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Gas extraction from the Groningen gas field resulted in significant induced seismicity. We analyze the magnitude‐frequency distribution of these earthquakes in space, time and in view of stress changes calculated based on gas production and reservoir properties. Previous studies suggested variations related to reservoir geometry and stress. While we confirm the spatial variations, we do not detect a clear sensitivity of b‐value to Coulomb stress changes. However, we find that b‐value correlates positively with the rate of Coulomb stress changes. This correlation is statistically significant and robust to uncertainties related to stress change calculation. This study thus points to a possible influence of stress change rate on the probability of the magnitude of induced earthquakes. Plain Language Summary: Gas extraction from an underground reservoir in the Netherlands has induced significant seismicity. We analyze how stress changes and the rate of stress changes influence the magnitude of these earthquakes. We find that more smaller earthquakes tend to occur at higher stress change rates. Earthquakes triggered at a lower stress change rate may thus grow to larger magnitudes than those triggered at high stress change rate. This observation is statistically significant and independent of the method used to calculate stress change. Key Points: We report a positive correlation between the b‐value and the rate of Coulomb stress change of induced earthquakes in GroningenThis trend is statistically significant and robust to changes in the mechanical model used to calculate the stress changesWe interpret earthquake growth inhibition through a decrease of nucleation lengths at high stress change rates [ABSTRACT FROM AUTHOR]

The accuracy of Interferometric Synthetic Aperture Radar surface deformation solutions depends on the quality of the chosen interferogram subset. We present a method to select interferogram subsets based on unwrapping errors rather than temporal baseline thresholds. Using Sentinel‐1 interferograms over the Tulare Basin, California, we show that subtle phase noise can lead to up to 31.5 mm/yr line‐of‐sight (LOS) errors in short temporal baseline subset solutions, while decorrelation leads to a systematic underestimation of LOS rates (up to 92.3 mm/yr) in long temporal baseline subset solutions. Our new workflow better mitigates these noise sources at the same time. In the Eagle Ford region, Texas, our strategy better reconstructed up to 11.9 cm of cumulative LOS deformation between 2017 and 2021 over a ∼ ${\sim} $900 km2 $\mathrm{k}{\mathrm{m}}^{\mathrm{2}}$ region. This deformation feature can be linked to the total volume of produced oil and water through a linear relationship. Plain Language Summary: Surface deformation estimates derived from spaceborne imaging radar data are often impacted by weather conditions and surface vegetation changes. While it is common to select high‐quality data based on the time separation between two image passes, we designed a new data selection strategy based on measurable phase artifacts. We applied the new method to two vegetated regions that experienced land subsidence due to agricultural groundwater pumping (Tulare Basin, California) or oil and gas production (Eagle Ford region, Texas). In both cases, we show that surface deformation estimates can vary substantially depending on the choice of data subsets. Our strategy better mitigates different InSAR measurement noise terms at the same time. We showed that the observed subsidence in the Eagle Ford region is linearly proportional to the volume of produced oil and water from unconventional oil and gas resources exploitation. Furthermore, ongoing production activities have led to an increase in human‐induced earthquakes. Based on these findings, accurate surface deformation derived from InSAR data is now achievable in densely vegetated regions and can play an important role in future induced seismicity studies. Key Points: InSAR phase coherence does not always decrease with temporal baselinesChoosing interferograms based on phase unwrapping errors rather than temporal baselines better mitigates InSAR measurement noiseThe improved InSAR analysis strategy reveals up to 11.9 cm of LOS deformation signals related to oil and gas operations over the Eagle Ford [ABSTRACT FROM AUTHOR]

In areas of induced seismicity, earthquakes can be triggered by stress changes due to fluid injection and static deformation from fault slip. Here we present a method to distinguish between injection‐driven and earthquake‐driven triggering of induced seismicity by combining a calibrated, fully coupled, poroelastic stress model of wastewater injection with interpretation of a machine learning algorithm trained on both earthquake catalog and modeled stress features. We investigate seismicity from Paradox Valley, Colorado as an ideal test case: a single, high‐pressure injector that has induced thousands of earthquakes since 1991. Using feature importance analysis, we find that injection‐driven earthquakes are approximately 22± $\pm $5% of the total catalog but act as background events that can trigger subsequent aftershocks. Injection‐driven events also have distinct spatiotemporal clustering properties with a larger b‐value, closer proximity to the well, and earlier occurrence in the injection history. Generalization of our technique can help characterize triggering processes in other regions where induced seismicity occurs. Plain Language Summary: The Paradox Valley Unit, Colorado in the central United States has had a remarkable increase in seismicity coincident with over 8 million cubic meters of brine fluid injection since 1991, inducing thousands of earthquakes within an aquifer 4.5 km below the surface. We use a physics‐based model of the Earth combined with statistical and machine learning techniques to help discern which earthquakes are triggered by other earthquakes and which earthquakes are directly triggered by the stress changes from the well as well as their comparative characteristics. Discerning which earthquakes are directly caused from pressure changes due to the fluid injected by the well can inform our understanding of earthquake physics and provide useful information to operators of energy production sites. Key Points: Physics‐based models with interpretable machine learning can identify likely triggering mechanism of earthquakes within an induced sequenceWith this technique, we show that only 22土5% of earthquakes in Paradox Valley are primarily injection‐drivenInjection‐driven events have a larger b‐value, are closer to the well, and occur earlier in the injection history [ABSTRACT FROM AUTHOR]

Fault normal stress (σn) changes dynamically during earthquakes. However, the impact of these changes on fault strength is poorly understood. We explore the effects of rapidly varying σn by conducting rotary‐shear experiments on simulated fault gouges at 1 μm/s, under well‐drained, hydrothermal conditions. Our results show both elastic and anelastic (time‐dependent but recoverable) changes in gouge layer thickness in response to step changes and sinusoidal oscillations in σn. In particular, we observe dilation associated with marked weakening during ongoing σn‐oscillations at frequencies >0.1 Hz. Moreover, recovery of shear stress after such oscillations is accompanied by transient (anelastic) compaction. We propose a microphysically based friction model that explains most of the observations made, including the effects of temperature and step versus sinusoidal perturbation modes. Our results highlight that σn‐oscillations above a specific frequency threshold, controlled by the loading regime and frictional properties of the fault, may enhance seismic hazards. Plain Language Summary: Faults in the crust sometimes experience rapid stress changes, caused by nearby or remote earthquakes, by seasonal impoundment and discharge of reservoirs, by hydrocarbon or geothermal energy production, or by reservoir stimulation. The impact of these stress changes on the earthquake potential of faults is poorly understood. This study explores such effects through laboratory experiments on simulated faults under upper crustal PT conditions, perturbing the normal stress on the fault in various ways. Our results show that the shear stress supported by the fault, and the fault thickness, respond instantly to normal stress changes, followed by a transient evolution. In particular, we observed dilation (fault‐normal expansion) associated with marked weakening during fast oscillation. We propose a micromechanical model that can qualitatively explain the general experimental observations. Our results indicate that varying the normal stress on a fault at frequencies above a specific threshold may enhance seismic hazard. Key Points: We observed dynamic weakening and dilation of shearing gouges subjected to rapid oscillations in normal stressFault compaction/dilation explains the shear stress evolution in response to different modes of normal stress perturbationWe propose a micromechanical model for gouge friction under time‐variable normal stress conditions [ABSTRACT FROM AUTHOR]

Abstract Gas extraction from the Groningen gas field resulted in significant induced seismicity. We analyze the magnitude‐frequency distribution of these earthquakes in space, time and in view of stress changes calculated based on gas production and reservoir properties. Previous studies suggested variations related to reservoir geometry and stress. While we confirm the spatial variations, we do not detect a clear sensitivity of b‐value to Coulomb stress changes. However, we find that b‐value correlates positively with the rate of Coulomb stress changes. This correlation is statistically significant and robust to uncertainties related to stress change calculation. This study thus points to a possible influence of stress change rate on the probability of the magnitude of induced earthquakes.

