Determining source parameters of earthquakes, such as focal mechanisms, rupture directivity, and stress drop, is important in understanding subsurface fault structures, rupture kinematics and dynamics, and regional seismic hazard assessment. Focal mechanisms can be applied to illuminate the geometry of active faults and regional stress field orientations (e.g., Kisslinger, 1980), rupture directivity patterns reveal the dynamic evolution of fault systems and their influences on seismic energy distribution (e.g., Somerville et al., 1997;Abercrombie et al., 2017), and stress drops provide critical information about fault mechanics and energy radiation efficiency during rupture (e.g., Boatwright, 1978;Allmann and Shearer, 2007). The detailed earthquake source characterization is particularly crucial for induced seismicity, dominated by small magnitude earthquakes while impacts profoundly in the operational safety of hydrocarbon and minerals production (Ellsworth, 2013;Yang et al., 2017). The numerous aftershocks of intermediate and large induced earthquakes provide valuable opportunities to identify and characterize the spatial and temporal seismicity patterns and the source properties (e.g. Zi et al., 2025). Such comprehensive analysis of aftershock sequences is essential for improving seismic hazard assessment and developing more effective risk mitigation strategies in industrial operations.
Intermediate and large earthquakes have the abundant low-frequency waveform information and good station coverage, benefiting for the source parameter determination.
Reliable focal mechanisms for intermediate to large earthquakes can be obtained through broadband seismic waveform inversion methods such as Cut and Paste (CAP) (Zhu & Helmberger, 1996) and ISOLated Asperities (ISOLA) (Sokos et al., 2008), while rupture directivity can be effectively characterized through azimuthal variations in source duration (Ni et al., 2005) and wave amplitude (Tan and Helmberger, 2010). However, determining source parameters for small-magnitude events is significantly challenging due to the higher frequency signals scattering by small-scale heterogeneities, limited azimuthal differences in ground motion energy, and smaller rupture areas.
Previous studies have demonstrated that rupture characterization is a critical source feature across a wide range of earthquake magnitude (e.g., Chen et al., 2021;Lin et al., 2025;Tang et al., 2024;Yang et al, 2022;Jia et al, 2025). Recent advances in seismic monitoring, particularly the deployment of dense seismic networks, have enabled more detailed studies of fault complexity (e.g., López-Comino and Cesca, 2018;Trugman et al., 2021) and rupture directivity of small earthquakes (e.g., Folesky et al., 2016;Király-Proag et al., 2019). Focal mechanisms of small events can now be determined using first-motion polarity and S/P amplitude ratio methods (Hardebeck andShearer, 2002, 2003). In the Sichuan basin, such methods reveal that the faulting mechanisms of induced earthquakes in the Weiyuan shale gas field are primarily dominated by reverse rupturing on preexisting NNE-trending faults, with a minority of normal faulting events co-located with reverse faulting ones (Song et al., 2025). The divergence in faulting styles indicates distinct structural controls or stress conditions, highlighting the critical role of focal mechanism determination of small earthquakes in understanding local fault complexity and stress heterogeneity. On the other hand, estimating the rupture directivity of small earthquakes primarily relies on analyzing the durations and spectra of body waves (e.g., Boore and Joyner, 1978;Warren and Shearer, 2006;Cesca et al., 2011;Chen et al. 2021). Recently, using only one near-source strong motion station, the polarization of S waves also can infer the rupture directivity of small earthquakes (Yao et al., 2025). The combination of the focal mechanism inversion and directivity estimation can reduce nodal plane ambiguities for small events, especially for a complex fault network where such ambiguities cannot be resolved by earthquake locations and focal mechanism solutions.
Here we focus on the Changning region of the Sichuan Basin where massive shale gas exploration and salt mining have been conducted (Fig. 1), accompanied by gradually larger earthquakes over time in recent years (Lei et al., 2013). The June 2019 MS 6.0 Changning earthquake, probably the largest induced event by industrial exploitation in the world to date (Liu and Zahradník, 2020), was followed by four ML > 4 earthquakes and a series of aftershocks in neighboring regions, covering a span of approximately 20 km in space (Fig. 1a). While the mainshock’s source mechanism and directivity have been systematically investigated (e.g. Li et al., 2020;Anyiam et al., 2024), the aftershocks’ rupture characteristics remain poorly constrained. If the focal mechanism and the rupture directivity were unaccounted for, it may cause biases
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