Rock mechanical analysis of a M1 = 4.0 seismic event induced by mining in the Saar District, Germany

SUMMARY On 2008 February 23 a 4.0 magnitude seismic event occurred in close proximity to a deep underground longwall coal mine in the German Saar mining district. Preceding this event was a series of smaller seismic events, which can be traced back to geological structures. A seismic network was installed to observe and analyse the seismic events. Underground core drilling was performed to obtain rock samples for rock mechanical testing. Possible causes for the events, failure of rock mass or slip on unknown faults, are evaluated by numerical methods. Failure of rock mass is excluded as a cause since the stresses are well below the strength of the rock mass. Seismic events are of similar fault plane strike and are organized in several clusters, associated with local and regional tectonic elements. It was found from backcalculation that fault planes with friction angles as low as 8 ◦ must have been present. The role of pore pressure on the fault stability is discussed and found to play no role in generating the seismic events. Finally, the question of mining in a rock mass obviously close to failure is addressed and it was concluded that any mining activity would lead to a significant seismic response.