Laumontite is a common and potentially frictionally unstable hydrothermal alteration product present in deep faults of the Gonghe EGS reservoir. We characterize the friction‐stability characteristics of synthetic laumontite gouge under in situ reservoir conditions. The pure laumontite gouge is frictionally strong (μ = 0.73–0.98) and the quartz/laumontite mixture (1:1) is generally less strong (μ = 0.73–0.78) under experimental conditions (Pc = 95 MPa, T = 90–250°C, Pf = 0–90 MPa). The shear velocity was stepped between 6.1, 0.61, then 0.061 μm/s for our experiments. For both gouges, the friction coefficient is independent of temperature and increases with elevated pore pressures. The pure gouge and mixture are strongly velocity‐weakening over a broad range in temperatures (∼90–220°C) and excess pore pressures (0–90 MPa) relevant to the Gonghe stimulation. Microearthquakes (MEQs) observed during stimulation are confined to within the broad depth range of inferred frictional instability—although fluid overpressures are also limited to this region. The observation that laumontite mixtures are frictionally unstable over a broad range of pressures and especially temperatures representative of EGS reservoirs and insensitive to the presence of the coexisting mineral phase (quartz) suggests its presence is a strong indicator of potential seismic hazard. Plain Language Summary: Laumontite is a very low‐grade altered mineral that can easily occur in fractures or faults in granite, basalt, or sandstone. Laumontite is widely developed in the Gonghe geothermal reservoir of Western China. Fluid injection into deep geothermal rock mass may reactivate subsurface faults containing altered minerals and cause earthquakes. Hence, we conducted laboratory experimental analysis on the frictional characteristics of simulated laumontite gouge to further understand the impact of fluid injection on the triggering of deep fault earthquakes. These experiments were performed at conditions reflecting the temperature and pressure of the water injection depth of the Gonghe geothermal reservoir. The results showed that the fault's strength and friction stability strongly depend on pore pressure and temperature. Our study emphasizes the significant role of the altered mineral laumontite in controlling fault strength and stability, as well as its potential for inducing earthquakes. A possible implication of this work is that when selecting geothermal resource targets that require fluid‐injection operations, it is best to avoid laumontite‐rich sites or reservoir sections. Key Points: We report the first evaluations of frictional stability properties for laumontite gouge and mixtures under hydrothermal conditionsGouges are strongly velocity‐weakening over a broad range of temperatures and insensitive to pressures and relative proportionsThis instability field is coincident with P‐T conditions typical for EGS and hence an indicator of ubiquitous seismic hazard [ABSTRACT FROM AUTHOR]

Unprecedented levels of seismicity have been seen in southern Sichuan, China, since the large‐scale exploitation of shale gas. Fluid and pore pressure transported through hydrological channel are thought as pivotal elements in the induction of earthquakes. Our high‐resolution tomography results reveal two inclined seismic anomalies featured by low Vs and high Vp/Vs at different depth range. The deeper anomaly extends 15 km from NE to SE and connects the well g048 from 3 km depth to the vicinity of the Ms 4.7 Gongxian earthquake 5.4 km deep, which is hinted to be a hydrological channel inferred from the high fluid overpressure of 28 Mpa calculated from focal mechanism solution. The injection operation of multiple shale gas wells along the channel may potentially accumulate the pore pressure and cause the fault near the end of the channel to reach critical stress state through various mechanisms. Plain Language Summary: Increasing seismicity in southern Sichuan is suspected to be linked to hydraulic fracturing stimulation for shale gas extraction. Induced earthquakes resulting from fracturing operations at nearby wells can be verified using in‐situ fracturing data, while remote‐induced factors are hard to prove. In this study, a seismic anomaly of low Vs and high Vp/Vs inferred to be a long‐distance hydrological channel connecting the 2021 Ms 4.7 Gongxian earthquake to the active shale gas well 15 km away. Our results indicate that pore fluid pressure generated by hydraulic fracturing in multiple wells may continuously accumulated at the end of the channel, ultimately resulting in high pore pressure and triggering the fault rupture. Therefore, it is urgent to plug this channel and prevent the filtration loss. Key Points: Two long‐distance hydrological channels were observed in southern SichuanThe injection operation of multiple wells along the channel may increase the pore pressure in the hydrological channelThe accumulation of pore pressure at the end of the hydrological channel was confirmed by the occurrence of Ms 4.7 Gongxian earthquake [ABSTRACT FROM AUTHOR]

Induced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken. Plain Language Summary: In July 2013, a sequence of more than 340 earthquakes was induced during the exploitation of the subsurface for energy production in St. Gallen geothermal field (Switzerland). To understand the mechanisms underlying the evolution of the sequence, we investigated the role of fluids and elastic stress transfer. The excess pore fluid pressure measurements suggest that the main triggering mechanism is related to high‐pressure fluids. The high values of the pore fluid pressure may be due to an already existing in situ overpressure condition from which a documented unexpected gas kick occurred. Key Points: The main earthquake triggering mechanism is the effect of high‐pressure fluidsThe high pore fluid pressure values may be due to an already existing in situ overpressure condition [ABSTRACT FROM AUTHOR]

Abstract Fault normal stress (σn) changes dynamically during earthquakes. However, the impact of these changes on fault strength is poorly understood. We explore the effects of rapidly varying σn by conducting rotary‐shear experiments on simulated fault gouges at 1 μm/s, under well‐drained, hydrothermal conditions. Our results show both elastic and anelastic (time‐dependent but recoverable) changes in gouge layer thickness in response to step changes and sinusoidal oscillations in σn. In particular, we observe dilation associated with marked weakening during ongoing σn‐oscillations at frequencies >0.1 Hz. Moreover, recovery of shear stress after such oscillations is accompanied by transient (anelastic) compaction. We propose a microphysically based friction model that explains most of the observations made, including the effects of temperature and step versus sinusoidal perturbation modes. Our results highlight that σn‐oscillations above a specific frequency threshold, controlled by the loading regime and frictional properties of the fault, may enhance seismic hazards.

Pore fluids are ubiquitous throughout the lithosphere and are commonly invoked as the cause of induced seismicity and slow earthquakes. We perform lab experiments to address these questions for drained fault conditions and low pore pressure. We shear simulated faults at effective normal stress σn′ $\left({\sigma }_{n}^{\prime }\right)$ of 20 MPa and pore pressures Pp from 1 to 4 MPa. We document the full range of lab earthquake behaviors from slow slip to elasto‐dynamic rupture and show that slow slip can be explained by the slip rate dependence of the critical rheologic stiffness without dilatancy hardening or other fluid effects. Our fault permeabilities ranges from 10−18 to 10−17 m2 with an initial porosity of 0.1 and estimated fluid diffusion time ≈1 s. Slow slip and quasi‐dynamic fault motion may arise from high Pp at higher pressures but dilatancy strengthening is not a general requirement. Plain Language Summary: Earthquakes begin and propagate within the fluid‐saturated rocks of Earth's crust. Many investigators have suggested that high pore fluid pressure (Pp) is essential for slow earthquakes and tremor. These studies rely on the idea that changes in Pp can impact rupture propagation speed by dilatant volume increase during faulting with concurrent increases in fault effective normal stress. Thus, understanding the processes that produce slow‐slip versus dynamically propagating rupture is integral to seismic hazard forecasting. Here, we describe experiments on granular faults that produce the full spectrum of slip observed in nature. We measure the mechanical and hydraulic behavior of the faults and determine that frictional and fluid‐driven processes occur in conjunction. Importantly, we demonstrate that frictional processes are sufficient to explain slow‐slip when fluid migration is not inhibited. We demonstrate that for low pore fluid pressures, the full transition from slow slip to dynamic rupture events can be explained as a frictional effect via the critical rheologic stiffness. Key Points: The frictional stability transition does not require dilatant hardening for granular fault zones sheared at low pore pressuresSlow earthquakes and quasi‐dynamic fault slip can be explained by the strain‐rate dependence of the critical fault stiffness (Kc)For the effective normal stresses studied, pore pressure has a negligible impact on frictional stability and the mode of fault slip [ABSTRACT FROM AUTHOR]

Abstract Induced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken.

