Induced seismicity due to fluid injection, including hydraulic fracturing, is an increasingly common phenomenon worldwide. Yet, the mechanisms by which hydraulic fracturing causes fault activation remain unclear. Here we show that pre-existing fracture networks are instrumental in transferring fluid pressures to larger faults on which dynamic rupture occurs. To date, studies of hydraulic fracturing-induced seismicity have used observations from regional seismograph networks at distances of 10's km, and as such lack the resolution to answer some of the key questions currently in the field. A high-quality dataset acquired at a hydraulic fracturing site in Alberta, Canada that experienced several events over MW 2.0 is presented for the purpose of analysing detailed mechanisms of fault activation. Both event hypocentres and measurements of seismic anisotropy reveal the presence of pre-existing fracture corridors that allowed communication of fluid-pressure perturbations to larger faults, over distances of up to a km or more. The presence of pre-existing permeable fracture networks can significantly increase the volume of rock affected by the pore pressure pulse, thereby increasing the probability of induced seismicity. This study demonstrates the importance of understanding the connectivity of pre-existing fracture networks as a tool for assessing potential seismic hazards associated with hydraulic fracturing of shale formations, and offers a conceptual understanding of induced seismicity due to hydraulic fracturing.

Post-injection seismicity associated with hydraulic stimulation has posed great challenges to hydraulic fracturing operations. This work aims to identify the causal mechanism of the post shut-in ML 2.9 earthquake in August 2019 at the Preston New Road, UK, amongst three plausible mechanisms, i.e., the post shut-in pore pressure diffusion, poroelastic stressing on a non-overpressurised fault, and poroelastic stressing on an overpressurised fault. A 3D fully-coupled poroelastic model that considers the poroelastic solid deformation, fluid flow in both porous rocks and fracture structures, and hydraulic fracture propagation was developed to simulate the hydromechanical response of the shale reservoir formation to hydraulic fracturing operations at the site. Based on the model results, Coulomb stress changes and seismicity rate were further evaluated on the PNR-2 fault responsible for the earthquake. Model results have shown that increased pore pressure plays a dominant role in triggering the fault slippage, although the poroelastic stress may have acted to promote the slippage. Amongst the three plausible mechanisms, the post shut-in pore pressure diffusion is the most favoured in terms of Coulomb stress change, seismicity rate, timing of fault slippage and rupture area. The coupled modelling results suggested that the occurrence of the post shut-in ML 2.9 earthquake was a three-staged process, involving first propagation of fracture tips that stimulated surrounding reservoir formations, then hydraulic connection with and subsequent pore pressure diffusion to the partially-sealing PNR-2 fault, and eventually fault activation primarily under the direct impact of increased pore pressure.