What is the earthquake intensity of 7 degrees?

In the past when science was underdeveloped, people often relied on the power of gods to explain the causes of earthquakes. In our country, there is a popular legend among the people. They say that there is a big turtle living underground. After a long time, the big turtle wants to turn over. As long as the big turtle turns over, the earth will tremble. Analyzing this legend from a modern perspective is simply absurd. But China is not the only country that holds this view.

For example, in ancient Greek mythology, Psheton, the sea god, was the god of earthquakes. There is also a legend in South America that the movement of the giant who supports the world caused earthquakes. In ancient Japan, it was believed that there was a big catfish living under the Japanese island. Once the catfish was unhappy, it would just sweep its tail away, and an earthquake would occur in Japan. In addition, Egypt and India also have legends about animals living underground and causing mischief.

With the advancement of science, no one will believe such superstitious statements now.

In fact, an earthquake is an earthquake, a vibration on the earth's surface. There are many causes of vibrations on the earth's surface, which can be man-made, such as nuclear explosions, cannon fire, mechanical vibrations, etc.; they can also be natural causes, such as tectonic earthquakes, volcanic earthquakes, collapse earthquakes, etc.

According to the different causes of earthquakes, we can divide earthquakes into five categories:

1. Tectonic earthquakes: The cause of tectonic earthquakes is that underground rock layers are affected by ground stress. When the geostress is too great and the rock layer cannot bear it, sudden and rapid rupture or dislocation will occur. When the rock layer ruptures or displaces, it will stimulate a seismic wave that spreads around. When the seismic wave reaches the surface, it will Cause the ground to shake. 85%-90% of the world's earthquakes and all earthquakes that cause major disasters are tectonic earthquakes.

2. Volcanic earthquake: An earthquake caused by a volcanic eruption.

3. Reservoir earthquake: Earthquakes occur in the reservoir area due to the storage and release of water from the reservoir.

4. Subsidence earthquake: An earthquake caused by the collapse of the ground.

5. Artificial earthquakes: Earthquakes caused by man-made activities such as nuclear explosions and artillery fires

Abbreviated as intensity, it refers to the intensity of ground vibration at a certain location within the affected area when an earthquake occurs. (Or interpreted as the degree of earthquake impact and damage). The strength of ground vibration directly affects the strength of people's feelings, the degree of reaction of objects, the degree of damage or destruction of houses, changes in ground landscape, etc. Therefore, the identification of intensity mainly relies on macroscopic inspection and qualitative description of the above aspects.

Conceptually, earthquake intensity and earthquake magnitude are strictly different and should not be confused with each other. Magnitude represents the size of the earthquake itself. It is determined by the energy of seismic waves emitted by the earthquake source. There should only be one value for the same earthquake. The intensity of an earthquake varies from place to place and is affected by various local natural and man-made conditions. For earthquakes of the same magnitude, if the source is shallower and the epicentral distance is shorter, the intensity will generally be higher. Similarly, whether the local geological structure is stable, whether the soil structure is solid, and whether houses and other structures are strong and earthquake-resistant are directly related to the high or low local intensity. (Five factors that affect the intensity of an earthquake in a place: magnitude, focal depth, epicentral distance, geological structure, and buildings). During an earthquake, people often emphasize the intensity of the epicenter (or the epicenter).

In order to assess the intensity in actual work, it is necessary to formulate a unified assessment standard. This prescribed standard is called the seismic intensity scale. There are several different intensity scales used in countries around the world. What is more popular in Western countries is the modified Mercalli intensity scale, referred to as the M.M. intensity scale, which has 12 intensity levels from degree I to degree ***. In Japan, no sensitivity is rated as 0 degrees, and sensitivity is divided into degrees I to VII, with a maximum of 8 levels. The former Soviet Union and China both divided intensity scales into 12 intensity levels. China revised its earthquake intensity scale in 1980 (see table).

