Seismological Evidence for the Existence of Long-Distance Hydrological Channel and Its Implication for Fluid Overpressure in Southern Sichuan, China

Unprecedented levels of seismicity have been seen in southern Sichuan, China, since the large-scale exploitation of shale gas. Fluid and pore pressure transported through hydrological channel are thought as pivotal elements in the induction of earthquakes. Our high-resolution tomography results reveal two inclined seismic anomalies featured by low Vs and high Vp/Vs at different depth range. The deeper anomaly extends 15 km from NE to SE and connects the well g048 from 3 km depth to the vicinity of the Ms 4.7 Gongxian earthquake 5.4 km deep, which is hinted to be a hydrological channel inferred from the high fluid overpressure of 28 Mpa calculated from focal mechanism solution. The injection operation of multiple shale gas wells along the channel may potentially accumulate the pore pressure and cause the fault near the end of the channel to reach critical stress state through various mechanisms. Increasing seismicity in southern Sichuan is suspected to be linked to hydraulic fracturing stimulation for shale gas extraction. Induced earthquakes resulting from fracturing operations at nearby wells can be verified using in-situ fracturing data, while remote-induced factors are hard to prove. In this study, a seismic anomaly of low Vs and high Vp/Vs inferred to be a long-distance hydrological channel connecting the 2021 Ms 4.7 Gongxian earthquake to the active shale gas well 15 km away. Our results indicate that pore fluid pressure generated by hydraulic fracturing in multiple wells may continuously accumulated at the end of the channel, ultimately resulting in high pore pressure and triggering the fault rupture. Therefore, it is urgent to plug this channel and prevent the filtration loss. Two long-distance hydrological channels were observed in southern Sichuan The injection operation of multiple wells along the channel may increase the pore pressure in the hydrological channel The accumulation of pore pressure at the end of the hydrological channel was confirmed by the occurrence of Ms 4.7 Gongxian earthquake