Chapter 2 2 Basic knowledge of earthquakes

Earthquake is the rapid vibration of the earth's surface, also known as ground motion in ancient times.Like wind, rain, lightning, landslides, and volcanic eruptions, it is a natural phenomenon that often occurs on the earth. There are many reasons for the vibration of the earth's surface. According to the cause of the earthquake, the earthquake can be divided into the following types: Earthquakes caused by dislocation and rupture of deep underground rock formations are called tectonic earthquakes.Such earthquakes occur most frequently and are most destructive, accounting for more than 90% of all earthquakes in the world.

Earthquakes caused by volcanic action, such as magma activity and gas explosions, are called volcanic earthquakes.Volcanic earthquakes can only occur in volcanic active areas, and such earthquakes only account for about 7% of the world's earthquakes. Earthquakes caused by the collapse of the roof of an underground cavern or mine are called subsidence earthquakes.The scale of this type of earthquake is relatively small, and the frequency is very small. Even if there are, they often occur in limestone areas with dense caves or large-scale underground mining areas. Earthquakes caused by activities such as reservoir water storage and oilfield water injection are called induced earthquakes.This type of earthquake only occurs in some specific reservoir areas or oilfield areas.

Ground vibrations caused by humans such as underground nuclear explosions and explosive blasting are called artificial earthquakes. In the process of unraveling the mystery of earthquakes, human beings, accompanied by rich imagination, produced various myths and legends. Around the 12th century, there were descriptions of so-called "earthquake bugs" in Japanese ancient almanacs. In 1710, when a Japanese book talked about the relationship between catfish and earthquakes, it was believed that the big catfish lay low on the ground, covering the land of Japan. When the catfish was angry, it moved its tail and fins, thus causing earthquakes.In ancient my country, there was also a special description of the special disaster of earthquake.There is a folk legend: There is a big turtle fish in the ground, carrying the earth on its back, and it will turn over after a long time, so the earth will shake, and the turtle fish turning over will cause an earthquake.

With the development of science, people's understanding of earthquakes has come out of mythology.Epicurus in ancient Greece believed that earthquakes were caused by the wind being sealed in the crust, resulting in the crust being divided into small pieces and moving continuously, that is, the wind caused the earthquake to shake the ground.Then came the Aeolian theory of Lucretius, that some great force of wind and air from outside or from the earth itself suddenly entered the void of the earth, and in this vast void there was first moaning and commotion and a whirlwind, Then the resulting power is ejected to the outside world, and at the same time, deep cracks appear in the earth, forming huge cracks, which is an earthquake.Aristotle also proposed that earthquakes are caused by sudden underground winds and hot flammable objects underground.

At the beginning of the 20th century, scientists began to study seismic waves in depth, which opened a new page for earthquake science and the entire earth science.There are three more influential hypotheses that have been put forward in succession: First, in 1911, Reed proposed that when the accumulated strain energy in the earth exceeds the strength of the rock, faults will be formed. After the fault is formed, the rock will bounce back elastically and return to its original state, so the accumulated energy will be released suddenly. This is the so-called "elastic rebound theory"; secondly, in 1955, Takeo Matsuzawa of Japan proposed that the heat conduction of underground rocks was not uniform, and part of the molten volume expanded, which squeezed the surrounding rocks, causing an exception to generate earthquakes in the surrounding rocks. This is The so-called "magma impact theory"; the third is the "phase transition theory" proposed by the American scholar Bridgman that the underground material transforms from one crystallization state to another crystallization state under a certain critical temperature and pressure, and the volume suddenly changes and earthquakes occur ".

Although the mystery of earthquakes has not been completely solved so far, with the cross-penetration and in-depth development of multiple disciplines such as physics, chemistry, paleontology, geology, mathematics and astronomy, the discipline of earthquakes has made great progress. Epicentre: The place within the Earth where an earthquake occurs. Focal Depth: The distance from the source vertically upwards to the surface is the focal depth.We call earthquakes that occur within 60 kilometers shallow-focus earthquakes; 60-300 kilometers are medium-focus earthquakes; and more than 300 kilometers are deep-focus earthquakes.At present, the deepest earthquake source on record is 720 kilometers.

Epicenter: The ground directly above the epicenter is called the epicenter.The place near the epicenter and its vicinity is called the epicenter area, also known as the extreme earthquake area.The distance from the epicenter to any point on the ground is called the epicentral distance (referred to as epicentral distance).Earthquakes within 100 kilometers of the epicenter are called local earthquakes; those within 1,000 kilometers are called near-earthquakes; Seismic waves: During an earthquake, the elastic waves that appear inside the earth are called seismic waves.It's like throwing a stone into the water, and the water waves will spread around in circles.

