Earthquakes are localized sharp rupture of the medium inside the Earth, generating seismic waves, thus causing ground vibration within a certain range of phenomena. Earthquake (earthquake) is the rapid vibration of the Earth's surface, in ancient times also known as the ground movement. It is like wind, rain, lightning, landslides, volcanic eruptions, is a natural phenomenon that often occurs on earth. Earth vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes that occur under the sea or in coastal areas can cause huge waves called tsunamis. Earthquakes are extremely frequent, with about 5 million earthquakes occurring globally each year, and have a great impact on society as a whole.
The place where an earthquake wave originates is called the source (focus). Vertical projection of the source on the ground, the ground from the source of the nearest point is called the epicenter. It is to receive the earliest part of the vibration. The depth from the epicenter to the epicenter is called the depth of the epicenter. Generally, earthquakes with a depth of less than 70 kilometers are called shallow earthquakes, earthquakes with a depth of 70-300 kilometers are called medium earthquakes, and earthquakes with a depth greater than 300 kilometers are called deep earthquakes. For earthquakes of the same size, since the depth of the epicenter is not the same, it is also not the same, and the degree of damage caused to the ground is also not the same. The shallower the source, the greater the damage, but the smaller the reach, and vice versa.
Destructive earthquakes are generally shallow. For example, the 1976 Tangshan earthquake had a depth of 12 kilometers.
The most intense ground vibration of a destructive earthquake is called the extreme seismic zone, which is often the epicenter of the region.
The distance between a place and the epicenter is called the epicenter distance. Earthquakes with an epicenter distance of less than 100 kilometers are called local earthquakes, earthquakes between 100-1000 kilometers are called near earthquakes, earthquakes greater than 1000 kilometers are called distant earthquakes, of which the farther the distance from the epicenter of the place to be affected by the impact of the destruction of the smaller.
The ground vibration caused by earthquakes is a complex movement, which is caused by longitudinal and transverse waves **** with the results of the action. In the epicenter area, the vertical wave makes the ground up and down. Transverse waves make the ground shake horizontally. As the longitudinal wave propagation speed is faster, the attenuation is faster, the transverse wave propagation speed is slower, the attenuation is slower, so far from the epicenter of the place, often do not feel up and down, but can feel up to the horizontal shaking.
When a larger earthquake occurs in a certain place, within a period of time, a series of earthquakes tend to occur, of which the largest one is called the main shock, the main shock before the earthquake is called a foreshock, the main shock after the earthquake is called an aftershock.
Earthquakes have a certain spatial and temporal distribution pattern.
From the time point of view, earthquakes have an active period and a quiet period of alternating cyclical phenomena.
From a spatial point of view, the distribution of earthquakes in a certain band, called the seismic zone, mainly concentrated in the Pacific Rim and the Mediterranean - Himalayan two major seismic zones. The Pacific Ocean seismic belt almost concentrated in the world more than 80% of the shallow source earthquake (0 km ~ 70 km), all of the medium source (70 km ~ 300 km) and deep source earthquake, the release of seismic energy accounted for about 80% of all the energy.
Earthquake magnitude and intensity
Earthquake research department in the report of an earthquake in a region, often to be crowned with the occurrence of XX magnitude of the earthquake, the intensity of X degrees and so on. The magnitude and intensity of an earthquake are not the same thing.
Magnitude
Magnitude refers to the size of the earthquake, is characterized by the strength of the earthquake measure, is measured by seismographs to determine the amount of energy released by each seismic activity to determine. Magnitude is usually indicated by the letter M. China is currently using the earthquake magnitude standard, is the international common Richter scale, *** divided into 9 levels. Usually, earthquakes with a magnitude less than 2.5 are called small earthquakes, earthquakes with a magnitude of 2.5-4.7 are called felt earthquakes, and earthquakes with a magnitude greater than 4.7 are called destructive earthquakes. For every 1.0 magnitude difference in magnitude, the energy difference is about 30 times; for every 2.0 magnitude difference, the energy difference is about 900 times. For example, a magnitude 6 earthquake releases energy equivalent to that of the atomic bomb dropped by the United States on Hiroshima, Japan. A magnitude 7 earthquake is equivalent to 30 magnitude 6 earthquakes, or 900 magnitude 5 earthquakes, with a difference of 0.1 magnitude, releasing an average of 1.4 times the difference in energy.
