 8.8 on the Richter scale, which occurred off the coast of Ecuador and Colombia. The earthquake triggered a strong tsunami, resulting in more than 1000 deaths. Shocks were felt along the coast of Central America, San Francisco and Japan.
6. (Parallel) Indonesia Earthquake (March 28th, 2005): 8.7 on the Richter scale. The epicenter was located in the waters north of Sumatra Island, Indonesia, not far from the place where the 9.0-magnitude earthquake occurred three months ago. At present, it has caused 1000 deaths, but it has not caused a tsunami.
6. (tied) Alaska earthquake (1February 4, 965): 8.7 on the Richter scale. The earthquake triggered a tsunami as high as 10.7 meters, which swept the whole Sumatra island.
7. China Tibet Earthquake (1August 950 15): 8.6 on the Richter scale. More than 2000 houses and temples were destroyed. The Yarlung Zangbo River in India was the worst hit, with at least 1.500 deaths.
8. (tied) Russian earthquake (1February 3, 923): 8.5 on the Richter scale, which occurred in kamchatka peninsula, Russia.
9. (Parallel) Indonesia Earthquake (1February 3, 938): 8.5 on the Richter scale, which occurred in the waters near Banda, Indonesia. Earthquakes triggered tsunamis and volcanic eruptions, causing great losses to people and property.
10, (tied) Kuril Islands earthquake in Russia (196310/0/0/3): 8.5 on the Richter scale, and affected Japan and Russia.
1 1, Wenchuan earthquake in Sichuan, China (May 12, 2008): Richter scale 8, the epicenter was located in Wenchuan County, Aba Prefecture, and it affected more than half of China and overseas. Heavy casualties and property losses.
? The earthquake with the largest magnitude in the world is1May 22, 960, Chile's M8.9 earthquake.
? The biggest earthquake in China was the August 1950 earthquake with magnitude 8.6 in Tibet.
? 1The earthquake of magnitude 8 in hua county, Shaanxi Province on October 23rd,1556,830 people died, with the largest number of deaths.
Followed by the Tangshan earthquake of magnitude 7.6 on July 27th, 1976, with official data of 255,000 deaths (estimated to reach 655,000).