The 2016 Mw5.1 Fairview, Oklahoma earthquakes: Evidence for long-range poroelastic triggering at >40 km from fluid disposal wells

Wastewater disposal in the central U.S. is likely responsible for an unprecedented surge in earthquake activity. Much of this activity is thought to be driven by induced pore pressure changes and slip on pre-stressed faults, which requires a hydraulic connection between faults and injection wells. However, direct pressure effects and hydraulic connectivity are questionable for earthquakes located at large distances and depths from the injectors. Here, we examine triggering mechanisms of induced earthquakes, which occurred at more than 40 km from wastewater disposal wells in the greater Fairview region, northwest Oklahoma, employing numerical and semi-analytical poroelastic models. The region exhibited few earthquakes before 2013, when background seismicity started to accelerate rapidly, culminating in the Mw5.1 Fairview earthquake in February 2016. Injection rates in the ∼2–2.5 km deep Arbuckle formation started to increase rapidly in 2012, about two years before the start of seismicity-increase. Most of the injection activity was concentrated toward the northeast of the study region, generating a relatively cohesive zone of pressure perturbations between 0.1 and 1 MPa. Much of the near-injection seismicity was likely triggered by pressure effects and fault-assisted pressure diffusion to seismogenic depth. Outside of the high-pressure zone, we observed two remarkably detached, linear seismicity clusters, which occurred at 20 to 50 km distance from the initial seismicity and 10 to 40 km from the nearest, high-rate injector. Semi-analytical models reveal that poroelastically-induced Coulomb-stress-changes surpass pore pressure changes at these distances, providing a plausible triggering mechanism in the far-field of injection wells. These results indicate that both pore-pressures and poroelastic stresses can play a significant role in triggering deep and distant earthquakes by fluid injection and should be considered for seismic hazard assessment beyond the targeted reservoir. • A fluid injection field induced moderate earthquakes as far away as 40 km. • Poroelastic solid stresses are larger than fluid pressures at such large distances. • Densely spaced wells form a coherent source extending the reach of poroelasticity. • Resulting Coulomb-stress-changes are comparable to static triggering stresses.