# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
## 4 Stress Field Inversion  
### Focal Mechanism Types  
The focal mechanism type is indicative of the stress and deformation state of geological structures. Initially, we decomposed the double-couple components from the full moment tensor solutions of 118 earthquakes with magnitudes Ms \(\geq\) 3.5 (Figure 3). To enhance the statistical significance of our analysis, we compiled an additional 91 focal mechanism solutions derived using HASH [52] for earthquakes with magnitudes ranging from 2.5 to 3.4, recorded between 2018 and 2022. These data were classified using the FMC program (Version 1.3) [53], which employs a hierarchical clustering algorithm and supports various distance metrics, facilitating the identification of spatial or mechanical clustering signatures associated with seismic activity (Figure 4).  
Figure 3: **(a)** Map of moment tensor solutions, colored by the percentage of double-couple. The black beach balls illustrate the focal mechanism solutions of 91 earthquakes with magnitudes of 2.5–3.4 derived by HASH [52] and evaluated as quality “A” during the period of 2018–2022. The colored beach balls in **(a)** represent the full moment tensor solutions of 118 earthquakes with magnitudes Ms \(\geq\) 3.5, where different colors indicate the degree of the double-couple (DC) component (refer to the color scale below **(a)**. **(b)** The Tk source-type plot after Tape and Tape [45]. The yellow dots in **(b)** represent individual earthquake events. The white region within the two dashed lines indicates areas with significant double-couple (DC) components. The gray regions outside the dashed lines correspond to non-DC events, which are further divided into two parts: the upper part represents the +ISO component region, indicating volumetric expansion, while the lower part corresponds to the -ISO component region, indicating volumetric contraction.  
In the southeast Sichuan region, the seismic source mechanism solutions predominantly feature the reverse type (approximately 51%), followed by the strike-slip type (about 11%), with only a small fraction exhibiting the normal fault type (around 7%). The remaining solutions are of mixed types (Figure 4). Concurrently, the type of seismic mechanism exhibits significant spatial variability. Region A, in particular, displays the greatest complexity (Figure 5a), with a predominance of thrust and strike-slip types and a minor occurrence of normal fault and mixed types. This complexity is echoed in the studies by [PERSON] et al. (2018).  
Figure 4: Map of focal mechanism types, colored by focal mechanism types classified by FMC. (**a**) shows the distribution of the double-couple component decomposed from the full moment tensor solutions of 118 earthquakes with magnitudes Ms \(\geq\) 3.5. Insets (**b–d**) illustrate detailed clustering analyses for Region A, Region B, and Region C, respectively.  
[PERSON] et al. [5] and [PERSON] et al. [54], which also highlight substantial differences in seismic mechanisms and deformation characteristics within this area. This may be attributed to the southern margin of the Sichuan Basin being situated in a transition zone of tectonic deformation modes, leading to a complex array of source mechanism solutions due to varying tectonic environments [3].  
Region B is characterized by a dominance of the reverse type, with a minor normal component (Figure 5b), while Region C is primarily composed of reverse-type earthquakes, albeit with a concurrent presence of a small number of strike-slip events (Figure 5c). The main focal mechanism types in each region align with the findings of previous classifications based on smaller sample sizes [7; 55]. However, our analysis, which is based on a larger sample size, reveals a more nuanced depiction of focal mechanism solution types in each region, suggesting the involvement of multiple faults in the rupture process rather than a single fault or a few.