# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
## 6 Conclusions  
This research focuses on seismic cluster areas in the shale gas extraction regions of southeastern Sichuan. We computed moment tensor solutions for M \(\geq\) 3.5 earthquakes using the GCAP method based on a regional three-dimensional velocity model. Meanwhile, using focal mechanism solutions for smaller earthquakes obtained via the HASH method, we classified the focal mechanisms with the FMC method and inverted the regional stress field using the MSATSI (Version 1.0.9) approach. Based on the above results and considering the interaction between fluids and faults, a comprehensive analysis was conducted on the characteristics of earthquake moment tensors and the stress field in southeastern Sichuan. Furthermore, the stress state and fluid interaction mechanisms in different subregions of southeastern Sichuan were discussed, leading to the following conclusions:  
1. The seismic activity in the southeastern Sichuan region is predominantly characterized by double-couple (DC) mechanisms, indicating that shear faulting is the primary mode of fault movement, consistent with the typical features of tectonic earthquakes. However, some earthquake events still exhibit non-double-couple components, particularly negative isotropic (ISO) components. This is speculated to be related to fluid intrusion caused by hydraulic fracturing and water injection activities during shale gas extraction. 2. The focal mechanism solutions in the southeastern Sichuan region predominantly exhibit thrust faulting patterns (approximately 51%), followed by strike-slip mechanisms (about 11%), with normal faulting representing a minor proportion (around 7%). The remaining solutions correspond to mixed-type mechanisms. Significant spatial heterogeneity characterizes the distribution of seismic mechanism types. In region A, thrust and strike-slip mechanisms dominate while minor normal faulting and mixed-type solutions coexist. Region B primarily features thrust faulting with subordinate normal faulting components. Region C consists mainly of thrust-faulting earthquakes accompanied by occasional strike-slip events. The complexity of these mechanism solutions primarily reflects the involvement of multiple faults in seismic rupture processes rather than single or limited fault systems in the southeastern Sichuan region. 3. The stress field in southeastern Sichuan, determined through seismic focal mechanism solutions, demonstrates fundamental consistency in orientation across subregions, predominantly governed by near-horizontal compressional stress with thrust faulting as the principal seismogenic structure. Regional variations nevertheless emerge in stress orientations: the \(\sigma_{1}\) axis in the northern segment (Region B) trends WNW-ESE, aligning with both the principal compressive stress field of the Sichuan Basin and GNSS station velocity vectors, whereas the southern segment (Region A) exhibits an E-W oriented \(\sigma_{1}\) axis displaying counterclockwise rotation relative to the background principal compressive stress field. This rotational pattern indicates that superimposed perturbations on the regional stress field have modified local stress orientations in Area A. * The inversion of Mohr circles using STRESSINVERSE software (Version 1.1.3) for subregions in southeastern Sichuan reveals that Areas A and B possess similar stress shape ratios and fault plane distributions. However, Region A exhibits a relatively higher proportion of fault planes located within Mohr circles compared to Region B. This observation indicates that Region B demonstrates higher levels of fault instability, whereas the initiation of fault ruptures in Region A is primarily controlled by pore pressure effects. The above analysis indicates that the seismicity in southeastern Sichuan is generally characterized by thrust faulting governed by tectonic stress, while high-pressure fluid injection during shale gas extraction exerts a crucial influence on the non-double-couple component of earthquakes.