Seismic waves mainly include longitudinal waves and shear waves.The wave whose direction of vibration is consistent with the direction of propagation is a longitudinal wave (P wave).The longitudinal waves from the ground cause the ground to vibrate up and down.A wave whose vibration direction is perpendicular to the direction of propagation is a transverse wave (S wave).Shear waves from the subsurface can cause horizontal shaking of the ground.Shear waves are the main cause of building damage during earthquakes. Since the longitudinal wave propagates faster than the transverse wave in the interior of the earth, the longitudinal wave always reaches the surface first during an earthquake, while the transverse wave always lags one step behind.In this way, when a relatively large near-seismic event occurs, people generally feel a jolt up and down first, and then feel strong horizontal shaking after a few seconds to more than ten seconds.This is very important, because the longitudinal wave gives us a warning, telling us that the shear wave that will damage the building is coming soon, so take precautions quickly.

During the Tangshan earthquake in 1976, a cadre living in a building was suddenly awakened by the earthquake.Since this cadre usually knows a little about earthquakes, when he felt the shaking of the earthquake, he quickly got under the table. Five or six seconds later, the roof collapsed.Until noon, after he was rescued, he deeply felt that if he hadn't resolutely got under the table, he would have died long ago.He said earthquake knowledge saved his life. We cannot directly observe the conditions inside the Earth.However, human beings can speculate and analyze according to the law of seismic wave propagation in the earth's inner medium.

The outer layer of the earth is the crust, and below the crust, from the outside to the inside, are the mantle and the core.Their layered structure is like the shell, white and yolk of an egg.The core is divided into inner core and outer core.The outer core is in a liquid molten state, mainly composed of iron, nickel and some light elements, which can flow (convection). This layer of liquid outer core provides conditions for the rotation of the inner core.The core is solid, the composition is mainly iron, the internal pressure is extremely high, and the temperature is extremely high. The average thickness of the crust is 30-40 km

