According to the chronology of earthquakes in China in 1990s, there were 8 137 recorded earthquakes in China, among which

Destructive earthquakes 1004, 22 earthquakes with magnitude 8 or above, accounting for about 35% of the earthquakes recorded in the global history. In the 20th century, there were 650 earthquakes of magnitude 6 or above in China, including magnitude 7-7.9.

There are more than 8 earthquakes 100, with an average of 9 earthquakes 1 00 per year. The largest earthquake is magnitude 8. The Chayu M _ S 6 earthquake in Tibet (1August 950 15).

According to statistics, the area of high intensity area of earthquakes of magnitude 7 or above in China is 3. 12 × 106km2, 70% of the big cities with a population of over one million are located in this area, especially some important cities such as Beijing, Tianjin, Xi, Taiyuan, Hohhot, Kunming, Urumqi, Yinchuan, Lhasa and Shantou.

(2) the west is strong and the east is weak, and the east is strong and the north is weak.

Generally speaking, the earthquakes in Chinese mainland have obvious distribution law that the west is strong and the east is weak, and the west is more than the east. Mainly as follows:

(1) During the 20th century 100, there were nine large earthquakes with magnitude of 8 or above, except for two in the sea area near Taiwan Province Province, the other seven occurred in western provinces such as Xinjiang, Tibet, Ningxia and Gansu.

(2) 1949 From 2000 to 2000, there were 52 earthquakes of magnitude 7 or above, 20 occurred in the eastern waters of Taiwan Province Province, and 25 occurred in Xinjiang, Tibet, Inner Mongolia, Gansu, Qinghai, Yunnan, Sichuan and other provinces.

(3) The areas with earthquake intensity above 7 degrees in China are mainly distributed in the west, and the areas with earthquake intensity above 7 degrees in the east are much less than those in Taiwan Province Province.

(4) According to the measurement results of fault displacement and ground stress, the intensity of tectonic activity in the west of China is 5-6 times higher than that in the east, and the total energy released by the earthquake is about 70 times that in the east.

(5) During the 20th century 100, there were more than 300 earthquakes of magnitude 6 or above in Chinese mainland, and more than 250 earthquakes in the western region, accounting for 83%.

Figure 1-5 Death toll from earthquake disaster in affected countries (1949 ~ 2000)

The seismic activity in North China is obviously stronger than that in South China, which is caused by many factors. Among them, the different properties of basement rocks are an important factor. The basement of North China is mainly crystalline granite, which is brittle. The basement age in South China is relatively new, and the rocks are mainly ductile shallow metamorphic rocks.

(3) Disaster losses are characterized by heavy east and light west.

The population, buildings and cities in the eastern part of China are dense, the economy is developed, and the population in the western part is sparse, which determines that the earthquake disaster losses in the eastern part are bound to be serious, especially in the north-south earthquake belt, North China and Taiwan Province Province (Figure 1-5).

Subordinate graph

1-5 shows that the death toll in areas with population density ≥400 persons /km2 is 10 times that in areas with population density of 200 ~ 400 persons /km2; It's population density

100 ~ 200 people /km2 area is 100 times. Generally speaking, the number of earthquake deaths is directly proportional to the population density. Therefore, small earthquakes may also cause great disasters in eastern China.

Since 1949, the development and changes of the death population and economic losses in China show that the overall death population has shown a downward trend, while the economic losses have shown a rapid upward trend.

China's earthquakes are shallow in focus and high in intensity, which have been suffered by other provinces and regions except Zhejiang and Guizhou provinces.

Earthquakes of magnitude 6 or above. China's seismic intensity zoning map shows that areas with seismic intensity ⅷ or above account for about 1/2 of the national area. Cities located in this intensity zone account for the total number of cities in China.

46%, of which megacities with a population of 6.5438+0 million account for 70%; Judging from the population threatened by the earthquake, the population living in the vast high-intensity area is close to 9.

One billion. It can be seen that the severity of hidden dangers of earthquake disasters.

(d) The occurrence of earthquakes has obvious periodicity.

Due to the difference of tectonic movement between the east and the west, the period of seismic activity in the east is generally longer than that in the west. The research shows that the earthquake period in the eastern region is about

300 years, West 100 ~ 200 years, Taiwan Province Province for several decades. Since the 20th century, Mainland China has experienced four active periods of major earthquakes. Each active scene generally lasts 10.

From 0 to 20 years, there is a phenomenon of alternating high tide and low tide. Now it has entered a new activity curtain.

Take the earthquake activity in the eastern region as an example. 15 ~ 18

Earthquakes in North China in the 20th century were mainly concentrated in Wei Fen, Shanxi-Yanqing-Huairou, Yanshan piedmont and Qinling Mountains, but there were no earthquakes of magnitude 6 or above in North China Plain. 19

At the beginning of the century, the active area of strong earthquakes shifted, and all earthquakes of magnitude 6 or above occurred in North China Plain and Yanshan piedmont area. According to many in-situ stress measurements, it is found that in the 1960s and 1970s, the direction of stress was NNE.

The stress in the strike structural belt decreases, while the stress in the east-west structural belt increases, resulting in earthquakes of magnitude 6 or above, such as Bohai 7. 4 (1970) and Haicheng 7. 3 (

1975) and Tangshan M7.8 (1976). At the age of 20

At the end of the century, according to the in-situ stress measurement results of Tangshan, Xingtai, Luanxian and Shunyi, Beijing, the intensity of crustal activity in this area has weakened.