Unloading-induced rock fracture activation and maximum seismic moment prediction

Hundreds of anthropogenic earthquakes have recently occurred worldwide due to underground space creation and energy extraction. The mechanism behind the human-induced geohazards is most likely associated with the reduction of normal stress on pre-existing fractures and faults. This study reports a series of laboratory experiments to investigate the mechanism of unloading-induced fracture activation, and proposes a simple approach to predict the maximum seismic moment for a critically stressed fracture. The unloading-driven shear test results exhibit that the unloading process induces the stress states of the sawcut and natural fractures to approach the Mohr-Coulomb failure envelope, and the normal stress unloading rate influences the peak slip rate. The fracture instability is dependent on the relationship between the stiffness of the system and the slip weakening rate of the fracture, and the shear dilation mainly occurs after the fracture activation. The test results also show that the critical shear stress of the sawcut fracture during the unloading-driven shear test is approximately equal to the residual shear strength after the displacement-driven fracture slip. This relationship inspires us to develop a new approach to estimate the maximum seismic moment. Our data demonstrate that the maximum seismic moments for both the fractures obtained from the unloading-driven shear tests are all below the upper limit lines, indicating that the proposed approach is reasonable. The uncertainty analysis shows that the accurate estimation of fault size can improve the maximum seismic moment prediction.