The Crooked Lake area in Central Alberta, Canada, became seismically active in December 2013 with a sequence of earthquakes Mw 2.0 and higher. The earthquakes are suspected to be induced by hydraulic fracturing in nearby horizontal wells due to their strong spatiotemporal correlation. To investigate the physical mechanism of the induced seismicity near Crooked Lake, we calculate stress and pore pressure perturbations resulting from high‐rate multistage fluid injection in the framework of linear poroelasticity. The calculated perturbations are used for seismic risk analysis based on the Coulomb failure criterion. Results show that most seismicity is within a positive Coulomb stress change regime, indicating that failure is promoted by injection of fluid. By comparing Coulomb stress results for different parameter settings, we show that elastic response of the solid matrix, instead of fluid diffusion, is more likely the dominant factor for the induced earthquakes shortly after fluid injection.

According to the Coulomb failure criterion the variation of either shear stress, normal stress, or pore pressure can affect the occurrence, or not, of earthquakes. Abnormal seismicity increases around reservoirs are often thought to be induced by the water impounded behind the dam, which leads to nearby increases in crustal pore pressure and Coulomb stress, and so may promote the nearby faults to fail. To investigate how much the Zipingpu reservoir, whose dam is just a few hundred meters from the Longmen Shan fault, influenced the 12 May 2008 Wenchuan earthquake M w 7.9, we calculated the Coulomb stress variation induced by the filling of the Zipingpu reservoir, which began in October 2005. We also analyzed the correlation between local seismicity variations and the induced Coulomb stress variations. Both the calculated Coulomb stress variations and the observed seismicity analysis suggest that the probability that the huge Wenchuan earthquake, M w 7.9, was induced by the Zipingpu reservoir is very low. The filling of the Zipingpu reservoir could only result in an increase in the rate of shallow earthquakes with hypocenter depth smaller than 5 km near the reservoir region.

Over 100 small-to moderate-sized earthquakes, including an Mw 5.0 event, were detected during September 2015 to November 2016 near the town of Cushing, Oklahoma. The seismic sequence was spatial-temporally linked to four wastewater disposal wells within 4 km. We calculate pore pressure and stress perturbations caused by fluid injection at multiple wells and analyze seismic risk in a Coulomb failure stress framework. Despite being more than an order of magnitude smaller than the pore pressure perturbation, the sign of shear stress change, in the sense of assumed right-lateral fault motion, dictates where earthquakes are induced. Most of the relocated earthquakes are located within areas of positive shear stress changes. Our results suggest that poroelastic stress changes also play an essential role in the wastewater disposal environment, and a strategic design of well locations with respect to fault orientation and direction of motion can help mitigate induced seismic hazard. Plain Language Summary Fluid injected into the subsurface is known to induce earthquakes. This study analyzes the relationship between the 2015/2016 Cushing earthquake sequence and wastewater disposal at four wells within 4 km of the sequence. Our results reveal that while pore pressure increase due to fluid diffusion makes the dominant contribution to promote fault slip toward instability, shear stress change in the sense of motion on a preexisting fault is a critical factor that determines where earthquakes can occur. This study suggests that a strategic design of disposal wells locations, with respect to mapped faults, may be an effective way to mitigate injection-induced seismicity.

The 2017 Mw 5.5 Pohang earthquake in South Korea, the first reported and largest magnitude-induced earthquake, occurred near the enhanced geothermal power plant in Pohang on 15 November 2017. We compute the spatiotemporal changes in poroelastic stresses perturbed by injected fluid under various conditions to better understand the occurrences of the Pohang earthquake and the small-magnitude earthquakes preceding it. Space-time variation of the earthquakes that occurred before the Pohang earthquake correlates significantly with fluid injection history between January 2016 and September 2017. We attribute the timing in earthquake occurrence to slow fluid diffusion, making hydraulic diffusivity of bedrock the critical model parameter for representing this slow process. In this context, the delay between the injection and the Pohang earthquake requires diffusivity estimates within a range of 1 × 10 −4-5 × 10 −4 m 2 /s for damaged granodiorite at 4-5 km, corresponding to the depth range between the well and the focal depth. According to these estimates, the pore pressure and thus the Coulomb failure stress changes are further enhanced by each injection with minimum stress dissipation. We find fluid injection can result in a change of the Coulomb stress of up to 0.4-1.1 bar, exceeding those associated with the 2016 Mw 5.5 Gyeongju earthquake by 2 orders of magnitude.

The mechanism of why the rupture terminated sharply on both sides of the seismic gap between the Wenchuan and Lushan earthquakes is not yet known along the southwestern Longmen Shan fault zone. The accommodation modes of compressional strain and stress are still not known well in the gap subregion. Using the Match and Locate method, we obtained a detailed catalog of background microearthquakes around the seismic gap from May to December 2015. The characteristics of spatiotemporal distribution of the events is highlighted with a complete magnitude of 0.2. These small events tend to concentrate at different depth ranges on two sides of the seismic gap, indicating that the gap serves as a transition zone between the Wenchuan and Lushan source regions. Strain release through microearthquakes is an unlikely candidate for accommodating modes in the gap, because of the sustained deficit of microseismicity. A strong anticorrelation between background seismicity and monthly precipitation variation rate is observed for different magnitude scales. Enhanced seismicity may be activated by the terrestrial water storage reduction next to a precipitation peak with a time lag of 1-2 months during the summer and autumn, indicating a seasonal modulation of seismicity by the water storage cycle. The background microseismicity exhibits prominent responses to precipitation variation due to a short nucleation duration of less than one quarter. (1) The seismic activity is distinctively lower in the gap subarea compared to that on the two sides of it. (2) Another intriguing phenomenon is that the background seismicity may be modulated by the seasonal rainfall. The seismicity peaks correspond very well with sharp decrease of rainfall. The dramatic variation of water storage in mountain regions, which is controlled by rainfall, is a primary factor of background seismicity modulation in the study region. Plain Language Summary