During the development of shale gas, the high-pressure fluid injected into the shale formation changes the formation fluid pressure, which causes the change of the in-situ stress environment, and thus producing fracture (fracture system) in some directions, and this is the basic principle of hydraulic fracturing. These fractures may cause microseisms (M < 3.0), but such microseisms are transient, not persistent, and the level of microseisms is generally low. In the petroleum industry,

micro seismic monitoring is used to judge the effect of hydraulic fracturing, and the signal generated by micro seismic is used for fracture monitoring and shale gas development, and the micro seismic level caused by hydraulic fracturing is generally much smaller than 0.5 on the Richter scale (M).

There is much debate about whether hydraulic fracturing causes earthquakes of moderate intensity (5.0≤M<7.0) (Note: the classification standard of earthquake magnitude is GB/ t18207.1-2008). Atkinson et al.^[1], Bao and Eaton^[2], Lei et al.^[3,4] and Meng et al.^[5] believe that hydraulic fracturing can cause earthquakes of magnitude 3-5. In states with lots of hydraulic fracturing operations in the central United States, such as Ohio and Oklahoma, some earthquakes are considered to be fracturing-induced earthquakes^[6,7], and even wastewater treatment in development management is thought to be likely to cause earthquakes^[3,8]. Bao and Eaton^[2] show that fracturing is common in the western Canada basin. However, it is still unclear whether induced earthquakes can directly cause earthquakes of moderate intensity or above.

The Changning anticline at the southwest margin of the Sichuan Basin has been a natural earthquake-prone area since the first seismic records were available, but also a key zone for shale gas exploitation since 2011 and undergoing large-scale shale hydraulic fracturing since 2014. In recent years, there have been many moderate-intensity earthquakes and the cause of these earthquakes is currently highly controversial. In this paper, the Changning anticline is taken as the research area to analyze whether shale hydraulic fracturing can induce earthquakes of above moderate intensity. Earthquakes of magnitude M5.7 and M5.3 occurred on December 16, 2018 and January 3, 2019, respectively. It is of great significance for shale gas development in this area to explore whether these earthquakes are natural earthquakes or fracturing induced earthquakes. In this paper, the double-difference earthquake location algorithm were used to accurately relocate the seismic sequences and indicate the characteristics of seismic distribution based on seismic phase reports of Xingwen (Dec. 16, 2018) and Gongxian earthquake series (Jan. 3, 2019). Based on the shale gas exploration drilling and high-resolution seismic reflection data in this area, the structural geometry and kinematics model of the Changning anticline are established, and the distribution characteristics of faults are analyzed. The paper, from the perspective of structural geology, intends to explore the geometry and kinematics of seismogenic faults and to discuss the seismogenic mechanism in this area.

The North-South Seismic Belt in China, extending from the western edge of the Ordos block to Myanmar in the south through western Qinling tectonic belt, Longmenshan tectonic belt and Anninghe-Xiaojiang Fault zones, is an active tectonic belt dividing the relatively stable Ordos plateau, Sichuan Basin and South China block in the eastern part of China mainland from the eastern and southern margin of the Tibet Plateau^[9,10]. Southwestern Sichuan Basin is at the eastern edge of the southern part of the North-South Seismic Belt (Fig. 1 and Table 1).

The main active fault zones in the Sichuan Basin are distributed in orogenic belt and basin-mountain transition belt in the northern and western Sichuan Basin and the fault activity in southwest Sichuan is relatively weak^{[11,12,13,14,15]} (Fig. 1). However, destructive earthquakes of moderate intensity may occur in some areas of China where Neotectonic tectonic activities are weak but persistent^[14].

Catalogue of historical Chinese earthquakes (M≥4.0) between 1831 BC to 1969 AC collected by Gu^[16] indicate that up to 9 earthquakes with a magnitude of 4.0 or above were recorded before 1970 in southwestern Sichuan Basin, with a maximum magnitude of 5.8 (Table 1). The specific magnitude and depth of earthquakes recorded in historical documents in Table 1 cannot be completely accurate, but the epicenters are macroscopic epicenter, supported by records of seismic geological hazard and survey of magnitude intensity of local earthquakes.

Based on the earthquake catalog provided by China Earthquake Data Center, this paper analyzed the distribution of earthquakes (M≥3.0) in the Sichuan Basin and its surrounding areas from January 1970 to December 2010. The results show that during the 30 years, several large earthquakes of magnitude 7.0 or above occurred in this region, such as the Yongshan Earthquake in 1974, Songpan Earthquake in 1976 and Wenchuan Earthquake in 2008. Earthquakes of magnitude 3.0 or above mainly occurred in the southwestern Sichuan Basin (Fig. 2).