The evolution of a fault's frictional strength during the interseismic period is a critical component of the earthquake cycle, yet there have been relatively few studies that examine the time‐dependent evolution of strength at conditions representative of seismogenic depths. Using a simulated fault in Westerly granite, we examined how frictional strength evolves under hydrothermal conditions up to 250°C during slide‐hold‐slide experiments. At temperatures ≤100°C, frictional strength generally increases with hold duration but, at 200 and 250°C, an initial increase in strength transitions to rapid time‐dependent weakening for holds longer than 14 hr. Forward modeling of long hold periods at 250°C using the rate and state friction constitutive equations requires a second, strongly negative, state variable with a long evolution distance. This implies that significant hydrothermal alteration is occurring at 250°C, consistent with microstructural observations of dissolution and secondary mineral precipitation. Plain Language Summary: A fault can regain strength following an earthquake. The rates, conditions, and mechanisms by which this recovery of strength occurs are not well understood but they are critical to understanding earthquake recurrence and induced seismicity. By conducting a series of experiments in which a simulated fault alternates between shear displacement and quasi‐stationary hold periods, we examine how fault strength evolves with time at temperatures up to 250°C. At elevated temperatures the fault initially strengthens but then weakens over long timescales. This weakening appears to be due to hydrothermal alteration, potentially including the precipitation of secondary minerals in the fault. Relating this observed weakening behavior and associated mechanisms to constitutive equations will facilitate modeling earthquake behavior in different materials and at variety of environmental conditions. Key Points: Time‐dependent weakening of simulated fault surfaces at hydrothermal conditions is observedWeakening mechanism is water‐assisted, temperature‐dependent, and depends on host rock compositionCapturing frictional behavior across all time scales requires a rate and state model with two state variables and a time delay parameter [ABSTRACT FROM AUTHOR]

Deterministic earthquake prediction remains elusive, but time‐dependent probabilistic seismicity forecasting seems within reach thanks to the development of physics‐based models relating seismicity to stress changes. Difficulties include constraining the earthquake nucleation model and fault initial stress state. Here, we analyze induced earthquakes from the Groningen gas field, where production is strongly seasonal, and seismicity began 3 decades after production started. We use the seismicity response to stress variations to constrain the earthquake nucleation process and calibrate models for time‐dependent forecasting of induced earthquakes. Remarkable agreements of modeled and observed seismicity are obtained when we consider (a) the initial strength excess, (b) the finite duration of earthquake nucleation, and (c) the seasonal variations of gas production. We propose a novel metric to quantify the nucleation model's ability to capture the damped amplitude and the phase of the seismicity response to short‐timescale (seasonal) stress variations which allows further tightening the model's parameters. Plain Language Summary: Earthquakes are difficult to predict with certainty, but progress in forecasting their likelihood using probabilistic models based on stress changes has been made. However, challenges remain in understanding how earthquakes start and the initial conditions of faults. Here, we analyzed induced earthquakes in the Groningen gas field, where production is seasonal and seismic activity began 34 years after gas production started. By studying how the earthquakes respond to rapid changes in stress, we could better understand how they start and develop models to forecast their temporal occurrence. By considering factors like the initial strength of the faults, the duration of earthquake initiation, and seasonal variations in gas production we could accurately match the observed seismic activity. We introduced a new measure to evaluate how well the models captured the dampened strength and timing of seismic activity in response to short‐term stress changes (such as seasonal variations), which helped refine the model's parameters. Key Points: An improved reservoir, geomechanical, and seismicity modeling workflow is proposed for forecasting induced seismicity at various timescalesShort‐timescale stress variations allow constraining the characteristics of the earthquake nucleation process using Groningen as case studyInitial strength excess and finite duration of the nucleation process allow reproducing long‐and‐short timescale characteristics of seismicity [ABSTRACT FROM AUTHOR]

We present a series of controlled fluid injection experiments in the laboratory on a pre‐stressed natural rough fracture with a high initial permeability (∼10−13 m2) in granite using different fluid pressurization rates. Our results show that fluid injection on a fracture with a slight velocity‐strengthening frictional behavior exhibits dilatant slow slip in association with a permeability increase up to ∼41 times attained at the maximum slip velocity of 0.085 mm/s for the highest‐rate injection case. Under these conditions, the slip velocity‐dependent change in hydraulic aperture is a dominant process to explain the transient evolution of fracture permeability, which is modulated by fluid pressurization rate and fracture surface asperities. This leads to the conclusion that permeability evolution can be engineered for subsurface geoenergy applications by controlling the fluid pressurization rate on slowly slipping fractures. Plain Language Summary: Understanding the evolution of fracture permeability during hydraulic stimulation of subsurface reservoirs is the key to characterizing fluid transport and formulating strategies to limit induced seismicity. Accordingly, there is a significant interest in deciphering how the fluid pressurization rate, a constitutive operational parameter during injection, influences the transient permeability change during fracture slip. We conducted a series of experiments in the laboratory using different fluid pressurization rates on a natural rough fracture in granite under a pre‐stressed state. The fracture had a high initial permeability. Our findings show that when fluid is injected into a fracture with a slight velocity‐strengthening frictional behavior, it causes slow slipping with significant permeability enhancement. The change in hydraulic aperture caused by slip velocity is the main reason for the temporary change in permeability, and this effect is modulated by fluid pressurization rate and fracture surface irregularities. Our results suggest that we can modulate the permeability of subsurface geoenergy reservoirs by controlling the fluid pressurization rate on slowly slipping fractures. Key Points: We conducted fluid injection experiments on a pre‐stressed natural rough fracture in granite at different pressurization ratesThe velocity‐strengthening fracture exhibits slow slip accompanied by a significant increase in permeability during fluid injectionTransient fracture permeability is controlled by injection‐induced slip velocity, modulated by pressurization rate and surface asperities [ABSTRACT FROM AUTHOR]

We provide direct observations of the three-dimensional displacements of a fluid-driven fracture during water injections in a borehole at ∼1.5 km depth in the crystalline rock at the Sanford Underground Research Facility (USA). Micro-shearing of the borehole initiates on a foliation plane at 61% of the minimum principal stress σ3. As the fluid pressure increases further up to 112% of σ3, borehole axial and radial displacements increase with injection time highlighting the opening and sliding of a new hydrofracture growing ∼10 m away from the borehole, in accordance with the ambient normal stress regime and in alignment with the microseismicity. Our study reveals how important it is to consider monitoring zonal deformation in conjunction with pressure and flow to better manage the complex hydromechanical evolution of the growing fracture.

Abstract Deterministic earthquake prediction remains elusive, but time‐dependent probabilistic seismicity forecasting seems within reach thanks to the development of physics‐based models relating seismicity to stress changes. Difficulties include constraining the earthquake nucleation model and fault initial stress state. Here, we analyze induced earthquakes from the Groningen gas field, where production is strongly seasonal, and seismicity began 3 decades after production started. We use the seismicity response to stress variations to constrain the earthquake nucleation process and calibrate models for time‐dependent forecasting of induced earthquakes. Remarkable agreements of modeled and observed seismicity are obtained when we consider (a) the initial strength excess, (b) the finite duration of earthquake nucleation, and (c) the seasonal variations of gas production. We propose a novel metric to quantify the nucleation model's ability to capture the damped amplitude and the phase of the seismicity response to short‐timescale (seasonal) stress variations which allows further tightening the model's parameters.

Microseismic data acquired during hydraulic stimulations is routinely used to characterize hydraulic fracture (HF) propagation away from a wellbore. When the data include induced seismicity (IS) related to induced fault slip, separating HF‐related and IS‐related events is essential for HF treatment optimization and IS mitigation. Linear and non‐linear analytical diffusivity models can be used to interpret microseismic data and quantify the propagation of HFs, but their accuracy reduces when significant induced microseismicity is present. A Bayesian quantile regression is used to extend these existing diffusivity models to data contaminated with IS. A plausible ellipsoid filters events that are clearly anomalous prior to the quantile regression. The regression effectively separates HF‐related microseismic events from induced events in a case study for all stages without interpretative bias. This reveals faults that are directed connected to HFs, as well as those solely related with induced seismicity. Plain Language Summary: We present a method for analyzing point clouds of small seismic events observed during hydraulic fracturing (microseismicity). The method separates events that are closely related with hydraulic fractures from those that are anomalous, far from hydraulic fractures, or triggered on preexisting faults. The method uses a statistical regression to infer the diffusivity, or rate of propagation, of hydraulic fractures. We account for a prior expectation of how quickly the hydraulic fracture propagates in length, width, and height. Events occurring beyond the inferred hydraulic fracture diffusivity are classified as induced seismicity. A case study is reviewed, and important parts of the regression are discussed. By separating hydraulic fracture related microseismicity from triggered events, it is possible to clearly understand each and how they relate to each other in a data‐driven, automated way. Key Points: Diffusivity can separate microseismicity created by hydraulic fracturing from induced seismicity to permit structural analysisBayesian quantile regression robustly estimates the linear and non‐linear diffusivity parameters required for this segregationWe reveal a direct connection between hydraulic fracturing and faults associated with induced seismicity [ABSTRACT FROM AUTHOR]