Earthquakes can be divided into the following categories according to magnitude:
Weak earthquakes have a magnitude of less than 3. If the epicenter is not very shallow, this kind of earthquake is usually not easy for people to notice.
A felt earthquake has a magnitude equal to or greater than 3, and less than or equal to 4.5. These earthquakes can be felt, but generally do not cause damage.
A moderately strong earthquake has a magnitude greater than 4.5 and less than 6. They are earthquakes that can cause damage, but the severity of the damage is also related to the depth of the epicenter, the epicenter distance, and many other factors.
A strong earthquake has a magnitude equal to or greater than 6. Those with a magnitude greater than or equal to 8 are also known as giant earthquakes.
Three elements of earthquakes:
Moment of onset, magnitude, and epicenter
Seismic intensity
Earthquakes of the same size do not necessarily cause the same amount of damage, and the same earthquake may cause different damage in different places. In order to measure the damage caused by earthquakes, scientists have created another "ruler", the seismic intensity scale. In the Chinese earthquake intensity scale, the human feeling, general housing damage and other phenomena are described, which can be used as the basic basis for determining the intensity. Factors affecting the intensity are the magnitude, depth of the epicenter, distance from the epicenter, ground conditions and stratigraphic structure.
Generally speaking, only on the intensity and source, the relationship between the magnitude, the larger the magnitude of the epicenter, the shallower the source, the greater the intensity. Generally speaking, after an earthquake, the epicenter of the area of the heaviest damage, the highest intensity; this intensity is called the epicenter of the intensity. From the epicenter to the surrounding area, the intensity of the earthquake gradually decreases. Therefore, an earthquake has only one magnitude, but the damage it causes is different in different areas. In other words, an earthquake can be divided into several areas with different intensities. This is the same as a bomb exploding, near and far the degree of destruction is different. The amount of explosives of the bomb, as if the magnitude of the earthquake; bomb damage to different locations, as if the intensity.
For example, on February 10, 1990, Changshu - Taicang occurred 5.1 magnitude earthquake, some people say in Suzhou is 4, in Wuxi is 3, this is wrong. No matter where, it can only be said that a 5.1 magnitude earthquake occurred in Changshu-Taicang, but this earthquake, in Shaxi Town of Taicang, the seismic intensity was 6 degrees, in Suzhou the seismic intensity was 4 degrees, and in Wuxi the seismic intensity was 3 degrees.
China's intensity is divided into twelve degrees, different intensity of the earthquake, its impact and damage are roughly as follows:
Less than three degrees of people do not feel, only the instrument can be recorded;
three degrees of the people in the dead of night when people feel;
four to five degrees of the sleeper will be woken up, the chandelier shaking;
six degrees of utensils toppled over, the house slightly damaged;
seven to five degrees of the earthquake will be the first time in the world that the earthquake has occurred in the past two years.
Seven to eight degrees of housing damage, cracks in the ground;
nine to ten degrees of housing collapse, ground damage serious;
eleven to twelve degrees of devastating destruction;
For example, the 1976 Tangshan earthquake, the magnitude of the magnitude of 7.8, the epicenter of the intensity of the eleventh degree; affected by the earthquake in Tangshan, the seismic intensity of the city of Tianjin for the eighth degree, the Beijing City Intensity of Six degrees, and then farther to Shijiazhuang, Taiyuan, etc. is only four to five degrees.
Earthquake phenomena
When an earthquake occurs, the most basic phenomenon is the continuous vibration of the ground, mainly obvious shaking.
People in extreme seismic zones sometimes first feel a jump up and down before they feel a big shake. This is because seismic waves are traveling from the ground to the ground, and the longitudinal waves arrive first. Horizontal waves are followed by large-amplitude horizontal shaking, which is the main cause of seismic hazards. 1960 Chilean earthquake, the largest shaking lasted 3 minutes. Earthquake disasters caused by the first is the destruction of houses and structures, resulting in casualties, such as the 1976 earthquake in Tangshan, Hebei, China, 70% to 80% of the buildings collapsed, heavy casualties.
