(A) the image characteristics of active faults
In the interpretation of remote sensing images of active faults, firstly, according to the image characteristics of active faults such as hue, particle shape and water system, combined with necessary field verification, the image interpretation marks of active faults in this province are established, and the image structure framework of active faults in this area is preliminarily understood. Then the active fault is interpreted by image, and its mechanical properties are preliminarily determined according to the overall image characteristics of various structural features, such as slow wave, straight line or zigzag tracking. First judge the main cracks, then judge the fine cracks, and make directional grouping respectively. When the connection is broken, pay attention to the image intensity, extension direction and handover relationship. For some vague and hidden phenomena, the extended interpretation method from clear image to adjacent image is used to get better interpretation effect.
(2) Other signs of active faults.
(1) Landform: According to statistics, six terraces are generally developed in Sishui River Basin of Hunan Province, showing the characteristics of regional intermittent rise. Modern negative landform combination, the contour lines of mountains, plains and basins are linear, broken and echelon, reflecting the existence of active faults. Cracking point is a common geomorphological phenomenon in mountainous areas, which is often related to active faults. For example, the crack point in the Aizhai valley in Jishou is related to the new activity of faults in this area, so the valley hangs high on the landform and forms a high waterfall.
(2) Stratigraphic inversion: As one of the important signs of neotectonic movement, Quaternary series and formations in Hunan Province are often in false conformity or unconformity contact, which is manifested by the appearance of pedestal terraces and accumulation terraces on the landform, reflecting the characteristics of neotectonic oscillation, especially the inversion of old and new strata, which is often caused by active faults. For example, the shallow metamorphic rock series of Jianbanxi Group in western Hunan covers the Cretaceous-Tertiary red rock series, and the middle part of Gongtian-Ningxiang-Xinning fault covers the Pleistocene gravel layer, which reflects that the fault is still in a relatively active stage since Quaternary.
(3) Earthquakes and hot springs: According to historical records, there have been more than 200 large and small earthquakes in Hunan Province since AD 288, including 20 destructive earthquakes of magnitude 4.8 or above, others of magnitude 3-4, with magnitude 3 being the majority. The distribution of earthquake epicenters is often banded, which is consistent with the distribution direction of active faults. Epicenters are mostly located in special parts such as fault inflection points, end points or the intersection of two groups of faults. Obviously, there is a genetic relationship between them. For example, the Changde M6.5 earthquake occurred on the Taiyangshan fault zone, and the Jianghua Tuojiang M5 earthquake was also located at the intersection of two sets of faults.
According to the statistics of the Hydrological Team of the Provincial Geological Bureau, there are more than 60 hot springs in our province, and the regional distribution is often concentrated in the southeast and northwest of Hunan. Judging from the occurrence conditions and formation of underground hot water, there is a genetic relationship between hot springs and active faults, and hot springs are often exposed on both sides, ends, bends or fault intersections of active faults.
9.4.2 Main Active Faults and Their Seismic Activities in Hunan Province
According to the development history and structural scale of active faults in Hunan Province, they can be divided into two types: one type of active faults was formed in Mesozoic or earlier, and there were activities in Cenozoic, especially in Quaternary, that is, inherited active faults, which were large in number, large in scale and deep in cutting, mainly in NE and NE; Another type of active fault was formed in Cenozoic, which is small in scale and shallow in cutting, and has a certain control effect on water system, micro-topography and Quaternary strata. NNE-trending inherited active fault zones are mainly distributed in western Hunan, central Hunan and eastern Hunan. These fault zones are active during the NEO period, which are closely related to earthquakes and directly control seismic activity. They are the main seismic tectonic belts in our province.
(1) Inherited active faults
Qihe-Mayang active fault: It consists of Luxi fault, Wusu fault and Yuanling-Qihe fault. It has been very active since Cenozoic, forming fault scarps and valleys. Some sections of Shui Yuan, Youshui and Xixi River are developed along the fault zone, indicating that the fault zone is not cemented and shows a clear linear structure on the remote sensing image. This fault intersects with Yuanjiang fault, and an earthquake of magnitude 5 occurred in Luxi 193 1 year, indicating that this fault zone has not stopped its activity since Quaternary.
