Methodology for the interpretation of fault-slip seismicity in a weak shear zone

Abstract

Fault-slip related seismic events that occur in underground mines could inflict severe damage to underground openings; thus a proper estimation of fault-slip potential in active mining areas is of paramount importance in assessing its risk. It is not uncommon in underground mines that large seismic events take place away from stopes being extracted, where fault-slip potential is presumed not to be high enough to result in those seismic events. In the present paper, fault-slip related seismic events taking place within a weak shear zone in Garson Mine, Sudbury, Canada are investigated. First, in order to understand the stress states of rockmass in the mine, numerical analysis is carried out with a 3D mine-wide model whilst assuming isotropic elasticity. The result obtained from the analysis reveals that the shear stress of rockmass in a weak shear zone does not reach the maximum shear strength determined by Mohr–Coulomb failure criterion with basic friction angles of the rockmass. The result contradicts a fact that quite a few seismic events have been actually recorded in the regions with micro seismic monitoring systems installed in the mine. As an interpretation of that, it is postulated that variations in shear stiffness within the shear zone contribute to the generation of high slip potential resulting in the occurrence of those seismic events. In order to justify the postulation, numerical analysis is additionally carried out, in which the shear zone is modelled with transversely isotropic models, of which shear stiffness is decreased in the same direction as a measured joint orientation in the shear zone. For source regions of those seismic events, isotropic models are used without decreasing its shear stiffness, thus resulting in the discrepancy in shear stiffness between the source regions and other areas in the shear zone. The result obtained from the analysis verifies that fault-slip potential drastically increases within the source regions due to the difference in shear stiffness. It is further found out from dynamic analysis in which fault-slip is simulated with Barton's shear strength model that the increasing slip potential is high enough to cause large seismic events in the regions. In the present study, the interpretation of seismic events occurring within a weak shear zone is provided, and a methodology to simulate high fault-slip potential that could be generated within the shear zone is developed. The methodology can be used with back analysis to determine the mechanical properties of the weak shear zone, which lead to the better estimation of fault-slip potential.