Statistical characteristics and causal mechanisms of hydraulic fracturing induced seismicity in the Southern Sichuan Basin, China

In the Southern Sichuan Basin, China (SSBC), some moderate-sized seismic events (local magnitude M L ranging between 4 and 5) have affected the safe production of shale gas. In this study, we used the recorded seismic data from China national and temporary networks within the SSBC to obtain the relocated seismic hypocenter distribution between January 2016 and May 2017 based on the hypocenter double-difference (HypoDD) method. The statistical characteristics of microseismicity resulting from water injection in SSBC were analyzed, and the potential correlation between the event rate and statistical parameters, such as Gutenberg-Richter b -value, spatial correlation length, and fractal dimension, was quantified. Based on spatial variations of b -value and fractal dimension of event distribution, we identified two potential risk areas in the East and West of the Zhaotong shale gas block (YS108), respectively. The focal mechanism solutions (FMSs) of the observed seismic events ( M L >2.5) near the H7 well pad were calculated utilizing the generalized cut-and-paste (gCAP) technique combined with P-wave polarity. The FMSs’ results show reverse faults, and some of them have fault planes oriented in the N-S direction, causing oblique slip movement. In addition, we also inverted the regional stress field using high-quality FMSs, revealing that the maximum principal stress ( σ 1 ) trends NW–SE and lies nearly horizontal, in agreement with the World Stress Map and borehole breakout records. Considering geological structures and regional stress distribution, the reasons for induced seismicity were mainly linked to pore pressure diffusion. Our obtained findings may provide insights for future seismic risk assessment and mitigation strategies.