Source Model and Seismogenic Environment of the Ms 6.4 Yangbi Earthquake in Yunnan, China—Based on InSAR Observation

: On 21 May 2021, an Ms 6.4 earthquake struck Yangbi County, Dali Prefecture, Yunnan Province, China, which is the largest earthquake to hit this area since 1976. In this paper, we obtained the coseismic deformation of the Yangbi earthquake in Yunnan Province based on the interferometric synthetic aperture radar (InSAR) observation. After that, we obtained the fault geometry and slip distribution model of this earthquake via the two-step inversion method. The maximum deformation in the ascending orbit along the line of sight (LOS) direction was 7.3 cm, and the maximum deformation in the descending orbit along the LOS direction was 8.9 cm; the slip distribution model showed that the slip distribution of this earthquake was concentrated at a depth of 1–14 km, and the maximum slip was 0.6 m at a depth of 5 km. Based on the modeling result, it was inferred that the seismogenic fault of this earthquake was a dextral strike-slip fault on the west side of the Weixi-Qiaohou–Weishan fault. Combining the existing geological data and the changes in Coulomb stress caused by this earthquake, the seismic hazard and seismogenic structure in the area near the epicenter were analyzed and discussed, and the results showed that, in the northwest of the Weixi-Qiaohou fault zone, there will be an increased hazard of a future earthquake in the NW trend; thus, we should pay attention to this area. Fault; WX-QHF;


Introduction
At 21:48 on 21 May 2021, Beijing time, an Ms 6.4 earthquake struck Yangbi County in the central part of the Dali Bai Autonomous Prefecture, in Yunnan Province in China. According to the China Earthquake Network, the epicenter was located at 25.67 • N and 99.87 • E. The earthquake caused three deaths and thirty-four injuries. Many houses in Dali City and Eryuan County and other surrounding areas collapsed, the earthquake caused some certain economic loss, and it was obviously felt in many places in Yunnan Province. According to the distribution of historical earthquakes, taking the epicenter of the Yangbi Ms 6.4 earthquake as the center and within a radius of 100 km, there are 112 earthquakes with magnitudes over 3.0 that have occurred in history in total, of which, 21 earthquakes had a magnitude above 5.0. The maximum is an earthquake that occurred 25 km away from the NE direction of Dali City on 16 March 1925. This earthquake event 2 of 14 is the largest since one in 1976 that occurred near the Weixi-Qiaohou fracture. After the earthquake, domestic and foreign research institutions and scholars rapidly carried out research work on the earthquake based on different geophysical methods: Fang's team from the Institute of Geophysics, China Earthquake Administration used artificial intelligence seismic monitoring to produce a relatively complete earthquake catalog and relocation results for this earthquake sequence. Using the observation data of the Yunnan Seismic Network, Zhao et al. [1] from the Yunnan Seismological Bureau analyzed the spatiotemporal evolution characteristics and strong aftershock activity characteristics of the Yangbi 2021 Ms 6.4 earthquake sequence, and the results show that, after the relocation, the epicenter of the Yangbi earthquake sequence was distributed in the NW-SE direction as a whole, the long axis was about 135 • , and the total length was about 25 km. The largest aftershock of the Yangbi Ms 6.4 earthquake was the Ms 5.2 earthquake that occurred at 22:31 on 21 May. Yang et al. [2] used the broadband seismic waveform data of the Yunnan Seismic Network to calculate the focal mechanism solutions of the Yangbi mainshock and aftershocks. The preliminary analysis of the seismogenic tectonic features was performed with the Cut and Paste method focal mechanism solution inversion software, and it revealed that the seismic fault was characterized by a dextral strike-slip [3,4]. Zhang [5] used the 3D coseismic deformation field data observed by the Global Navigation Satellite System (GNSS) to invert the Yangbi earthquake, and the results showed that the maximum slip was located in the area south of the mainshock source. Based on the finite fault model inversion method, Zhu et al. [6] used regional broadband data to invert the source rupture process of the earthquake. The results showed that the maximum dislocation amount reached 0.55 m, located at a depth of 9 km, and the seismogenic fault was in the SE direction and was dominated by a dextral strike-slip, and the rupture mainly occurred on the southeast side of the epicenter. The results obtained by different research methods are still different in the slip direction and the distribution of the maximum slip on the fault plane.
In recent years, moderate and strong earthquakes have occurred frequently in Yangbi County and the surrounding areas: an Ms 5.5 earthquake occurred in Eryuan in 2013, an Ms 5.0 earthquake occurred in Yunlong in 2016, and Ms 5.1 and Ms 4.8 earthquakes occurred in the Yangbi area in 2017. The epicenter of the Yangbi earthquake was Dali City, Yunnan Province. It is a tourist destination in China. It is densely populated and actively developed. It is also an area inhabited by ethnic minorities. Therefore, the future earthquake trend in this area is of great significance for maintaining local social stability. Seismic hazard analysis and research in this area have always attracted widespread attention in the field of earthquakes. After the earthquake, through the results of the coseismic displacement field, and after discussion of the source slip model and the seismogenic mechanism, we can better understand the cause of the earthquake and the characteristics of the earthquake. It has important guiding significance for the work of crustal tectonic movement, seismic dynamic characteristics, rapid emergency response after an earthquake, engineering seismic design, and post-earthquake trend determination. In order to be able to assess the future seismic hazard of this area, we need to study the tectonic origin of this earthquake and its associated activity. As an advanced and rapidly improved geodetic technique, InSAR [7] can obtain terrain elevation and surface deformation with high precision and continuous coverage in all weather based on existing intensive satellite observations. Therefore, in this paper, high-precision and high-resolution coseismic deformations related to the 2021 Yangbi Ms 6.4 earthquake were obtained from the ascending and descending orbit of Sentinel-1A data provided by the European Space Agency (ESA). Based on the InSAR observation results, the Okada elastic half-space dislocation model was used to determine the fault geometric parameters through a two-step inversion method and to reverse the fine slip distribution. Finally, the static Coulomb failure stress was calculated so that we can further understand the physical process of the earthquake source. Combined with the existing geological data, the fault activity characteristics, seismogenic structure, and the regional earthquake potential risk were discussed and analyzed to provide a reference basis for disaster assessment.

