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. Just like wind, rain, lightning, landslides and volcanic eruptions, they are natural phenomena that often occur 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. There are about 5 million earthquakes around the world every year, which have a great impact on the whole society.
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 to the ground because of 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 the jump up and down, but you can feel the 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.
Earthquake magnitude and intensity
When the earthquake research department reports an earthquake in a certain area, it is often called an earthquake of magnitude XX, with intensity reaching X, etc. The magnitude and intensity of an earthquake are not the same thing.
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 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.
According to the magnitude, earthquakes can be divided into the following categories:
The magnitude of a weak earthquake is less than 3. This kind of earthquake is generally not easy to detect if the source is not very shallow.
The magnitude of the felt earthquake is equal to or greater than 3 and less than or equal to 4.5. People can feel this kind of earthquake, but generally it won't cause damage.
The magnitude of moderate-strong earthquakes is greater than 4.5 and less than 6. It belongs to an earthquake that can cause damage, but the degree of damage is also related to many factors such as focal depth and epicentral distance.
The magnitude of a strong earthquake is equal to or greater than 6. Among them, the magnitude greater than or equal to 8 is also called a giant earthquake.
Three elements of an earthquake:
Time, magnitude and epicenter of the 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.
In China, the intensity is divided into 12 degrees, and the impact and damage of earthquakes with different intensities are generally as follows:
People below three degrees have no feelings, only instruments can record them;
Third, people have feelings in the dead of night;
People who sleep at four to five degrees will wake up and the chandelier will shake;
Six times the ship capsized and the house was slightly damaged;
Seven to eight degrees houses are damaged and cracks appear on the ground;
Nine to ten degrees houses collapsed and the ground was seriously damaged;
Eleven to twelve degrees of devastating damage;
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 structures, causing human and animal casualties. 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.
Two major earthquake zones in the world
Pacific Rim Seismic Belt: It is distributed around the Pacific Ocean, like a huge garland, separating the mainland from the ocean.
Mediterranean-Himalayan seismic belt: From the Mediterranean Sea to the east, one branch passes through Central Asia to the Himalayas, then passes through the Hengduan Mountains in China to the south, passes through Myanmar, turns in an arc to the east, and reaches Indonesia, while the other branch extends from Central Asia to the northeast to kamchatka peninsula, showing scattered distribution.
The seismic activity in China is mainly distributed in 23 seismic zones in five regions. These five areas are: ① Taiwan Province Province and its adjacent waters; (2) Southwest China, mainly Tibet, western Sichuan and central and western Yunnan; (3) Northwest China, mainly distributed in Hexi Corridor of Gansu, Qinghai and Ningxia and the foothills of Tianshan Mountain; ④ North China, mainly located on both sides of Taihang Mountain, Wei Fen Valley, Yinshan-Yanshan area, Luzhong and Bohai Bay; ⑤ Southeast coastal areas such as Guangdong and Fujian. Taiwan Province province in China is located in the Pacific Rim seismic belt, Tibet, Xinjiang, Yunnan, Sichuan and Qinghai provinces are located in the Himalayan-Mediterranean seismic belt, and other provinces are located in related seismic belts.
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. Crustal 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.
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.
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.
Suppose an elastic body, such as a rock, is hit, which will produce two kinds of elastic waves to propagate from the sound source.
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 the water quality plane with 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 elasticity is called obeying Hooke's law, which is named after Newton's contemporary British mathematician robert hooke (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
Ten Earthquakes in China
No. Earthquake Name Date Time Magnitude (ms) Epicenter Intensity Focal Depth (km)
1 Xingtai earthquake in Hebei province1966.3.8 05: 29:14.06.8ix10.
Dongwang, Ningjin, Hebei1966.3.2216:19: 46.0 7.2x10
2 Yunnan Tonghai Earthquake1970.1.501:00: 37.07.7x13.
Sichuan Luhuo earthquake1973.2.618: 37: 08.37.9x17.
4 Zhaotong earthquake in Yunnan1974.5.103: 25:18.37.1IX14.
Haicheng earthquake in Liaoning province1975.2.0419: 36: 06.07.3ix12.
6 Yunnan Longling Earthquake1976.5.29 20: 23:18.07.3 Ix24
1976 . 5 . 29 22:00:22.5 7.4 IX 20
7 Tangshan earthquake in Hebei1976.7.28 03: 42: 53.8 7.8 Xi12
8 Sichuan Songpan Earthquake1976.8.16 22: 06: 46.27.2x24
1976 . 8 . 23 1 1:30: 10.0 7.2 VIII 23
9 Taiwan Province 92 1 Earthquake1999.9.2101:47.38.
10 Wenchuan earthquake, Sichuan, May 20081214: 28: 04.08.0x19.
The strongest earthquake 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 eight strongest earthquakes in human history since 1900. The following are the basic information of 8 major earthquakes (in order of magnitude):
1, Chile earthquake (1May 22, 960): 8.9 on the Richter scale (later revised to 9.5 on the Richter scale). 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.
2. Alaska Earthquake (1March 28th, 964): 9.2 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): 9. 1 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): 9.0 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 4th): 9.0 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, Sichuan, China (May 12, 2008): Richter scale 8, the epicenter was located in Wenchuan County, Aba Prefecture, which affected more than half of China and overseas. Heavy casualties and property losses.
★ Encyclopedia of earthquake self-help
There are likely to be aftershocks after the earthquake, and the location of the aftershocks may not be very close to the source. So learning to save yourself is one of the most important measures after the earthquake.
When an earthquake happens, it is very important to keep a clear head and a calm attitude. Only when you are calm, it is possible to judge the magnitude and distance of an earthquake with the earthquake knowledge you usually learn. Recent earthquakes often start with ups and downs and then swing from side to side. The teleseism is not so bumpy, but it is mainly shaking from side to side, with clear voice and small vibration. Generally, small earthquakes and distant earthquakes do not have to flee.
The latest self-help advice: Don't hide under the table.
There are ten items of knowledge about earthquake prevention in Japan's Earthquake Handbook, the first of which clearly says "hide under solid furniture". Therefore, Japanese teachers firmly believe that the best way is to "hide under the table". This idea is based on the premise that the earthquake in Japan ended dozens of seconds later and the ceiling did not fall.
When the ceiling of a building collapses due to a strong earthquake, it will crush tables, beds and other furniture. If people hide inside, the consequences are unimaginable. If people hide next to the furniture in a low posture, the furniture can be forced by the collapsed objects first, so that the people next to it can get living space.
When you encounter an earthquake while driving, you should leave the car as soon as possible. Many people who died in the parking lot during the earthquake were crushed to death in the car, but the people between the two cars were not injured at all. When a strong earthquake occurs, if you are in the parking lot, don't stay in the car, lest the collapsed ceiling crush the car and cause injury; You should lie prone beside the car, because the falling smallpox will not directly hit people, and may form a "living space" to increase your chances of survival.
School shock absorption
When you are in the playground or outdoors, you can squat down in place, protect your head with your hands, and pay attention to avoiding tall buildings or dangerous objects.
Don't go back to the classroom
Evacuation should be organized after the earthquake.
Never jump off a building! Don't stand outside the window! Don't go to the balcony
Classes should be held outdoors when necessary.