Regional Characteristics of Seismicity Associated with Hydraulic Fracturing in the Southern Sichuan Basin of China

In the southern Sichuan Basin of China (SSBC), hydraulic fracturing (HF) production of shale gas and surface buildings have been seriously affected by some medium scale (local magnitude M_L > 4.0) earthquake events. In this study, focusing on relocated hypocenters in SSBC from January 2016 to May 2017, the Density-Based Spatial Clustering of Applications with Noise algorithm was introduced to divide it into four seismic regions. The temporal evolution of seismicity parameters, detailed views of potential faults near injection wells and spatial migration of seismic activity in each region were obtained. Moreover, the statistical correlation between seismicity and engineering disturbance was calculated by adopting epidemic type aftershock sequence model (ETAS); the relationship between corner frequency, stress drop and seismic moment of each region were analyzed, and the spatial distribution of the stress drop, the relationship with the focal mechanism solutions (M_L > 3.5) and the fault slip-tendency under the regional stress pattern were discussed. The results show that the seismic diffusion rate of each region can be well described by the square root envelope (0.3–0.8 m²/s) and seismicity has a strong cluster mode (triggered events) and a weak background mode (independent events). Most (more than 97.5%) of the stress drops are in the range of 0.1–10 MPa, and the high stress drops is mainly located in the YS108 shale block; under the current regional stress pattern, the faults in region YS108 are unfavorablely oriented, while those in the south of N201 are favorablely oriented, with a high sliding trend. This knowledge may provide time to take mitigating actions to reduce the potential for damaging earthquakes.