Earthquakes also have a great impact on the natural landscape. The most important consequence is the emergence of faults and cracks on the ground. The surface faults of large earthquakes often extend tens to hundreds of kilometers, often with more obvious vertical and horizontal misalignment, reflecting the tectonic changes at the epicenter of the characteristics (see the thick tail earthquake, the San Francisco earthquake). However, not all surface ruptures are directly linked to the movement of the earthquake source; they may also be secondary effects due to seismic waves. Especially in areas with thick surface sedimentary layers, ground fractures often occur at the edges of slopes, riverbanks, and along roads, often due to topographic factors, where shaking loosens and disintegrates the topsoil under conditions where there is no support on one side. The shaking of an earthquake causes the topsoil to sink, and shallow groundwater is squeezed and rises to the surface along the cracks, creating the phenomenon of sand and water bubbling up. Large earthquakes can change the local topography, or uplift, or sink. It can cause urban and rural roads to chap, railroad tracks to twist, and bridges to break. In modern cities, water and power outages and communication disruptions are caused by ruptured underground pipes and severed cables. Leakage of gas, toxic gases and radioactive substances can lead to fires and secondary disasters such as toxic and radioactive pollution. In mountainous areas, earthquakes can also cause landslides and landscapes, often resulting in tragedies that bury villages and towns. Collapsing rocks clog rivers and form seismic lakes upstream, and during the Great Kanto Earthquake in Japan in 1923, mudslides occurred in Kanagawa Prefecture, sliding down valleys as far as 5 kilometers.
Two major seismic zones around the world
The Pacific Rim Seismic Zone: Distributed around the Pacific Ocean, like a giant wreath separating the continents from the ocean.
Mediterranean-Himalayan Seismic Belt: from the Mediterranean Sea to the east, one through Central Asia to the Himalayas, and then south through China's Hengduan Mountain Range, over Myanmar, in an arc turning east to Indonesia, the other from Central Asia to the northeast, to Kamchatka, the distribution of the more sporadic.
China's seismic activity is mainly distributed in five regions of the 23 seismic zones. These five regions are: ① Taiwan Province and its adjacent waters; ② Southwest region, mainly Tibet, western Sichuan and west-central Yunnan; ③ Northwest region, mainly in Gansu Hexi Corridor, Qinghai, Ningxia, the north and south foothills of the Tianshan Mountains; ④ North China, mainly in the two sides of the Taihang Mountains, Fenwei Valley, Yinshan - Yanshan area, central Shandong and Bohai Bay; ⑤ Southeast coast of Guangdong, Fujian and other places. China's Taiwan Province is located on the Pacific Rim seismic belt, Tibet, Xinjiang, Yunnan, Sichuan, Qinghai and other provinces and regions are located in the Himalayan-Mediterranean seismic belt, and other provinces and regions are in the related seismic belt.
Types of earthquakes
Earthquakes are divided into two categories: natural earthquakes and artificial earthquakes. In addition, some special circumstances can also produce earthquakes, such as a large meteorite impact on the ground (meteorite impact earthquake). Cause the earth's surface vibration of many reasons, according to the causes of earthquakes, earthquakes can be divided into the following types:
1. tectonic earthquakes
Due to the depth of the underground rock rupture, the fault to the long-term accumulation of the energy dramatically released, in the form of seismic waves spread out in all directions, to the ground caused by the shaking of the ground is called tectonic earthquakes. These earthquakes occur most often, the destructive force is also the largest, accounting for about 90% of the world's earthquakes.
2. Volcanic earthquakes
Due to volcanic effects, such as magma activity, gas explosions caused by earthquakes known as volcanic earthquakes. Volcanic earthquakes can only occur in the volcanic activity area, such earthquakes only account for about 7% of the world's earthquakes.
3. collapse earthquakes
Due to the collapse of underground caverns or the top of the mine caused by earthquakes known as collapse earthquakes. The scale of such earthquakes is relatively small, the number of times is also very small, even if there is, but also tends to occur in the cavern densely populated limestone areas or large-scale underground mining mining areas.
4. Induced earthquakes
As a result of reservoir storage, oil field water injection and other activities triggered by earthquakes known as induced earthquakes. These earthquakes occur only in certain specific reservoir reservoirs or oil field areas.
5. Artificial earthquakes
Underground nuclear explosions, explosives blasting and other human-induced ground vibration is called artificial earthquakes. Artificial earthquakes are caused by man-made activities. Such as industrial blasting, underground nuclear explosions caused by vibration; high-pressure water injection in deep wells as well as large reservoirs to increase the pressure of the earth's crust after storage, and sometimes induced earthquakes.
