For example, in ancient Greek mythology, Poseidon was the god of earthquakes. There is also a saying in South America that the giants supporting the world move and cause earthquakes. In ancient Japan, it was believed that there lived a big catfish under the island of Japan. Once the catfish is unhappy, as long as the tail is swept away, there will be an earthquake in Japan. In addition, there are legends about animals living underground in Egypt and India.
With the progress of science, no one will believe this superstition now.
In fact, an earthquake is a kind of ground movement, that is, the vibration of the earth's surface. There are many reasons for the vibration of the earth's surface, which can be man-made, such as nuclear explosion, artillery fire, mechanical vibration and so on. It can also be caused by nature, such as tectonic earthquake, volcanic earthquake, collapse earthquake and so on.
According to the different causes of earthquakes, we can divide them into five categories:
1. Tectonic earthquake: The reason of tectonic earthquake is that underground strata are subjected to geostress. When the local stress is so great that the rock stratum can't bear it, it will suddenly break or dislocation quickly. When the rock stratum is broken or dislocated, it will stimulate seismic waves that spread around, and when local seismic waves reach the surface, it will cause ground vibration. 85%-90% of earthquakes in the world and all earthquakes that cause major disasters belong to tectonic earthquakes.
2. Volcanic earthquake: an earthquake caused by volcanic eruption.
3. Reservoir earthquake: an earthquake occurred in the reservoir area due to water storage and discharge.
4. Collapse earthquake: earthquake caused by stratum collapse.
5. Artificial earthquake: an earthquake caused by human activities such as nuclear explosion and artillery fire.
The abbreviation of intensity, that is, the ground vibration intensity of a certain place within the earthquake range. (or interpreted as the degree of earthquake impact and damage). The strength of ground vibration directly affects the strength of people's feelings, the reaction degree of objects, the damage or destruction of houses, the changes of ground landscape and so on. Therefore, the determination of strength mainly depends on the macroscopic investigation and qualitative description of the above aspects.
Conceptually, earthquake intensity and earthquake magnitude are strictly different and should not be confused. The magnitude represents the magnitude of the earthquake itself, which is determined by the seismic wave energy emitted by the source. The same earthquake should have only one value. The same earthquake has different intensity in different places and is influenced by various local natural and man-made conditions. For earthquakes of the same magnitude, if the source is shallow, the epicentral distance is short and the intensity is generally high. Similarly, whether the local geological structure is stable, whether the soil structure is solid, and whether the buildings and other structures are strong and earthquake-resistant are also directly related to the local intensity. There are five factors that affect the earthquake intensity in a place: magnitude, focal depth, epicentral distance, geological structure and buildings. In an earthquake, people often emphasize the intensity of the epicenter (or extreme earthquake zone).
In order to evaluate the strength in practical work, it is necessary to formulate a unified evaluation standard. This specified standard is called seismic intensity scale. 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 Table (Brief)
I degree; No sense-only instruments can record;
Second degree; Micro-feeling-individual sensitive person feels in complete stillness;
ⅲ degree; Feeling less-several people in the room feel resting and the clothes rack swings slightly;
ⅳ degree; A lot of feelings-most people are indoors, a few people are outdoors, hanging objects are swaying, and unstable utensils are creaking;
ⅴ degree; Wake up-most people can feel it outdoors, animals are restless, doors and windows are rustling, and cracks appear on the wall.
ⅵ degree; Panic-people stand unsteadily, livestock flee, utensils fall, simple sheds are damaged, and steep slopes slide;
ⅶ degree; Damage to the house-slight damage to the house, damage to the archway and chimney, cracks on the surface, sand blasting and bubbling;
ⅷ degree; Building damage-many houses were damaged, a few damaged roadbed collapsed, and underground pipelines were broken;
ⅸ degree; Buildings are generally destroyed-most houses are destroyed, a few collapse, arches and chimneys collapse, and railway tracks bend;
X degrees; Buildings were generally destroyed-houses were pushed down, roads were destroyed, a lot of rocks collapsed, and water waves rushed to the shore;
ⅶ degree; Destruction-a large number of houses collapsed, a large section of levee collapsed, and the surface changed greatly;
ⅹ Ⅱ degree; Landscape changes-all buildings are generally destroyed, the terrain changes dramatically, and animals and plants are destroyed;
In the early days, the intensity scale was based entirely on the macro consequences of earthquakes. However, the perfect macro-intensity table, after all, uses qualitative standards, and can not rule out the subjective factors of observers. For this reason, people have been looking for a physical standard to evaluate the intensity, which is not only closely related to the earthquake damage phenomenon, but also convenient for instrument measurement. The first physical quantity studied is the peak ground acceleration during the earthquake. Because it is generally believed that the damage caused by earthquake is caused by seismic inertia force, which is determined by ground acceleration. In this way, the peak ground acceleration is associated with each intensity level. The results show that for every degree of intensity increase, the acceleration is about doubled. The physical quantity added to the intensity table later is the peak ground speed. China's current intensity table adds acceleration and velocity data.
