Potential triggers for large earthquakes in open-pit mines: A case study from Kuzbass, Siberia

The extraction and movement of rock during mining operations is considered a possible trigger for slip along the fault. In this study, possible anthropogenic causes were analyzed for triggering the earthquake with the local magnitude of ML = 6.1 (at a depth of 4 km just beneath the quarry), in the vicinity of the large coal open-pit mine in Russia. This event was the largest earthquake associated with a quarry (an open pit). A sufficiently deep occurrence of the source testified that seismic vibrations were produced in a dynamic slip along a preexisting and prestressed tectonic fault. Analytical calculations were conducted of increments of normal and shear stresses at fault planes with several dip angles at depths corresponding to the probable location of the hypocenter of the Bachat earthquake. As the results show, long-term rock excavations bring a prestressed thrust fault closer to the ultimate Coulomb strength, and stress variations at those depths may suffice to initiate a movement along the fault. By measuring seismic vibrations at different quarries and mines in Russia, the dynamic effect at the supposed depth of the source occurrence could be reliably estimated. As is shown, the ultimate anticipated dynamic deformations are noticeably lower than corresponding values produced by seismic waves of distant earthquakes in the case when the effects of dynamic triggering were observed. Accordingly, the seismic effect of explosions cannot trigger a large earthquake with deep source occurrence. Operations in open-pit mines can only bring forward the moment of an earthquake at a potentially seismogenic fault. At the same time, the numerical calculations reveal that a branching network of underground tunnels located at several horizons can noticeably reduce the effective shear modulus of the host rock. This effect can even provoke a dynamic movement at a previously aseismic fault. By analyzing the available seismic data and analytical modeling, we came to the conclusion that open-pit operations can trigger (bring closer) an earthquake that has already been "prepared" by natural conditions. The key point is that to trigger the seismic fault slip, the direct influence of anthropogenic factors should occur over a sufficiently large area, obviously larger than the size of the earthquake nucleation zone. However, an underground mining operation can provoke a dynamic movement at a previously aseismic fault. image