Coupled hydromechanical modeling of mine water rebound-induced seismicity at the Gardanne mine, France

After the Gardanne mine (France) closure in 2003, the mine experienced a substantial rise in mine water level, from −1100 m MSL to −10 m MSL, which resulted in prolonged seismicity observed between 2008 and 2021. To analyze the processes causing the induced seismicity, we developed a 2D coupled hydromechanical (HM) model to simulate the evolution of pore pressure and associated stress redistribution and deformation within the rock mass. The numerically modeled surface subsidence during the post-2015 water pumping program was calibrated with Interferometric Synthetic-Aperture Radar observations in the Fuveau area. The model results demonstrate that the recovery of mine water levels triggered a significant increase in pore pressure, causing large-scale volumetric expansion within the flooded mine that led to surface deformation and stress changes in shallow areas, even before the arrival of the flooding front. Integrated interpretation of the model results, seismic observations, and mine geometry indicate that induced seismicity was jointly controlled by mine flooding and poroelasticity. The HM coupling and poroelastic effect associated with mine water rebound substantially altered the stress state of the mine, reducing effective normal stress while increasing Coulomb failure stress changes and the slip tendency along unfavorably oriented pre-existing fractures. The spatio-temporal distribution of seismicity was also driven by the direct fluid-to-solid HM coupling. These results highlight the importance of comprehensive monitoring programs and coupled HM analyses for the effective assessment and management of seismicity risks associated with post-mining flooding.