Fluid-injection processes can induce earthquakes by increasing pore pressure and/or shear stress on faults. Natural processes, including transformation of organic material (kerogen) into hydrocarbon, can similarly cause fluid overpressure. Here we document examples where earthquakes induced by hydraulic fracturing are strongly clustered within areas characterized by pore-pressure gradient in excess of 15 kPa/m. By contrast, induced earthquakes are virtually absent in the same formations elsewhere. Monte Carlo analysis indicates that there is negligible probability that this spatial correlation developed by chance. A detailed analysis was undertaken within a region in Alberta, Canada where uniquely comprehensive data characterize dynamic interactions between seismicity and well completions. Seismicity is strongly clustered in space and time, exhibiting spatially varying persistence and activation threshold. The largest event (ML 4.4) can be reconciled with a previously postulated upper bound on magnitude, only if the cumulative effect of multiple treatment stages is considered. Induced seismicity from hydraulic fracturing reveals contrasting signatures of fault activation by stress effects and fluid diffusion. Patterns of seismicity indicate that stress changes during operations can activate fault slip to an offset distance of > 1 km, whereas pressurization by hydraulic fracturing into a fault yields episodic seismicity that can persist for months.


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