Earthquakes induced by human activities can impede the development of underground resources. Significant induced events (M5) have caused both economic and human losses. The recent ML 5.6 (MW 5.1) event near Peace River, Alberta occurs in a region of in situ bitumen recovery. We find that 3.4 cm of ground deformation was caused by reverse fault slip (∼29 cm), possibly related to Peace River Arch faulting. Events are located within the shallow basement, nearby to significant wastewater injection into Paleozoic strata. We find a statistical relationship between earthquakes and injection operations. These events were likely related to the in situ bitumen development: dominantly from wastewater disposal induced pore pressure increases, with smaller poroelastic contributions from bitumen recovery. The assessment of this earthquake as induced will likely have implications for future energy development, management, and regulation—including carbon capture and blue hydrogen. Plain Language Summary: Earthquakes can be caused by underground fluid injection; cases of M5 induced events have caused damage and harm. One of the largest recorded earthquakes in Alberta (ML 5.6) occurred in a region of underground oil sand development. Here, ground shaking and deformation information are combined into an interpreted result: that ancient faults were reactivated with reverse slip. The fault slip is largely within the crystalline basement, with a small portion extending into basal sediments. Nearby injection operations dispose of petroleum‐related wastewater in these basal sediments. This earthquake was likely triggered by the injection process: injection increases pore pressure, which diffuses laterally along permeable sediments, until encountering fractured rock, which channelizes flow into the crystalline basement—the increase of pore pressure within the fault continues until reaching a critical point for slip initiation. This event likely being induced will have important implications for future operations. Key Points: On 30 November 2022 one of Alberta's largest recorded events (ML 5.6, MW 5.1) occurred in a region of in situ bitumen recoveryA well oriented fault was reactivated with reverse slip (29 cm), causing up to 3.4 cm of ground deformationThe event was likely induced by pore pressure from disposal, with smaller poroelastic stress changes from bitumen production [ABSTRACT FROM AUTHOR]

Abstract Microseismic data acquired during hydraulic stimulations is routinely used to characterize hydraulic fracture (HF) propagation away from a wellbore. When the data include induced seismicity (IS) related to induced fault slip, separating HF‐related and IS‐related events is essential for HF treatment optimization and IS mitigation. Linear and non‐linear analytical diffusivity models can be used to interpret microseismic data and quantify the propagation of HFs, but their accuracy reduces when significant induced microseismicity is present. A Bayesian quantile regression is used to extend these existing diffusivity models to data contaminated with IS. A plausible ellipsoid filters events that are clearly anomalous prior to the quantile regression. The regression effectively separates HF‐related microseismic events from induced events in a case study for all stages without interpretative bias. This reveals faults that are directed connected to HFs, as well as those solely related with induced seismicity.

Pore pressure in aquifers confined below a cryosphere will increase as Mars cools and the cryosphere thickens. Increased pore pressure decreases the effective stress and hence promotes seismicity. We calculate the rate of pore pressure change from cooling of Mars's interior and the modulation of pore pressure from solar and Phobos tides and barometric loading. Using the time-varying pressure and tidal stresses, we compute Coulomb stress changes and the expected seismicity rate from a rate-and-state friction model. Seismicity rate will vary by several tens of percent to 2 orders of magnitude if the mean pore pressure is within 0.2 and 0.01 MPa of lithostatic, respectively. Seismic events promoted by high pore pressure may be tremor-like. Documenting (or not) tidally modulated shallow seismicity would provide evidence for (or against) water-filled confined aquifers, that pore pressure is high, and that the state of stress is close to failure—with implications for processes that can deliver water to the Martian surface.

Abstract Earthquakes induced by human activities can impede the development of underground resources. Significant induced events (M5) have caused both economic and human losses. The recent ML 5.6 (MW 5.1) event near Peace River, Alberta occurs in a region of in situ bitumen recovery. We find that 3.4 cm of ground deformation was caused by reverse fault slip (∼29 cm), possibly related to Peace River Arch faulting. Events are located within the shallow basement, nearby to significant wastewater injection into Paleozoic strata. We find a statistical relationship between earthquakes and injection operations. These events were likely related to the in situ bitumen development: dominantly from wastewater disposal induced pore pressure increases, with smaller poroelastic contributions from bitumen recovery. The assessment of this earthquake as induced will likely have implications for future energy development, management, and regulation—including carbon capture and blue hydrogen.

Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut‐in. Understanding the extent and duration of the post‐injection microseismicity is critical to limit the risk of fluid‐induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut‐in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone toward induced seismicity mitigation strategies. Plain Language Summary: Injection of fluid in the subsurface for energy and storage applications can lead to the onset of microseismicity, and possibly to major induced seismic events. Fluid pressurization decreases the shear strength of surrounding faults and slip occurs when the in situ shear stress on a fault reaches its shear strength. The nature of slip (aseismic or seismic) depends on the rate at which it occurs and thus on the stability of the deformation. Understanding the mechanics controlling the onset and arrest of aseismic slip and the transition to seismic slip is therefore key to design mitigation strategies for the safe utilization of the subsurface. In this contribution, we investigate using theoretical and numerical techniques how aseismic slip on a fault plane nucleates and evolves in response to fluid injection and how it stops after injection shut‐in when fluid pressure relaxes. We demonstrate that critically stressed faults prior to injection can slip for a longer time after injection shut‐in than during injection and that the extent of rupture can double in size after the end of injection. These results help to quantify the duration and sphere of influence of fluid injection where microseismicity can occur during and after injection. Key Points: Quantification of fault aseismic slip that continues to propagate away from the former injection point after injection shut‐inArrest time of aseismic slip is proportional to the pressurization durationCritically stressed faults slip significantly longer and at a greater distance than marginally pressurized faults after injection shut‐in [ABSTRACT FROM AUTHOR]

We conducted fluid injection experiments on cylindrical low‐permeability granite samples with a critically stressed sawcut fault at local injection rates of 0.2 and 0.8 mL/min and confining pressures of 31 and 61 MPa. A local array of six strain gauges attached close to the faults allows us to estimate the nucleation length of each injection‐induced dynamic slip event (i.e., laboratory earthquake). We find nucleation lengths decrease from approximately 90% to <15% of the fault length with higher injection rate and increased effective normal stress. Injection‐induced laboratory earthquakes with smaller nucleation lengths show generally higher peak slip rates and larger fault slip displacements, signifying an intensified seismic hazard. Our results also indicate that initially stable fault patches may be reactivated to slip seismically by increasing injection rates. This study systematically demonstrates that higher injection rates constitute dynamic loading, which increase the seismic hazard by shrinking the earthquake nucleation length. Plain Language Summary: Earthquakes associated with fluid injection in various geo‐energy settings, such as shale gas and deep geothermal energy, have shelved many projects with great potential. However, the injection‐rate dependence of earthquake nucleation length, which characterizes the length of a slowly slipping (creeping) fault patch in preparation for a subsequent earthquake, remains to be studied. To this end, we performed fluid injection experiments on low‐permeability granite samples containing a critically stressed sawcut fault. We demonstrate that the nucleation length of injection‐induced laboratory earthquakes shrinks with higher effective normal stress and larger injection rate. We show that laboratory earthquakes induced by higher fluid injection rates are characterized by smaller nucleation lengths, faster peak slip rates, and larger fault slip displacements. Our results also suggest that a stable fault patch subject to low‐rate injection may be induced to slip seismically by increasing injection rates. Our findings may explain the higher seismic hazards associated with higher injection rates as has been frequently documented in field cases. Key Points: The nucleation length of injection‐induced laboratory earthquakes reduces with higher injection rate and greater effective normal stressInjection‐induced laboratory earthquakes with smaller nucleation lengths exhibit higher peak slip rates and larger fault slip displacementsFast‐rate fluid injection may cause initially stable fault patches of smaller sizes to slip seismically, leading to higher seismic hazards [ABSTRACT FROM AUTHOR]

The influence of elevated temperature on injection‐induced fault slip is poorly constrained. In this study, at steady‐state elevated temperatures, triaxial shear‐flow experiments on a sawcut fault in granite were conducted to simulate injection‐induced slip of a critically stressed fault. Our results suggest that an elevated temperature favors a more uniform fluid pressure distribution over the fault surface mainly by reducing water viscosity. At temperatures above ambient, a larger perturbation force from the injected fluid is required to reactivate the fault primarily because of the enhanced thermally activated fault healing processes, resulting in a faster fault slip rate upon failure. This study may partially explain the causal link between higher reservoir temperature and higher maximum magnitude of injection‐induced earthquakes in geothermal systems, and the observation that larger magnitude seismic events concentrate near the deeper part of the reservoir, where temperature is higher. Plain Language Summary: It is imperative to explore the poorly constrained influence of elevated temperature on injection‐induced fault slip to explore the implications for injection‐induced seismicity in geothermal reservoirs. To this end, we injected water with a similar temperature to the host rock into a laboratory‐scale fault in granite under a critically stressed condition at constant elevated temperatures to simulate injection‐induced fault slip beyond the limited cooling zone near the injection well in geothermal reservoirs. The reduced water viscosity promotes a more uniform fluid pressure distribution on the fault surface at elevated temperature. The perturbation force upon fault reactivation provided by the injected fluid increases with higher temperature, indicating an accelerated and enhanced fault healing at higher temperature. Fault friction reduces under the stress relaxation experimental condition, while fault healing can partially compensate for the friction reduction associated with relaxation. The fault strength recovery at elevated temperature may have caused the faster fault slip rate upon failure. The higher injection‐induced seismic hazard associated with higher geothermal reservoir temperature, and the concentration of larger magnitude events near the deeper part of the Basel geothermal reservoir could be partially explained by the findings of this study. Key Points: Elevated temperature promotes a more uniform fluid pressure distribution over the fault surface mainly by reducing water viscosityAccelerated and enhanced fault healing at elevated temperature causes faster fault slip rates upon injection‐induced fault failureThe injection‐induced seismic hazard is enhanced in higher‐temperature geothermal reservoirs and deeper part of the reservoirs [ABSTRACT FROM AUTHOR]