Study Area
The epicenter of the earthquake was in Yangbi County, Dali Bai Autonomous Prefecture in Yunnan Province. Yunnan Province is located on the southeastern edge of the Tibet Plateau, which is at the junction of the Sichuan-Yunnan rhombic block and the western Yunnan block, as illustrated in Figure 1. There are multiple block boundaries and large fault zones that produce destructive earthquakes [8]. The topography of western Yunnan, where the epicenter of the Ms 6.4 earthquake was located, is highly undulating, and there are several active ruptures, mainly distributed in the SN, NE, or NW direction [9], including the NW direction Red River fault, Weixi-Qiaohou fault, Jinshajiang fault, and Longpan-Qiaohou fault near the NS direction [10]. The Red River fault has a total length of over 1000 km and about 600 km of it is in Yunnan. It is an important large-scale strike-slip fault in the Himalayan-Tibet collision zone [11], which has experienced two earthquakes of a magnitude 7.0 or higher in history, and seven earthquakes of a magnitude of 6.0-6.9. The Red River fault is one of the most active faults in Yunnan Province, with a long-term average slip rate of 2-5 mm/a [12][13][14]. Yangbi County is located in the northwest of the Red River fault zone and has a complex regional geology and active tectonic development, and there was an Ms 5.4 earthquake in 2017. In addition, the SN trend ruptures in the western part of Yunnan include the Nujiang fault, Lancangjiang fault, Chenghai-Binchuan fault, Xiaojiang fault, Anninghe fault, etc. The Weixi-Qiaohou fault is a branch fault extending in the northwest section of the Red River fault zone, with a total length of about 280 km [15]. The active nature of this fault in the early period was mainly extrusion, and since the Late Quaternary, it was mainly a dextral strike-slip and also had a normal and fractal extension [16]. These faults crisscross and divide the entire western Yunnan area into multiple blocks: Baoshan block, Tengchong block, and the Lanping-Simao block near the location, where an Mw 6.4 Ning'er earthquake occurred in Pu'er in 2007. There are also tectonic units such as the secondary block in central Yunnan [17]. After the Lijiang Ms 7.0 earthquake that occurred in 1996 and the Mojiang Ms 5.9 earthquake that occurred in 2018, this earthquake is the strongest earthquake to occur in the Yangbi area in recent years. The risk of strong earthquakes in the western Yunnan area has always been one of the focuses of attention in the field of seismic geology [18]. Therefore, in this paper, we studied and discuss the characteristics of the deformation field, fault activity, and seismogenic structure of this earthquake.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 14 seismogenic structure, and the regional earthquake potential risk were discussed and analyzed to provide a reference basis for disaster assessment.