The place where the seismic wave originates is called the epicenter. The vertical projection of the epicenter on the ground is called the epicenter. The depth from the epicenter to the epicenter is called the depth of the epicenter. Usually the depth of the epicenter is less than 70 kilometers called shallow earthquakes, the depth of 70-300 kilometers called medium-source earthquakes, the depth of more than 300 kilometers called deep earthquakes. Destructive earthquakes are usually shallow earthquakes. For example, the 1976 Tangshan earthquake had a depth of 12 kilometers.
Earthquake expertise
The fluctuations we are most familiar with are observed in water waves. When a stone is thrown into a pond the water is disturbed and a ripple extends outward centered on the point where the stone enters the water. This wave train is caused by the movement of particles of water near the water wave. However the water does not flow in the direction in which the water wave propagates; if a cork floats on the surface of the water, it will bounce up and down, but will not be removed from its original position. This perturbation is transmitted continuously by the simple back-and-forth motion of the water particles, passing the motion from one particle to the one further ahead. In this way, the water wave carries the energy of the stone striking the broken surface of the water towards the edge of the pool and stirs up waves on the shore. Earthquake motion is quite similar. The shaking we feel is the vibration of elastic rock produced by the energy of seismic waves.
Assuming that an elastic body, such as a rock, is struck, two types of elastic waves are generated that propagate outward from the source.
The first type of wave has physical properties exactly like sound waves. Sound waves, and even ultrasound, are transmitted in air by alternating squeezing (pushing) and expanding (pulling). Because liquids, gases, and solid rock are just as capable of being compressed, the same type of wave can travel through bodies of water such as oceans and lakes as well as the solid earth. During earthquakes, this type of wave travels outward from a fracture at equal speed in all directions, alternately compressing and tensioning the rock through which they pass, with its particles moving forward and backward in the direction in which these waves are propagating; in other words, these particles move perpendicular to the wave front. The amount of forward and backward displacement is called amplitude. In seismology, this type of wave is called a P-wave, or longitudinal wave, and it is the first wave to arrive.
Unlike air, which can be compressed but not sheared, elastic rock allows a second type of wave to propagate by causing objects to shear and twist. Earthquakes produce this second arriving wave called an S-wave. During the passage of an S-wave, rocks behave quite differently than they do during the propagation of a P-wave. This is because S-waves involve shearing rather than crushing, causing rock particles to move across the direction of transport. These rock motions can be in a vertical or horizontal plane, and they are similar to the transverse motions of light waves.The simultaneous presence of P and S waves makes the seismic wave train a unique combination of properties that make it physically different from light or sound waves. Because shear motion is not possible in liquids or gases, S-waves cannot propagate through them.
The distinct properties of P and S waves can be used to detect the presence of fluid zones deep within the Earth.
S-waves are polarized, and only those light waves that vibrate laterally (up and down, horizontally, etc.) in a particular plane can pass through a polarizing lens. The light waves that pass through are called plane-polarized light. Sunlight passing through the atmosphere is not polarized, i.e., there is no preferred transverse direction of vibration of light waves. However the refraction of crystals or through specially manufactured plastics such as polarizing eyes can make non-polarized light plane-polarized.
As S-waves travel through the Earth, they are refracted or reflected when they encounter a tectonic discontinuity and polarize their direction of vibration. When the polarized S-wave rock particles move only in the horizontal plane, they are called SH waves. When the rock particles move in a water plane containing the direction of wave propagation, the S-wave is called an SV-wave.
Most rocks, if not forced to vibrate at too large an amplitude, have linear elasticity, i.e., the deformation due to an applied force varies linearly with the applied force. This linear elasticity manifestation is called obeying Hooke's law, and is based on the English mathematician Robert Hooke (1635~1635), who was a contemporary of Isaac Newton. It is named after Robert Hooke (1635~1703), an English mathematician who was a contemporary of Isaac Newton. Similarly, rocks will deform proportionally more in response to increased forces during earthquakes. In most cases, the deformation will remain in the linear elastic range and the rock will return to its original position at the end of the shaking. However, important exceptions sometimes occur during seismic events, such as when strong shaking occurs in soft soils, where permanent deformation remains and fluctuating deformation is not always followed by a return of the soil to its original position, in which case seismic intensity is more difficult to predict.