Earthquake is the vibration of the earth's surface caused by the sudden release of slowly accumulated energy in the earth. When the huge pressure on the earth's crust caused by the energy accumulated in the internal movement of the earth exceeds the limit that the rock stratum can bear, the rock stratum will suddenly break or dislocate, so that the accumulated energy will be released sharply and spread in all directions in the form of seismic waves, and an earthquake will be formed. A series of smaller aftershocks often occur after a strong earthquake.
Earthquakes are also divided into natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes, which are caused by the fracture and dislocation of rocks in the deep underground, so that the long-term accumulated energy is suddenly released and spread outward in the form of seismic waves, causing house shaking and ground vibration. Tectonic earthquakes account for more than 90% of the total number of earthquakes. Followed by earthquakes caused by volcanic eruptions, known as volcanic earthquakes, accounting for about 7% of the total number of earthquakes. In addition, earthquakes will also occur in some special circumstances, such as cave collapse (collapse earthquake) and large meteorites hitting the ground (meteorite impact earthquake).
Earthquake self-help:
First, do a good job in earthquake prediction.
There are omens before the earthquake. Now we know that this omen is: the groundwater becomes turbid, the flowers turn over, bubbles, and tastes bad; Chickens, ducks, pigs and sheep run around screaming; The mouse escaped and the fish jumped around on the water. This situation indicates that the earth's crust will bend, fold and fracture, and earthquakes will occur. At present, it is difficult for any country in the world to accurately predict earthquakes, and China's earthquake prediction can be said to be in the leading position in the world. Most earthquake predictions can show that earthquakes may occur in a certain area within a certain period of time. 1976, before the Tangshan earthquake, there was an earthquake of magnitude 6 or above in Haicheng, Liaoning Province. Before the earthquake, the government repeatedly urged all indoor personnel to be placed in the earthquake shed and repeatedly preached earthquake knowledge to minimize casualties. Therefore, the key to earthquake prevention lies in timely prediction, full preparation, and mastering the knowledge of earthquake rescue.
Second, make a family 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 normal life. Therefore, it is necessary to make a good family earthquake prevention plan.
First of all, it is necessary to eliminate hanging objects in high places indoors, items placed vertically on cabinets and wooden frames, and change their placement positions and ways so that they are not easily knocked down and injured. At the same time, all inflammable and explosive articles should be removed. The bed should be removed from the glass window. Stick a piece of shatterproof tape on the window. Shockproof equipment should be packed in a place where it is easy to grab. In order to get out of danger, you may only have time to grab one item, and all the emergency items are in it, which will help you tide over the difficulties. These items include cash, drinking water and medicines to prevent influenza and dysentery.
Third, schools should be prepared before the earthquake.
Knowledge of earthquake prevention should be popularized in primary and secondary schools, and schools in earthquake-stricken areas should have earthquake prevention training programs. If there is an earthquake in class, the teacher should immediately give students a simple and clear instruction to close their eyes and squat under the desk. When an earthquake is likely to occur in an area within a certain period of time. Families and schools should communicate frequently. Schools with conditions in the earthquake zone should prepare disaster relief items, such as first-aid medical supplies, cold-proof and rain-proof supplies and tools. , and distributed by class or classroom until reaching individuals.
How can we protect ourselves if there is an earthquake?
Once an earthquake happens, if you are outdoors, don't go near any tall buildings or trees that may collapse, such as buildings, chimneys, telephone poles, etc. It is safer to leave bridges and interchange roads and go to open fields. Although earthquakes are natural disasters that cause casualties, they are not inevitable. If we can grasp the opportunity and use the knowledge of earthquake prevention, we can protect ourselves, such as observing the abnormal restlessness of birds and animals before the earthquake; Squatting under the table during an earthquake can reduce the damage caused by the earthquake. It can be seen that it is very important to learn earthquake knowledge. & lt/CA & gt； earthquake
abstract
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. It originates from a point underground, which is called the focus.