Study Area
The epicenter of the earthquake was in Yangbi County, Dali Bai Autonomous Prefecture in Yunnan Province. Yunnan Province is located on the southeastern edge of the Tibet Plateau, which is at the junction of the Sichuan-Yunnan rhombic block and the western Yunnan block, as illustrated in Figure 1. There are multiple block boundaries and large fault zones that produce destructive earthquakes [8]. The topography of western Yunnan, where the epicenter of the Ms 6.4 earthquake was located, is highly undulating, and there are several active ruptures, mainly distributed in the SN, NE, or NW direction [9], including the NW direction Red River fault, Weixi-Qiaohou fault, Jinshajiang fault, and Longpan-Qiaohou fault near the NS direction [10]. The Red River fault has a total length of over 1000 km and about 600 km of it is in Yunnan. It is an important large-scale strikeslip fault in the Himalayan-Tibet collision zone [11], which has experienced two earthquakes of a magnitude 7.0 or higher in history, and seven earthquakes of a magnitude of 6.0-6.9. The Red River fault is one of the most active faults in Yunnan Province, with a long-term average slip rate of 2-5 mm/a [12][13][14]. Yangbi County is located in the northwest of the Red River fault zone and has a complex regional geology and active tectonic development, and there was an Ms 5.4 earthquake in 2017. In addition, the SN trend ruptures in the western part of Yunnan include the Nujiang fault, Lancangjiang fault, Chenghai-Binchuan fault, Xiaojiang fault, Anninghe fault, etc. The Weixi-Qiaohou fault is a branch fault extending in the northwest section of the Red River fault zone, with a total length of about 280 km [15]. The active nature of this fault in the early period was mainly extrusion, and since the Late Quaternary, it was mainly a dextral strike-slip and also had a normal and fractal extension [16]. These faults crisscross and divide the entire western Yunnan area into multiple blocks: Baoshan block, Tengchong block, and the Lanping-Simao block near the location, where an Mw 6.4 Ning'er earthquake occurred in Pu'er in 2007. There are also tectonic units such as the secondary block in central Yunnan [17]. After the Lijiang Ms 7.0 earthquake that occurred in 1996 and the Mojiang Ms 5.9 earthquake that occurred in 2018, this earthquake is the strongest earthquake to occur in the Yangbi area in recent years. The risk of strong earthquakes in the western Yunnan area has always been one of the focuses of attention in the field of seismic geology [18]. Therefore, in this paper, we studied and discuss the characteristics of the deformation field, fault activity, and seismogenic structure of this earthquake.

InSAR Observations
This study benefitted from the abundant SAR images over the epicentral region, providing us the opportunity to explore this event. We employed InSAR technology to process observation data, InSAR technology has received extensive attention in recent years with its ability to obtain high-precision, continuous coverage of ground elevation and surface information, and it is widely used in disaster monitoring and evaluation, topographic surveying, mapping, resource and energy extraction, etc. [19][20][21]. Since 1993, Massonnet et al. used two ERS-1 images of pre and post the Mw 7.3 earthquake in Landers, CA, USA, and successfully obtained the coseismic deformation field using the two-track differential interferometry method [7]. InSAR technology is widely used in seismic deformation monitoring research and to invert the geometric parameters of seismic faults and slip distribution [22][23][24][25][26][27]. In this study, we obtained the InSAR observation data of the Sentinel-1A IW mode satellite data provided by the European Space Agency (ESA). The working band is C-band, including the image data of the InSAR ascending orbit from 8 May to 1 June 2021, and the descending orbiting image data from 10 May to 22 May 2021. The time baseline was 24 days and 12 days, respectively, and the time interval for acquiring image data was relatively short, which can ensure that the interferogram has a good correlation. We obtained the line-of-sight (LOS) surface displacements by the two-pass differential InSAR (DInSAR) method using the GMTSAR software [28,29]. The satellite orbital parameters are listed in Table 1.

