# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
## 3 Moment Tensor Inversion
The subtle distinctions lie in the fact that, compared to the 1-D model, the full moment tensor based on the 3-D model exhibited an overall better fitting quality (higher variance reduction, VR) and a more stable depth distribution of the full moment tensor solutions. However, we did not obtain a large number of moment tensor results in the study area calculated by other researchers using GCAP or similar methods, making it difficult for us to systematically compare and analyze the validity of our results. In fact, there is no significant difference between the double-couple solutions extracted from our moment tensor solutions and those directly reversed using the CAP method [5; 6; 7]. Figure 2: (**a**) Variance reduction with focal depth during the moment tensor inversion for an \(M_{5}3.6\) event on 24 September 2021. The highest VR indicates the best centroid depth; the blue and green beach balls are derived from the 3-D and 1-D velocity models, respectively. (**b**) The full moment tensor results and the waveform fit diagram of the same event when its focal depth is 4 km based on the 3-D velocity model. Observed (black) and synthetic (red) waveforms for the station observations that were used to constrain the moment tensor inversion. The letters on the left represent station names. The numbers below the station names, separated by a “/”, represent the epicentral distance (in km) and azimuth (in degrees), respectively. Numbers below the waveforms are the time shifts (in s) in the synthetic waveforms relative to the observed waveforms (upper) and the cross-correlation coefficients (in %; lower). The FM at the top of the diagram represents the optimal double-couple solution, while Mw indicates the moment magnitude. MT refers to the full moment tensor solution for this seismic event, and the six numbers following MT correspond to the six parameters of the complete moment tensor solution (\(M_{rr},M_{tt},M_{pp},M_{rt},M_{rp},M_{tp}\)). The computational outcomes presented in Figure 3a reveal that the majority of the earthquakes within the study area manifest as double-couple (DC) events, with 62% of the seismic occurrences exhibiting a DC component proportion exceeding 70%. However, a minority of the seismic events display a non-double-couple signature, a characteristic observed across all three regions under investigation. To facilitate a more direct visualization of the distribution of various moment tensor components, we constructed the Tk source-type lune [45], as depicted in Figure 3b. The lune analysis indicates that the predominant cluster of earthquakes is centered within the DC region, illustrating that the focal process is dominated by shear deformation, which is a typical characteristic of tectonic earthquakes. Nonetheless, a subset of the seismic events extends towards the negative ISO region, implying that these earthquakes involved volume contraction during the source rupture process. The observed non-double-couple (non-DC) components in mining-induced seismic events can be attributed to intricate interactions involving fluid injection, pre-existing fault networks, and the anisotropic characteristics of the rock formations. The process of hydraulic fracturing introduces high-pressure fluids into shale strata, which diminishes the effective normal stress on fault planes and facilitates a combination of shear and tensile failure modes. This leads to volumetric alterations--either expansion or contraction--that are superimposed upon shear displacement, thereby giving rise to non-DC seismic mechanisms [38; 46; 47]. The pre-existing faults within shale reservoirs frequently display geometric intricacies, such as intersecting fractures and curved fault planes. Fluid injection has the potential to initiate slip along these complex structures, thereby producing non-DC moment tensors [48; 49]. The intrinsic anisotropy of shale, exemplified by bedding planes and aligned fractures, induces asymmetric stress responses during the hydraulic fracturing process.