Xinhuang-Zhijiang-Huaihua active fault: along the fault triangle and cliff, there are obvious differences between the two sides, and there are many hot springs exposed. On the Bouguer gravity anomaly map, the southwest section of the fault shows a series of low gravity zones protruding from northeast to east, and the seismic activity along the fault zone is weak. Earthquakes with magnitude above 3.5 have occurred many times in Zhijiang and other places, which may be related to the late fault activity.
Hongjiang-Xupu active fault: it may have been formed in pre-Sinian, obviously revived in Yanshan and Himalayan periods. The Cretaceous-Tertiary basins of Tuanhe, Luchong, Qianyang, Wantong and Xupu are distributed in a beaded shape along the fault and cut by the fault. The Anhua and Xupu earthquakes with M ≥ 3 and the exposure of hot springs reflect the recent activities of the fault.
Luoweng-Suining fault: it extends from Ganxitang area of the passage to the south in the direction of NE28, and after passing through Suining to Bamian Mountain of Luoweng, it deflects in the direction of 50, and disappears in the southeast of Shenshan rock mass, with a length of 330 km. The cross section is inclined to SE, which is a normal fault, which is clearly reflected in remote sensing images. The main peak of Xuefeng Mountain is generally distributed to the east of the fault, and the main peak is as high as 2 147 m, which becomes the watershed between Shui Yuan and Zishui, showing the geomorphological characteristics of young people. It shows that there is obvious reverse activity along the main fault during the neotectonic period.
Gongtian-Ningxiang-Xinning active fault: It is composed of Miluo-Huitang fault in the north section and Shaoyang-Xinning fault in the south section, with a total length of about 430 km, extending in the northeast, with a cross section inclined to the northwest with an inclination of 35 ~ 45. From the landscape of remote sensing images, the river cuts down, forming a new river terrace, and the river changes course, forming a pirate river and quaternary sediments overlapping. There is a sharp contrast between the two sides of the fault, especially in the northern section. Based on the perspective effect of TM6 image, it is found that the fault zone is a groundwater-rich zone, and its groundwater level is higher than its surroundings. Earthquakes are frequent along fault lines. There was an earthquake of magnitude 5 in Loudi, and small earthquake swarms have become more active in recent years. There was also an earthquake of magnitude 3.5 in Xinning. Ningxiang is a region with dense small earthquake swarms, which may be related to the activity of faults.
Honghu-Yueyang-Taojiang active fault: There are obvious differences between the two sides of the fault in the landform displayed by remote sensing images. The Holocene in the west is covered with a vast lacustrine plain, and the Pleistocene in the east is widely exposed, forming low mountains and hills. On the seismic profile, it is found that the depth of T3 and T4 reflection surfaces on both sides of the fault is very different, with the west side falling and the east side rising. According to the sedimentary thickness and distribution of early, middle and late Pleistocene and Holocene, there are also great differences between the two sides of the fault, indicating that the fault is still active in Quaternary.
Xinhua-Dafuping active fault: Due to the left-lateral translation of the fault, a pull-apart basin was formed in Xinhua area. Especially in Himalayan period, the activity was so obvious that the Cretaceous and Tertiary red rock series at both ends (Xinhua and Hongyanzui) were cut. 19 10 and 193 1 year, earthquakes of magnitude 5 and 4.8 occurred in the northwest margin of the pull-apart basin, respectively, showing the modern tectonic activity of the fault zone.
Xiangxiang-Shaodong active fault: located in Shaodong, Xiangxiang and Xiangtan, it is composed of roughly parallel staggered faults, generally trending northeast, inclined northwest, with an inclination of 55. There are 30-40 m fault zones along the fault, which control the Cretaceous and Paleogene basins. It develops along the fault triangle with cliffs, and it can be seen moving to the right in Xiangxiang. The fan is inclined from northeast to southwest with a gentle angle. Above the reverse river, the river developed along the west side of the basin, forming a meander, indicating that the fault was still active in Quaternary. There have been five earthquakes of magnitude 4 ~ 4.8 along the fault, including two earthquakes of magnitude 4 193 1 and 1938, and there have been microseismic activities recently.
Taiyangshan active fault zone: It is composed of many NE-trending high-angle normal faults on the east and west sides of Taiyangshan, which controls the Taiyangshan uplift in Dongting Lake Depression, with a total length of more than 60 kilometers and is divided into two faults on the east and west sides of Taiyangshan. The largest earthquake related to the Taiyangshan fault is 163 1 year Changde M6.5 earthquake with a focal depth of 20km and an epicentral intensity of 8 degrees. The earthquake-affected area is NNE oval, which is consistent with the extension direction of Taiyangshan fault zone.
