# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
## 5 Discussion  
The inversion results of the full moment tensor for earthquakes occurring in southeastern Sichuan indicate that a significant number of these seismic events feature a pronounced proportion of non-double-couple (non-DC) components, necessitating further inquiry into their origins. The relatively shallow focal depths (<5 km), the strong anisotropy in the near-surface velocity structure [67], and the extensive fracturing within the shallow crust [6] in the region may all contribute to the increased non-DC components. Nevertheless, we suggest that fluid activity is a pivotal factor in this occurrence. This hypothesis is substantiated by several key observations: the earthquake source locations show a strong spatial correlation with the locations and operational depths of the fluid injection wells involved in shale gas extraction [4]; there is compelling evidence of fluid infiltration in the shallow velocity structures [67]; and both experimental and theoretical research confirm that shale dissolution during seismic events results in a decrease in source volume, which aligns with the higher negative isotropic (ISO) component identified in our computed full moment tensors. Furthermore, both Regions A and B exhibit pore pressure signatures within their respective Mohr circles. These collective findings imply that fluid activities exert direct or indirect influences on the emergence of non-DC components in the study area. The prevalence of thrust faulting activity in the study area is anticipated, given that the Sichuan Basin, located within the stable Yangtze Platform, is subject to compressional forces emanating from the Tibetan Plateau. Within the basin, a network of NE-SW trending faults has formed, aligning with the Longmen Shan Fault Zone. Some studies suggest that these faults might be continuations of the Longmen Shan thrust structures into the basin, potentially linked by a common detachment surface at greater depths [68]. However, due to the differential slip rates along various segments of these faults, new structures akin to \"transform faults\" have developed, orienting perpendicular to the main fault strike. These structures appear to accommodate stress release associated with the varying slip velocities. In the context of an overall NW-SE oriented tectonic stress field, the structural elements parallel to the Longmen Shan Fault Zone exhibit thrust faulting characteristics, whereas those oriented perpendicular to it display strike-slip faulting traits. In the study area, two distinct regions, A and B, are situated at the northern and southern extremities, respectively. A comparison of the stress fields in these regions reveals two primary differences:  
Firstly, Region B, located to the north, aligns with the orientation of the maximum horizontal stress (\(\sigma_{\rm{Hmax}}\)) observed across the Sichuan Basin. In contrast, Region A, to the south, exhibits a noticeable counterclockwise rotation from this common orientation. This discrepancy suggests that Region B adheres to the sliding rupture assumption under a uniform stress field [69; 70], while Region A does not conform to this model. The extensive hydraulic fracturing operations for shale gas extraction in Region A may lead to stress transfer due to high-pressure fluid injection, potentially creating new resultant forces that interact with the existing background stress field [71], thereby modifying the regional stress field direction. However, this raises a conundrum, as similar shale gas extraction activities in Region B have not induced comparable alterations. A plausible explanation for this discrepancy may be attributed to the spatial heterogeneity in the background stress levels within the basin. The tectonic forces exerted by the Tibetan Plateau generate a notable stress gradient across the Sichuan Basin.