Fig. 3 shows the distribution of earthquakes (M≥3.0) in the Sichuan Basin and its surrounding from January 2011 to May 2019. There have been two 7.0 magnitude earthquakes in the past nine years, including the Lushan Earthquake in 2013 and the Jiuzhaigou Earthquake in 2017.

According to the statistical analysis of 922 seismic data from January 2011 to May 2019 in the southwestern Sichuan Basin, it is found (Fig. 4) that the focal depth of earthquakes below magnitude 2.0 are mainly distributed at a shallow depth of 5 km, and 115 earthquakes above magnitude 3.0 are mainly distributed at a shallow depth of 10 km. Earthquakes of magnitude 3.0 or above in the Sichuan Basin and its surrounding are mainly distributed in the depth range of 10-20 km (Fig. 5).

Figs. 1 and 2 show that the seismic distribution in the southwestern Sichuan Basin has the characteristics of planar distribution, that is, the earthquakes are not limited to one or several structural belts, and the distributed deformation control the seismic activity in the southwestern Sichuan Basin. Earthquakes tend to be concentrated along several active fault zones. Earthquakes are mainly developed along the Longmenshan, Xianshuihe and Sanjiang belts in basin margin. In the interior of the basin, earthquakes are more occurred along the buried fault zone in front of Longmenshan belt, Pujiang-Xinjin fault zone, Longquanshan fault, Yingjing-Mabian fault and the west of Huaying belt (Fig. 2). From the analysis of seismic profile in the basin, it can be seen that the above faults all have the property of detachment and most of them occur in the Lower Triassic Jialingjiang Formation or the Middle Triassic Leikoupo Formation, not deep faults cutting down to the basement.

The distribution statistics of earthquake source show that the focal depth is mainly 10-20 km (Fig. 5), indicating that the earthquakes occurred in the basement of the Sichuan Basin. Many earthquakes occur at depths of 20 to 30 km, while shallow earthquakes (5 km or less) tend to have magnitudes of less than 2.0, and intermediate or higher earthquakes are rare

We have downloaded the seismic phase reports of the 5.7-magnitude earthquake on December 16, 2018 and 5.3- magnitude earthquake on January 3, 2019 from the Earthquake Cataloging System in Changning, Sichuan Province, and relocated the two main earthquakes and their aftershock sequences with the double-difference earthquake location algorithm which mainly uses the arrival time of seismic phase for relocation^[17,18], with the longitudinal wave arrival time weight of 1.0 and the shear wave’s weight of 0.5.

The seismic velocity model is divided into shallow (5 km or less) and deep (5 km or more) comprehensive models. The shallow velocity model is based on the longitudinal wave velocity information synthesized from the seismic reflection data and vast oil wells in the Sichuan Basin^[19]. The deep velocity model refers to the average crustal and upper mantle velocity model of 1990 earthquakes in the eastern Sichuan Basin during December 16, 2018 solstice March 31, 2019^[20]. It is found that the residual value decreases after the relocating, which indicates that the accuracy is improved (Fig. 6).

The relocation results (Table 2) show that the focal depth of the 5.7 magnitude earthquake in Changning on December 16, 2018 was 10.5 km, close to the 12 km reported by the CENC^[21] (Fig. 7), the focal depth of the 5.3-magnitude earthquake on January 3, 2019 was 15.1 km, which was basically consistent with the result of the CENS of 15 km. In addition, GCMT has a depth of 14 km and 12 km for the focal depth of two earthquakes respectively^[22], which has some errors, but the overall focal depth should be more than 10 km. The focal depth results show that the focal depth of smaller earthquakes (M<4.0) are mainly 5-10 km, but the larger magnitude (M≥ 4.0) are mainly distributed in the basement with a depth of 8 km (Fig. 7).

The Changning anticline is located at the junction of Sichuan, Yunnan and Guizhou. And its tectonic location is the transition zone between the south Sichuan low-slow fold belt and the Daliangshan-Daloushan fault fold belt^[23,24,25], with multi-stage structural deformation superposition characteristics in different directions^[25,26,27]. Its east side is affected by the compression stress from the eastern Sichuan-the western Hunan and Hubei fault zone, its west side is affected by the remote transfer of direction of Longmenshan, the northern area is limited by the Sichuan Basin and Huayingshan fault zone, and the southern area is superimposed with compression and uplift caused by tectonic transformation of Ziyun-Luodian fault zone, which finally forms the present tectonic framework.