Nanxian-Hanshou active fault: According to the data of drilling and artificial seismic sounding, it is a high-angle normal fault with a west dip of NE20, with a length of 70 km. The vertical fault distance between the two plates of the fault is 800 ~ 1400 m, which controls the secondary structural unit of the Cretaceous-Tertiary Dongting Lake depression and the thickness of the Quaternary strata. Recently, there has been a small earthquake activity at the intersection of the fault end and the NW fault.
Wutuan-Shimen active fault: it is very eye-catching in remote sensing images. It runs north-south, extending intermittently, cutting a series of NE-trending faults and Wutuan and Wawutang rock masses, and controlling modern sedimentation. The Neogene in Wawutang rock mass was cut by late active faults. There is frequent seismic activity along this torsional fault zone, especially in the north where it meets other structures (such as Cili and Shimen). Taiyangshan fault parallel to this torsional fault and Yueyang-Xiangyin fault on the east bank of Dongting Lake are the products of simultaneous activities, which are one of the most important earthquake-generating and earthquake-controlling structures in our province.
Except the NNE fault is the main active fault in our province, some parts of the EW and NW structural belts in our province, especially the compound junction with NNE, showed obvious active nature in Neogene. For example, the east-west structural belt in southern Hunan at 2610' north latitude, the east-west structural belt in central Hunan at 27 40' ~ 28 40' north latitude (equivalent to Anhua-Ningxiang-Liuyang east-west structural belt) and the east-west structural belt in northern Hunan at 29 20' north latitude (equivalent to Shimen-Huarong-Linxiang east-west structural belt). Especially, the composite part with the NNE fault zone is often an earthquake-intensive area. From the remote sensing images, Huarong Uplift in the north of Hunan, baimashan-Longshan Uplift in the middle of Hunan and Yufeng Uplift in the south of Hunan all show the characteristics of east-west extension, forming a series of east-west mountains, controlling the direction of regional water flow, indicating that the east-west structure has new active properties in the Neogene, resulting in new uplift in some areas and becoming favorable structural parts for earthquakes.
(2) Quaternary active faults
Yiyang Heshan Miao fault: it occurs in the middle Pleistocene red soil gravel layer, with a small scale, the occurrence of 335 ∠ 47 and the nature of tension and torsion.
Hanshou Shibantan Fault: Found near Shibantan Town, the fault strike is NNE, and the exposed strata of the two plates are completely different. The west panel is a gray-white clay layer of Miluo Formation of Lower Pleistocene, covered with laterite gravel layer; Only the Meso-Pleistocene reticular laterite is exposed in the east plate, and no underlying stratum is found. Obviously, horizontally, two different horizons are in fault contact. The landform reflects the straight and steep lakeshore line.
Longtanpu thrust fault in Liutangpu, Xiangyin: the strike of the fault is 295, the dip angle is 45, and it tends to SW, which is tensional and torsional. The broken sand layer of Miluo Formation in ne plate is obliquely connected with the reticular red soil layer of Pleistocene in SW plate, and the sand layer traction phenomenon is seen near SW plate, indicating that NE plate moves upward.
There are similar faults in Changsha, Xiangtan, Hengyang, Ningxiang, Xinning, Dongting Lake and other places, but they are generally small in scale and have little impact. They usually occur near large inherited active faults.
9.4.3 Basic Features of Neotectonic Movement in Hunan Province
Geomorphological characteristics of (1) neotectonic movement
The outline of modern landform in Hunan Province is generally high in the east, west and south, and low in the middle and north. Neotectonic movement occurred at the end of Tertiary and Quaternary, showing intermittent and oblique movement. V-shaped valleys or barrier Gu Duo are found in mountainous areas, and box valleys are mostly found in hilly and plain areas. The red basin rises, for example, the red basin at the northern foot of Nanling Mountain rises at least 60 ~ 70 m. The farther north it goes, the less it rises, but it falls to Dongting Lake. The terrain of our province is high in the south and low in the north, forming a slope inclined to the north. Due to the influence of neotectonic movement, the surrounding mountains are eroded and destroyed, which are multi-level (500~300 meters denudation surface) peaks and hills with uniform peak lines. Affected by the tilting movement of the earth's crust, the mountainous areas have been eroded, and deep ditches, narrow valleys and deep bends have appeared, which have become today's landforms. On the other hand, after uplifting, valley terraces and floodplains with 5 ~ 6 grades appear under cutting, and the surface fluctuation gradually increases. Dongting Lake continued to decline in modern times. Judging from the general flow direction of Sishui and its tributaries, it is obviously controlled by neotectonic movement, and it converges northward in Dongting Lake along the east, west and south directions, and then flows into the Yangtze River from Chenglingji. From the point of view of Sishui estuary and lake, it has been in a state of change for a long time, reflecting the geomorphological characteristics of neotectonic movement.
