It is important to study the characteristics of the tectonic stress field studies which could provide a deeper understanding of the internal stress environment of the crust. It can provide useful assistance for exploring the relationship between the tectonic stress field and earthquake development. At the same time, it plays an important role in understanding block interactions, fault movement, tectonic deformation, and revealing the dynamic mechanical processes of the continent. The focal mechanism solutions contain abundant information reflecting the stress field.

In this paper, using the broadband records from 128 permanent and temporary regional stations from the Chinese National Seismic Network(CNSN)deployed in the Sichuan-Yunnan Province and its adjacent, we determined the focal mechanisms of 3 951 earthquakes by the cut-and-paste(CAP)method and the HASH method. The friction coefficient and stress properties of the main active fault and characteristics of the tectonic stress field in this area are analyzed by using two different methods which are the damped inversion method(STASI)and iterative joint inversion method from focal mechanisms.

The results of the focal mechanisms show that: there are 2 512 strike-slip earthquakes in the study area, accounting for 63.58% of all earthquakes; there are 818 normal fault type and normal strike-slip type earthquakes, accounting for 20.70% of all earthquakes; there are 621 reverse strike slip and reverse thrust earthquakes, accounting for 15.72% of all earthquakes. The most of earthquakes in the study area are distributed in active fault zones, the strike of the fault plane is consistent with the orientation of active fault zones. It revealed predominantly strike-slip faulting characteristics of earthquakes in the Eastern Boundary of the Sichuan-Yunnan Block, while the reverse thrust of earthquakes is mainly concentrated in the Longmenshan fault zone, as well as the NW trending Mabian-Yanjin Fault and the NE trending of Ludian-Zhaotong and Lianfeng faults which lied on the eastern boundary of the Sichuan-Yunnan block. Overall, the characteristics of the source mechanism are consistent with the regional tectonic background.

Results of the stress field inversion confirmed main active fault in the Eastern Boundary of the Sichuan-Yunnan Block is under a strike-slip stress regime, maximum and minimum compressional stress axes are nearly horizontal. The maximum compressional axes are primarily oriented in NW-SE and NWW-SEE direction, and they experience a clockwise rotation from north to south. Against the strike-slip background, normal faulting stress regimes and reverse faulting stress can be seen in the regional areas. The most prominent is the Daliangshan fault zone, which has obvious differences from the overall characteristics of the stress field with the eastern boundary of the Sichuan Yunnan Block. The maximum horizontal principal stress in the northern section shows a nearly EW direction, with a strike-slip type stress property, and the NW-SE direction in the southern section, with a thrust type stress property. The distribution characteristics of the stress field are consistent with the fault type of sinistral strike-slip and thrust on the eastern boundary of the Sichuan Yunnan block

The shape ratio R-value varies significantly, the R-value in the Sanchakou area is relatively high, with obvious extrusion characteristics, the R-values of the Xianshuihe fault zone, Anninghe fault zone and Xiaojiang fault zone are all between 0.25-0.5, showing NE-SW compression and NW-SE tension, and the tensile stress may be much less than the compressive stress(strike-slip type). The R values of the northern segment of the Daliangshan fault zone, the southern segment of the Anninghe fault zone, and Zemuhe fault zone are all between 0.5-1, showing NW-SE compression and NE-SW tension, and the compressive stress is greater than the tensile stress. To sum up, the current stress characteristics of the eastern boundary of the Sichuan Yunnan rhombic block are shear strain and local compression or tension.

There are different friction coefficients of the main faults in the study area: The Anninghe fault zone is 0.60, the Xianshuihe and Zemuhe fault zones are 0.80, the Xiaojiang fault zone is 0.75 and northern and southern sections of the Daliangshan fault zone are 0.65 and 0.85. The friction coefficients of the Xianshuihe Fault, the southern section of the Daliangshan Fault, and the Zemuhe Fault are above 0.75. The high friction coefficients of these fault zones may be because they are strike-slip faults, and the friction coefficients themselves are relatively high. The southern section of the Xiaojiang fault zone may be related to the development of fault gouges in the fault zone.

