On March 19, 1962, the Xinfengjiang Reservoir in Heyuan City, Guangdong Province, induced an
M6.1 earthquake as it approached its first full-capacity impoundment peak. This event stands as one of the largest reservoir-induced earthquakes recorded in China and one of the few globally exceeding magnitude 6. Since this mainshock, moderate and small earthquakes have persisted within the reservoir area, characterized by high frequency and widespread distribution. The seismogenic mechanism remains unclear due to the complex geological structure of the reservoir area, featuring intersecting faults and well-developed joints and fractures. While most existing research focuses on earthquakes with
M 
≥4.0, systematic studies on
M 3-4 events are still lacking. To reveal the primary seismogenic structures and the complex genesis mechanism of reservoir earthquakes in this region, this paper conducts a systematic study on source parameters of moderate and small earthquakes occurring between 2008 and 2023. We employed the double-difference earthquake relocation method to relocate events of
M ≥1.0. The Cut and Paste (CAP) method was applied to invert for focal mechanism solutions and source depths of events with
M ≥3.0. Additionally, we obtained the regional stress field with the damped regional-scale stress tensor inversion method. Furthermore, to overcome the ambiguity of the two nodal planes in focal mechanism solutions and identify the true causative fault, rupture directivity parameters for events of
M ≥3.0 were inverted using the source time-frequency signature method. The results indicate distinct spatial heterogeneity in the seismicity of the Xinfengjiang Reservoir area, with the overall seismogenic structures exhibiting a NW-SE trend. Current earthquake types near Xichang Town and Yukeng Village are predominantly strike-slip, while the dam area is dominated by normal faulting. The overall stress field in the reservoir area aligns with that of South China, characterized by a NW-SE oriented maximum principal compressive stress (
\sigma _1 
). However, local stress field variations exist, showing an approximately 10° clockwise rotation in the orientations of the maximum and minimum principal stresses from south to north. The
R-values are approximately 0.26 and 0.22 near Xichang Town, 0.32 near Yukeng Village, and 0.1 and 0.06 in the dam area, indicating a regional stress state biased towards extension, which is more pronounced near the dam. Combined with the analysis of rupture directivity parameters, it is revealed that the Xichang seismic zone contains two intersecting faults at approximately 10 km depth—a NW-trending dextral strike-slip fault and a NEE-trending sinistral strike-slip fault; the Yukeng Village area features a pre-existing NNW-trending sinistral strike-slip fault at 8–10 km depth; the Laohuilong Village area exhibits a NW-trending fracture at about 5 km depth; the northern dam area is characterized by a NW-trending strike-slip fault at ~10 km depth intersecting with the Renzishi Fault; the southern dam area contains a WNW-trending strike-slip fault at 6–10 km depth; the southeastern dam area presents a NEE-trending sinistral strike-slip fault at ~10 km depth intersecting with the Heyuan Fault. Comprehensive research indicates that current seismic activities in the Xinfengjiang Reservoir area are jointly influenced by regional tectonic background and long-term reservoir impoundment, leading to reactivation or kinematic transformation of pre-existing faults, thereby triggering frequent small earthquake swarms. These findings provide important seismological evidence for understanding the mechanisms of reservoir-induced seismicity and assessing regional seismic hazards.
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