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Using geodetic methods, significant static ground deformation has been observed for many large natural earthquakes. Some of the largest earthquakes induced by hydraulic-fracturing operations have been observed in the Western Canada Sedimentary Basin; however, because of the size and depths of these events, the associated static ground deformations have not yet been observed using traditional geodetic techniques. A seismic processing technique, developed for small volcano-seismic events, has the potential to resolve micrometer-scale static displacements using broadband seismic data. In this study, we test this processing method using vertical component broadband recordings of an Mw 4.1 event acquired at four nearby broadband seismometers. Estimated static displacements at the four stations are compared with the theoretical surface displacement field for a dislocation on a finite rectangular source within a homogeneous, elastic half-space. The theoretical displacements have the same polarities as the measured displacements across the seismic network and have similar amplitudes for three of the four stations. However, one station yielded unstable results, which shows that care must be taken when using this method. These results suggest that this processing method has potential for obtaining surface deformation for small to moderate-sized earthquakes using broadband data. [ABSTRACT FROM AUTHOR]

Seismic swarms are defined as an increase in seismicity that does not show a clear mainshock-aftershock sequence. Typically, swarms are primarily associated with either fluid migration or slow earthquakes (aseismic slip). In this study, we analyze a swarm induced by hydraulic fracturing (HF) that persisted for an unusually long duration of more than 10 months. Swarms ascribed to fluid injection are usually characterized by an expanding seismicity front; in this case, however, characteristics such as a relatively steady seismicity rate over time and lack of hypocenter migration cannot be readily explained by a fluid-diffusion model. Here, we show that a different model for HF-induced seismicity, wherein an unstable region of a fault is loaded by proximal, pore-pressure-driven aseismic slip, better explains our observations. According to this model, the steady seismicity rate can be explained by a steady slip velocity, while the spatial stationarity of the event distribution is due to lateral confinement of the creeping region of the fault with increased pore pressure. Our results may have important implications for other induced or natural seismic swarms, which could be similarly explained by aseismic loading of asperities driven by fluid overpressure rather than the often-attributed fluid-migration model. [ABSTRACT FROM AUTHOR]

One of the largest induced seismic events attributed to hydraulic-fracturing operations, with a moment magnitude (M w) of 4.1, occurred on 12 January 2016 in Alberta, Canada. The hydraulic-fracturing treatment was monitored using a dense array of 93 shallow wellbores, each containing three geophones, producing a high-resolution data set that is presented here as a case study. Over 9700 smaller events were observed both before and after the event, with a magnitude of completeness of approximately M w −1.9. The use of perforation shots with known locations enabled refinement of the velocity model, such that event hypocentres could be located with high accuracy and precision. Most of the hypocentres of induced events were located within the sedimentary section, 100s of metres above the treatment zone. Event locations of the foreshocks and aftershocks reveal fault activation along both small and large structures. The mechanisms and locations suggest a complex north–south oriented strike-slip fault system that we interpret to be part of a regional flower structure. The b -value for the data set is close to unity, as expected for fault activation events; however, the three largest magnitude events deviate significantly from the fit, suggesting that it would be difficult to forecast these larger events based on the Gutenberg–Richter relation. This has significant implications for the mitigation of hydraulic-fracturing induced seismicity. The three largest events appear to be located on fault segments that are less optimally oriented for slip than those imaged by other event clusters, suggesting that larger events require a larger perturbation to the stress state in order to nucleate. Monitoring setups with the capability of the system presented here, if operated in real time, could be of great value to discern fault activation due to seismicity aligning on planar structures prior to a main shock. This approach may have considerable potential to improve short-term forecasts of induced seismicity. [ABSTRACT FROM AUTHOR]

Models for hydraulic fracturing-induced earthquakes in shales typically ascribe fault activation to elevated pore pressure or increased shear stress; however, these mechanisms are incompatible with experiments and rate-state frictional models, which predict stable sliding (aseismic slip) on faults that penetrate rocks with high clay or total organic carbon. Recent studies further indicate that the earthquakes tend to nucleate over relatively short injection time scales and sufficiently far from the injection zone that triggering by either poroelastic stress changes or pore pressure diffusion is unlikely. Here, we invoke an alternative model based on recent laboratory and in situ experiments, wherein distal, unstable regions of a fault are progressively loaded by aseismic slip on proximal, stable regions stimulated by hydraulic fracturing. This model predicts that dynamic rupture initiates when the creep front impinges on a fault region where rock composition favors dynamic and slip rate weakening behavior.