The anticline of Changning extends northwest through Gongxian to Gaoxian area, and reaches Xuyong area in southeast (Fig. 8). The anticline is wider in the southeast and narrow in the northwest. The anticlinal axis shows the trend from NW to SE, and the northwest end curves to southwest. The core of anticline exposed the Cambrian system, and the surrounding strata successively exposed the Ordovician, Silurian, Permian, Triassic and Jurassic. A series of thrust faults are developed in the core of the anticline in Changning. The faults cut through the Cambrian and the secondary folds in the anticline are developed.

The Changning anticline gradually enters the Sichuan Basin in the north, characterized by bands of anticlines and synclines, mainly distributed in the near E-W direction, the core of the anticline is mainly composed of Middle and Lower Jurassic strata, and the thick cretaceous is often developed in the synclinal area. The west of the Changning anticline is limited by a series of NE -SW trending tectonic belts. The southern part of the Changning anticline is a series of complex structural belts, which often have the characteristics of multi- phase superposition, showing the distribution of anticline and syncline, and the characteristics of well-developed faults.

The strata in the anticline area of Changning are well developed, including Sinian, Cambrian, Ordovician, Silurian, Permian, Triassic, Jurassic and Cretaceous and missing Devonian, Carboniferous, Paleogene and Neogene, with strata thickness of more than 9 000 m^[23,24]. According to the stratigraphic profile comparison between well Ning 201, Ning 203 and Ning 2 (Fig. 9), the drilling of well Ning 2 in the core of the anticline of Changning met Cambrian and Sinian systems with a thickness of 3 300 m. Thick white mirabilite and salt rock layers were developed in the lower section of Sinian Dengying Formation, with thickness of 52 m and 240 m respectively. The Qiongzhusi Formation has developed black carbonaceous shale with thickness of 225 m, and it has developed several sets of gypsum layers cyclically in upper Gaotai Formation. Well Ning 203 and well Ning 201 are located in the southern slope of the anticline of Changning and the kink-band of syncline respectively. Both wells have been drilled Triassic, Permian, Silurian and partial Ordovician, which have similar stratigraphic characteristics. The upper Permian Longtan Formation and middle Permian Maokou Formation are in unconformity contact, Longtan Formation is thick basalt, and Maokou Formation is mainly limestone. There is an unconformity contact between the Permian Liangshan Formation and Silurian Hanjiadian Formation. The Liangshan Formation is mainly composed of thin layers of carbonaceous shale and mudstone sandwiched coal seam. The Hanjiadian Formation is mainly composed of sandstone and mudstone interbedded with some argillaceous limestone. Silurian Longmaxi Formation and Shiniulan Formation are mainly composed of shale mudstone and sandy mudstone. The Longmaxi Formation of the Lower Silurian to Wufeng Formation of the Upper Ordovician is dominated by shale and mudstone rich in organic matter^[26], and the black shale at the bottom of the Longmaxi Formation is rich in graptolite and pyrite, which is the main gas producing layer for shale gas exploitation. The Ordovician Baota Formation, which characterized by nodular limestone and turtle crack structure, is a set of stable shallow marine carbonate.

Shale, gypsum and salt are recognized by weak layer which often have the characteristics of low compression and shear strength and young's modulus and relatively high poison’s ratio. As a result, these strata are mainly characterized by plastic deformation in the structural deformation process, which plays a role in regulating the structural morphology of upper and lower strata and absorbing the deformation and displacement amount. Two sets of regional detachment layers of the Upper Sinian Dengying Formation and the Middle Cambrian Gaotai Formation are developed in the Changning anticline. The thin layers of carbonaceous shale, mudstone and coal seam in Liangshan Formation and Silurian Longmaxi

Formation can be used as local detachment layer to regulate the structural deformation in a small range. In addition, there are ductile shear layers in Pre-Sinian basement, which are also important regional detachment layer.

In this paper, a seismic reflection profile through well Ning 203, is selected for analysis. This profile starts from Luobiao town and passes the Jianwu syncline, and passes the epicenter of the 5.7 magnitude earthquake in Xingwen, Sichuan Province in 2018, through the Changning anticline (Fig. 8), and finally ends in the syncline area in the southern margin of the Sichuan Basin in the northeast direction (Fig. 10). This profile is 67 km long and the seismic reflection depth is 13 km. The velocity model of this section is constrained by logging data from several exploration wells in the anticline area with a relatively high accuracy. On the basis of synthetic seismic record of Ning 203 well, combined with digital elevation data and geological occurrence information, the seismic horizon is identified and tracked, and the structural interpretation of the section is carried out by applying fault-related folding theory.

