Interevent triggering in microseismicity induced by hydraulic fracturing

Permeability enhancing treatments such as hydraulic fracturing (HF) induce microseismic events with typical magnitudes in the -3.0 to -0.5 range, although significantly larger induced earthquakes up to 4.7 in moment magnitude have been reported. Diffusion of pore pressure away from the hydraulic fracture system is thought to be a primary controlling mechanism. Understanding other primary or secondary triggering mechanisms during HF is expected to furnish new insights regarding stress, strength of faults, and rupture initiation and propagation. Using novel methods from statistical seismology, we present evidence for the existence of event-event triggering cascades in microseismicity induced by HF. Although background seismicity dominates, we find that these triggering cascades exhibit features that also characterize tectonic aftershock sequences such as the empirical Omori-Utsu relation and the productivity relation. This suggests that the underlying physical earthquake-earthquake triggering mechanisms are similar in both cases, as also observed for other earthquake swarms. The presence of triggering cascades is of direct relevance for optimizing the effectiveness of the stimulation.