The motion of elasticity provides excellent insight into how energy changes as seismic waves pass through rock. The energy associated with the compression or extension of a spring is the elastic potential, and the energy associated with the motion of the spring's parts is the kinetic energy. The total energy at any given time is the sum of the elastic and kinetic energies. For an ideal elastic medium, the total energy is a constant. At the position of maximum wave amplitude, the energy is all elastic potential energy; when the spring oscillates to an intermediate equilibrium position, the energy is all kinetic energy. We have assumed that there is no friction or dissipative forces present, so that once the reciprocating elastic vibration begins, it will continue at the same amplitude. This is of course an ideal situation. In an earthquake, the friction between moving rocks gradually heats up and dissipates some of the fluctuating energy, and unless new energy is added, like a vibrating spring, the earth's vibrations will gradually cease. Measurements of the energy dissipation of seismic waves provide important information about the inelastic properties of the Earth's interior, but there are factors other than frictional dissipation that contribute to the phenomenon of seismic shaking diminishing with distance.
As sound waves propagate with an expanding spherical surface in front of their wavefront, they carry sound that diminishes with distance. Similar to the expanding water waves in a pond, we observe that the height, or amplitude, of the water waves also decreases outward. The decrease in wave amplitude is due to the attenuation of the initial energy as it spreads wider and wider; this is called geometric diffusion. This type of diffusion also weakens seismic waves passing through the Earth's rocks. Unless there are special circumstances, the farther seismic waves travel outward from the source, the more their energy decays.
Famous earthquakes
China's top 10 earthquakes
No. Earthquake name Date Time Magnitude (Ms) Intensity at epicenter Depth at epicenter (Km)
1 Xingtai, Hebei, China Earthquake 1966.3.8 05:29:14.0 6.8 IX 10
Dongwang, Ningjin, Hebei, China 1966.3.22 16:19:46.0 7.2 X 10
2 Yunnan Tonghai Earthquake 1970.1.5 01:00:37.0 7.7 X 13
3 Sichuan Fuhuo Earthquake 1973.2.6 18:37:08.3 7.9 X 17
4 Yunnan Zhaotong Earthquake 1974.5.11 03:25:18.3 7.1 IX 14
5 Liaoning Haicheng Earthquake 1975.2.04 19:36:06.0 7.3 IX 12
6 Yunnan Longling Earthquake 1976.5.29 20:23:18.0 7.3 IX 24
1976.5.29 22:00:22.5 7.4 IX 20
7 Hebei Tangshan Earthquake 1976.7.28 03 : 42:53.8 7.8 XI 12
8 Sichuan Songpan Earthquake 1976.8.16 22:06:46.2 7.2 IX 24
1976.8.23 11:30:10.0 7.2 VIII 23
9 Taiwan 921 Earthquake 1999.9.21 01:47 7.3 8
10 Sichuan Wenchuan Earthquake 2008.5.12 14:28:04.0 8.0 X 19
The Strongest Earthquakes of the Twentieth Century
An earthquake measuring 8.5 on the Richter Scale struck the waters off the island of Sumatra on March 28, 2005 (09:09 GMT on 29 March 2005), which was one of the eight strongest earthquakes to occur in the history of mankind since 1900. The following is the basic situation of the eight major earthquakes (in order of magnitude):
1, the Chilean earthquake (May 22, 1960): 8.9 on the Richter scale (later revised to 9.5 on the Richter scale). It occurred in the central Chilean sea and triggered a tsunami and volcanic eruption. The earthquake*** killed 5,000 people and left 2 million homeless.
2. The Great Alaskan Earthquake (March 28, 1964): 9.2 on the Richter scale. This triggered a tsunami that killed 125 people and caused $311 million in property damage. Strong tremors were felt in most of Alaska, the Yukon Territory in Canada, and Columbia.
3, the United States, Alaska earthquake (March 9, 1957): 9.1 on the Richter scale, occurred in the United States, Alaska, Andrea Island and Unak Island in the vicinity of the sea. The earthquake caused the 200-year dormant volcano Visevidov to erupt and triggered a massive 15-meter-high tsunami that affected as far as the island of Hawaii.