Accurate identification of fault ramp location is the key to fault plane interpretation. The seismic reflection profile of Changning anticline (Fig. 11) can be seen by local amplification: (1) The footwall ramp in the left column (Fig. 11) deep basement fault ramp up for the spread of cut ramp position, there is an obvious phenomenon of cutting layer, namely: the upper fault seismic reflection wave group continues the surface strata occurrence characteristics, has a tendency to southwest, northeast direction gradually raise the trend, and almost parallel faults. However, the underlying layer of the fault is nearly horizontal and inclined to the northeast, which is opposite to the trend of the upper wall strata and has obvious crosscutting relationship, which is a typical seismic reflection feature of the lower wall fault ramp. (2) In the north limb of the Changning anticline (Fig. 11), it can also be observed that the occurrence of the strata of the front limb of the fold changes significantly from southwest to northeast, showing the characteristics of synclinal reflection. The southwest limb of the syncline tends to the northeast, and cut down by a basement fault, after passing through the axial plane of the syncline in the northeast direction, the stratum becomes nearly horizontal, which is consistent with the occurrence of the underlying layer under the fault, this is the seismic reflection characteristic of the front limb strata of the fault-bend fold, The section where the anticlinal axial plane of the north limb intersects with the synclinal axial plane and the fault is called the hanging wall ramp (The fault plane has a cross relationship with the hanging wall). (3) The middle flat, the southern and northern limbs of the Changning anticline are separated by a relatively narrow flat roof, which is the development of a double anticlinal axial plane. The intersection of the axial plane of the anticlines of the front and rear limbs and the fault plane is the position of the middle flat, and the upper and lower seismic events in the section are nearly parallel (Fig. 12). Thus, the hanging fault ramp, the middle flat and footwall fault ramp define the fault as a simple step-like section, and the Changning anticline act as a fault-bend fold anticline (Fig. 10).

According to the structural model established, it can be found that a wide and slow syncline-Jianwu syncline is developed in the southwest of the Changning anticline, and the outcropping of its core is Shaximiao Formation of the Middle Jurassic, and the stratigraphic dip is relatively gentle, ranging from 5° to 10°, and the outcrops to the two limbs are getting older and the stratigraphic dip is getting steeper. The dips of the Permian and Silurian in the southwest limb of the Changning anticline are 15° and 10° respectively, and the stratigraphic dip gradually decreased to near level to the core of the outcropping Ordovician. One or two secondary faults are developed each in the Ordovician and Silurian in the NE direction. The dip angle of the surface strata can reach 48°. The northeast limb of the Changning anticline is steeper, and the dip angle of the Jialingjiang Formation and Xujiahe Formation can reach 60° and 57°, respectively. The dip angle of Middle Jurassic strata decreased sharply, the dip angle of Cretaceous which near the syncline is 17°, and the stratigraphic dip gradually become horizontal to the core of syncline. Through the identification of seismic profile faults, it can be found that there are large thrust faults in the underlying basement of the Changning anticline (Fig. 11). Three back thrusts developed above the Cambrian detachment beds in the northeast limb of the anticline, and the faults gradually steepened upward, cut through the Cambrian, Ordovician and Silurian, and gradually converged downward to the Cambrian detachment beds. The basement faults are deep in southwest and shallow in northeast, with ramp-flat distribution, and the ramp of the faults is about 13° (Fig. 10).

The structural model in Fig. 10 shows that the Changning anticline is a large fault-bend fold anticline, and the development of the anticline is at the stage of top widening. The faults are developed in the pre-Sinian and extend into the basin in a step-like manner, the structures of the back fault flat, footwall fault ramp, hanging wall fault ramp and front fault flat are restrained by the overlying strata, and the bed cutting point of the fault ramp is more obvious. The displacement of the anticline is 18.01 km, which is transmitted from the south side. After the formation of the Changning anticline by folding, the displacement continued to transfer to the north is 11.90 km, that is, another displacement of 11.90 km from the north of the Changning anticline was transferred to the interior of the Sichuan Basin, and continued to form the anticline and syncline (Fig. 8) The Changning anticline absorbs about 33% of the displacement, which is mainly accomplished by parallel shear, small fault and micro-fracture system.