The thickness of the mantle is about 2900 km The radius of the earth's core is about 3400 kilometers Earthquakes are the result of movement of matter within the earth.This movement is reflected on the crust, which causes the crust to rupture and promotes the generation, development and activity of faults. "Where there are earthquakes, there must be faults, and where there are faults, there must be earthquakes." Fault activity induces earthquakes, and earthquakes promote the generation and development of faults. Therefore, earthquakes and faults are closely related. The faults in the earth's crust are dense like a network.Faults range in size and depth from small ruptures to thousands of kilometers of fault zones, which are aggregations of multiple faults. Faults associated with earthquakes are active faults, while faults that have moved in the past but are now stable are called dormant faults, or "dead" faults. Earthquakes are strong and weak.There are two rulers used to measure the intensity of earthquakes, one is called earthquake magnitude; the other is called earthquake intensity.For example, earthquake magnitudes are like light bulbs of different wattages, the higher the wattage, the brighter the light.Intensity is like the degree of light received in a room. For the same lamp, the closer it is to the lamp, the greater the luminosity, and the farther it is from the lamp, the smaller the luminosity. Earthquake magnitude is a measure of the size of an earthquake.Each earthquake has only one magnitude.It is divided according to the amount of energy released during an earthquake. The magnitude can be calculated from the records of seismic instruments. The higher the magnitude, the more energy released.The magnitude standard used in my country is the international common magnitude standard, called "Richter Scale". Earthquake grading standards vary from country to region. Generally, earthquakes less than magnitude 1 are called super-microseisms: those greater than or equal to magnitude 1 and less than magnitude 3 are called weak or microseismic; those greater than or equal to magnitude 3 and less than magnitude 4.5 are called felt earthquakes; those greater than or equal to magnitude 4.5 , Those with magnitude less than 6 are called medium-strong earthquakes; those with magnitudes greater than or equal to 6 and less than 7 are called strong earthquakes; those with magnitudes greater than or equal to 7 are called major earthquakes, and those with magnitude 8 and above are called giant earthquakes. So far, the largest earthquake recorded in the world is 8.9, which occurred in Chile in South America in 1960. Seismic Intensity: Seismic intensity refers to the degree to which the ground, buildings and other structures are damaged by an earthquake.For the same earthquake, the magnitude of the intensity is different in different regions.The closer to the source, the greater the damage and the higher the intensity; the farther away from the source, the smaller the damage and the lower the intensity. Less than three degrees: people have no feelings, only instruments can record; Three degrees: people have feelings in the dead of night; Four to five degrees: the sleeping person wakes up, the chandelier swings; Sixth degree: the container is dumped, and the house is slightly damaged; Sixth-seventh degrees: house damage, cracks in the ground; Nine-ten degrees: houses collapsed, ground damage is serious; Tenth-twelfth degrees: Devastating damage. China is located at the intersection of two major seismic belts in the world—the circum-Pacific seismic belt and the Eurasian seismic belt. It is squeezed by the Pacific plate, the Indian plate and the Philippine Sea plate, and the seismic fault zone is very developed.The tectonic location determines that earthquakes are frequent and severe.In the 20th century, there were three strong earthquakes above magnitude 8.5 in the world, two of which occurred in my country; two strong earthquakes that killed 200,000 people also occurred in my country, one was the Haiyuan earthquake in Ningxia in 1920, More than 230,000 people died; one was the 1976 Tangshan earthquake in Hebei, which killed more than 240,000 people.The death toll in these two earthquakes is also unique in the world. Since the last century, nearly 800 earthquakes of magnitude 6 or above have occurred in China, covering all provinces, autonomous regions, and municipalities except Guizhou, Zhejiang, Hong Kong, and Macao Special Administrative Regions. China is a country severely affected by earthquakes with high frequency, high intensity, shallow focus and wide distribution. Since 1900, more than 550,000 people have died in earthquakes in China, accounting for 53% of the global earthquake death toll; since 1949, more than 100 destructive earthquakes have hit 22 provinces (autonomous regions, municipalities directly under the central government), including the eastern region In 14 provinces, more than 270,000 people were killed, accounting for 54% of the death toll of various disasters in the country. The earthquake affected an area of ​​more than 300,000 square kilometers, and 7 million houses collapsed.The severity of earthquakes and other natural disasters constitutes one of China's basic national conditions. Statistics show that China's land area accounts for one-fifteenth of the world's land area, or about 6 percent; China's population accounts for about one-fifth of the world's population, or about 20 percent, which is less than 20%, but China's land earthquakes accounted for one-third of the world's land earthquakes, that is, about 33%, and the number of deaths caused by earthquakes reached more than 1/2 of the world's total.Of course, there are special reasons for this. First, China is densely populated and has a large population; China's economy is backward, and houses are not strong, easy to collapse, and easy to break; third, it is closely related to China's strong and frequent earthquake activities. According to statistics, since the 20th century, the number of deaths caused by earthquakes in China has accounted for 54% of all natural disasters in China, including floods, wildfires, mudslides, landslides, etc., more than 1/2.From the point of view of the death of personnel, the earthquake is the first of the group disasters; and the economic losses are caused by meteorological disasters (floods), and the economic losses caused by meteorological disasters are much larger than earthquakes. Earthquakes in China are mainly distributed in five regions: Taiwan, Southwest China, Northwest China, North China, southeast coastal areas and 23 seismic belts. "North China Earthquake Zone".Including all or part of Hebei, Henan, Shandong, Inner Mongolia, Shanxi, Shaanxi, Ningxia, Jiangsu, Anhui and other provinces.Among the five earthquake zones, its earthquake intensity and frequency rank second only to the "Qinghai-Tibet Plateau Earthquake Zone" in the country.Since the metropolitan area is located in this area, it is particularly interesting.According to statistics, there have been 5 well-documented earthquakes of magnitude 8 in this area; 18 earthquakes of magnitude 7-7.9 have occurred.In addition, it is located in an area