(2) The main types of neotectonic movements.
(1) Arched uplift: The neotectonic movement in Hunan Province is dominated by large-scale slow uplift and subsidence, which has obvious inheritance in the nature of activity. For example, western Hunan, southern Hunan and eastern Hunan were uplifted into mountains in a large area before Himalayan period, mainly rising, and the Neotectonic period was still in the uplift stage. Huarong Uplift and Wuqiangxi Uplift are clearly displayed on remote sensing images.
Huarong Uplift: It is located in the north of Dongting Lake, bordering the Yangtze River in the north and Dongting Lake in the south, and it is a nearly east-west uplift. Since the tertiary period, it has shown the characteristics of continuous uplift. Judging from the uplift range, the east and west are obviously unbalanced. The eastern part has a large amount of rising, and the bedrock is exposed in a large area, which is a low mountain and hill; The western uplift is small, and the bedrock is less exposed, showing an isolated hill shape. Gravity and aeromagnetic anomalies are reflected as gravity cascade zone and alternating zone of magnetic positive and negative anomalies, respectively, and geothermal flow is shown as east-west linear anomaly zone. In Neogene, Chensuokou sag in the north sank strongly, and the maximum thickness of sedimentation exceeded 500 m, but this area basically did not accept sedimentation. Since the Quaternary, the channel of the Lower Jingjiang River has been winding, with the Quaternary thickness of 200 m in the north and only a few meters in this area, showing strong differential activities.
Wuqiangxi uplift is located between Yuanling and Taoyuan. It is estimated that Yangshan in the north and Montenegro in the south, with an altitude of 500-700 meters and a maximum of 900 meters. Pre-Sinian shallow metamorphic rocks and lower Paleozoic carbonate strata constitute the NNE fold uplift mountain. On the remote sensing image, Shui Yuan crosses Wuqiangxi Uplift, forming a canyon. Above the erosion platform preserved at the altitude of 170 m, you can also see multi-level erosion surface, and there is still red bed on the slope at the top of the uplift, covering the Banxi Group. The Cretaceous-Tertiary red beds in Tao Chang Basin and Ma Yuan Basin on the east and west sides of the uplift form low hills, with circuitous rivers and Quaternary development, and six-level pedestal terraces are formed along the banks of the river, which is a plain landscape between rivers. Therefore, since Tertiary, Wuqiangxi began to uplift due to neotectonic movement, which caused the separation of two basins and formed Wuqiangxi Gorge, and affected the Shui Yuan estuary to move eastward gradually and the west Dongting Lake to retreat eastward. According to the analysis of Quaternary data in Shui Yuan, the Yuanshui estuary was still in Damashan area of Taoyuan in the early Middle Pleistocene, and moved to Deshan area in the late Middle Pleistocene. After the late Pleistocene, the Yuanshui estuary continued to move eastward to Niubitan area.
(2) Tilting movement: The overall landform of Hunan Province reflects the characteristics of tilting movement, especially around Dongting Lake. Judging from the landform and stratum distribution, the overall structural characteristics of Dongting Lake basin in recent years are peripheral uplift and middle subsidence, with old strata distributed around and new strata distributed in the middle. That is to say, in the landform, there are low mountains, hills, hills and plains distributed in a stepped ring belt from the periphery to the hinterland. Compared with the basement elevation, the terrace elevation in the lower reaches of Li, Yuan and Zi on the north bank of Dongting Lake tends to decrease from north to south. The Yangtze River is divided into four ports, and part of the Yangtze River water flows back to Dongting Lake in the south, showing a trend of high in the north and low in the south. Due to the tilting movement of the south bank, the downstream tributaries of Zishui River are asymmetric, the south bank tributaries are long, the sediments are widely distributed, and the north bank tributaries are short, which is undeveloped in Quaternary.