The 2022 M_S6.0 Maerkang earthquake swarm in Sichuan Province is the first rare strong swarm activity with high frequency, concentrated spatial and temporal distribution, strong explosive and strong magnitude in Maerkang area in the eastern segment of Bayan Har block in China seismic network records. It is also another significantly strong earthquake event in Bayan Har block after the M_S7.4 Maduo earthquake on May 22, 2021. The M_S6.0 Maerkang earthquake on June 10, 2022 not only broke the 33-year record without M_S≥6.0 earthquakes within 100km of the epicenter, but also broke the historical record without M_S≥6.0 earthquakes within 50km of the epicenter. The earthquake swarm is mainly located in the nearly “T” shaped conjugate fault structure area composed of the NW strike Maerkang fault and NE strike Longriba fault in the Bayan Har block. This area is a relatively rare region for moderate and strong earthquakes in the history. Therefore, it is of great significance to analyze and discuss the possible seismogenic faults of the Maerkang strong earthquake sequence for the study of seismogenic structures and the risk of strong earthquakes in the weak seismic region of Bayan Har block.

The earthquake swarm was relocated by double-difference method, and focal mechanisms and centriod depths of M_S≥3.6 earthquakes were calculated by using gCAP inversion method. Then the relationship between the stress system in the Malkang area and these earthquake focal mechanisms was analyzed, and fault plane was fitted by using relocation results. Maerkang earthquake swarm is mainly distributed along NW direction, and the initial rupture depth is 9.8km on average. Depth profiles show that earthquakes are mainly concentrated at depth between 0km to 15km. The most earthquakes of early-stage occurred in 48 hours. The mid-stage and late-stage earthquakes are located less than 15km in depth and move to the northwest of the epicenters. Initial rupture depth of the largest M_S6.0 earthquake is 12.5km, which is almost at the bottom of the dense area. The focal mechanism of M_S6.0 earthquake is 150° in strike, 79° in dip, and 7° in rake on nodal plane Ⅰ, and 59° in strike, 83° in dip, and 169° in rake on nodal plane Ⅱ, with the centroid depth of 9km. Other focal mechanisms of M_S≥3.6 earthquake are strike-slip types. Dips of nodal plane of focal mechanism range from 71° to 86°, and there exist different dip directions for one strike of every nodal plane. All azimuths of P axis are in NWW direction, and the plunges are nearly horizontal. The focal mechanisms of M_S≥3.6 earthquakes show that the tectonic environment is very favorable for NE or NW strike faults to generate the strike-slip movement. Centriod depths range from 5 to 9km, which are lower than the average depth of 9.8km of relocation, indicating that these earthquakes mainly ruptured from deep to shallow. The relative shear stress of the NW nodal plane are significantly greater than that of the NE nodal plane, and the normal stress of the NW nodal plane was smaller than that of the NE nodal plane, indicating more possibility of strike-slip dislocation on the NW nodal plane. The fault plane fitting results reveal that there are obviously two nearly parallel and nearly NW strike earthquake belts in the epicenter area. Fitted fault plane parameters of the belt in the north branch show the strike 333°, the dip 88°, the slide -22°, and the belt in the south branch show the strike 331°, dip 88°, and slide -23°. It is indicated that the fault properties of these two earthquake belts are basically the same, revealing that most of earthquake activities of the swarm may be controlled by at least two parallel structures near the Maerkang fault with the NW strike, dip 88° and left-lateral strike-slip. Combined with the existing regional geological structure, it is inferred that the Maerkang earthquake swarm may be induced by the NW and NE strike conjugate faults, and the NW strike faults control most of the earthquake activities.