China Earthquake Intensity Scale (brief)

I degree; no sense - only instruments can record;

II degree; slightly sensitive - some are sensitive People feel it when they are completely still;

Ⅲ degree; rarely feel it - a few people in the room feel it when they are still, and the hanging object swings slightly;

IV degree; mostly feel it - Most people indoors, but a few people outdoors feel it, hanging objects swing, unstable utensils rattle;

V degree; awakening - most people outdoors feel it, domestic animals are restless, doors and windows rattle, and wall surfaces Cracks appear

VI degree; Panic - people stand unsteadily, livestock escape, utensils fall, simple shacks are damaged, steep landslides;

VII degree; house damage - the house is minor Damage, archway, chimney damage, cracks on the ground surface, sand blasting and water leakage;

VIII degree; damage to buildings - many houses were damaged, a few damaged roadbeds, collapsed, and underground pipes ruptured;

Ⅸ degree; general destruction of buildings - most houses were destroyed, a few collapsed, archways, chimneys, etc. collapsed, and railway tracks were bent;

Ⅹ degree; general destruction of buildings - houses collapsed, roads were damaged, and a large number of rocks collapsed , big waves crashing onto the shore;

Ⅺ degree; destruction - a large number of houses collapsed, large sections of roadbeds and embankments collapsed, and great changes occurred on the ground surface;

ⅩⅡ degree; mountains and rivers changed scenery - everything Buildings were generally destroyed, and the terrain changed dramatically and animals and plants were destroyed;

The early intensity scale was entirely based on the macroscopic consequences of earthquakes to classify intensity levels. However, no matter how perfect the macro-intensity scale is formulated, it uses qualitative criteria after all, and the subjective factors of the observer cannot be eliminated. For this reason, people have been looking for a physical standard to evaluate the intensity. This physical standard should not only be closely related to the earthquake damage phenomenon, but also be easy to measure with instruments. The first physical quantity studied is the peak ground acceleration during an earthquake. Because it is generally believed that the damage caused by earthquakes is caused by the force of seismic inertia, and this force (which cannot be called inertial force) is determined by the ground acceleration. This adds a peak ground acceleration to each level of intensity. The results show that for each degree increase in intensity, acceleration approximately doubles. The physical quantity later added to the intensity scale was the peak ground speed. China's current intensity scale has included two physical quantities: acceleration and speed.

An earthquake is a vibration of the earth's surface caused by the sudden release of slowly accumulated energy in the earth's interior. When the energy accumulated in the movement of the earth exerts a huge pressure on the earth's crust that exceeds the limit that the rock layers can withstand, the rock layers will suddenly break or dislocate, causing the accumulated energy to be released sharply and spread in all directions in the form of seismic waves. , forming an earthquake. A strong earthquake is often followed by a series of smaller aftershocks.

Earthquakes are also divided into two categories: natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes. They are caused by the cracking and dislocation of rocks deep underground, which rapidly releases the energy accumulated over a long period of time and spreads out in the form of seismic waves, causing shaking and ground shaking on the ground. Tectonic earthquakes account for more than 90% of the total earthquakes. Followed by earthquakes caused by volcanic eruptions, called volcanic earthquakes, accounting for about 7% of the total number of earthquakes. In addition, earthquakes may also occur under certain special circumstances, such as cave collapse (collapse earthquakes), large meteorites impacting the ground (meteor impact earthquakes), etc.

Earthquake self-rescue:

1. Make good earthquake forecasts.

There are warning signs before earthquakes. The omens we now know are: the groundwater becomes muddy, turbulent, bubbling, and smells bad; chickens, ducks, pigs, and sheep run around and scream; rats escape, and fish jump around on the surface of the water. This situation indicates that the earth's crust will bend, wrinkle and break, and an earthquake will occur. At present, it is difficult for any country in the world to accurately predict earthquakes. my country's earthquake prediction can be said to be in a leading position in the world. Most earthquake forecasts can indicate that an earthquake is likely to occur in a certain area within a certain period of time. In 1976, before the Tangshan earthquake, an earthquake of magnitude 6 or above occurred in the Haicheng area of ??Liaoning Province. Before the earthquake, the government repeatedly urged everyone indoors to be placed in earthquake sheds and repeatedly preached earthquake awareness. As a result, there were no casualties. Numbers were reduced to a minimum. Therefore, the key to preventing earthquakes lies in timely forecasting, adequate preparation, and mastering the knowledge of earthquake rescue.

2. Develop a home earthquake prevention plan.

Once an earthquake occurs, our power supply, water supply, heating system, transportation system, daily necessities supply system, information system, and medical and health system may be damaged to some extent, affecting people's lives. Normal life, therefore, we should develop a home earthquake prevention plan.