Coseismic Deformation
The coseismic deformation displacement fields based on the ascending and descending tracks of Sentinel-1A observations are shown in Figure 2, respectively. The shapes of the two coseismic deformation fields are elliptical and extend in the NW direction of the long axis, which is consistent with the surface fissure zone found in the field survey results of the Institute of Geology, China Earthquake Administration [30]. The deformation field of the long axis modification shows a significant difference, and the sign of the deformation field between the ascending and descending track is opposite. A positive value indicates movement close to the satellite, and the surface shows uplift or westward movement; a negative value indicates movement away from the satellite, and the surface shows subsidence or eastward movement. The ascending orbit coseismic deformation field was along the line of sight to the east disk with a maximum deformation value of −7.3 cm, indicating that it was dominated by subsidence and far away from the satellite, and the noise was obvious,. The reason may be that the study area is located in the Sichuan-Yunnan area with high vegetation coverage, with large terrain undulation and atmospheric influence. The coseismic deformation field of the descending orbit was dominated by subsidence along the line of sight to the west, with a maximum deformation value of −6.5 cm, and the east disk was dominated by uplift, with a maximum deformation value of 8.9 cm. Combining with the ascending and descending tracks' imaging mode and the deformation field showing the main deformation trend with horizontal deformation, it was preliminarily inferred that the deformation field of the Yangbi earthquake conformed to the characteristics of the dextral strike-slip earthquake deformation. Specifically, it needs to be further determined whether the seismogenic fault is the Weixi-Qiaohou fault in the NW direction.
Combining with the ascending and descending tracks' imaging mode and the deformation field showing the main deformation trend with horizontal deformation, it was preliminarily inferred that the deformation field of the Yangbi earthquake conformed to the characteristics of the dextral strike-slip earthquake deformation. Specifically, it needs to be further determined whether the seismogenic fault is the Weixi-Qiaohou fault in the NW direction.

Source Parameter Inversion
Studying the focal mechanism and exploring the types of earthquakes can help deepen understanding of the nature of earthquake disasters and likewise facilitate the government in conducting post-earthquake disaster relief work that can be carried out smoothly [31]. The determination of focal depth is of great significance for evaluating earthquake disasters, determining the cause and dynamic environment of earthquakes, and judging the development trend and danger of aftershocks [32]. Based on the obtained line-of-sight lifting orbit coseismic deformation field (Figure 2), the coseismic deformation field had good coherence and the deformation area was completely covered in the image range. If all the observation data in the deformation area are involved in the inversion, the huge amount of data will reduce the inversion efficiency and may affect the accuracy of the inversion result [33]. Therefore, in order to improve the reliability of the inversion and enhance the spatial correlation of the observation, in this paper, we performed downsampling processing on the InSAR deformation field, and the down-sampling results are shown in Figure 3.