The occurrence of strong earthquake is closely related to the distribution of crustal velocity anomalies. Some studies have shown that strong earthquakes occur in the transition zone between high-velocity anomalies and low-velocity anomalies in the middle and upper crust or inside the low-velocity anomaly zone. Thus, high-resolution imaging of the velocity structure in the seismic source area and accurate earthquake location can assist the evaluation of seismogenic settings of strong earthquakes. On May 21, 2021, an M_S6.4 earthquake occurred in Yangbi, Yunnan with casualties and property losses. The epicenter region of the Yangbi earthquake is in the western Yunnan area of the Sichuan-Yunnan block, which is located on the southeastern edge of the Qinghai-Tibet Plateau and characterized with intensive tectonic activity. Previous studies in this area are mostly on regional scales, and lacking on the three-dimensional fine crustal velocity structure in the Yangbi earthquakes area. To investigate the seismogenic environment and source characteristics of the 2021 Yangbi M_S6.4 sequence in Yunnan, we used the P-wave and S-wave arrival data of 12 652 earthquakes recorded by both the Yunnan regional digital network and the mobile observation arrays over a 10-year period(May 1, 2011, to May 31, 2021) and obtained the average V_P/V_S ratio of 1.79 via fitting the P-wave and S-wave arrival-time curves with the Wadati method. The magnitude ranges from M_S0.0 to M_S6.4, and the original focal depth ranges from 0 to 35km. To ensure the reliability of the calculation results, at least 4 stations records are required, and the maximum station azimuth gap allowed is 120°. Furthermore, the event-station distance is restricted to 400km and only earthquakes with travel time residuals<0.5s are retained. Our final velocity model is further refined via gridding(i.e., nodes)with an optimal horizontal grid of 0.25°×0.25° and a range between 0~65km vertically. A checkerboard test is also conduced to validate our inversion results. The test results showed that the recovery degree is high except for the depths of 0 and 65km, which were impacted by the uneven seismic distribution and rays. The high degree of recovery of 5~45km suggests high-resolution and robust imaging at these depths. Finally, the double-difference tomography method(TomoDD)was used to invert the three-dimensional P-wave and S-wave velocity structures in the Yangbi and its surrounding areas(24.5°~26.5°N, 99°~101°E). According to the result of precise location, the M_S6.4 main shock is located at 99.89°E, 25.70°N with a focal depth of 7.9km. The Yangbi M_S6.4 earthquake sequence is mainly distributed along the NW direction. Least-squares fitting prefers a~20km long axis with a strike of 312°, and the hypocenter depths are 5~20km. In general, the studied sequence is shallow and located within the upper crust, consistent with the depth distribution characteristics of historical earthquakes in this area. According to the spatio-temporal evolution characteristics of the aftershock sequence, the aftershocks of the M_S6.4 earthquake mainly spread unilaterally toward SE direction. Thus, we speculate that the overall medium in the NW of the mainshock is rigid and hinders aftershocks evolution. On the north side of the M_S6.4 mainshock epicenter, a group of earthquakes spread along the NNE direction and extended to the Weixi-Qiaohou Fault that hosted the M_S4.1 earthquake on May 27, 2021. Considering the geological and structural background, we believe this earthquake occurred on a parallel but unmapped fault on the SE side of the Weixi-Qiaohou Fault. In contrast, the earthquakes spreading in the NNE direction on the north side of the main shock maybe occurred on an unknown fault in the NNE direction. Therefore, the two faults form a conjugate structure. From the imaging results, the upper crustal velocity structure in the study area is consistent with the geological structure changes and the active faults, where the velocities are low. At 0km depth, the extremely low P-wave and S-wave velocities may reflect impacts from surface sediments. A velocity contrast is observed at a depth of 5km near the mainshock. In addition, a high-velocity anomaly was observed to the southeast side of the mainshock at 10-km depth, with a length of about 0.6°(EW)and a width of about 0.2°(SN). Within the depth range of 10~20km, the distribution of earthquakes near the mainshock shows a clear strip-like distribution, delineating the geometry of the fault. The velocity structure and seismic relocation results at 10-km depth suggest that majority of the events locate around the high-velocity anomaly on the west side of the Weixi-Qiaohou Fault. From the AA' profile, both P- and S-wave velocities suggest high-velocity anomalies in the SE direction of the mainshock. Combining with the distribution characteristics of aftershocks, the non-uniform variations of velocity structure are probably the major factor controlling the distribution of aftershocks, leading to the aftershock distribution extending along the SE direction.

