Shallow Lingering and Deep Transient Seismicity Related to Hydraulic Fracturing in the Changning Shale Gas Field, Sichuan Basin, China

Characterizing seismic responses to hydraulic fracturing (HF) in shale-gas development is crucial for seismic-hazard assessment and mitigation-strategy design. Although intensive HF operations have led to severe induced seismic hazards in the Changning shale gas field (CSF) in China for over a decade, the typical spatiotemporal characteristics of induced seismicity during and after HF in this region remain unclear, due to a lack of detailed fluid-injection data. Using a 70-day-long dense deployment of 336 nodal-sensors in 2019, we develop an enhanced seismicity catalog and combine it with focal mechanism solutions, fluid-injection time series, seismic-reflection profiles, and geomechanical models to identify the distinct shallow and deep seismicity responses to HF. The first pattern consists of deep earthquake clusters that migrate along strike-slip faults in the limestone formation ∼1 km below the treatment depth. These clusters contain frequent $M_L>2$ earthquakes, including the largest $M_{L}3.3$ event, and exhibit transient seismicity-rate changes in rapid response to HF. In contrast, the second pattern consists of shallow clusters in the target shale formation that persist for over a year following HF. The shallow clusters include smaller earthquakes and exhibit thrust-style faulting with no discernible spatial migration. Our geomechanical simulations suggest the deep fault reactivation is best explained by the combined effects of poroelastic-stress loading and pore-pressure increases. Stable seismicity rate and frequent casing deformation indicate post-HF, long-term aseismic deformation may drive the shallow seismicity. These distinct seismic responses during and after HF operations underscore the need for a spatiotemporally adaptive hazard mitigation strategy for the CSF.