First of all, remove hanging objects from high places in the room, items placed vertically on cabinets and wooden shelves, and change their placement and placement to make them less likely to knock over and injure people. At the same time, all flammable and explosive items must be removed. The bed should be moved farther away from the glass window. Put anti-shatter strips on the windows. Keep your shock gear bag somewhere easily accessible. To get out of danger, you may only have time to grab one item. Urgent items are among them, which will help you get through the difficulties. These items include cash, drinking water, and medicines to prevent influenza and dysentery.

3. Schools should be prepared for earthquakes.

Earthquake prevention knowledge should be popularized in primary and secondary schools. Schools in earthquake areas should have earthquake prevention training plans. If an earthquake occurs during class, teachers should immediately give students a simple and clear instruction to close their eyes on the spot. Bend under the desk.

An earthquake may occur in an area within a certain period of time. Families and schools should communicate frequently. Qualified schools in earthquake areas should prepare disaster relief items, such as first-aid medical supplies, cold and rainproof supplies, tools, etc., and distribute them according to classes or classrooms until they are in the hands of individuals.

How to protect yourself if an earthquake occurs?

Once an earthquake occurs, if you are outside, do not get close to buildings, chimneys, telephone poles or any other tall buildings or trees that may collapse. It is safer to leave bridges and overpasses and go to open fields. . Although earthquakes are natural disasters that cause casualties, they are not unpreventable. If you can seize the opportunity and use earthquake prevention knowledge, you can protect yourself. For example, you can observe the abnormal restlessness of birds and animals before an earthquake occurs; squatting under a table when an earthquake occurs can reduce the damage caused by the earthquake. It can be seen that learning earthquake knowledge is very important. Earthquake

Overview

An earthquake (earthquake) is a rapid vibration of the earth's surface. In ancient times, it was also called an earthquake. It is a natural phenomenon that often occurs on the earth, just like wind, rain, lightning, landslides, and volcanic eruptions. It originates from a certain point underground, which is called the focus.

Vibrations are emitted from the source and propagate through the Earth. The point on the ground closest to the earthquake source is called the epicenter, and it is the earliest point to receive vibrations. 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. About 5 million earthquakes occur around the world every year, which has a great impact on the entire society.

The occurrence of earthquakes

A sudden local rupture of the earth's internal medium, generating seismic waves, thereby causing ground vibration within a certain range. The place where an earthquake begins is called the source, and the ground directly above the source is called the epicenter. The place where the ground vibrations of a destructive earthquake is strongest is called the epicenter, and the epicenter is often the area where the epicenter is located.

Earthquake Phenomenon

Earthquake Phenomenon When an earthquake occurs, the most basic phenomenon is continuous vibration of the ground, mainly obvious shaking. People in extreme earthquake areas sometimes feel the first thing they feel is jumping up and down before they feel big shaking. This is because seismic waves are transmitted from the ground to the surface, and longitudinal waves arrive first. The transverse waves then produce large-amplitude horizontal shaking, which is the main cause of earthquake disasters. During the 1960 Chilean earthquake, the largest shaking lasted for 3 minutes. Disasters caused by earthquakes first destroy houses and structures, causing casualties of humans and animals. For example, in the 1976 Tangshan earthquake in Hebei, China, 70% to 80% of buildings collapsed, causing heavy casualties. Earthquakes also have a great impact on the natural landscape. The main consequence is faults and cracks in the ground. The surface faults of major earthquakes often extend for tens to hundreds of kilometers, and often have obvious vertical and horizontal offsets, which can reflect the characteristics of tectonic changes at the source (see the Nobi Earthquake and the San Francisco Earthquake). But not all surface fractures are directly linked to the movement of the earthquake source. They may also be due to secondary effects caused by seismic waves. Especially in areas with thick surface sedimentary layers, ground cracks often appear on the edges of slopes, river banks, and on both sides of roads. This is often due to topographic factors. Shaking without support on one side causes the topsoil to loosen and crack. The shaking of the earthquake causes the topsoil to sink, and the shallow groundwater will be squeezed and rise to the surface along the ground fissures, causing the phenomenon of sand blasting and water eruption. Major earthquakes can cause local terrain changes, uplift or subsidence. Cracked urban and rural roads, twisted railroad tracks, and broken bridges. In modern cities, water outages, power outages, and communication disruptions occur due to ruptured underground pipes and cut 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 landslides, often burying villages and towns. Collapsed rocks blocked rivers and formed earthquake lakes upstream. During the Great Kanto Earthquake in Japan in 1923, a mudslide occurred in Kanagawa Prefecture, sliding down the valley as far as 5 kilometers.