Source Parameter Inversion
Studying the focal mechanism and exploring the types of earthquakes can help deepen understanding of the nature of earthquake disasters and likewise facilitate the government in conducting post-earthquake disaster relief work that can be carried out smoothly [31]. The determination of focal depth is of great significance for evaluating earthquake disasters, determining the cause and dynamic environment of earthquakes, and judging the development trend and danger of aftershocks [32]. Based on the obtained line-of-sight lifting orbit coseismic deformation field (Figure 2), the coseismic deformation field had good coherence and the deformation area was completely covered in the image range. If all the observation data in the deformation area are involved in the inversion, the huge amount of data will reduce the inversion efficiency and may affect the accuracy of the inversion result [33]. Therefore, in order to improve the reliability of the inversion and enhance the spatial correlation of the observation, in this paper, we performed down-sampling processing on the InSAR deformation field, and the down-sampling results are shown in Figure 3.
In this paper, based on the single fault and Okada elastic half-space dislocation model [34], we used the simplex algorithm for uniform inversion in order to make the simulated deformation value and the observation value best fit. According to the dislocation theory, there is a highly nonlinear relationship between the geometric parameters of the fault and the surface deformation. In the inversion process, the Okada uniform elastic half-space dislocation model was used to simulate the observed interference deformation field, and the fault parameters to be estimated include longitude, latitude, length, depth, strike, dip, and slip angle. The equation to be solved is: where d represents the observed value of surface deformation, m represents the slip parameters of faults (including strike, dip angle, slip angle, length, width, depth, and slip value), G is the Green's function matrix, and ε indicates observation error. We used the Multi-peak Particle Swarm Optimization (MPSO) algorithm [35] to determine the above parameters by searching for the optimal solution of the parameters; the algorithm has the characteristics of fast convergence speed and high computational efficiency. Based on previous studies of motion characteristics and geometric shapes, combined with the focal mechanism solution of the Yunnan Yangbi earthquake given by the Global Centroid Moment Tensor (GCMT) and the United States Geological Survey (USGS) and the coseismic deformation obtained from InSAR observations, we established the seismogenic fault of this Yangbi earthquake model.  In this paper, based on the single fault and Okada elastic half-space dislocation model [34], we used the simplex algorithm for uniform inversion in order to make the simulated deformation value and the observation value best fit. According to the dislocation theory, there is a highly nonlinear relationship between the geometric parameters of the fault and the surface deformation. In the inversion process, the Okada uniform elastic half-space dislocation model was used to simulate the observed interference deformation field, and the fault parameters to be estimated include longitude, latitude, length, depth, strike, dip, and slip angle. The equation to be solved is: where d represents the observed value of surface deformation, m represents the slip parameters of faults (including strike, dip angle, slip angle, length, width, depth, and slip value), G is the Green's function matrix, and ε indicates observation error.
We used the Multi-peak Particle Swarm Optimization (MPSO) algorithm [35] to determine the above parameters by searching for the optimal solution of the parameters; the algorithm has the characteristics of fast convergence speed and high computational efficiency. Based on previous studies of motion characteristics and geometric shapes, combined with the focal mechanism solution of the Yunnan Yangbi earthquake given by the Global Centroid Moment Tensor (GCMT) and the United States Geological Survey (USGS) and the coseismic deformation obtained from InSAR observations, we established the seismogenic fault of this Yangbi earthquake model.

Distribution Slip Inversion
Based on the geometric parameters of the fault determined by nonlinear inversion, the fault plane was divided into uniform and continuous rectangular sub-units to obtain the fine slip distribution on the fault plane. When the geometric parameters of the fault were determined, the motion parameters on the fault plane and the surface deformation were transformed into a generally linear relationship. The linear model of the parameters of the slip distribution and the deformation and displacement of the ground was related to Green's function matrix. After determining the parameters of the fault geometry model by nonlinear inversion, the fault plane was divided into uniform and continuous rectangular sub-units, and the slip amount of each sub-unit was solved separately; the ground deformation and displacement caused by the strike-slip and tilt-slip of each sub-unit constituted the Green's function matrix. In this paper, the length and width of the fault plane were extended to 30 km and 20 km along the strike and downdip directions and were divided into rectangular units of 1 km × 1 km, and a total of 600 units were obtained. Consistent with the uniform slip inversion, the Okada uniform elastic half-space dislocation model was used to simulate the surface deformation field. In the inversion process, in order to avoid the oscillation of the sliding distribution solution, a Laplace smoothing constraint was added to obtain a reliable and stable solution: where κ 2 is the smoothing factor, H is a Laplacian, and Hm 2 is the slip roughness.
The fault slip distribution model is shown in Figure 4. The predominant slip mainly occurred along the fault at the depth of 1-14 km, the peak slip was able to approach 0.6 m, which is at the depth of 5 km, and the results showed that the coseismic rupture did not extend to the surface. The results indicate that the fault model had a dip angle of 76.2 • and an average slip angle of −176.3 • , which is consistent with the focal mechanism solution given by USGS, and it belongs to the dextral strike-slip earthquake, as represented in Table 2. In order to verify the reliability of the above fault model, the residual error between the simulated deformation and the real deformation was calculated based on the Okada model, and Figure 5 shows the simulated deformations and residuals for the slip model. From the overall perspective, the simulated deformation field was similar to the shape and size of the deformation field obtained by InSAR observation. Figure 5c,d shows that the standard deviations of the ascending and descending data were about 3 cm, which shows that the simulated deformation had a good fit with the real observation, and the fault geometry model obtained by the inversion can better explain the surface deformation of the Yangbi earthquake. In other words, it can be reasonably concluded that the general coseismic displacement obtained by the Sentinel-1A satellite can be accurately reproduced by the best slip distribution model.

