Chongqing Fengjie earthquake in the reservoir area?

1) Formation lithology and engineering geological rocks

Except for Lower Devonian, Upper Carboniferous, Cretaceous and Tertiary, strata in the reservoir area are exposed from Pre-Sinian to Quaternary (Table 3- 1). Its distribution is from east to west, from old to new. Pre-Sinian crystalline rocks exposed from Sandouping to He Miao reach; He Miao to Xiangxi is Sinian to Triassic to Jurassic strata; The Middle and Lower Triassic from Niukou to Guanwu Town is exposed in a large area; Jurassic strata are widely distributed in the reservoir area nearly 400 kilometers west of Guanwu town, and only Triassic strata and a small amount of Permian strata are exposed in the core of anticline. Quaternary sediments are scattered on river terraces, stripping surfaces and slopes. The Quaternary accumulations with relatively concentrated distribution and large volume are mostly collapses and landslides.

According to the characteristics of lithofacies structure and rock mass structure, the strata in this area can be divided into four kinds of engineering geological rocks:

Massive crystalline rocks: including pre-Sinian massive magmatic rocks and medium-deep metamorphic rocks. Only distributed in He Miao-Sandouping area.

Stratified clastic rocks: mainly the red beds of Middle-Upper Triassic and Jurassic, which are the main sliding rocks in this area, mainly distributed in the areas from Xiangxi to Zigui and Fengjie to Kuwei.

Layered carbonate rocks: Upper Sinian, Cambrian, Lower Ordovician, Carboniferous, Permian and Lower Triassic. Mainly distributed in the main tributaries from He Miao to Fengjie and Wujiang section.

Loose soft rock (soil): Quaternary loose soft accumulation, mostly urban residual slope accumulation, landslide accumulation and artificial accumulation. It belongs to the sliding rock (soil) in this area.

(2) Geological structure

The three Gorges reservoir area is all located in the Yangtze platform area, adjacent to the Qinling geosyncline in the north.

It is bounded by Qiyueshan basement fault between Wushan and Fengjie (roughly distributed between Fengjie and Shizhu in the southwest direction), bordering Sichuan Taiao (Sichuan-Yunnan block depression) in the west and Upper Yangtze Platform fold belt (western Hubei block uplift) in the east. The section near the reservoir head (Sandouping-Badong) is located in the upper Yangtze platform fold belt, and Badong to Fengjie is located in the transition zone between the upper Yangtze platform fold belt and Sichuan platform depression. This area has experienced three strong tectonic movements, namely Jinning movement before Sinian, Yanshan movement at the end of Jurassic and Himalayan movement at the end of Paleogene. Jinning movement caused strong folding and metamorphism of pre-Sinian strata, accompanied by multi-stage magmatic activity, forming an ancient crystalline basement. Yanshan movement is mainly characterized by folds and fractures of the caprock, with little damage to the basement. The marine sedimentary caprocks in the Yangtze platform fold belt developed well from Paleozoic to Middle Triassic, and the Yanshan movement was completely folded. Sichuan Tai 'ao was a relatively uplift area of Yangtze paraplatform in Paleozoic, lacking Devonian and Carboniferous strata. Himalayan movement changed into a new tectonic movement characterized by overall uplift, but the caprock was slightly deformed and a few faults were weakly inherited.

fold

The Three Gorges reservoir area spans the middle section of the Sichuan-Hubei fold belt and the eastern section of the eastern Sichuan arc fold belt, with Dabashan arc fold belt in the north, Changyang east-west structural belt in the southeast, Sichuan-Guizhou south-north structural belt in the southwest and quasi-Yangshan structure in the east. The characteristics of folds in the reservoir area are that a series of NNE arc folds from west to east are blocked by Bashan arc, protruding from northwest to outside, turning from northeast to northeast, and finally intersecting with Zigui syncline in the east-west direction and embedded in Zigui syncline.

The fold shape is bounded by Fengjie, and the anticline east of Fengjie is closed with inversion. Syncline is a composite fold with secondary folds, which are mostly distributed in parallel diagonal lines along both sides of the main trough. Fengjie west syncline is wide and gentle, and the anticline is closed, forming a "blocking" structure.

According to the strike, the folds in this area can be divided into four groups:

(1) axial nearly north-south fold

Huangling anticline and Zigui syncline, which are nearly north-south in the axial direction, are located in the east of Badong and strike nearly orthogonal to the Yangtze River.