Combined with regional tectonic evolution history and low-temperature thermochronology data, the formation and evolution process of the Changning anticline was reconstructed on the basis of balanced profile analysis (Fig. 12). It can be found that: before Yanshanian period, the research area had relatively stable tectonic setting, and deposited Sinian to Jurassic successively from bottom to top. At the beginning of Yanshanian period, the compressive stress in the southwest direction resulted in the formation of an initial thrust fault and two active axial planes in the undeformed strata. During the early Yanshanian period, the fault sliding northeast led to the folding of the hanging wall strata along the active axial plane, and the Changning anticline appeared the initial uplift form, limited the development range of Cretaceous only to the depression area of anticlinal front limb, and the hanging wall strata are pushed from southwest to northeast, resulting in the fold and uplift of the hanging wall, thus forming the fault- bend fold. The whole Changning anticline is characterized by a gentle southwest limb and a steep northeast limb, and the formation shortening of early Yanshanian was about 5.6 km. In the late Yanshanian period, the anticline was gradually raised as the strata continued to thrust along the hanging wall fault ramp. Plastic deformation occurred in soft layers such as gypsum, mud and shale of Cambrian, and the slip surface in layers formed a back thrust opposite to the sliding direction of basement fault in anticlinal forelimb. The back-thrust cut upward through the Ordovician, Silurian, Permian, Triassic and Jurassic, forming the secondary fault-propagation fold, and the overall shortening amount of this period was about 8.0 km. In the early Himalayan period, as the footwall continued to extrude and push to the northeast along the basement fault, the anticline kept widening and the anticline forelimb back-trust fault continued to develop, making the deformation of footwall more severe. With the development of the new tectonic wedge of the southwest limb of the anticline, the early flat strata were folded and uplifted to form a symmetrical Jianwu syncline. In the early Himalayan period, the formation shortening reached 6.5 km. In general, Changning anticline is formed by controlling basement fault and has the characteristics of continuous activity. In the late Himalayan period, with the weathering and denudation of the earth's surface, the Silurian and Ordovician in the core exposed gradually, forming the present appearance of the Changning anticline.

By comparing Fig. 8 and Fig. 10, the anticline of the southern limb of Jianwu syncline absorbed some displacement during the formation process. In Fig. 10, the absorption displacement of the control section at the left end of the profile (only show the anticlinal front limb) is 2 km long, to make the overlying strata suffered from fold shortening and Silurian flexural-slip fold and parallel shear.

The producing layer of the Changning shale gas field has an altitude of -1200-600 m and a shallow buried depth of 0- 2 600 m. The main production area is in the synclinal area of the southern anticline of Changning. The pressure coefficient of Longmaxi Formation in Changning area is mainly 1.2-2.0, showing overpressure or abnormal high pressure. That is to say, the Longmaxi Formation is characterized by overpressure before high-pressure fluid injection in shale hydraulic fracturing. The formation of overpressure in Longmaxi Formation is mainly caused by the formation and accumulation of natural gas in the formation. Even in the setting of intense uplift since Late cretaceous, the overpressure state is still maintained. In the process of uplift, although the confining pressure is reduced, the adsorbed and desorbed part of shale becomes free gas, which increases the formation pressure continuously.

The in-situ stress test data show that^[28,29], the two-direction stress difference in the Changning anticline area is 21.4-22.3 MPa. According to the logging interpretation, the maximum horizontal principal stress is 57 MPa, the minimum horizontal principal stress is 44.6 MPa, and the difference of horizontal stress is 12.4 MPa, which is smaller than the measured value. The maximum horizontal principal stress direction measured in this area is 100°-155° north by east. It is obvious that the direction of maximum principal stress is oblique to the axis trace of the Changning anticline, which is mainly formed in Yanshanian period and is an "ancient" anticline.

Natural fractures are relatively developed, and the fracture trend is generally consistent with the direction of maximum horizontal principal stress. During fracturing, the net pressure of the formation is much larger than the horizontal stress difference (12.4 MPa), which can form a complex fracture network. In the initial stage of shale gas exploitation in the Changning anticline, casing pressure is about 70 MPa (such as well H3-4 in Changning). In the past 2 years, according to micro seismic monitoring, construction pressure response, pressure monitoring of adjacent Wells and other conditions, combined with 3D seismic prediction and geological model, the fracturing scheme was adjusted in real time to ensure that the reconstruction scheme could adapt to the formation characteristics as much as possible, so as to improve the fracturing effect. The construction pressure is generally 56-66 MPa, such as the oil pressure of well H6-1 in Changning is 50-60 MPa and casing pressure is 12 MPa. The formation fluid pressure of Silurian Longmaxi Formation is mostly less than 55 MPa, and the fluid pressure has a great influence on the ground stress.

