Geomechanics of Flooding-Induced Microseismicity—Implications for Post-Mining Environments

Seismicity related to mining has gained considerable public attention in the last decades and is one of the reasons for mine closures in Germany. The seismicity in the Ruhr coal mining district of Germany has been monitored by different regional and (temporal) local networks and is classified as purely mining-induced without evidence for movement of major geological faults due to the contemporary tectonic stress field. During active mining, water was continuously pumped out of the mine to enable safe mining conditions. Mine closure was followed by a seismic gap. When pumping was reduced the mines were gradually flooded and microseismicity developed. This study investigates flooding-induced seismicity at the Heinrich Robert mine in the Eastern Ruhr coal district, where water levels rise from ∼1,150 m to ∼380 m depth as part of post-mining flooding operations. By applying event relocalization methods we detected concentrations of microseismicity at about 300 m below the lowest mine levels, and there especially in sections with pillars above. Classical diffusion models cannot be applied due to the non-linearities in the increase of pore pressure and water volume, different flow processes and the heterogeneity of the subsurface. We performed 3D numerical geomechanical modeling for different mine geometries, hydraulic behaviors and regional state of stress. Our results show that flooding-induced seismicity in mines is less influenced by the ambient stress state and the flooding water volume. Instead, the locations, the temporal evolution and the segments of potential fault reactivation during flooding are controlled by a combination of local stress concentrations in pillar zones below the lowest mining level and pore pressure rise. Both are governed by a complex mine geometry and the hydraulic connections therein. Our findings support that the magnitude of flooding-related earthquakes is controlled and limited by the size of critical stress concentrations and thus the dimensions of the mined panels.