Seismogenic Fault
The determination of the seismogenic faults of moderate and strong earthquakes can

Seismogenic Fault
The determination of the seismogenic faults of moderate and strong earthquakes can well identify the activity and distribution of faults in the surrounding area and play an important role in the assessment of geological disasters. The Yangbi Ms 6.4 earthquake area is located in the Sanjiang tectonic belt in western Yunnan on the southeastern margin of the Qinghai-Tibet Plateau, which is part of the Himalayan orogenic belt. Many scholars at home and abroad have studied the deep structure of the crust in this area through geological and different geophysical methods. Western Yunnan is a region with strong tectonic activity, frequent seismic activity, and complex deformation. In recent years, Ninglang, Tengchong, Longling, Eryuan, and Yingjiang in western Yunnan have successively experienced earthquakes of an Ms 5.0 or above. The study of the crustal structure characteristics in this area can provide scientific information for the analysis of the seismic structure and future seismic activity trends.
Before the main shock of the Ms 6.4 earthquake that occurred in Yangbi, the largest foreshock of Ms 5.6 had occurred, and aftershocks of Ms 5.0 and Ms 5.2 occurred after the mainshock. Several earthquakes were located on the west side of the Sichuan-Yunnan block; therefore, from the perspective of time and space, the Yangbi earthquake was not isolated. It was a foreshock-mainshock-aftershock type earthquake, which is similar to the characteristics of multiple large earthquakes in a concentrated seismic activity with shallow focal depth and large destructive force [38]. In addition, on 27 March in 2017, there were two moderately strong earthquakes of an Ms 4.8 and Ms 5.1 that occurred at 7:40 and 7:55 in Yangbi County, respectively, with focal depths of 8 km. Based on previous studies on the activity characteristics of seismic sequence, the sequence of this earthquake belonged to a typical earthquake sequence of 'seismic swarm (multi-seismic) type' (or 'double earthquake type') [39]. The relocation results, the focal mechanism solutions of the mainshock, and the early strong aftershocks all show that the seismogenic structure of the 2017 Yangbi earthquake was the Weixi-Qiaohou fault [40]. The Yangbi Ms 6.4 earthquake had a large number of foreshocks and aftershocks, according to InSAR observations and inversion results, and the seismogenic fault is an active fault in the NW trend and is consistent with the strike of the Weixi-Qiaohou-Weishan fault, which is the closest to the epicenter of the earthquake. The results of the slip distribution obtained from the inversion in this paper are shown in Figure 4. the earthquake did not rupture to the ground surface, and at the same time, according to the results of the Coulomb failure stress change calculation, it was found that there was a certain distance between the changing area and the Weixi-Qiaohou fault.
After the earthquake, many researchers [41,42] studied the 2021 Yangbi earthquake, and their results suggested that the Weixi-Qiaohou fault was the seismogenic fault of this event. The Weixi-Qiaohou fault, the closest to the epicenter of this earthquake, is located to the east of Yangbi County, and is a large-scale boundary fault with a total length of about 280 km, with strong Cenozoic activity characteristics, showing strong activity in the Late Quaternary, and the nature of motion is mainly a dextral strike-slip with a positive fault component [43]. In addition to the Ms 5.1 earthquake in 2017, an earthquake with a magnitude above 6.0 occurred in the area near the rupture in 1925, which indicates that the Weixi-Qiaohou fault has a background of medium and strong earthquakes. In order to better determine the seismogenic structure and activity characteristics of the Yangbi earthquake, Li et al. [30] conducted a field investigation on the Weixi-Qiaohou and surrounding faults in the Yangbi research area on 23 May 2021. The results showed that there are only a few surface fractures near the Weixi-Qiaohou fault, and it was possible that they were influenced by the gravity of the side slopes. The surrounding buildings were relatively lightly damaged in the earthquake, which indicates that the Weixi-Qiaohou fault is not the seismogenic fault of this earthquake; at the same time, it was found that the deformation distribution was concentrated and continuous, the fracture trend was NW, and the dip trend was in the SW, which is close to vertical. According to the inversion results of Yang et al. [37] and others, the slip is mainly concentrated in the west of the surface trace of the Weixi-Qiaohou-Weishan fault; moreover, if the Weixi-Qiaohou fault is regarded as a seismogenic fault, in order to realize the phenomenon of fracture, the fault must extend downward at a high dip angle (parallel to the Weixi-Qiaohou fault at an angle of about 80 • ), which is inconsistent with the strike-slip characteristics of the actual fault. According to the location of aftershocks, the small earthquake swarms related to the Yangbi Ms 6.4 earthquake happened to be concentrated on such an NW-trending belt. In addition, houses and buildings in the west of the Weixi-Qiaohou fault were seriously damaged in the earthquake, which further proves that the seismogenic fault is a fault in the west of the Weixi-Qiaohou fault.