Observations on the surface outcropping and core of Silurian Longmaxi Formation in the anticlinal area of Changning indicate (Fig. 13) that the shale deformation of Longmaxi Formation is relatively common, with clear sliding surface and visible mirror, scratches and cracks. The organic mature- rich shale segment is stratified (Fig. 13a). Horizontal stratification of Longmaxi Formation in well 2 515.79-2 515.89 m of well 2 515.201, which is the sedimentary characteristics of deep water continental shelf environment. Core observation shows that there are various types of natural fractures (Fig. 13). The Longmaxi Formation filling fractures (Fig. 13b) are perpendicular to, parallel to, or oblique to the plane at 2 521m of well Ning 201, showing the characteristics of multi-stage fracture crosscutting development. Structural open fractures (Fig. 13d) are found in Longmaxi Formation at 2 228 m of Ning 203 well, with high-angle oblique fractures. At the same time, under a high-power microscope, hydrocarbon generation fractures can be seen in Longmaxi Formation at no. 2 492.59 m of well Ning 201 (Fig. 13c), which is a direct evidence of micro fracture development caused by hydrocarbon generation pressurization. The slip interface between shale and underlying Ordovician limestone floor can be clearly seen in the field.

The Silurian Longmaxi Formation in well Ning 201 has silica, clay and carbonate mineral contents of 54.9%, 21.0% and 21.0%, respectively, and the brittleness index reaches 61, showing a great brittleness. It indicates that this kind of formation rock is prone to crack under stress.

Based on the acoustic emission experiment, Ding et al.^[30] obtained that the maximum tectonic stress suffered by the shale of Silurian Longmaxi Formation in well Yuye 1, located in the east of Qiyueshan fault, the core of the Guochangba anticline, was 148.8 MPa in the Yanshanian stage and 122.5 MPa in the Himalayan stage. Fan et al.^[31] obtained that the maximum tectonic stress of the shale samples of the Long-maxi Formation in well Dingye 2, Dingshan area, in the middle and late stage of Yanshanian stage was 164.32 MPa, and the finite element simulation results showed that the regional maximum effective main stress was 97.06 MPa. At the end of Yanshanian-Himalayan period, the maximum tectonic stress of the samples was 152.05 MPa, and the maximum effective regional principal stress was 90.71 MPa. That is to say, the maximum paleo stress of the shale in the Longmaxi Formation is 150-160 MPa, which is lower than the compressive strength of the rock (generally 150-180 MPa), that is, fracture should not occur. However, in geological history, the buried depth of Longmaxi Formation once reached 6 000 m. Due to hydrocarbon generation pressurization, fluid retention depth was 1 500-1 800 m, and overpressure was developed in the underlying strata^[32], with a pressure coefficient of 1.6 or above, thus reducing effective stress and making shale prone to shear fracture^[33]. Fig. 13 is a direct reflection of this phenomenon, which indicates that the shale of Longmaxi Formation has been fractured repeatedly in geological history (Fig. 13b).

Figs. 4 and 6 show the distribution map of earthquake epicenter in the anticline and adjacent areas of Changning. The region surrounded by the Gaoxian, Gongxian, Xingwen, Junlian, Yanjin, Jingyan, Rongxian, Weiyuan, Yongshun, Longchang and Rongchang areas, has two seismic concentration regions, which are also the current shale gas exploration concentration regions. According to the seismic source depth given so far, the seismic source depth of M>3.0 is mainly about 5 km deep (Fig. 7, Table 2), concentrated in 5-15 km, which happens to be located in the pre-Sinian basement of the Changning anticline. In the shallow region of 2 km or so, earthquakes are mainly those with a magnitude less than 2.0.

Ellsworth^[34] proposed two mechanisms that induce earthquakes: (1) Direct fluid effect of injection (fluid pressure diffusion). Permeability reservoir/aquifer attached directly to the fault, fault zone caused by the pore pressure increases, thus weakening fault and make a critical stress state of the fault fracture, thus induced earthquake, its premise is the high permeability fluid migration channel along the fault development, the model can be called directly induce model (high pressure fluid). (2) A change in solid state stress caused by injection or withdrawal of a fluid. Due to reservoir permeability/aquifer volume or quality change caused the change of gravity load and pore-thermoelastic effect, cause underlying fault activity and the induced earthquake again, when the fault with overlying permeability reservoir/aquifer is not directly connected, is due to the changing load conditions and the normal stress change of fault induced earthquake, the model can be referred to as indirect induce model, in the face of this two cases are discussed respectively.

