Stimulate recent tectonic activities

In the last chapter, we discussed the location of geotectonic units in the area where the pipeline project is located, and mainly studied the Yanshan movement and the structural development and evolution of different geotectonic units in previous geological historical periods, which had a certain impact on the formation and activities of geological disasters in pipeline engineering. However, the modern tectonic activities since Cenozoic are more closely related to the formation and activities of geological disasters, so this issue will be discussed emphatically.

(a) Recent plate activity near the Chinese mainland block.

Yanshan Movement laid the basic structural pattern of Chinese mainland and ended the long-term "North-South confrontation" structural differentiation pattern between North China and South China, but a bigger new geological event is brewing. The China block at the eastern end of Eurasia plate, the Indian subcontinent plate in the southwest and the Pacific plate in the east came from SSW and NEE directions respectively, and collided with the China block in the late Eocene and swooped down. The continuous tectonic activity led to the compression and uplift of the Qinghai-Tibet block, which stimulated the Himalayan orogeny; However, the eastern block was stretched and collapsed, forming rifts and plains, which made the structural framework of Chinese mainland in a situation of "East-West confrontation" recently.

The ancient China block was originally surrounded by the ocean. At the end of Paleozoic, China block collided with Siberia block, and the Central Asia-Mongolia Ocean closed, which aroused the Variscan orogeny and led to the formation of Eurasian ancient land. Due to the poor rigidity of Chinese mainland block and the huge and hard Siberian block in the north, the Chinese mainland block sandwiched between several plates has accumulated particularly strong tectonic stress, which has become one of the regions with the strongest tectonic activity in modern plates and plays a leading role in restricting the formation of geological disasters in Chinese mainland.

According to satellite photo interpretation, theoretical analysis of Molnar slip line field and in-situ stress measurement data, the spatial distribution of modern tectonic stress field in Chinese mainland has obvious zoning phenomenon (Figure 4- 1). The Qinghai-Tibet Plateau and Xinjiang in the west are mainly potential reverse faults. The maximum principal stress σ 1 is generally in the north-south direction, and the minimum principal stress σ3 is generally vertical, forming a series of huge arc-shaped folded mountains and reverse fault zones in the NWW—SEE direction. Up to now, the crust of the Qinghai-Tibet Plateau has been rising continuously, with a thickness of 70 kilometers, making it the largest crustal block in the world. The north China block in the east and southeast coast belongs to the tension-shear strike-slip type, and the maximum principal stress σ 1 is distributed horizontally, and its direction is NE- near E-W-NW, which is distributed radially from the inland to the coast. The minimum principal stress σ3 is also horizontally distributed, and it is regularly distributed in a circular shape along the arc protruding eastward, and it is often in a state of tensile stress. In a few areas, σ 1 is nearly vertical, which is a potential normal fault type, such as Wei Fen Graben. Normal faults, rift basins and graben are developed in this area, and hills, low mountains, plains and basins are often distributed alternately in landform, and the direction is NE-NNE. The central part of Chinese mainland is located at the turning point of the maximum principal stress trace from near north to near east-west, and the σ 1 direction changes greatly. In-situ stress state belongs to potential strike-slip type and is characterized by huge left-lateral active faults. From south to north, its strike changes from near S-N direction, northwest direction and northwest direction, and gradually turns to near east-west direction or even NEE direction, forming a north-south active structural belt, which is the area with the strongest seismic activity in Chinese mainland.

Figure 4- 1 Modern tectonic stress state in Chinese mainland and its adjacent areas

1. Strong compression region; 2. Medium extrusion zone; 3. Tensile stress zone; 4. Active reverse fault; 5. Active strike-slip faults; 6. Fault rift and active normal fault; 7. Direction of plate action

The interaction of plates leads to the formation of different modern tectonic stress fields and structural patterns in Chinese mainland from west to east, which plays a leading role in the formation of geological disasters in different regions of China. First, internal dynamic geological disasters such as active faults and earthquakes are controlled. Because plate action restricts topography, climate and meteorological environment, it also controls various external dynamic mountain geological disasters such as collapse, landslide and debris flow. Judging from the intensity and degree of geological disasters, the central and western regions are obviously greater than the eastern regions.

(b) Impact of plate activity on topography and climate

Due to the interaction of plates, China has formed a ladder-like topographic pattern with high west and low east, which can be clearly divided into three steps. The highest step is the Qinghai-Tibet Plateau, with an average elevation of more than 4,000 meters. The second step is Tarim Basin, Inner Mongolia Plateau, Loess Plateau, Sichuan Basin and Yunnan-Guizhou Plateau, with an altitude of1000 ~ 2000m; The third step is dominated by Songliao Plain, Huanghuaihai Plain, the middle and lower reaches of the Yangtze River and low hills. The elevation of the plain area is less than 200m, and the low mountains and hills generally do not exceed1000 m. Between the second and third steps are towering mountains in the northeast, such as Daxinganling and Taihang Mountain.