Estimates of intruded magma volume are critical for forecasting volcanic unrest. Geodetic modeling can provide such estimates but is of limited use in submarine and highly vegetated settings. A complementary approach could be to use estimates of seismic moment release. In this study, we examine the moment‐volume scaling of several proximal volcanic earthquake sequences and compare it to that of injection‐induced seismicity. We find a notable similarity in scaling between the volcanic sequences, which contrasts with the broad range of responses exhibited by anthropogenic injection‐induced sequences. This may imply an underlying similarity in the geologic conditions for volcanoes that is distinct from induced seismicity settings. It could also allow for estimates of intruded volume to be made without geodetic information. This provides further insight into the factors controlling seismogenesis in these different settings and has implications for volcano seismology and injection‐induced seismicity hazard estimation. Plain Language Summary: Knowing how much magma is beneath a volcano before or during an eruption can help forecast its behavior and aid evacuation or relief efforts. Currently, satellite images or sensitive measurements of the ground inflation are used to model the amount of magma, but this isn't always possible if the volcano is underwater or shrouded by tree cover. A proposed alternative to this is to use the size and number of earthquakes as a gauge on the amount of volume change occurring underground. The relationship between the total amount of energy released by earthquakes and volume change has been a key research area of earthquakes triggered in the injection of fluids by industries like geothermal energy or hydraulic fracturing (i.e., fracking). We study several well‐recorded volcanoes and see that there is a similarity in the observed volume change and the amount of energy released from associated earthquakes. This is very different from the many examples of anthropogenic injection‐induced seismicity, which show a very broad spread. This implies that the similar conditions under volcanoes lead to similar responses to volume changes. This helps us better understand and thus potentially mitigate the hazards of both volcanoes and industrial fluid injection. Key Points: We observe a similar moment‐volume scaling for volcano seismicity, contrasting with the range exhibited by injection‐induced examplesThe results show that scaling between moment release and volume change can be used to estimate intruded volumesThis also provides insights into the geologic factors which control the crust's response to volume changes [ABSTRACT FROM AUTHOR]

Abstract Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut‐in. Understanding the extent and duration of the post‐injection microseismicity is critical to limit the risk of fluid‐induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut‐in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone toward induced seismicity mitigation strategies.

Hydraulic fracturing (HF) operations are widely associated with induced seismicity in the Western Canadian Sedimentary Basin. This study correlates injection parameters of 12,903 HF stages in the Kiskatinaw area in northeast British Columbia with an enhanced catalog containing 40,046 earthquakes using a supervised machine learning approach. It identifies relevant combinations of geological and operational parameters related to individual HF stages in efforts to decipher fault activation mechanisms. Our results suggest that stages targeting specific geological units (here, the Lower Montney formation) are more likely to induce an earthquake. Additional parameters positively correlated with earthquake likelihood include target formation thickness, injection volume, and completion date. Furthermore, the COVID‐19 lockdown may have reduced the potential cumulative effect of HF operations. Our results demonstrate the value of machine learning approaches for implementation as guidance tools that help facilitate safe development of unconventional energy technologies. Plain Language Summary: Hydraulic fracturing (HF), a technique used in unconventional energy production, increases rock permeability to enhance fluid movement. Its use has led to an unprecedented increase of associated earthquakes in the Western Canadian Sedimentary Basin in the last decade, among other regions. Numerous studies have investigated the relationship between induced earthquakes and HF operations, but the connection between specific geological and operational parameters and earthquake occurrence is only partly understood. Here, we use a supervised machine learning approach with publicly available injection data from the British Columbia Oil and Gas Commission to identify influential HF parameters for increasing the likelihood of a specific operation inducing an earthquake. We find that geological parameters, such as the target formation and its thickness, are most influential. A small number of operational parameters are also important, such as the injected fluid volume and the operation date. Our findings demonstrate an approach with the potential to develop tools to help enable the continued development of alternative energy technology. They also emphasize the need for public access to operational data to estimate and reduce the hazard and associated risk of induced seismicity. Key Points: We use supervised machine learning to investigate the relationship between hydraulic fracturing operation parameters and induced seismicityGeological properties and a limited number of operational parameters predominantly influence the probability of an induced earthquakeThe approach has the potential to guide detailed investigations of injection parameters critical for inducing earthquakes [ABSTRACT FROM AUTHOR]

Large‐scale CO2 sequestration into geological formations has been suggested to reduce CO2 emissions from industrial activities. However, much like enhanced geothermal stimulation and wastewater injection, CO2 sequestration has a potential to induce earthquake along weak faults, which can be considered a negative impact on safety and public opinion. This study shows the physical mechanisms of potential seismic hazards along basement faults driven by CO2 sequestration under variation in geological and operational constraints. Specifically we compare the poroelastic behaviors between multiphase flow and single‐phase flow cases, highlighting specific needs of evaluating induced seismicity associated with CO2 sequestration. In contrast to single‐phase injection scenario, slower migration of the CO2 plume than pressure pulse may delay accumulation of pressure and stress along basement faults that may not be mitigated immediately by shut‐in of injection. The impact of multiphase flow system, therefore, needs to be considered for proper monitoring and mitigation strategies. Plain Language Summary: Large‐scale geological CO2 storage has been considered one of key mitigation strategies to reduce greenhouse gas emission. Geological formation is evaluated to decide the presence of preexisting faults prone to failure prior to CO2 injection, but the hidden and/or undetected faults are sometimes observed due to induced seismicity detected after fluid injection. Hence, comprehensive mechanistic study is required to study the impact of CO2 injection into geological formations in the presence of faults. This study demonstrates that earthquakes can strike unexpectedly along the faults at large distances and depths even after terminating well operations, caused by stress transfer through rock pores or interaction among injected CO2, preexisting pore fluids and rock matrix. Contrast of multiphase fluid properties between CO2 and brine can generate larger and delayed perturbations in pressure and stress fields along faults that may maintain the potential of seismicity after shut‐in. Elastic energy can transfer to low‐permeability or hydraulically isolated basement faults, such that geomechanical simulations based on comprehensive geophysical data are required to detect deformation‐driven signals and mitigate potential seismic hazards associated with geological CO2 sequestration. Key Points: Hydro‐mechanical deformation of the CO2‐saturated reservoir can destabilize basement faults, even hydraulically isolated from the reservoirTwo‐phase flow system can generate prolonged and large stress changes along the faults due to contrast of flow and transport characteristicsTerminating injection may not mitigate elastic energy in a basement immediately that can induce post shut‐in earthquakes along deep faults [ABSTRACT FROM AUTHOR]

Despite public concerns, only a few CO2 injections into saline aquifers have reported microseismicity. We analyze passive seismic monitoring of a small (15,000 tonnes and 0.15 MPa pressure) injection of supercritical CO2‐rich mixture for Stage 2C of the CO2CRC Otway Project (Victoria, Australia), which induced 19 detectable events with maximum moment magnitude MW‐0.5. The locations and dynamic parameters of the triggered events indicate a reactivation of a small fault patch where CO2 flowed through the fault. Time‐lapse seismic images of the plume and reservoir simulations show that the reactivation occurred when the CO2 plume reached this fault. This might be indicative of a fault weakening by the plume that enabled subsequent reactivation by pressure variations. Our observations suggest that a leakage from a commercial‐scale storage may trigger felt seismicity in the overburden without strong overpressure, thus, the de‐risking workflows should involve a detailed study of small faults. Plain Language Summary: Geological carbon storage is one of the key technologies for reducing the greenhouse gas effects. The storage projects may reactivate subsurface faults and lead to felt earthquakes and even surface deformations. We present a new case study, Stage 2C of the CO2CRC Otway Project (Victoria, Australia), where a small leakage‐like injection triggered seismicity that could be detected by high‐sensitivity sensors but is way below the "felt" levels. Typically, pre‐existing faults and fractures are triggered by pressure build‐up from an injection. Timing and locations of the triggered micro‐earthquakes at the Otway Project may suggest that the injected gas might have weakened the rocks filling the fault gouge thus making the fault more prone to reactivation. Our study implies that even a small leakage into such a reservoir, driven mainly by buoyancy, has seismogenic potential and thus this risk should be carefully considered by site operators for geological carbon storage projects. Key Points: A low‐pressure injection of 3,000 tonnes of supercritical CO2 into a high‐quality sandstone aquifer induced detectable seismicityAll of the seismic events are located on a small fault undetected in the seismic images prior to the injectionThe seismic monitoring data suggest that a fluid‐rock interaction enabled the fault's reactivation by the increased pore pressure [ABSTRACT FROM AUTHOR]