4.2.1. Direct induced type by overpressure

From January 2016 to January 2019, 11 earthquakes of M≥4.0 and 3 earthquakes of M≥5.0 occurred in the Changning shale gas block. These earthquakes may not be directly induced by overpressure, for the following reasons.

(1) There are large "pre-existing faults" in the Silurian or adjacent layers. According to the current high-resolution 2D and 3D seismic data, no large faults exist in the shale gas producing areas (syncline areas) identified by the seismic data (resolution within 30 m) (Figs. 8 and 10). The above analysis shows that the Changning anticline is a large anticline formed by the basement fault transition, and the Silurian fold reflects the activity along the basement fault. The Silurian System itself is only prone to the sliding of parallel faults. In this process, parallel shear of layers occurs to form micro fracture systems, and no large to medium-sized faults develop in the Silurian System (Figs. 8 and 10). The exposed faults in the core of the anticline of Changning take the Middle Cambrian salt rocks as the detachment layer and cut upward through the Ordovician and above strata (Figs. 8 and 10), but these faults are not active at present (Figs. 6 and 8).

(2) The above earthquake source repositioned (Table 2) is mainly located in the pre-Sinian basement, not in the Silurian shale.

(3) Different structural deformation mechanism. The deformation of Longmaxi Formation is only the repeated development of micro-cracks in the parallel shear process of layers (Fig. 13), while most earthquakes are shear failures. There are no major faults in Longmaxi Formation, which cannot accumulate the stress required by major earthquakes, and the magnitude of micro-earthquakes detected is far less than 0.5. The change of coulomb failure stress caused by overpressure is generally small, and it is difficult to reach the level of moderate scale earthquake.

(4) Before 2011, there were many earthquakes in the anticlinal area of Changning (Figs. 3 and 4). At that time, there was no fracturing operation.

4.2.2. Indirect induced type by change in gravity load

If the fault is developed in the basement or underlying brittle layer (Fig. 10), can the fluid pressurization in the Longmaxi Formation caused by shale fracturing be transmitted downward, leading to the weakening and re-activity of underlying faults? At present, this possibility is difficult to hold, mainly because there is no such fluid channel between the two.

Further evaluation is needed of the gravity changes caused by shale gas production, hydraulic fracturing fluid injection and removal. However, in general, the "in" and "out" may be balanced, and the normal stress changes caused by underlying faults (such as basement faults) may not be significant (which remains to be verified by numerical simulation).

In large scale shale gas exploitation areas such as Ohio, Oklahoma and British Columbia, Canada, there have been no reported cases of hydraulic fracturing operations themselves directly causing magnitude over 3^[8,35-36]. Federal geological survey on shale gas in the interpretation of the relationship between hydraulic fracturing and induced earthquake, points out that the waste water produced in the process of oil and gas exploration, the booster injection or injected directly into the water well, is to produce most of the main causes of induced earthquake, induced earthquake in deeper water injection point position, the greater distances (more than 3 km), but the hydraulic fracturing of shale gas is not the principal causes of induced earthquake, waste water containing hydraulic fracturing fluid also not clear relationship with induced earthquake^{[8, 35-37]}.

If the earthquake is not induced by overpressure or gravity load change, what causes the earthquake in this area?

It should be pointed out that earthquakes are tectonic activities on the time scale of present or recent decades and belong to the category of active structure. The Changning anticline belongs to the "paleo structure" of Yanshanian-Himalayan period. It is difficult to answer this question. The source depth and distribution characteristics can be intuitively understood by projecting the 1990 seismic events repositioned on the depth seismic profile within 30 km of the above seismic profile (Fig. 14). The Fig. 14 shows that Xingwen 5.7 earthquake focal projection on the basement fault ramp break, Gongxian region projection near a fault point below the 5.3 magnitude earthquake, while other projection is less than 5.0 magnitude earthquake are mainly distributed in the basal fault nearby, and mostly distributed in the bottom of the Sinian, especially the two hinge zone of the fault of intensive earthquake distribution. Although there are earthquakes in the Sinian and above strata above the basement, most of them are small- scale earthquakes. Among them, there was a 3.0-4.0 earthquake near the Cambrian/Sinian interface, and a 3.0-4.0 earthquake in the Lower Triassic Feixianguan formation. There are more earthquakes less than 2.0 in Silurian, and the overall trend is that the magnitude of earthquakes decreases gradually and the number of earthquakes decreases gradually. Earthquakes rarely occur on the northern flank of the anticline.