The second step and its transitional zone on both sides have large ground undulations, especially in marginal mountainous areas, where the ground elevation difference is wide and the valley slope is steep, which is often the prone or dangerous area of geological disasters.

The rise of the Qinghai-Tibet Plateau has a great influence on the climate of China. In Pliocene, the Qinghai-Tibet block was only about 1000 meters above sea level, belonging to a humid subtropical environment. However, since the early Pleistocene, the Qinghai-Tibet block has risen sharply, and the Himalayas, Kunlun Mountains and other giant mountains have risen to more than 4000m m, and the southwest wind circulation in the upper air has been blocked, and the plateau climate has developed to dry and cold, gradually forming the Siberian high.

After the uplift of the Qinghai-Tibet Plateau, the atmospheric circulation situation changed, monsoon prevailed in Chinese mainland, and the regional and seasonal changes of temperature and precipitation were great. In winter, the northwest wind prevails, which is cold and dry, and the temperature difference between north and south is very large; However, in summer, the country is generally hot and rainy, with concentrated precipitation and heavy rain. In the vast areas of northwest and Inner Mongolia, the climate is very dry, the land desertification is serious, and sand blowing and sandstorms are frequent.

(3) Constraints of recent plate activities on geological disasters in pipeline engineering

The recent plate activities have caused great changes in China's geology, topography and climate environment, which obviously restricted the formation and activities of geological disasters, mainly reflecting the regional distribution law of geological disasters. The regional distribution characteristics of geological disasters in the crossing section of Lanzhou-Zhengzhou-Changsha pipeline project are analyzed.

According to the geological, topographic and climatic environment where the pipeline is located, the distribution of geological disasters can be clearly divided into two sections, namely Lanzhou-Zhengzhou section and Zhengzhou-Changsha section.

1. Lanzhou-Zhengzhou section

The Lanzhou-Zhengzhou section (including the west branch of the mountain) is located in the second step (including the eastern step) of the large-scale geomorphic unit in China, and its overall characteristics are plateau, faulted basin and hilly and low mountains where loess is widely distributed. The terrain gradually decreases from west to east, from more than 2400 meters to about 200 meters.

The westernmost Gansu section is located in the Qilian Mountain fold belt on the northeast edge of the Qinghai-Tibet block, which is strongly pushed by the Indian plate. Modern tectonic stress belongs to the potential strike-slip thrust type, and the trace of principal compressive stress turns from near north-south to near northeast. Thrust strike-slip faults are well developed. The main representative faults are Qilian Mountain fault zone, Liupanshan fault zone and Tianshui-Baoji fault zone. There have been many earthquakes of magnitude above 7 in history, and the strongest earthquake is 1920 Haiyuan earthquake of magnitude 8.5. This is the hinterland of the Loess Plateau, which is located at the second level of the large-scale landform unit in China, close to the first level of the Qinghai-Tibet Plateau. Due to the blocking of the south west Qinling Mountains, the collapsible loess is thick and the engineering geological properties are poor. The climate is dry, with an annual precipitation of 350~500mm, which is uneven. The heavy rainfall in rainy season in July and August every year induces the collapse, landslide, debris flow, collapse and potential erosion of loess. In addition, the remains of collapses and landslides caused by several major earthquakes are still vivid, and the geological disasters are more serious on the whole.

Shaanxi section and Shanxi section are located in Weifen new rift zone of North China platform, and Qinling fold system is in the south. The rift zone was formed by the subduction of the Pacific plate under the Eurasian plate. Modern tectonic stress belongs to latent normal fault type and extensional strike-slip type. The maximum principal stress is vertical or nearly east-west, and normal faults are well developed. The main representative faults are Weihe fault, Qishan-Ganxian fault, Huashan piedmont fault, Zhongtiaoshan northern foot fault, Luoyunshan piedmont fault, Huoshan-Dayang fault, Taigu fault and Jiaocheng fault. Rift zone has complex structure and strong modern tectonic activity. Since the classical records, there have been many strong earthquakes of magnitude 7 or above. The strongest earthquakes were the Zhao Cheng and Hong Tong M8.0/303 on September 25th and the Huaxian M8.25 earthquake on February 2nd 1556. The seismic intensity in the south of rift zone is higher than that in the north. Collapsible loess is widely distributed here with poor engineering geological properties. Topographically, it is in the second step of China's large landform unit. Due to the Qinling Mountains in the south, it belongs to the semi-arid continental monsoon climate, and the annual precipitation is 450-600 mm, which is uneven, mostly concentrated in the rainy season in summer and autumn, and there is often heavy rainfall, which leads to collapse, landslide and loess collapse, and undercurrent surges. Under the action of structure and pumping activities, this section is the most serious area of ground fissure disaster in China. In addition, coal and other mineral resources are mined in Shanxi, and goaf collapse is a prominent geological disaster. Generally speaking, there are many types of geological disasters in Shaanxi and Shanxi, and the disasters are more serious.