One of the challenges associated with subsurface high‐pressure fluid injections is the estimation of the seismic hazard and its spatial footprint. Field data have shown that the spatial footprint typically varies significantly between injections into the basement and injections above basement. Here, we show that varying degrees of spatial correlations in porosity or log(permeability) can explain this observation. Using high‐resolution well‐log data, we first show that porosity within the basement tends to follow a power‐law scaling, S(k) ∼ 1/kβ, with β ≈ 0.9, while above basement β > 1.4. Using this in a novel conceptual model, we show that β controls the spatial footprint of fluid‐induced seismicity such that large values of β lead to more seismic activity at large distances and a higher variability in the spatio‐temporal migration of seismic events, hence, explaining the field observations. Our findings indicate that correlations in log(permeability) need to be incorporated in the seismic hazard assessment. Plain Language Summary: Fluid‐induced earthquakes are a side effect of industrial operations such as hydraulic fracturing and enhanced geothermal systems, where high‐pressure fluids are pumped into the earth's crust to increase oil and/or gas flow to a well from petroleum‐bearing rock formations or to improve permeability in underground geothermal reservoirs. The induced seismicity has become a concern for industry and nearby residents. One of the challenges associated with this seismic hazard is answering the question of how far away from the injection site seismicity can be induced. Previous observations showed that if the fluids are pumped into the crystalline basement, the occurrence of the seismicity is typically very localized in space, while otherwise it is not. Here, we introduce a novel model that explains this behavior as a consequence of a varying degree of correlations in permeability. By analyzing high‐quality permeability data, we directly show that the degree of correlations in the crystalline basement is indeed significantly different from formations above the basement. Our findings can be directly incorporated in any seismic hazard assessment to improve the safety of workers and residents. Key Points: Spatial correlations in permeability can control the spatial localization of fluid‐induced seismicitySpatial correlations in permeability tend to be different within the basement and above basementThe variability in the migration profile of fluid‐induced seismicity is controlled by the permeability field [ABSTRACT FROM AUTHOR]

Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with the majority of M ≥ 4 mainshocks occurring near/after well completion. This poses serious challenges for risk mitigation and hazard assessment. Here, we document a high‐resolution, three‐dimensional source migration process with delayed mainshock triggering that is controlled by local hydrogeological conditions near Fox Creek, Alberta, Canada. Our results reveal that poroelastic effects might contribute to induced seismicity, but are probably insufficient to activate a large fault segment not critically stressed. The rapid pore‐pressure build‐up from HF can be very localized and capable of producing large, felt earthquakes if adequate hydrological paths exist. We interpret the delayed triggering as a manifestation of pore‐pressure build‐up along pre‐existing faults needed to facilitate seismic failure. Our findings can explain why so few injection operations are seismogenic. Plain Language Summary: Fluid injection‐induced earthquakes (IIE), especially those M ≥ 4 mainshocks, are often observed to occur near or after well completion. Such delayed triggering relative to injection commencement poses serious challenges for both regulators and the energy industry to establish an effective mitigation strategy for the potential seismic risk. In this study, we reveal a high‐resolution, complex three‐dimensional pattern of IIE migration near Fox Creek, Alberta, Canada. The observed first‐outward‐then‐inward IIE sequence highlights the significance of hydrogeological networks in facilitating fluid pressure migration and the associated seismic failure. The detailed spatiotemporal distribution of IIE suggests that the effect of pore‐pressure build‐up from hydraulic fracturing (HF) can be very localized. The delayed triggering is a combined result of the fluid pressure migration and the current stress state of the hosting fault system away from the HF wells. The findings from this study also provide plausible explanations on why only a very limited number of fluid injections are seismogenic. Key Points: We document a complex 3D source migration process with delayed mainshock triggering that is controlled by a local hydrogeological settingPoroelastic effects contribute to induced events but are probably insufficient to activate a large fault segment not critically stressedRapid pore‐pressure build‐up can be very localized and lead to large earthquakes if adequate hydrological paths exist [ABSTRACT FROM AUTHOR]

The 2011 Mw 5.7 Prague earthquake is the second largest induced earthquake in Oklahoma, and occurred after decades of wastewater disposal. The local geological structure that led to the triggering of this large earthquake is not well understood. In this study, tomographic inversion of seismic data recorded by a dense local seismic network resulted in a high‐resolution 3D velocity model with three major layers. The model clearly illuminates the geometry and characteristics of the Meeker‐Prague Fault that hosted the 2011 Prague sequence. A conceptual model is proposed to link the tomographic structure to the triggering process of the sequence. The low‐permeability second layer at ∼1.5–3.5 km may be the key that delays the occurrence of the first sizeable earthquake after decades of wastewater injection. However, a low‐shear‐velocity zone within this layer at the intersection of two major faults could have provided a fluid pathway to facilitate downward fluid propagation. Plain Language Summary: The recent surge of seismicity in the central United States since 2008 has largely been attributed to wastewater disposal from oil and gas production. The 6 November 2011 Mw 5.7 earthquake near Prague, is the second largest earthquake ever recorded instrumentally in the state of Oklahoma. Previous studies mainly focused on the characterization of seismicity in the 2011 Prague earthquake sequence and found that the majority of earthquakes occurred along the Meeker‐Prague fault, which is a ∼20 km splay fault off the Wilzetta fault zone. In this study, we apply the seismic traveltime tomography technique on data recorded by a dense local seismograph network to image the fine upper crustal velocity structure in the Prague fault zone. Our results clearly mapped zones with significant velocity anomalies in the fault zone compared to the surrounding area. And the velocity anomalies coincide with the clusters of seismicity, indicating that fluid injection probably played an important role in altering the properties of the fault zone and therefore promoting the occurrence of earthquakes. This study highlights that it is critical to investigate the fault zone structure to advance our understanding of the process of pore pressure diffusion and fluid migration in triggering of induced seismicity. Key Points: We develop a fine fault zone velocity structure for the 2011 Mw 5.7 Prague earthquake and relocate the earthquakes using the 3D modelsThe results depict the Meeker‐Prague Fault that hosted the majority of the larger events in the Prague sequenceWe propose a conceptual model that links velocity structure to the triggering process of the Prague sequence due to wastewater injection [ABSTRACT FROM AUTHOR]

The possibility of controlling induced earthquake magnitude through managed metering of water injection has yet to be rationalized. Mechanisms of reducing magnitudes of induced events through cycled fluid injection remain unclear. To explore such mechanisms and this possibility, we report experiments with water injection into laboratory faults. Water injection results in early triggering for both single and cycled injection. However, the maximum moment magnitude and total energy of the repeating induced earthquakes during cycled water injection are both lower than those of induced earthquakes induced with continuous injection and for natural earthquakes without water injection. Higher permeability of the host reduces the number of injection‐induced earthquakes but increases their moment. With an increasing number of water injection cycles, both maximum moment and total energy decrease, particularly as permeability decreases, while the number of induced events increases. The moment magnitude of induced events can thereby be controlled through cycled fluid injection. Plain Language Summary: Fluid injection‐triggered earthquakes have been documented worldwide. Although it is clear that fluid pressure triggers fault slip and results in induced earthquakes, the possibility of controlling induced earthquakes through managed metering of water injection has not been rigorously examined. We performed experiments with single and cycled injection into laboratory faults. Our results show that cycled water injection into a fault could successfully reduce both the maximum magnitude of the individual induced seismic events and the total radiated energy—by setting appropriate water injection schedules. The magnitude and number of induced events are dependent on the number of water injection cycles and rock permeability. These results may provide a better understanding of managing and preventing large induced earthquakes. Key Points: Water injection results in early occurrence of the fault seismic slip for both single and cycled injectionCycled water injection could reduce both the maximum magnitude of the individual induced seismic events and the total released energyBoth maximum moment and total energy decrease with an increasing number of water injection cycles [ABSTRACT FROM AUTHOR]