The structural model in Fig. 14 provides the tectonic setting for the occurrence of earthquakes in the Changning anticline area. First, along the ramp of the basement fault (and its upper and lower parts), the earthquake is relatively concentrated, manifested as shear failure along the fault ramp; Secondly, at the hinge zones of Jianwu syncline and the hinge zones of the Changning anticline (knee fold zone), affected by the change of the sliding direction along the fault, there were two relatively concentrated earthquake zones, which were obviously two parallel shear zones. The earthquake magnitude was less than 5.0, and the magnitude was smaller along the knee fold zone. At present, there are no large- to medium-sized faults and no large seismic activity in the shale gas producing area. According to the conjecture in Fig. 14, the basement fault that formed the Changning anticline is resurrected at present, and it mainly occurred in the part of the footwall ramp, which has not been transferred to the part of the hanging wall ramp on the north side, and the north limb of the anticline is relatively “calm”. The current GPS observation results show that^[38], the crust in the southwest of Sichuan has a movement trend in the direction of NW-SE. The tectonic setting of this resurrection may be related to the extrusion of the southeastern margin of the present Qinghai-Tibet Plateau. However, the specific factors or boundary conditions driving the tectonic activity still need to be further studied.

The velocity model constrained by logging data from many Wells in the Changning anticline area and southwest Sichuan area was used for the extracted wave data over 2.0 magnitude, and the results of repositioning were tested. The precise location results can constrain the spatial distribution of small earthquakes. Is it in the Longmaxi Formation or in the Sinian - Ordovician or pre-Sinian? What are their distribution probabilities?

It is necessary to elaborate the interpretation of the seismic profile passing the epicenter in the Changning and Weiyuan- Rongxian exploration blocks, implement the geometry and kinematics characteristics of the old fault/closed fault, establish the initial model of the fault^{[39,40,41,42,43,44,45]}, and analyze the spatial distribution and kinematics rules of the preexisting fault.

Collect fluid pressure, engineering mechanical data, the establishment of structural stress field model, fluid pressure (including injection pressure), stress difference, Cullen rupture stress and other quantitative calculations, simulation of the main boundary conditions of seismic shock, to clarify the possible relationship between overpressure and seismic magnitude.

Systematic study of the tectonic geological setting of distributed earthquakes in southwestern Sichuan, and assessment of the possibility of active fault rupture^{[46,47,48,49,50]}; conducting long-term scientific detection of seismic activity in shale gas mining areas, analyzing the relationship between fluid injection pressure, frequency, injection and the micro earthquakes and the possibility of inducing an earthquake.

The Changning anticline is a large basement fault-bend fold, the geometry of basement fault is ramp-flat and its displacement of the fault forming the anticline is 18 km, the displacement transferred to the north is 11.9 km and the Changning anticline absorbs 33% of the fault slip.

The Silurian Longmaxi Formation of the Changning anticline experienced larger-parallel shearing along underlying basement faults, forming a micro-fracture system. Hydrocarbon generation and fluid overpressure lead to fruquent occurrence of fractures.

The earthquakes in the Changning anticline are mainly natural. The footwall ramp of the basement fault is reactivated at present, earthquakes in this area mostly occur along the footwall ramp of the basement fault and above and below it. The anticlinal and synclinal hinge zones are also the earthquake concentration areas, but the earthquake magnitude decreases upwards along the kink-band, and small earthquakes below M2.0 occur in the Silurian Longmaxi Formation. Seismic activities in the Changning anticline need to be monitored and analyzed in a long term.

CNPC Southwest Oil and Gas Field Company provided drilling and seismic exploration data of the Changning anticline and adjacent areas. China Earthquake Administration Institute of Geology “Active Fault Detection Data Exchange and Sharing Management Center” provided fault data for this study. The National Earthquake Science Data Sharing Center provided earthquake catalogs and data. In the writing process, we also received the guidance of Academician Li Desheng, Jia Chengzao, Ma Yongsheng, Zhang Guowei, and discussed with Prof. Cai Xunyu, Xu Xuhui, Zhang Jian, Liu Bo, Liu Shugen, Yang Hongzhi, etc. Additionally, Academician Zhao Wenzhi, Zou Caineng and Prof. Xu Huaixian have also made some contributions to this paper.