with dense population, concentrated large cities, and developed politics, economy, culture, and transportation in China, so the threat of earthquake disasters is extremely serious. The North China seismic region is divided into four seismic zones. (1) Tancheng-Yingkou seismic zone.Including most or part of Liaoning, Hebei, Shandong, Jiangsu and other provinces from Suqian to Tieling.It is a strong seismic activity zone in the eastern mainland of my country. The Shandong Tancheng M8.5 earthquake in 1668, the Bohai M7.4 earthquake in 1969, and the Haicheng M7.4 earthquake in 1974 occurred in this seismic belt. According to records, there have been more than 60 earthquakes above M4.7 in this belt.Among them, there were 6 earthquakes of magnitude 7-7.9 and 1 earthquake of magnitude 8 or above. (2) North China Plain seismic zone.The southern boundary is roughly located on the Xinxiang-Bengbu line, the northern boundary is located on the south side of Yanshan Mountain, the west boundary is located on the east side of Taihang Mountain, and the east boundary is located on the western edge of the Lower Liaohe-Liaodong Bay Depression, extending southward to the southeast of Tianjin, and the economically southeast to the Suzhou area .It is the seismic belt that threatens the Beijing, Tianjin and Tang areas the most. The 8.0-magnitude Sanhe earthquake in Hebei in 1679 and the 7.8-magnitude Tangshan earthquake in 1976 occurred in this belt.According to statistics, more than 140 earthquakes with a magnitude of 4.7 or above occurred in this area.Among them, there were 5 earthquakes of magnitude 7-7.9, and one earthquake of magnitude 8 or above. (3) Fenwei earthquake zone.It starts from Xuanhua-Huai'an Basin and Huailai-Yanqing Basin in Hebei Province in the north, and passes through Yangyuan Basin, Yuxian Basin, Datong Basin, Xinding Basin, Lingqiu Basin, Taiyuan Basin, Linfen Basin, Yuncheng Basin to Weihe Basin in the south.It is another strong seismic activity zone in eastern my country. The 8.0-magnitude Hongdong earthquake in Shanxi in 1303 and the 8.0-magnitude earthquake in Huaxian, Shaanxi in 1556 both occurred in this belt. The Zhangbei M6.2 earthquake in January 1998 is also near this zone.Since records began, there have been about 160 earthquakes with a magnitude of 4.7 or above in this seismic belt.Among them, there were 7 earthquakes of magnitude 7-7.9 and 2 earthquakes of magnitude 8 or above. (4) Yinchuan-Hetao seismic zone.Yinchuan, Wuda, Dengkou to the west of Hohhot in the west and north of the Hetao area. In 1739, the 8.0-magnitude Yinchuan earthquake in Ningxia occurred in this belt.In this seismic zone, historical earthquake records began in 849 AD. Due to the lack of historical records, according to the available data, there are a total of about 40 earthquakes of magnitude 4.7 or above recorded in this zone.Among them, there were 9 earthquakes of magnitude 6-6.9 and 1 earthquake of magnitude 8. "Qinghai-Tibet Plateau Earthquake Zone".Including the vast plateau area surrounded by the Hindu Kush Mountains, West Kunlun Mountains, Altun Mountains, Qilian Mountains, Helan Mountains-Liupan Mountains, Longmen Mountains, Himalayas and the east wing of the Hengduan Mountains.It involves all or parts of Qinghai, Tibet, Xinjiang, Gansu, Ningxia, Sichuan, and Yunnan, as well as parts of the former Soviet Union, Afghanistan, Pakistan, India, Bangladesh, Myanmar, Laos and other countries. This earthquake area is the largest earthquake area in my country, and it is also the area with the strongest seismic activity and frequent occurrence of major earthquakes.According to statistics, there have been 9 earthquakes with magnitudes above 8 and 78 earthquakes with magnitudes 7-7.9.Both rank first in the country. In addition, the "Xinjiang earthquake area" and "Taiwan earthquake area" are also two earthquake areas in my country where earthquakes of magnitude 8 occurred.It is also well known that strong and destructive earthquakes frequently occur here.Generally speaking, the earthquake zone in Xinjiang is sparsely populated and economically underdeveloped.Although there are more and more frequent strong earthquakes, most of them occurred in mountainous areas, and the loss of people and property caused by them is much smaller than that of several earthquake zones in eastern my country. It is worth mentioning the "Southeast Coastal Outer Seismic Belt" in the "South China Seismic Zone", where the 8.0-magnitude earthquake in Quanzhou, Fujian Province in 1604 and the 7.5-magnitude earthquake in Qiongshan, Guangdong Province in 1605 occurred in history.But from then to now more than 300 years, no significant destructive earthquake occurred. The current situation of earthquake prediction level in my country can be summarized as follows: We have some understanding of the principles and laws of earthquake breeding and occurrence, but we have not yet fully understood; we can predict certain types of earthquakes to a certain extent, but we cannot predict all earthquakes. Mid- and long-term forecasts have a certain degree of credibility, but the success rate of short-term forecasts is still relatively low. Due to the country's emphasis on earthquake prediction and its clear mission, my country's earthquake prediction has been in the advanced ranks of the world after a generation of efforts.In the fourth active period of earthquakes, it has successfully made short-term forecasts for several major earthquakes such as Haicheng. Therefore, it has been listed as the only country that has successfully made short-term forecasts for earthquakes after being reviewed by UNESCO. history. However, from a global perspective, earthquake prediction is still in the exploratory stage, and the law of earthquake development has not been fully grasped. Our prediction is mainly based on the observation data and earthquake cases accumulated over the years to make empirical predictions.Therefore, it inevitably has great limitations.For this reason, Article 16 of the "Law of the People's Republic of China on Earthquake Prevention and Disaster Mitigation" stipulates that the state implements a unified release system for earthquake forecasts. Short-term earthquake forecasts and impending earthquake forecasts are issued by the people's governments of provinces, autonomous regions, and municipalities directly under the Central Government in accordance with the procedures prescribed by the State Council. Opinions of any unit or professional engaged in earthquake work concerning short-term earthquake prediction or imminent earthquake prediction shall be reported to the earthquake administrative department of the State Council or the department or institution in charge of earthquake work of the local people's government at or above the county level for handling in accordance with the provisions of the preceding paragraph, and shall not be handled without authorization. spread to society. In our country, the right to issue earthquake forecasts rests with the government.Any level of administrative units, research units, observation stations, scientists and any individuals belonging to the earthquake system have no right to release news about earthquake prediction.