We investigate the relative importance of injection and production on the spatial‐temporal distribution of induced seismicity at the Raft River geothermal field. We use time‐series of InSAR measurements to document surface deformation and calibrate a hydro‐mechanical model to estimate effective stress changes imparted by injection and production. Seismicity, located predominantly in the basement, is induced primarily by poroelastic stresses from cold water reinjection into a shallower reservoir. The poroelastic effect of production from a deeper reservoir is minimal and inconsistent with observed seismicity, as is pore‐pressure‐diffusion in the basement and along reactivated faults. We estimate an initial strength excess of ∼20 kPa in the basement and sedimentary cover, but the seismicity rate in the sedimentary cover is four times lower, reflecting lower density of seed‐points for earthquake nucleation. Our modeling workflow could be used to assess the impact of fluid extraction or injection on seismicity and help design or guide operations. Plain Language Summary: It is important to understand the mechanisms behind human‐induced earthquakes, whether they are beneficial in the context of generating fractures for effective geothermal energy systems, or hazardous in the case of large earthquakes that may cause structural damage. Here, we present a case study of the Raft River geothermal field, which has operated since 2007 and generated earthquakes since 2010. Our objective is to understand how hot water, which is extracted from a deep sedimentary layer, and cold water, which is injected into a shallower layer, contribute to the observed seismicity, which have primarily occurred underneath the extraction layer. We construct a model to determine how the injection and extraction affect underground fluid pressures, and how these pressures impart stresses throughout the subsurface. This model is calibrated using InSAR, a satellite‐based technique which provides precise measurements of the Earth's surface deformation in time. The results show that the shallow cold water injection is the main culprit behind deep earthquakes, because of the stresses imparted by the fluid pressure increase, whereas the effects of deep extraction is negligible. Our modeling workflow could be used to assess the impact of fluid extraction or injection on seismicity and help guide operations. Key Points: A hydro‐mechanical model of effective stress changes from injection and production is calibrated using surface deformation from InSARSeismicity, located predominantly in the basement, is primarily induced by poroelastic stress changes from cold water re‐injectionSimilar stress changes trigger four times more earthquakes per unit volume in the basement than in the sedimentary cover [ABSTRACT FROM AUTHOR]

The Mw 7.2 El Mayor-Cucapah (EMC) earthquake ruptured a complex fault system in northern Baja California that was previously considered inactive. The Cerro Prieto Geothermal Field (CPGF), site of the world's second largest geothermal power plant, is located approximately 15 km to the northeast of the EMC hypocenter. We investigate whether anthropogenic fluid extraction at the CPGF caused a significant perturbation to the stress field in the EMC rupture zone. We use Advanced Land Observing Satellite interferometric synthetic aperture radar data to develop a laterally heterogeneous model of fluid extraction at the CPGF and estimate that this extraction generates positive Coulomb stressing rates of order 15 kPa/yr near the EMC hypocenter, a value which exceeds the local tectonic stressing rate. Although we cannot definitively conclude that production at the CPGF triggered the EMC earthquake, its influence on the local stress field is substantial and should not be neglected in local seismic hazard assessments. Key Points Geothermal energy production causes surface subsidence and crustal stressingProduction at CPGF generates positive Coulomb stresses in EMC rupture zoneAnthropogenic stresses exceed the tectonic loading rate at EMC hypocenter

Earthquakes were induced in the Fort Worth Basin from 2008 through 2020 by increase in pore pressure from injection of oilfield wastewater (SWD). In this region and elsewhere, a missing link in understanding the mechanics of causation has been a lack of comprehensive models of pore pressure evolution (ΔPp) from SWD. We integrate detailed earthquake catalogs, ΔPp, and probabilistic fault slip potential (FSP) and find that faults near large‐scale SWD operations became unstable early, when ΔPp reached ∼0.31 MPa and FSP reached 0.24. Faults farther from SWD became unstable later, when FSP reached 0.17 and at much smaller ΔPp. Earthquake sequences reactivated with mean ΔPp of ∼0.05 MPa. The response of faults shows strong variability, with many remaining stable at higher ΔPp and few that became seismogenic at smaller changes. As ΔPp spread regionally, an ever‐increasing number of faults were impacted and the most sensitive became unstable. Plain Language Summary: Increases in subsurface fluid pressure following years of deep disposal of oilfield wastewater are widely accepted to have caused earthquakes from 2008 through 2020 in the Fort Worth Basin (FWB) of north‐central Texas, underlying the population centers of the Dallas‐Fort Worth metropolitan area, were caused by increases in subsurface fluid pressure following years of deep disposal of oilfield wastewater. No earthquakes were reported in the area prior to 2008. Increasing the fluid pressure in subsurface reservoirs containing faults can cause the faults to become unstable and slip, producing earthquakes. A missing link to understanding the cause of these earthquakes is not knowing how much the fluid pressures have increased, and which geographic areas have been affected. This research combines several lines of evidence to show how the fluid pressure from deep injection increased over time and space leading to the increase in earthquakes. The changes in fluid pressure that made the faults unstable is surprisingly low, the subsurface region affected by the pressure change is large, and the natural sensitivity of the faults has significant variability. This work provides a critical component to understanding the specific conditions leading to induced seismicity in the FWB, which can be applied to similar cases elsewhere. Key Points: Onset of induced earthquakes in the Fort Worth Basin occurred with pore pressure increases of ∼0.01–0.36 MPa from wastewater injectionPore pressure increase in the slip potential of the seismogenic faults indicates that the instability threshold is 17%–24% for this systemSlip potential of faults that became seismogenic is similar to those that have not ruptured, indicating inherent differences in sensitivity [ABSTRACT FROM AUTHOR]

Aftershock cascades play an important role in forecasting seismicity in natural and human‐made situations. While their behavior including the spatial aftershock zone has been the focus of many studies in tectonic settings, this is not the case when fluid flows are involved. Using high‐quality seismic catalogs, we probe aftershocks dynamics in five settings influenced by fluids: (a) induced seismicity in Oklahoma and Kansas, (b) natural swarms in California and Nevada, and (c) suspected swarms in the Yuha Desert (California). All settings exhibit significant aftershock behavior highlighting the importance of event‐event triggering processes. The spatial aftershock zones scale with mainshock magnitude as expected based on the rupture length. While (a) and (b) show a rapid decay beyond their rupture length, (c) exhibits long‐range behavior suggesting that fluid migration might not be the dominant mechanism. We also find that the scaling of aftershock productivity with mainshock magnitude together with the Gutenberg‐Richter b‐value might allow to distinguish between natural swarms and induced seismicity. Plain Language Summary: While it is known that fluid injection operations can induce seismic activity, it has remained unclear how this activity compares to their natural counterpart, seismic swarms driven by natural fluid flows. The latter are typically characterized by the absence of a dominant event within the seismic sequence, while exhibiting other characteristics consistent with tectonic sequences including aftershock triggering. Our analysis of high‐quality seismic catalogs for both types of fluid‐driven seismicity shows that both exhibit a significant amount of aftershocks arising from "secondary" processes (i.e. stress‐based event‐event triggering as an indirect consequence of fluid injections) leading to spatially localized aftershocks zones. Yet, the trade‐off between the seismic productivity relation, which refers to the average increase in the number of aftershocks with the magnitude of their trigger, and the distribution of earthquake magnitudes controls the relative role of small compared to large triggers and we find that aftershock triggering is much more dominated by smaller events in the induced setting. Both findings are of direct importance for earthquake forecasting and seismic hazard assessment. Key Points: Significant event‐event triggering is present in both natural swarms and induced seismicityBoth fluid‐driven settings are characterized by narrow aftershock zonesAftershock triggering is dominated by smaller triggers in induced seismicity but much less so for natural swarms [ABSTRACT FROM AUTHOR]

Pore pressure diffusion along faults influences induced seismicity and rupture mechanics. In situ hydraulic diffusivity measurements along faults are rare and generally lower than inferred from seismicity migration. Here we use the tidal response of deep geothermal boreholes to measure fault diffusivity and permeability. Initial interpretations of the observation with a homogeneous confined aquifer model result in diffusivities of 10−3–10−1 m2/s. However, this model mixes signals from both the conduit and the host rock. We develop a model for tidal response with a fault passing through the aquifer based on the fault‐guided fracture network and solve for hydraulic properties in both the fault and the host rock. The resulting fault permeability is 2 × 10−14–7 × 10−14 m2 (90% confidence interval[CI]) and fault diffusivity is 0.08–0.33 m2/s (90% CI), which is 2 orders of magnitude higher than the host rock diffusivity in some wells, thus highlighting the role of faults as fluid conduits. Plain Language Summary: Hydraulic diffusivity is a critical factor determining how fast the fluid can flow and is necessary and useful for studying induced seismicity. However, measuring hydraulic properties of a fault system has long been a problem for hydrogeologists. As laboratory experiments may not be appropriate for large‐scale fault system studies, analyzing field data would be necessary for further research. In this study, we use tidal response to analyze our field data and determine hydraulic properties under a homogeneous isotropic aquifer model and extend our study to a fault‐guided model to better incorporate geological knowledge. This new model shows that consideration of heterogeneity brings the tidal response measurements more in line with inferences based on induced seismicity, but a discrepancy remains. Key Points: A new model with a fault‐guided hydrogeological channel is used to measure host rock and fault damage zone hydraulic propertiesFault diffusivity estimates between 0.08 and 0.33 m2/s are slightly higher than previous tidal forcing analysesA fault‐guided model is required for inferring diffusivity in systems with large difference between fault and host rock diffusivity [ABSTRACT FROM AUTHOR]