(2) Axial NE fold

Baifuping anticline, Nanmuyuan anticline, Guandu-Beishi anticline, Hengshixi anticline, Wushan anticline and Qiyueshan anticline. In the northeast, it is distributed between Fengjie and Badong, and the Yangtze River leans towards it.

(3) Arc-shaped folds protruding from northwest to outside in the axial direction.

The vast area from Fengjie to Fuling is a series of arc folds protruding to the northwest, including Fangdoushan syncline, Fengdu-Zhongxian syncline, Guling syncline, Dachi-Ganjing syncline, Wanxian syncline, Dongcun syncline, Tiefengshan syncline, Dianjiang-Liangping syncline and Qumahe syncline. The Yangtze River is basically parallel to the tectonic line.

(4) axial NNE fold

There are a series of NE-trending folds in the west of Fuling, including Xia Mingyue Syncline, Maliuchang Syncline, Guangfusi Syncline, Tongluo Syncline, NanWenquan Syncline, Jiangbei Syncline, Longwangdong Syncline, Yuelaichang Syncline and Guanyinxia Syncline, and most of the Yangtze River is nearly orthogonal to it.

break

Faults in this area are undeveloped. There are Jiuwanxi fault, fairy mountain fault and Xinhua fault in the first member of reservoir. Badong-Fengjie section includes Qiyueshan fault, Enshi fault, Yujiang fault and Qianjiang fault. Faults to the west of Fengjie are undeveloped. The larger fairy mountain and Xinhua faults in the area are far away from the reservoir area, and the main faults crossing the mainstream reservoir are only Jiuwanxi, Niukou, Hengshixi, Yangjiapeng and Huangcaoshan. In addition, Pingyangba fault and Beishi fault in the northern extension of Jianshi fault intersect with tributaries such as Longchuan River and Lengshuixi. These faults are not large in scale and have not caused large-scale rock mass damage.

(3) Neotectonic movement and earthquake

The neotectonic movement in the reservoir area belongs to the southwest uplift subregion of the Three Gorges. It is characterized by large-scale intermittent global uplift and differential fault activity in some areas in recent years. The center of the uplift is Fengjie-Wushan area, with the maximum rising range of 2000 meters. It is characterized by unevenness, inclination and intermittence of uplift, resulting in five-level stripping surfaces and six-level terraces on both sides of the Yangtze River. Since the Quaternary, the rising speed of the earth's crust has intensified, and rivers have been strongly incised, resulting in high and steep bank slopes and many landslides in the Three Gorges reach.

Another important manifestation of neotectonic movement is differential fault activity. The faults that are still active recently are fairy mountain fault and Jiuwanxi fault, both of which belong to active faults.

The earthquake level in the earthquake zone is not high, the intensity is small and the frequency is low. The crust is relatively stable and belongs to a weak earthquake environment. According to China Earthquake Intensity Zoning Map of State Seismological Bureau 1990 (1:4 million, with a 50-year exceeding probability of 10%), most of the reservoir areas are below VI.

Earthquake is an important manifestation of fault activity. The largest earthquake in the first section of the reservoir is the Zigui Longhuiguan earthquake, with the magnitude of 5. 1 (1979.5.2), and the epicenter is only 8 kilometers away from the Yangtze River. The Chongqing earthquake at the end of the reservoir reached 5.3 (1989.11.20), and the epicenter was away from the Yangtze River 15KM. According to the study of the earthquake history and seismic geological conditions in the reservoir area, it is impossible to rule out the recurrence of the same earthquake in history and the occurrence of larger earthquakes in the reservoir area. For the sake of safety, when evaluating the stability of landslide and reservoir bank, the degree above Fengjie is considered as VI, and the degree below Fengjie is considered as VII.

According to the research results of Three Gorges Project, there is the possibility of reservoir-induced earthquake caused by fault rupture in He Miao-Baidicheng reservoir area. There are two areas where earthquakes may be predicted: one is the intersection of Jiuxiwan fault at the junction of Xiling Gorge and Zigui Basin and the Yangtze River and Xiangxi River; The other is the section from Niukou in Zigui to Beishiku in Wushan. The maximum magnitude that may induce an earthquake is estimated not to exceed 6. In addition, there is the possibility of inducing small karst earthquakes. Except for carbonate rocks in the backwater area of tributaries Wujiang River and Jialing River, small karst earthquakes may occur, other reservoir sections generally do not have the conditions for reservoir-induced earthquakes.

