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Relevance of Seismic Risk Assessment in Abandoned Mining Districts : the Case of the Gardanne Coal Mine, Provence, France

Abstract : Mining shutdowns have increased significantly in last century, but seismic risk in post mining districts and consequent damage from ground shaking is still poorly understood. Large induced seismic events with M > 5 are known from active mining districts. Their origin is widely directly linked to stress perturbations related to mining activity. In post mining districts, especially when they are flooded, the bandwidth of potential seismic source origins is comparatively large and has been observed in the context of partial underground collapses, fluid induced redistribution of the environmental stresses, and reactivation of pre existing fault structures next to the mining district. The estimation of the associated seismic hazard is quite challenging, depending on many complexly interacting factors, such as the mine geometry and geological constitution, its long term alteration behaviour (modified by the presence of fluids), meteorological impacts and climate changes, triggering from regional or global natural earthquakes, and the presence of pre existing fault structures and tectonic stresses. Such challenges are today encountered in the case of the underground flooded, abandoned coal mine at Gardanne in the Provence region (in SE France). Local microseismic monitoring highlights the presence of significant periodic seismic swarming activity, including events of magnitudes close to 2 which have been several times felt by the nearby living population. Seismic analysis demonstrates that most of the events appear to be located below the excavated, flooded mine workings and seems to be spatially and temporally correlated with the flooding evolution, controlled by meteorological conditions and active pumping operations. Results from source mechanism analysis showed that swarming activity is probably related to rupture along a network of pre existing fault structures, which are favourably oriented with respect to the local tectonic stress field. Based on these observations, we suggest that some mine workings (especially room and pillar) act as a very efficient "anthropogenic" aquifer, whose water level fluctuations trigger reactivation of these faults, e.g. via a poroelastic effect or pore pressure increase. The nature of the detailed triggering mechanism remains however speculative and is part of currently ongoing investigations. This example shows the necessity and relevance of the understanding of this mechanism in order to reason if variations of the mine water level might potentially trigger larger local tectonic events, as recorded in the past (Mw 3.6, 1984), or simply episodes of swarm activity associated to small transient creep. In the light of these developments, seismic hazard analysis in post mining risk assessment, which is non standard today, could be assessed.
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Jannes Kinscher, Dajila Namjesnik, Isabelle Contrucci, P. Dominique, Emmanuelle Klein. Relevance of Seismic Risk Assessment in Abandoned Mining Districts : the Case of the Gardanne Coal Mine, Provence, France. 12th International conference on mine closure (Mine CLosure 2018), Sep 2018, Leipzig, Germany. pp.615-624. ⟨ineris-03239660⟩

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