Source characteristics and seismogenic structure of small-to-moderate earth-quakes in Xinfengjiang Reservoir area,Guangdong Province

Xu Shufeng; He Xiaohui; Hui Gege; Xu Binbin; Zhang Peizhen

doi:10.19975/j.dqyxx.2025-028

Abstract：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 fre-quency 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 exi-sting research focuses on earthquakes with M≥4.0,systematic studies on M3-4 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 prin-cipal compressive stress(σ1).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,in-dicating 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 con-tains 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 north-ern 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.Compre-hensive 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 transform-ation 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.