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Abstract

The surge of seismic activity in the recent years has been attributed, at least partly, to intense hydraulic stimulation activities associated with the exploitation of unconventional natural gas resources in the southern Sichuan Basin, China. We have deployed a dense seismic array, which consists of 30 broadband seismometers with station spacing about 2 km, from December 2020 to March 2022 in the Weiyuan shale gas block within the southern Sichuan Basin. In this study, we systematically detect and locate 52210 earthquake events using LOC-FLOW framework and compute focal mechanisms of small to medium events (M>2). The enhanced catalog, with a maximum ML of 3.6 and a completeness magnitude of 0.5, generally includes two distinct prolonged earthquake sequences. The temporal migration of the earthquake sequences is highly correlated with fracking operations of two well pads both spatially and temporally. The events have focal depth ranging from 2 to 6 km, extending from the injection zone downward into the underlying crystalline basement. The focal mechanisms exhibit predominant thrust faulting, with P-axis subparallel to the maximum horizontal compressional stress direction in the region. Moreover, the lineation of earthquake clusters is also clearly associated with the apparent fault zones identified by ant tracking algorithm of a 3D seismic reflection survey, indicate reactivation of a subsurface thrust fault system. These results suggest that the spatially clustered earthquake sequences are driven by pore pressure diffusion related to hydraulic stimulation and strongly controlled by pre-existing faults and fractures.