4. (Tied) The Great Indonesian Earthquake (December 26, 2004): 9.0 on the Richter scale, occurred in the province of Aceh, located on the island of Sumatra, Indonesia. The earthquake triggered a tsunami that swept through Sri Lanka, Thailand, Indonesia and India, leaving about 300,000 people missing or dead.
4, (tied) Russia earthquake (November 4, 1952): 9.0 on the Richter scale. The earthquake triggered a tsunami that rippled through the Hawaiian Islands, but caused no injuries.
5, the Great Ecuadorian Earthquake (January 31, 1906): 8.8 on the Richter scale, occurred in Ecuador and off the coast of Colombia. The earthquake triggered a strong tsunami, resulting in more than 1,000 deaths. The earthquake was felt along the coast of Central America, San Fransisco and Japan.
6, (tied) Indonesia earthquake (March 28, 2005): 8.7 on the Richter scale, the epicenter was located in the sea north of the Indonesian island of Sumatra, not far from the location of the magnitude 9.0 earthquake occurred three months ago. It has killed 1,000 people so far, but did not cause a tsunami.
6, (tied) the United States, Alaska earthquake (February 4, 1965): 8.7 on the Richter scale. The earthquake triggered a tsunami up to 10.7 meters high that swept across the island of Shumanya.
7, China's Tibet earthquake (August 15, 1950): 8.6 on the Richter scale. 2000 houses and temples were destroyed. India's Yarlung Tsangpo River suffered the worst damage, with at least 1,500 deaths.
8, (tied) Russian earthquake (February 3, 1923): 8.5 on the Richter scale, occurred in Kamchatka, Russia.
9, (tied) Indonesia earthquake (February 3, 1938): magnitude 8.5 on the Richter scale, occurred in the sea near Banda, Indonesia. The earthquake triggered a tsunami and volcanic eruption, resulting in heavy loss of life and property.
10, (tied) Russia's Kuril Islands earthquake (October 13, 1963): 8.5 on the Richter scale, and spread to Japan and Russia and other places.
11, China Sichuan Wenchuan earthquake (May 12, 2008): 8 on the Richter scale, the epicenter was located in Wenchuan County, Aba Prefecture, and spread to half of China and overseas and other places. There were heavy casualties among people and property.
★Earthquake self-rescue book
After the earthquake, there is a high probability of aftershocks, and the location of the aftershocks may not be very close to the epicenter. So learning to save yourself is one of the very important measures after an earthquake.
When an earthquake occurs, it is vital to have a clear head and a calm and collected attitude. Only calm, it is possible to use the earthquake knowledge usually learned to determine the size and proximity of the earthquake. Near quakes often start with up and down bumps, followed by swaying from side to side. Far from the quake but less up and down bumps feel, but to sway mainly left and right, and sound crisp, vibration is small.
Latest self-rescue advice: Don't hide under a table
The first of the 10 articles in Japan's Earthquake Handbook on how to avoid earthquakes clearly states, "Hide under sturdy furniture. Therefore, Japanese teachers firmly believe that the best way to avoid earthquakes is to "hide under the table". This idea is based on the premise that earthquakes in Japan tend to end after a few seconds, and ceilings don't fall.
When the ceiling of a building collapses due to a strong earthquake, furniture such as tables and beds will be crushed, and the consequences are unimaginable if a person hides in them. If a person hides next to a piece of furniture in a low position, the piece of furniture can be subjected to the force of the collapsed object first, allowing the person to get a space to live.
Driving in an earthquake, but also to quickly leave the car, many earthquakes in the parking lot of the people killed, are in the car was crushed alive, in the two cars between the people, but not hurt. When a strong earthquake occurs, if you are in the parking lot, do not stay in the car, so as not to collapse the ceiling flattened car, causing injury; should be lying down to hide in the car next to the falling ceiling pressure on the car, not directly hit the person, may form a piece of 'living space', increasing the chance of survival.
School earthquake avoidance
In the playground or outdoors, you can crouch in place, hands to protect the head, pay attention to avoid tall buildings or dangerous objects.
Do not return to the classroom.
There should be an organized evacuation after an earthquake.
Do not jump from buildings! Do not stand outside the window! Do not go out on the balcony!
Classes should be held outside if necessary.