Coal mining by the longwall method typically causes seismic events. This paper describes the research on mining induced seismicity in two mines. One mine experienced a steady occurrence of mining induced seismic events of magnitudes ML <2.4 around a specific longwall at 1200m depth under several old longwalls, the differently situated second longwall induced only few events. Seismic events were localized and different event types are described. The events are explained by different failure mechanisms, namely fault activation, remnant pillar punching and beam failure. In the second mine longwall mining was executed by a double panel. When driving the development gateroads an event of ML =3.3 was induced. After a long period of "quiet" resource extraction numerous seismic events were induced and culminated in aML =4.0 event which led to the closure of the mine. This research suggests that numerous local pre-existing fault systems of very low strength have been activated.
The German mining districts are well known for their deep longwall coal mining operations. Currently the average operating depth is below 1200m and due to the high stresses and strong rocks/rock masses there is an abundance of seismic events with considerable local magnitudes. Coal mining activity and the occurrence of seismic events show good correlation in space and time so there is little doubt about the causes for the seismicity. We report about the seismic events from one mine in the Ruhr area featuring two differently situated longwalls and from one mine in the Saar district.
The coal bearing strata in the Ruhr area belong to the Upper Carboniferous (Pennsylvanium, 318 to 299mya). The Ruhr coal district belongs to an external fold and thrust belt of the Variscan orogen, the so-called subvariscan through.