Western Henan is located in the transitional zone between the southern margin of North China Platform and Qinling Fold System, and its eastern part is the Henan-Anhui Depression. Obviously, under this regional tectonic background, the structural conditions of the pipeline section are complex, and the modern tectonic stress belongs to tensile strike-slip type. The representative faults are Wentang fault, Xin 'an-Linru fault, Fengmenkou-Wu Zhiling fault and Laoyachen fault, and the modern activity of these faults is relatively weak. There has never been an earthquake of magnitude above 7 in history. Most of the pipeline sections are covered with loess, and the collapsibility is weak. Topographically, it is located in the transition zone from the second stage to the third stage of the large geomorphic unit in China, characterized by hills and low mountains, and belongs to the warm temperate and semi-warm monsoon climate, with an annual precipitation of 600-650 mm. The main geological disasters are small loess landslides, collapses and undercurrents. In addition, there is ground collapse caused by coal mining. Generally speaking, the development of geological disasters in Henan section is relatively weak.

2. Zhengzhou-Changsha section

Zhengzhou-Changsha section is located in the third step of large-scale geomorphic units in China, and its overall characteristics are extensive distribution of alluvial-diluvial soil, coexistence of plains and hills, and low topography. Altitude plain: 30 ~ 200m (high in the north and low in the south), hilly and low mountains: 150 ~ 300m. Generally speaking, modern tectonic activity is not strong. According to the actual measurement and focal mechanism solution, the modern tectonic stress belongs to the potential normal fault or tension-shear strike-slip type, and the principal compressive stress trace from north to south changes from near east-west direction to SEE—NWW direction. According to the relationship between regional tectonic characteristics and geological disasters, it is divided into three sections: Henan section, Henan-Hubei section and Hubei-Hunan section.

The Henan section is mainly influenced by the subduction of the Pacific plate, resulting in the recent crustal extension and collapse. It is a part of North China depression, and its interior is divided into secondary structural units by deep faults. Representative faults related to pipelines include Yuzhou-Taikang fault, Lushan-Luohe fault and Suiping-Runan fault. These faults are nearly east-west and SE-NW, and intersect with the pipeline. There has never been an earthquake of magnitude 6 or above in history, so it should be said that modern tectonic activity is weak. Alluvial silt and silty clay are widely distributed on the surface of this section, of which loess is distributed in the northern section and gravel soil is developed in the valley of Huaihe River system. The terrain is flat and open, and it is a part of Huanghuai Plain and Huaihe Plain, with an altitude of 60 ~ 160m. The climate belongs to warm temperate semi-humid monsoon climate, with precipitation mostly concentrated in summer and autumn, with an average annual precipitation of 650 ~1000 mm. The main geological disasters are bank collapse, bunker collapse, goaf collapse, expansive soil expansion and contraction, etc., which are generally small and not serious.

The Henan-Hubei section is the Dabie Mountain (hilly and low mountain) area at the junction of the two provinces, and the regional structure is the Qinling-Dabie Mountain fold system, which is nearly east-west. The representative faults related to the pipeline are Xinyang-Ji Fang fault and Tongbai-Qingshankou fault, both of which intersect with the pipeline, and the modern activity of the faults is not strong, and there has never been an earthquake of magnitude 5 or above. The bedrock in this section is exposed and strongly weathered. The terrain fluctuates greatly, with an altitude of 150 ~ 300 m, belonging to a subtropical humid climate, with an average annual precipitation of1100 ~1200mm. Geological disasters are mainly small collapses, landslides, mudslides, and slope collapses in mining and sand mining pits.

The regional main structural unit in Hubei-Hunan section belongs to Yangtze platform. The representative faults related to pipelines mainly include Pingba fault (a branch of Xiangfan-Guangji fault), Xinshi-Ningxiang fault, Xiangxiang-Xiangtan fault and Lianyunshan-Hengyang-Lingling fault, all of which are normal faults. The modern activity of these faults is not strong, and the historical earthquake magnitude has not exceeded 5.5. Topographically, it is the plain in the middle reaches of the Yangtze River and the hilly and low mountains in the south of the Yangtze River. The plain area is 30 ~ 70m above sea level, and the hilly and low mountains are 150~250m ~ 250m above sea level. It belongs to subtropical humid climate, with an average annual precipitation of1200 ~1400 mm. Geological disasters include: karst ground collapse and goaf collapse, small landslides, collapses (including bank collapse and quarrying pits), debris flow disasters, expansive soil expansion and contraction, and soft soil subsidence disasters. Generally speaking, the risk of geological disasters in this section is relatively small.