Impact of Geological Heterogeneities on Induced-Seismicity in a Deep Sublevel Stoping Mine

With the aim of understanding the nature of mining-induced seismicity, microseismic activity in the deep metal mine of Garpenberg (Sweden) has been recorded during 2 years of excavations. The studied area of the mine is operated using sublevel stoping method with backfilling, between depths of around 1000 and 1300 meters. Spatiotemporal analysis of microseismic activity is presented and correlated with the occurrence of mining blasts. A clear dependence is observed between blasts and seismic sequences, even if the rock mass response to mining appears to be very variable across space and time. Two main clusters are observed: one located in the major production area (Central Cluster), while the second (Right Cluster) is located at some distance from the excavations, in a zone characterized by a heterogeneous distribution of weak materials and stiff rock masses. By analyzing seismic source parameters, we demonstrate that the two clusters are characterized by different dynamics. In addition, we show how Right Cluster events are mainly controlled by geological heterogeneities, which impose high stress concentrations in the stiff rock masses surrounding weak lenses. High apparent stresses and corner frequencies associated with the Right Cluster events agree with our proposed model. This suggests elevated stresses in the seismic source region and small source dimensions; indeed, fractures cannot propagate along great distances due to the presence of weak lenses interbedded with the breaking stiff rock mass.