Earthquake terminology and related knowledge

The structure of the earth is like an egg, which can be divided into three layers. The central layer is the "egg yolk" - the core; the middle layer is the "egg white" - the mantle; the outer layer is the "eggshell" - the crust. Earthquakes generally occur in the earth's crust. The earth is constantly rotating and revolving, and at the same time, the interior of the earth's crust is also constantly changing. The resulting force causes the crustal rock layers to deform, fracture, and move, causing earthquakes. The place where an earthquake occurs underground is called the source. The point vertically upward from the earthquake source to the surface is called the epicenter. The distance from the epicenter to the source of the earthquake is called the focal depth. Earthquakes with a focal concentration less than 70 kilometers are shallow earthquakes, earthquakes between 70 and 300 kilometers are intermediate earthquakes, and earthquakes more than 300 kilometers are deep earthquakes. The earthquake with the deepest focal depth was a magnitude 5.8 earthquake that occurred in the northern seas of Irian Jaya Province, Indonesia in 1963, with a focal depth of 786 kilometers. For earthquakes of the same size, due to different focal depths, the degree of damage to the ground is also different. The shallower the earthquake source, the greater the damage, but the smaller the spread, and vice versa.

The distance between a certain place and the epicenter is called the epicentral distance.

Earthquakes with an epicenter distance less than 100 kilometers are called local earthquakes, earthquakes between 100 and 1,000 kilometers are called near earthquakes, and earthquakes greater than 1,000 kilometers are called telequakes. The farther away the epicenter is, the greater the impact and damage. Small.

The ground vibration caused by earthquakes is a complex movement, which is the result of the simultaneous action of longitudinal waves and transverse waves. In the epicenter, longitudinal waves cause the ground to move up and down. Transverse waves cause the ground to shake horizontally. Since longitudinal waves propagate faster and attenuate faster, transverse waves propagate slower and attenuate slower, so in places far away from the epicenter, you often cannot feel up and down beating, but you can feel horizontal shaking.

The size of the earthquake itself is expressed by magnitude. The magnitude is determined based on the amount of elastic wave energy released during the earthquake. my country generally uses the Richter scale. Usually, earthquakes with a magnitude less than 2.5 are called minor 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 magnitude difference, the energy released by the earthquake differs by about 30 times. For example, a magnitude 7 earthquake is equivalent to 30 magnitude 6 earthquakes, or equivalent to 900 magnitude 5 earthquakes. The magnitude difference is 0.1 magnitude, and the energy released differs by an average of 1.4 times.

When a large earthquake occurs somewhere, a series of earthquakes often occur within a period of time. The largest earthquake is called a mainshock. The earthquake that occurs before the mainshock is called a foreshock. Earthquakes that occur after an earthquake are called aftershocks.

Earthquakes have certain spatiotemporal distribution patterns. From a time perspective, earthquakes have a periodic phenomenon of alternating active periods and quiet periods. From a spatial perspective, earthquakes are distributed in a certain belt shape, called seismic zones, mainly concentrated in the two major seismic zones of the Pacific Rim and the Mediterranean-Himalayas. The Pacific seismic zone concentrates almost 80% of the world's shallow source earthquakes (0 kilometers to 70 kilometers), all intermediate source earthquakes (70 kilometers to 300 kilometers) and deep source earthquakes, and the seismic energy released accounts for about 80% of all energy.

The intensity of the ground shaking at a certain point during an earthquake is called earthquake intensity. my country divides earthquake intensity into 12 degrees.

Although both magnitude and intensity can reflect the strength of an earthquake, they have different meanings. The same earthquake has only one magnitude, but its intensity varies from place to place. Different places have different intensity values. For example, on February 10, 1990, a magnitude 5.1 earthquake occurred in Changshu-Taicang. Some people said that it was magnitude 4 in Suzhou and magnitude 3 in Wuxi. This is wrong. No matter where it is, it can only be said that a 5.1-magnitude earthquake occurred in Changshu-Taicang. However, the earthquake intensity in Shaxi Town of Taicang was 6 degrees, in Suzhou it was 4 degrees, and in Wuxi it was 3 degrees.

