# Characterization of the Solution of the Seismic Source Mechanism in Southeastern Sichuan
###### Abstract  
Southeastern Sichuan has witnessed intensified seismic swarm activity since 2016, including events exceeding historical peak ground accelerations. This study integrates moment tensor solutions, stress field inversion, and Mohr-Coulomb analysis to investigate the interplay between tectonic processes and shale gas extraction in driving seismicity. Full-waveform moment tensor inversions of 118 earthquakes (M \(\geq\) 3.5) reveal predominant double-couple mechanisms (62% with DC \(>\) 70%), with minor non-double-couple components linked to fluid-induced volume contraction. Stress field inversions demonstrate spatial heterogeneity: Region A (south) exhibits a counterclockwise-rotated maximum horizontal stress direction compared to Region B (north), which aligns with the regional NW-SE tectonic compression. [PERSON]'s circle analysis highlights distinct failure regimes--40% of the events in Region A fall below the failure threshold (pore-pressure-influenced), while 60% in Region B exceed it (stress-dominated). These findings underscore the combined roles of tectonic inheritance (NE-SW basement faults) and anthropogenic perturbations (fluid injection) in modulating seismic hazards. Southeastern Sichuan Basin; moment tensor solutions; stress field; Mohr circles; fluid injection 2020 February 2025 27 March 2020 27 March 2020 27 March 2020 31 March 2020 31 March 2025 31 March 2025 31 March 2025 31 March 2025 31 March 2025 31 April 202 value recorded during the Changing Ms 6.0 earthquake in 2019 reached 599.16 gal [9], and the Luxian County Ms 6.0 earthquake in 2021 peaked at 756.02 gal [10]. These values not only exceeded regional seismic defense standards but also resulted in casualties and property damage. The seismogenic mechanisms underlying the earthquake swarm activities in the southeastern Sichuan Basin have sparked intense debate within the geophysical community. Two predominant hypotheses have emerged from the current research: (1) Under the Stress Transfer Hypothesis, postseismic stress transfer following major earthquakes, particularly the 2008 Mw 7.9 Wenchuan earthquake, may modulate regional seismicity patterns. Multiple studies demonstrate that the Wenchuan event significantly elevated Coulomb stress levels within the Sichuan Basin [11; 12; 13], establishing a stress perturbation field capable of triggering small-to-moderate earthquakes in adjacent regions [14; 15]. Notably, spatiotemporal correlations between stress field evolution and subsequent seismic swarm episodes provide empirical support for this mechanism [16]. (2) The Anthropogenic Triggering Hypothesis, an alternative perspective, posits that extensive shale gas extraction from the Silurian Longmaxi Formation in southeastern Sichuan's f  
Figure 1: (**a**) Structural geological map and earthquake epicenters of the Sichuan Basin and its surroundings. Black triangles represent the entire seismic network available in the region, and green triangles indicate stations used in full moment tensor inversion. Active faults are shown in red, with fault kinematics marked. The dashed box highlights the study area containing three earthquake clusters (Regions A, B, and C), which are further analyzed in panels (**b**,**c**) and will be studied in the next sections. The red rectangle in the upper-right subplot illustrates the location of the Sichuan Basin and its surroundings. (**b**) Zoomed-in map of the study area (dashed box in panel (**a**)) showing the detailed seismicity distribution. Earthquake epicenters are colored by magnitude, and Regions A, B, and C are outlined in green. (**c**) Temporal evolution of seismicity in the study area from 2008. Blue curves represent the cumulative frequency of earthquakes, with different line styles indicating the entire study area and Regions A, B, and C. Stems show magnitudes with time. role, alongside tectonic stresses, in contributing to recent seismicity.