Based on recent reports, there is little doubt that hydraulic fracturing can result in the generation of seismic events with magnitudes greater than zero as related to the activation of faults situated within or within proximity to reservoirs. The question that arises is whether fault activation is related to the presence of fluids and proppant or is related to perturbations in the local stress field. In other words, are these events triggered through stress perturbations or induced by pore pressure reducing effective stress?
Fault activation is also related to the degree of surface roughness, asperities and barriers to slip, frictional stress of the rock and the resulting rupture velocity. To derive a picture of the faulting and sub-faulting processes, we examine signals of a M>0 event associated with stimulation in the Marcellus formation as recorded near surface with low-frequency sensors along with the sub-faulting behaviour as observed in high frequency signals recorded close to the source. We investigate the dynamics of sub-fracture failures during the rupture process and growth of the overall fracture from initiation to rupture arrest, the role of asperities, roughness, fluids, the failure mechanisms (shearing versus tensile dominance of behavior), and the conditions under which induced-triggered failures may occur.
Based on our observations, fault activation was related to decreasing frictional resistance on the fault surface due to the presence of fluids. The increasing magnitude for sub-events during fault activation suggests that slip over a larger contact area such as an asperity and that fluid influence was dissipating.
Overall, failure was dominated by a shearing mode of failure, whereas the sub-fracture failures were mixed-mode, that is, more complex than a simple shearing process and included a strong tensile component of failure that evolved in time. However, from this example, it is difficult to ascertain if fault activation was triggered or induced; it appears both modes are plausible.