(4) topography

The reservoir area is located at the eastern edge of the second step of China's topography, and the country's topography is divided into mid-low mountains with intraplate uplift and erosion.

The landform of the reservoir area is obviously controlled by stratigraphic lithology, geological structure and neotectonic movement, and it is divided into two landform units, east and west, with Fengjie as the boundary.

Erosion and dissolution of the Three Gorges downstream of Zhongshan Gorge

To the east of Fengjie, the landform in the area is based on Daba Mountain and Wushan Mountain, forming Sichuan-Hubei folded mountain composed of carbonate rocks from Sinian to Triassic, which belongs to the landform landscape of low mountains and wide valleys in Zhongshan Gorge, with erosion and dissolution as the main features. This mountain range is roughly east-west and partly north-south. Most of the Yangtze River is oblique or horizontal, so most of the valleys are oblique or horizontal. The peak altitude is mostly 1000-2000m, and the relative height difference is about 1000m. The river valley is narrow, the bank slope is steep, and the river surface is generally 200-300 meters wide. The direction of the mountains is also controlled by the structure. The Daba Mountain Range stands in the north of the reservoir area in the northwest-northwest direction, and the main peak, Da Shennongjia, is 3050 meters high, which is the watershed between the Yangtze River and the Han River. Wushan mountain range starts from the junction of Hubei and Sichuan (now Chongqing) in the north, and runs northeast-northeast, from the green onion slope to the Yuntai desert, with an altitude of 1800-2000m, which is the watershed of the Yangtze River and Qingjiang River. The valley of the Yangtze River is deep, the peaks on both sides stand tall, the valley is narrow and the water is swift, forming the famous Three Gorges of the Yangtze River. Another feature of the landform in this section is the obvious layered landform, which gradually descends from the watershed to the Yangtze River basin, showing two stages and four levels of leveling. The tributaries on both sides of the Yangtze River are developed, with the north bank tributary in the northwest and the south bank tributary in the northeast.

Parallel "Ridge-Valley" Profile of Erosion-denudation Low Hills in East Sichuan

The west of Fengjie belongs to the east of Sichuan Basin, with low hills and wide valleys dominated by Jurassic clastic rocks. The mountain range is controlled by the structure, forming a series of narrow low mountains (syncline hills) distributed in the northeast-southwest direction, forming a unique parallel mountain valley landscape. The overall terrain gradually decreases from the edge of the basin to the center. The elevation of Fengjie area is nearly 1000 meters, and it gradually decreases to 300-500 meters near Changshou. The towering banded ridge is composed of hard limestone and sandstone, and the elevation of the ridge is generally 700-800 meters; Low hills are mostly composed of sandstone and clay rocks, and the elevation of the top of the mountain is generally 300-600 meters. The Yangtze River winds in the syncline valley, forming an open and gentle wide valley, and some sections often form short canyons when crossing the anticline.

There are various micro-landforms in the reservoir area, which are mainly the products of the transformation of mountainous areas by flowing water geology and gravity geology, such as gullies, alluvial fans, rockfill piles and landslides. Class Ⅱ-Ⅳ terraces can be seen from Wushan to Yunyang in the Yangtze River Basin, and Class Ⅰ-Ⅳ terraces can be seen from Liyongtuo in Chongqing. Karst landforms such as karst ditch, karst trough and karst funnel are also developed locally in the reservoir area.

(5) Hydrogeology

The groundwater types in the Three Gorges reservoir area mainly include:

Pore water of loose rocks, fissure water of clastic rocks, karst fissure water of carbonate rocks and fissure pore water of weathered zone of crystalline rocks. Among them, suspended water and phreatic water in vadose zone are the most common groundwater in the Three Gorges reservoir bank, which has the most direct influence on the stability of the reservoir bank. According to its groundwater dynamic model, it can be further divided into intermittent infiltration fissure diving, intermittent infiltration pore diving and fissure water-filled pressure-bearing type. The latter is an important driving force for the formation of rainfall-type landslides.

The interlayer water composed of interlayer sandstone in the slope belongs to confined water dynamic type. According to its circulation characteristics, it can be divided into weak circulation confined water and strong circulation confined water. Weakly circulating confined water mainly circulates in the recharge area, which has little effect on the evolution of bank slope. Strong circulating confined water is mostly developed in "gravity inclined ground", which has long recharge time and high water pressure and can be quickly transmitted downward. The uplift pressure of this water on the overlying strata is an important driving force for the deformation and failure of the reservoir bank slope.