Spatio-temporal distribution of induced seismicity in flooded mines in the Ruhr area - interpretation by geomechanical numerical modelling

Induced seismicity during mine flooding is the focus of the FloodRisk project. One of the study areas is the Ruhr area, which is characterised by centuries of intensive coal mining. After the closure of the last mines, controlled flooding began. Within the FloodRisk project, we investigate ground uplift, stress changes due to pore pressure changes and the reactivation potential of faults to explain induced seismicity. We concentrate on the seismicity monitoring and geomechanics of the Haus Aden catchment, for which we investigate the relationship between water rise, tectonic stress and induced seismicity. The monitoring of seismicity is based on a network of up to 30 short-period seismic stations installed by the Ruhr University in the area of the former "Bergwerk Ost", which exhibited the highest seismicity in the Ruhr area during active mining. The stations cover an area of about 160 km 2 and are spaced between 0.5 and 3.5 km apart. They allow continuous monitoring of seismicity. Since 2019, more than 2200 induced events have been localised. A prerequisite for the interpretation of seismicity is a detailed localisation of the events. ... mehrThe relative localisation of the induced earthquakes has significantly reduced the location uncertainty and allowed the spatial and temporal evolution of earthquake clusters due to the rise in mine water levels to be studied. The resulting pattern of seismicity was compared with known underground structures. This comparison indicates that most of the events occur approximately 300 m below the main pillars between the longwall panels in the already flooded deepest level of the mine. A generic FE numerical model was developed for a section of the Heinrich Robert mine based on the geometry of the pillars, shafts and longwall panels. The stress data for model calibration are based on a compilation of the regional stress state in the eastern Ruhr area. For this purpose, hydraulic fracture tests carried out in the mines to minimise rock bursts were re-evaluated and compared with stress orientations derived from independent sources such as borehole fractures and earthquake source mechanisms. Using this 3D numerical approach, we conclude that there is increased vertical stress within and below the pillars as a result of stress arching. As the horizontal stress changes below the mine levels are small, this results in increasing differential stresses that can lead to the observed events below the mine level when the mine water level rises.