(3) Neotectonic depression: The neotectonic depression in Hunan is a centripetal depression centered on Dongting Lake. Four rivers and four tributaries flow into Dongting Lake from different directions, forming a centripetal water system, from the surrounding mountains to hills, and then evolved into rivers and lakes plains, reflecting the general depression of the terrain. Quaternary thickness gradually widened and became thicker from mountainous area to hilly area and then to lake area. Repeated leveling data show that the mountain hills in the west of our province are still in a slow rising stage, and the Dongting Lake area continues to sink. In remote sensing images, these uplifts and depressions are controlled by active faults since Himalayan period, such as the east bank of Dongting Lake. The two depression lines overlap with the Gongtian-Ningxiang-Xinning fault and Yueyang-Xiangyin fault in this area, with the northern segment of Gongtian-Ningxiang-Xinning fault as the boundary, Mufushan strong rising area in the east and Yueyang-Miluo hilly valley in the west, which belong to intermittent rising area. The location of the fold line of NWW depression on the south bank of Dongting Lake is consistent with the extension of Changde-Yiyang-Changsha active fault.
(3) Crustal deformation characteristics of neotectonic movement
From the analysis of the relationship between crustal deformation and seismic activity, it can be seen that Hanshou-Yueyang deformation gradient zone is influenced by Nanxian subsidence area, and the deformation line is dense to the west, and Nanxian-Hanshou fault is distributed in it. The hidden structure of Qianjiang-Shimen makes Nanxian-Huarong-Shishou complex deformation zone easy to accumulate energy, which is a dangerous zone where earthquakes may occur in the future. There are NW-trending active faults in Changde-Changsha area along Hanshou-Ningxiang line. The Ningxiang-Pingjiang deformation gradient zone is NE-trending, and a composite part of the intersection of active faults and the turning of deformation step zone is formed near Yiyang. The Honghu-Yueyang-Taojiang eastward fault on the west side of Yiyang shows signs of activity on the Changde-Changsha vertical deformation profile.
(4) Analysis of tectonic stress field of neotectonic movement.
According to Deng Qidong's research, the tectonic stress field in the main areas of this province since Quaternary is as follows: under the action of near east-west compressive stress, the extensional shear movement of near east-west and north-northeast faults occurred, resulting in left-handed shear of northwest faults and right-handed shear of north-east faults. For example, in the active areas of NW-trending fault zones such as Xinkaipu and Nanjiao Park in Changsha, axial folds and NW-trending fault groups can be seen in the reticular laterite gravel layer in the Middle Pleistocene, and the intersection angle between the scratches of the latter profile and the horizontal plane is 24 ~ 30, with left-handed twisting steps. In the active area of NE-trending faults, such as Wusu fault, the unitary water is turned right by the fault into a right angle. The east-west fault active area, such as the Lishui fault zone centered in Tianjin, forms a modern subsidence zone of tens of kilometers in the near SW direction, which is one of the most serious subsidence areas in our province. In addition, the long axis of isoseismic line can basically reflect the direction of seismogenic tectonic line, and the long axis of isoseismic line is NNE, which is consistent with the shear dislocation surface of crustal medium under the action of regional tectonic stress field. The focal mechanism solution is similar to the geological analysis results, mainly in the following aspects: (1) the elevation angles of P axis and T axis in our province and its adjacent areas are mostly small, indicating that the geostress is mainly horizontal compression and horizontal extension; The direction of the P-axis changes gradually from SEE→ to near EW to →NEE, from northern Hunan to southern Anhui via northwest Jiangxi, but the direction of the P-axis is still near EW. The statistical data of the P-axis and the T-axis can roughly reflect the compression and torsion direction of the current tectonic stress.
9.4.4 Temporal and spatial distribution characteristics of seismic activity
(A) the spatial distribution characteristics of seismic activity
According to the comprehensive comparison of earthquake structures with M ≥ 4.7 in the seismic belt of our province, the earthquakes are mainly distributed in the NNE and NE fault zones with large scale and deep cutting, which have obvious directionality. Especially, the epicenter of strong earthquakes with M ≥ 5 is generally located on or next to the NNE fault zone, which constitutes the main seismic structural belt in our province. The intersection of NNE and NW or EW faults is an important part of seismogenic structures, such as Yueyang 1566M = 5.5 earthquake and Linxiang small earthquake swarm, which just happened at the intersection of the fault and Shimen-Huarong-Linxiang EW structural belt. 1509 The earthquake of magnitude 4.8 between Ningxiang and Linxiang is also located at the intersection of this fault zone and Anxiang-Ningxiang-Liuyang east-west structural belt.
