Seismicity in the Weiyuan-Rongxian area, Sichuan basin, SW China

Abstract

The Weiyuan-Rongxian shale gas field in the Sichuan basin, China, has experienced numerous earthquakes in recent years, causing significant casualties and property loss. To investigate the spatial–temporal evolution of seismicity and seismic fault structures, in this study, we first utilize a deep-learning-based picking algorithm –(PhaseNet) to identify seismic phases from raw seismic data between March 2019 and October 2020, and then employ a Rapid Earthquake Association and Location technique (REAL) to associate these seismic picks. Finally, the VELEST and GrowClust methods are performed to build a high-precision earthquake catalog containing 18,912 earthquakes. Our results reveal that the spatial–temporal distribution of earthquake swarms indicates a complex fault system. In the northern part of Weidong block (R1), the earthquakes illuminate a shovel-shaped structure that the dip angle of the fault plane gradually decreases with depth. In the south of Weidong block (R2), the natural fractures with dip angles of about 80° control the distribution pattern of earthquakes. In the Weixi block (R3), the Molin fault and basement fault form a typical structural wedge that bounds a triangular or wedge-shaped fault block. The majority of earthquakes located below the buried depth of the shale gas and spatio-temporal correlations between seismicity and industry operations both suggest that they are probably induced by hydraulic fracturing. The migration front of the earthquakes can be modeled by fluid diffusion with the diffusivity D between 0.2 m2/s and 3.5 m2/s. The Ms 5.2 earthquake occurring on 18 December 2019 is located in a positive ∼10 kpa Coulomb failure stress area generated by the 8 September 2019, Ms 5.4 earthquake, reaching the typical static triggering threshold. Therefore, in addition to fluid diffusion, the stress perturbation caused by previous seismic ruptures may be considered as another factor of earthquakes occurring in the Weiyuan-Rongxian. Our study provides evidences for better understanding the seismicity characteristics, seismogenic mechanism, and fault structure in Weiyuan-Rongxian shale gas field.