Static Coulomb Stress Changes
Seismic triggering is a process by which earthquake-related stress changes can induce or delay seismic activity in the surrounding area or trigger other earthquakes from a long distance. The calculation of static Coulomb failure stress change (∆CFS) related to earthquake slip has been applied to explain the seismic observation, including the distribution of aftershocks, the earthquake sequence, and the static state of areas that are usually active after major earthquakes [44]. The ∆CFS is an important indicator of earthquake stress triggering research [45]; according to previous studies, when the ∆CFS change is up to 0.01 MPa, it is enough to trigger an earthquake [46]. After the Yangbi earthquake, part of the accumulated stress was released due to the rupture of the seismogenic fault, and the remainder of the stress was transmitted throughout the surrounding areas, resulting in an aggregation of stress in the study area. Increasing Coulomb stress may promote fault rupture activity, while a decrease in the Coulomb stress may promote fault rupture [47]. From the perspective of regional structure, the eastward slip of the Qiangtang block in the central Qinghai-Tibet Plateau was blocked by the South China block, causing strong deformation [48]. In the area, there are the Weixi-Qiaohou fault and the northern section of the Red River fault. As a large right-lateral strike-slip fault in the southwest of the Sichuan-Yunnan diamond block, the Red River fault has an extremely complex tectonic background and frequent seismic activities in this area [49].
In order to further understand the impact of the Yangbi Ms 6.4 earthquake on the surrounding geological structures, in this article, we analyzed the coseismic rupture on the stress changes in the study area. Based on the results of the fault slip distribution, in this paper, the Matlab-based software Coulomb 3.3 was used to calculate the coseismic static Coulomb stress change triggered by the Yangbi event. The effective coefficient of [50] the friction and the shear modulus were set to 0.4 and 3.32 × 10 10 N/m 2 [51]. The ∆CFS at the depths of 3 km, 6 km, 9 km, and 12 km for the receiver fault with a strike of 132.4 • , a dip of 76.2 • , and a rake of −176.3 • were calculated, respectively. The Coulomb stress results are shown in Figure 6. In addition, we collected the aftershocks with magnitudes greater than 3.0 after 21 May 2021, combined with the slip distribution, the results indicated that: as the depth increases, the coulomb stress variation of the NW trend Weixi-Qiaohou fault is large, the ∆CFS in the north of the middle section of the Eryuan-Midu fault are positive, the Coulomb stress is positive indicating that the risk of subsequent earthquakes in this area will increase, and we should continue to pay more attention to this area. results are shown in Figure 6. In addition, we collected the aftershocks with magnitudes greater than 3.0 after 21 May 2021, combined with the slip distribution, the results indicated that: as the depth increases, the coulomb stress variation of the NW trend Weixi-Qiaohou fault is large, the ΔCFS in the north of the middle section of the Eryuan-Midu fault are positive, the Coulomb stress is positive indicating that the risk of subsequent earthquakes in this area will increase, and we should continue to pay more attention to this area.

Conclusions
In this paper, we used Sentinel-1A ascending and descending track satellite data as the data source to obtain the coseismic deformation field of the 2021 Yangbi Ms 6.4 earthquake in Yunnan. The maximum deformation values of the line of sight of the ascending and descending track were 7.3 cm and 8.9 cm, respectively, and the sign of the deformation field of the two tracks was the opposite. The results indicate that the Yangbi Ms 6.4 earthquake had obvious characteristics of the dextral strike-slip movement. The fault strike was 132.4 • and the dip was 76.2 • , and the results showed that: the fault slip was mainly distributed in the fault plane about 1-14 km from the surface, the maximum slip at 5 km was 0.6 m, the average slip angle was −176.3 • , and the coseismic rupture did not extend to the surface. The focal mechanism results were relatively close to the results given by the USGS. The seismogenic fault of the Ms 6.4 earthquake in Yangbi, Yunnan in 2021 was a fault located the northwest of the Weixi-Qiaohou fault; the ∆CFS in the north of the middle section of