According to the Unified Earthquake Catalogue of China Earthquake Networks, using the seismic phase data compiled by the Seismic Data Center, the observations of 101 fixed and mobile seismic stations in the Yunnan region and its surrounding seismic network from May 18 to 28, 2021, we conducted precise positioning research on the foreshock-mainshock-aftershock sequence of the Yangbi earthquake using the double-difference positioning method, and obtained the precise locations of 2 144 earthquakes. It is found that the distribution of the main aftershocks and the long axis orientation of the intensity isoseismal are not consistent with the image position of the Weixi-Qiaohou Fault, and the strike intersects with a small angle. The seismogenic fault of this earthquake may be a secondary fault of the Weixi-Qiaohou Fault. On the basis of the precision positioning results, the Bayesian Bootstrap Optimization(BABO)algorithm is used to perform a moment tensor inversion on the M6.4 earthquake and the M3.6 and above earthquake sequences in Yangbi, Yunnan from May 18 to 28, 2021. The results show that the sequence of Yangbi earthquake in Yunnan has obvious segmentation. The M6.4 Yangbi main shock is of right-handed strike-slip type with a small amount of normal dip-slip component, and the centroid depth is 5.9km. Most aftershocks have the same focal mechanism as the main shock, mainly right-handed strike-slip, except for the earthquakes in the west branch of the southeast section of the aftershock area, where the source property is obviously different, showing a normal strike-slip motion. The centroid depth of the entire earthquake sequence is 3.5~8.2km. The inversion results show that the principal compressive stress field of the earthquake area is in the near NS direction and the strike-slip dislocation is associated with a slight normal dip-slip component.

The spatial distribution of earthquakes shows that this earthquake sequence gradually developed from NW to SE, and the seismic density gradually dispersed from NW to SE. Therefore, it can be considered that the stress was mainly concentrated in the NW direction before the earthquake, and then gradually spread to the SE. Therefore, the main power source of this earthquake may be the southeastward extrusion of the Sichuan-Yunnan block. The rupture process and rupture pattern of this earthquake represents the “relaxation” process of the Sichuan-Yunnan rhombic block after being extruded.

The southern part of the Sichuan-Yunnan block is the material diffusion zone resulting from the eastward extrusion of Qinghai-Tibet Plateau. In this area, the subduction and westward retreat of the Indian plate led to the absence of lateral restraint on the Sichuan-Yunnan block, which may be the main reason causing the earthquake sequences in this area changing from convergence to dispersion, from strike-slip to normal fault type.

The Sichuan-Yunnan block is one of the most insensive areas where the Qinghai-Tibet Plateau squeezes out and escapes southeastward. Regarding the regional dynamic mode, we believe that under the background of continuous eastward extrusion of the material in the eastern part of the Qinghai-Tibet Plateau, and due to the lack of rigid blocks in the horizontal direction, it is more prone to velocity migration in the horizontal direction when the Sichuan-Yunnan block extrudes in the direction of SE and crosses the eastern structural junction and Longmen Mountains. The velocity migration in the study area may be caused by plate subduction or mantle underplating. The study of the lithospheric structure in the Sichuan-Yunnan area found that the crustal thickness of the sub-blocks in central Yunnan gradually thinned from north to south, and the lithospheric thickness in the area west of the Honghe fault zone shows a gradual thinning trend from east to west. It may be related to the intrusion of hot mantle material caused by the subduction and westward retreat of the Indian plate. Tomography results show that in the Ailaoshan-Honghe fault zone, the Yangtze block subducted downwards accompanied by mantle disturbance and asthenosphere upwelling, which led to magmatic activity and intrusion in the Cenozoic. Both of the above two effects can make the western boundary of the Sichuan-Yunnan block have a tensile stress background, and make the focal mechanism of major earthquakes on the nearby tectonic belt possible to appear normal. In summary, the dynamic source of earthquakes in the study area mainly comes from the escape of the Sichuan-Yunnan block to the southeast, and plate subduction or mantle underplating is possibly the deep-seated dynamic background for the lateral velocity migration of the study area.