# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
## 3 Moment Tensor Inversion  
The seismic moment tensor (MT) is an essential tool for elucidating the mechanisms of seismic sources. It is widely utilized in seismological studies to offer a comprehensive description of both double-couple and non-double-couple components at complex seismic sources. The decomposition of the moment tensor typically includes three independent components: the double-couple (DC), compensated linear vector dipole (CLVD), and isotropic (ISO) components [34]. Among these, the double-couple component is associated with shear rupture, while the CLVD and ISO components are linked to volumetric changes and fluid activity [34; 35]. In practical observations, tectonic earthquakes generally display a pure double-couple source mechanism, reflecting crustal deformation primarily caused by shear rupture. However, various specific seismic sources, such as mining-induced earthquakes, volcanic events, explosions, and fluid-triggered earthquakes, often exhibit significant non-double-couple components. These components may be indicative of specialized physical processes at the source, including complex geological environments, inelastic volumetric changes, or intense fluid-rock interactions [36; 37; 38; 39; 40; 41]. Consequently, analyzing the seismic moment tensor can profoundly investigate the quantitative relationship between seismic source mechanisms and crustal dynamic behaviors. Such analysis provides a scientific foundation for understanding the mechanisms underlying earthquake nucleation, triggering, and damage. By examining these relationships, we can gain deeper insights into how different factors, both natural and anthropogenic, contribute to seismic activity. In the current investigation, we meticulously selected 16 seismic stations situated in the southern region of Sichuan Province, all within a radius of 150 km from the earthquake epicenter. These stations have diligently recorded seismic waveform data for all earthquakes with a magnitude of M \(\geq\) 3.5 occurring in the area since 2010, as depicted in Figure 1a. For each seismic event, the waveform data were meticulously segregated into Pnl-wave and surface wave components, encompassing durations of 30 s and 60 s, respectively. Subsequent to this segmentation, the wave components underwent a rigorous analysis involving band-pass filtering within the frequency ranges of 0.05-0.3 Hz for Pnl-waves and 0.05-0.1 Hz for surface waves. Utilizing the grid point search technique, we successfully derived the full moment tensor parameters for each event [42]. In preparation for this analysis, we first acquired a comprehensive three-dimensional velocity model of the study region [43]. We then computed Green's functions by employing the fk method [44]. For each earthquake, we calculated the Green's functions at discrete depths ranging from 1 to 20 km. Applying the inversion outcomes, we ascertained the optimal centroid depth for each event, as illustrated in Figure 2a. To ensure data quality, we adhered to a strict criterion, selecting only data with a variance reduction (VR) of 60% or greater, as shown in Figure 2b. Ultimately, our study yielded complete moment tensor solutions for a total of 118 earthquakes within the defined research area. Certainly, the accuracy of the velocity model significantly influences the effectiveness of waveform modeling and ultimately determines the correctness of the full moment tensor inversion. Nevertheless, the waveform floating fitting method adopted by GCAP effectively mitigates the reliance on the precision of the velocity structure. Utilizing GCAP, we recalculated the full moment tensor for the Ms3.6 earthquake that occurred on 24 September 2021, based on a 1-D velocity model of the Sichuan Basin [5]. It was observed that the full moment tensors derived from both the 3-D (represented by blue beach balls in Figure 2a) and local 1-D velocity models (represented by green beach balls in Figure 2a) yielded similar optimal centroid depths and showed no significant differences in their optimal full moment tensors.