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Abstract

For around 700 hundred years, the greater Ruhr region has been the heart of the coal mining industry in Germany. In 2018, the last coal mine ceased operation in the country, and the process of abandonment and repurposing of the vast amounts of unused subsurface infrastructure has begun. The operations of mine flooding as well as the recent geothermal exploration campaigns, aiming to utilize old mine infrastructure as well as permeable Massenkalk formations found below mining depths, pose serious questions regarding safety and potential seismic risks in the densely populated greater Ruhr region. To alleviate these concerns, we assess the probability of reactivation of major fault in the region. This study benefits from the astonishing amount of 429 stress magnitude and 38 stress orientation data records derived from hydrofracturing tests and borehole logs carried out in six coal mines and two coal bed methane boreholes between 1986 and 1995. Due to the change in subsurface data regulations, this data has been just recently made available to the public. Our study summarizes the results of this extensive in situ stress test campaign, assigns quality to each data record using the established quality ranking schemes of the World Stress Map project for both stress orientations and magnitudes, and, in combination with already published material, provides the first comprehensive stress database of the region. This unique data set, together with geometries of major faults revealed from coal mining activities and already published results from laboratory studies, serves as a base for the probabilistic assessment of fault reactivation potential, incorporating uncertainty of each Mohr-Coulomb parameter (i.e., stress tensor, pore pressure, frictional properties, and fault geometry). The probabilistic assessment of fault reactivation potential, as presented in this study, aims to de-risk future subsurface operations facilitating the green transition of the greater Ruhr region.