Vibration comes from the source and propagates in the earth. The closest point on the ground to the earthquake source is called the epicenter, which is the place where the vibration was first received. 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 occurrence of earthquakes
The phenomenon that the earth's internal medium breaks violently locally, producing seismic waves, thus causing ground vibration in a certain range. The place where the earthquake started is called the source, and the ground directly above the source is called the epicenter. The strongest ground motion of a destructive earthquake is called the extreme earthquake zone, which is often the area where the epicenter is located.
seismism
Earthquake phenomenon 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, tracks 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.
Terminology and related knowledge of earthquakes
The structure of the earth is like an egg, which can be divided into three layers. The middle layer is the "yolk"-the core; In the middle is "egg white"-mantle; The outer layer is the "eggshell"-the shell. Earthquakes usually occur in the earth's crust. The earth is constantly rotating, and the interior of the crust is constantly changing. The resulting force led to the deformation, fracture and dislocation of the crust and strata, so an earthquake occurred. The place where an earthquake occurs underground is called the source. The place from the source vertically upward to the surface is called the epicenter. The distance from the epicenter to the source is called the focal depth. Earthquakes with a focal concentration of less than 70km are shallow earthquakes, earthquakes with a focal concentration of 70km to 300km are moderate earthquakes, and earthquakes with a focal concentration of more than 300km are deep earthquakes. The earthquake with the deepest focal depth was 1963, which occurred in the northern part of Irian Jaya province, Indonesia, with a focal depth of 786 kilometers. Earthquakes of the same size have different damage to the ground because of different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa.
The distance from a place to the epicenter is called epicentral distance. Earthquakes with epicentral distance less than 1000 km are called near earthquakes, earthquakes with epicentral distance between 100- 1000 km are called near earthquakes, and earthquakes with epicentral distance greater than1000 km are called teleseisms. Among them, the farther the epicentral distance, the smaller the impact and damage.
The ground vibration caused by earthquake is a complex movement, which is the result of the joint action of longitudinal wave and shear wave. In the epicenter, longitudinal waves made the ground jump up and down. Shear waves make the ground shake horizontally. Because longitudinal waves travel faster and decay faster, while shear waves travel slower and decay slower, they are far from the epicenter, so you often can't feel the jump up and down, but you can feel the horizontal shaking.
The magnitude of the earthquake itself is expressed by magnitude, and the magnitude is determined according to the energy of elastic waves released during the earthquake. China generally uses the Richter scale. Earthquakes with magnitude less than 2.5 are usually called small earthquakes, earthquakes with magnitude 2.5-4.7 are called inductive earthquakes, and earthquakes with magnitude greater than 4.7 are called destructive earthquakes. For every magnitude difference of 1, the energy released by the earthquake is about 30 times different. For example, an earthquake of magnitude 7 is equivalent to 30 earthquakes of magnitude 6 or 900 earthquakes with magnitude difference of 0. 1, and the average difference of energy released is 1.4 times.
When a large earthquake occurs in a certain place, a series of earthquakes often occur within a period of time, the largest of which is called the main earthquake, the earthquake before the main earthquake is called the foreshock, and the earthquake after the main earthquake is called the aftershock.
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.
The degree of ground vibration at a certain point during an earthquake is called seismic intensity. China divides the earthquake intensity into 12 degrees.
Although both magnitude and intensity can reflect the strength of an earthquake, their significance is the same. The same earthquake has only one magnitude, but the intensity varies from place to place, and the intensity value varies from place to place. For example, on1February 1990 10, an earthquake of magnitude 5. 1 occurred in Changshu-Taicang. Some people say that Suzhou is level 4 and Wuxi is level 3, which is wrong. No matter where it is, it can only be said that an earthquake of magnitude 5. 1 occurred in Changshu-Taicang, but this time, the earthquake intensities in shaxi town, Taicang, Suzhou and Wuxi were 6 degrees, 4 degrees and 3 degrees respectively.
Seismic intensity is a frequently used term. There are qualitative and quantitative standards for dividing strength.
Cause of 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
Earthquakes caused by dislocation and fracture of rock strata in deep underground are called tectonic earthquakes (Figure1-1). 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
The earthquake caused by the collapse of underground caves or the top of mines is called 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.