Earthquake intensity is a term often used. There are qualitative and quantitative standards for classifying intensity.

Causes of earthquakes

There are many reasons for vibrations on the earth's surface. According to the causes of earthquakes, earthquakes can be divided into the following types:

1. Tectonic earthquakes

Earthquakes caused by the dislocation and rupture of rock layers deep underground are called tectonic earthquakes (Figure 1-1). This type of earthquake occurs the most frequently and is the most destructive, accounting for more than 90% of earthquakes in the world.

2. Volcanic earthquakes

Earthquakes caused by volcanism, such as magma activity, gas explosions, etc., are called volcanic earthquakes. Volcanic earthquakes are only possible in volcanic active areas, and these earthquakes account for only about 7% of the world's earthquakes.

3. Collapse earthquake

An earthquake caused by the collapse of the roof of an underground cave or mine is called a collapse earthquake. Such earthquakes are relatively small in scale and infrequent. Even if they occur, they often occur in limestone areas with dense caves or mining areas with large-scale underground mining.

4. Induced earthquakes

Earthquakes caused by activities such as reservoir storage and oil field water injection are called induced earthquakes. This type of earthquake only occurs in certain reservoir areas or oil field areas.

5. Artificial earthquakes

Ground vibrations caused by underground nuclear explosions, explosive blasting, etc. are called artificial earthquakes. Artificial earthquakes are earthquakes caused by human activities. For example, vibrations caused by industrial blasting and underground nuclear explosions; 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 originate is called the earthquake source. The vertical projection of the earthquake source on the ground is called the epicenter. The depth from the epicenter to the source is called the focal depth. Usually, earthquakes with a focal depth less than 70 kilometers are called shallow earthquakes, earthquakes with a depth of 70-300 kilometers are called intermediate earthquakes, and earthquakes with a depth greater than 300 kilometers are called deep earthquakes. Destructive earthquakes are generally shallow earthquakes. For example, the focal depth of the 1976 Tangshan earthquake was 12 kilometers.

The most powerful earthquake

The Tangshan earthquake was one of the ten natural disasters in the 20th century. An earthquake occurred in Tangshan, Hebei Province, People's Republic of China at 3:42 on July 28, 1976. The magnitude of the earthquake was 7.8, and the epicenter intensity was XI. At 18:43 on the same day, another earthquake of magnitude 7.1 occurred in Luan County, more than 40 kilometers away from Tangshan, with an epicenter intensity of IX. This earthquake occurred in an industrial city with a dense population and caused very serious losses.

Most of the buildings in Tangshan urban area were basically flattened or seriously damaged, the railway tracks were twisted in a serpentine shape, a large number of cracks occurred on the ground surface, water sprayed out, sand and collapsed, and there was a glowing phenomenon before the earthquake. 242,769 people died and 164,851 were injured. Neighboring Tianjin also suffered eighth to ninth degree damage. The affected areas affected 14 provinces, municipalities and autonomous regions including Liaoning, Shanxi, Henan, Shandong and Inner Mongolia, with a radius of damage of approximately 250 kilometers. Research on earthquake source physics shows that the source dislocation process of this earthquake is complex.

The strong earthquake disrupted traffic, paralyzed communications, and cut off water and power in the city. Emergency repairs of lifeline projects such as communications, water supply, power supply, and traffic restoration are one of the most urgent tasks for disaster relief in Tangshan. Accordingly, the central government quickly arranged tasks for all professional systems to support each other. The post and telecommunications, railway, transportation, electric power, municipal construction and other departments took immediate action to ensure the smooth progress of the restoration and reconstruction of the above-mentioned system projects. It was midsummer at the time of the earthquake. The weather was hot and rainy. Human and animal corpses were rapidly decaying. The epidemic was very serious. The epidemic prevention work adopted comprehensive measures including surprise treatment, controlling the source of disease infection, improving the environment, eliminating germ vectors, vaccination, and improving personnel resistance. , the implementation of the combination of military and civilian, the combination of specialized groups, and the simultaneous development of local and foreign countries, to eliminate the epidemic before it occurs, thus creating a miracle of no major epidemic after the disaster.