Earthquakes often occur in special parts of active faults. Jianghua Tuojiang 1853M = 5 earthquake and Luxi 163 1M = 5 earthquake are located at the intersection of NE-trending faults and SN- trending faults. 1782M = 5 earthquakes are distributed in the composite parts of Ningyuan-Jianghua SN- trending structural belt, northeast Zhuqidong fault belt and Yangmingshan-Tashan east-west structural belt. Xinhua M5 earthquake at the turning point of Xinhua-Chengbu fault zone, Lianyuan Loudi earthquake on Gongtian-Ningxiang-Xinning fault zone, etc. , in the staggered position of the beginning and end of the fault.
(2) Temporal distribution characteristics of seismic activity.
The analysis of seismic records shows that there may be two periods of seismic activity in our province. The start time of the quiet period of the first seismicity cycle is unknown, but it ends at1468; The active period began at 1469 and ended at 1650, lasting 182 years. The largest earthquake in this section is 163 1 year Changde M6.8 earthquake, which occurred 19 years before the end of this active period 1650. The quiet period of the second earthquake cycle begins at 165 1, ends at 1842, and lasts for 192. The active period started from 1843 and lasted for 148 years. If the duration of the active segment of the first earthquake cycle (182) is used to estimate the duration of the active segment of the second earthquake cycle, the seismic activity time can last until around 2025, that is, there are still 36 years after 1990, for example, 25 years since 2000. The largest earthquake in the first seismic cycle occurred before the end of the active period 19 years, so the earthquake risk before the end of the active period in the second seismic cycle cannot be underestimated.
(3) seismic activity trend analysis
Judging from the temporal and spatial distribution of earthquakes, the intensity and frequency of seismic activity in our province may increase in the future, but it is unlikely that destructive earthquakes will occur in the short term. According to the structural characteristics of the crust and upper mantle provided by deep geophysical data, it can be known that the merging boundary of the subduction collision zone between Yangtze plate and South China plate in this province belongs to the merger of rigid mantle block and plastic mantle block, and the suture zone has early consolidation time and firm suture, which belongs to the low-temperature structural nature of thick crust and thick mantle in general, and the crust is firm. There is a weak, low-speed and low-resistance ductile "damping zone" in the crust, which belongs to a stable continental lithosphere block, so the probability of a destructive earthquake of magnitude 6 or above is small. In addition, it is new in our province. According to historical earthquake records, there are not many strong earthquakes in our province, the largest earthquake is only 6.8, and there are few earthquakes of magnitude 5, mainly small earthquakes, most of which are concentrated in Dongting Lake and its surrounding areas. Therefore, on the whole, seismic activity belongs to the weak earthquake area. However, from a local point of view, the seismic activity in Hunan is extremely unbalanced, especially in Dongting Lake and its surrounding areas. No matter in history or at present, this area is a relatively active area with frequent small earthquakes, and it is also a major earthquake distribution area. Dongting Lake area is a new faulted basin, surrounded by the transitional zone of neotectonic movement, which belongs to a fragile area and is easy to concentrate and release stress and cause earthquakes. Therefore, it is suggested that the future work should focus on Dongting Lake area, especially on the east and west sides. Comprehensive analysis shows that Taiyangshan fault has the geological and structural conditions for moderate and strong earthquakes. If a moderate earthquake occurs again, the earthquake location may still be somewhere in Changde-Lixian County, and the seismogenic fault may be the fault on the east side of Sun Mountain or the fault on the west side of Sun Mountain. Because the NE-trending fault in our province was very active in Neogene, it is one of the main earthquake-controlling structures in our province. In addition to the Taiyangshan fault that should be monitored, it is distributed on the east side of Dongting Lake and the NNE fault zone around Wutuan, Xupu, Wuqiangxi and Shimen, with distinctive characteristics. It runs through SN, extends intermittently, and cuts all other structures. There is frequent seismic activity along the torsional fault zone, especially in the structural joint area (Cili and Shimen areas). Regionally, Yueyang, Changde, Lianyuan and Loudi are areas with frequent earthquake activities in our province, and also areas with frequent earthquakes in the future, and should be one of the key areas for earthquake work in our province.