Native and fluid circulation-induced metamorphic products may affect the stability of faults in geothermal reservoirs-particularly epidote (Ep) and chlorite (Cl). Our laboratory experiments conducted at hydrothermal conditions show that these two minerals, when precipitated on natural faults, may promote fault instability or failure under conditions typifying a geothermal reservoir. Shear experiments on Ep-rich fault gouges indicate potentially unstable frictional behavior--more pronounced at elevated temperatures and pore fluid pressures--and indicate increased potential for the nucleation of slip instability relative to that of the host granodiorite gouge. Experimental results indicate in particular that Ep can increase the potential for induced seismicity on existing faults. Increased proportions of Cl in gouges stabilize the faults but apparently lower the frictional strength. Our results reveal that the metamorphic minerals exert a subtle but potentially significant control on fault strength and stability in geothermal reservoirs. [ABSTRACT FROM AUTHOR]

Rock materials show dramatic dynamic weakening in large-displacement (m), high-velocity (~1 m/s) friction experiments, providing a mechanism for the generation of large, natural earthquakes. However, whether such weakening occurs during induced M3-4 earthquakes (dm displacements) is unknown. We performed rotary-shear experiments on simulated fault gouges prepared from the source-, reservoir- and caprock formations present in the seismogenic Groningen gas field (Netherlands). Watersaturated gouges were subjected to a slip pulse reaching a peak circumferential velocity of 1.2-1.7 m/s and total displacements of 13-20 cm, at 2.5-20 MPa normal stress. The results show 22%-81% dynamic weakening within 5-12 cm of slip, depending on normal stress and gouge composition. At 20 MPa normal stress, dynamic weakening from peak friction coefficients of 0.4-0.9 to 0.19-0.27 was observed, probably through thermal pressurization. We infer that similar effects play a key role during induced seismic slip on faults in the Groningen and other reservoir systems. Plain Language Summary During large, natural earthquakes, rapid sliding motion on faults through several meters of displacement generates large amounts of frictional heat. This triggers mechanisms that cause crushed fault rocks to weaken drastically, allowing the rupture to propagate, and the earthquake to grow. However, whether this occurs during relatively small, human-induced seismicity, caused by hydrocarbon production or fluid injection in the subsurface, for example, is unknown. Here we simulate seismic fault slip during magnitude 3 to 4 earthquakes, using crushed rock materials derived from the seismogenic Groningen gas field (Netherlands). Our experiments show that even for shortdisplacement (dm), representative for such induced earthquakes, the fault rocks weaken significantly (22%-81%), depending on mineralogy and on the normal stress acting on the fault. These findings provide insights for the larger induced events recorded in Groningen (magnitude up to 3.6), and improve hazard assessment capabilities using numerical models. [ABSTRACT FROM AUTHOR]

Seismic hazard forecasts of induced seismicity often require estimates of the maximum possible magnitude (Mmax). Empirical models suggest that maximum magnitudes, or expected number of earthquakes, are related to the volume of injected fluid. We perform a suite of 3D physics-based earthquake simulations with rate- and state-dependent friction, systematically varying the area of the pressurized region and the amplitude of the initial homogeneous or heterogeneous shear stress. Using the resulting catalog we explore the conditions that result in pressure-controlled versus runaway ruptures that extend outside the pressurized zone. We find that proposed empirical scaling laws correctly predict Mmax when shear stresses are further from failure (=90% of maximum shear stress) and for high amplitude stress fields. Runaway ruptures are observed for higher initial shear stresses and smoother stress fields. In these cases, runaway ruptures occur early after the onset of injection and rarely preceded by foreshock activity. Plain Language Summary Industrial operations like wastewater disposal, geothermal energy production, and carbon sequestration that manipulate the subsurface cause earthquakes. Most earthquakes associated with such activities are small and imperceptible to humans. However, in some cases, earthquakes are larger and damaging. Currently, it is unclear how and why some earthquakes are larger than others. In this study, we use numerical simulations to systematically explore the stress and pressure conditions that distinguish "pressure-controlled" (and generally smaller) earthquakes from larger "runaway ruptures." Additionally, we investigate the conditions where previously proposed scaling relationships are validated and classify situations where these relationships break down. We find that scaling laws are most applicable when the stresses on fault surfaces are low or highly heterogeneous. Runaway ruptures generally occur early after operations begin and may not be preceded by foreshocks. [ABSTRACT FROM AUTHOR]

Geoenergy and geoengineering applications usually involve fluid injection into and production from fractured media. Accounting for fractures is important because of the strong poromechanical coupling that ties pore pressure changes and deformation. A possible approach to the problem uses equivalent porous media to reduce the computational cost and model complexity instead of explicitly including fractures in the models. We investigate the validity of this simplification by comparing these two approaches. Simulation results show that pore pressure distribution significantly differs between the two approaches even when both are calibrated to predict identical values at the injection and production wells. Additionally, changes in fracture stability are not well captured with the equivalent porous medium. We conclude that explicitly accounting for fractures in numerical models may be necessary under some circumstances to perform reliable coupled thermohydromechanical simulations, which could be used in conjunction with other tools for induced seismicity forecasting. Plain Language Summary: The subsurface will play an important role in decarbonizing the economy. The transition to carbon neutrality can be accelerated by utilizing geothermal energy, returning carbon underground, and storing energy in the subsurface to offset the fluctuations in production of renewables. These low‐carbon geoenergy technologies oftentimes deal with fractured rock masses. To minimize the risks related to geoenergy projects, numerical simulations are performed to predict the response of the subsurface to fluid injection and production. Given the complexity and high computational cost of simulating fractures, fractured rock is usually treated as an equivalent porous medium. Here we investigate the validity of this simplification by comparing these two approaches. We find that even though an equivalent porous medium can reproduce the fluid pressure changes at wells, the pressure distribution within the fractured media significantly differs between the two approaches. Equivalent porous media fail to reliably predict the rock behavior when fracture spacing is within the order of the reservoir size and computer simulations should explicitly include fractures to provide reliable forecasts and reduce the risks of induced seismicity. The latter is necessary to enable a successful deployment of geoenergies to mitigate the climate crisis. Key Points: Equivalent porous media can reproduce pressure and temperature evolution at wells but not their distributions within the fractured mediaConsideration of explicit fractures affects stability and induced seismicity in a way that cannot be represented by equivalent mediaSimulating thermo‐hydro‐mechanical processes requires explicit inclusion of fractures whenever they add inhomogeneities at reservoir scale [ABSTRACT FROM AUTHOR]

Like natural seismicity, induced seismicity caused by injection also shows a power law size distribution, and its gradient b‐value is used for seismic hazard analysis. Despite the well‐known result that b‐value is negatively correlated with differential stress for natural earthquakes, there is no similar correlation for b‐value variations because the differential stress is nearly constant for injection‐induced seismicity, we thus investigate the b‐value dependence on the relative shear stress acting on existing fractures, using the fault orientation and in situ stress information. The seismicity occurring along existing fractures having high shear stress has significantly lower b‐values than does that associated with lower shear stress fractures. We examine the b‐value dependency on slightly changing differential stress, but the relationship is not clear. The b‐value for injection‐induced seismicity is dependent on relative shear stress on faults, which is a novel physical explanation for the b‐value variations of induced seismicity. Plain Language Summary: Frequency magnitude distribution of a series of natural earthquakes and laboratory earthquakes correlates with the applied stress. However, the variation of the frequency magnitude distribution for injection‐induced seismicity has not been understood well since the stress state is rather constant in reservoir scale. This study investigates the stress state of the faults that caused the injection‐induced seismicity. We found that the events that occurred from the fault that oriented to have relatively higher shear stress caused the b‐value reduction. This finding provides the new perspective of the scaling law of frequency magnitude distribution for induced seismicity. This insight leads to seismic hazard mitigation due to fluid injection. Key Points: We estimate shear stresses on the faults from in situ stress and fault orientation, and study the correlation to magnitude and b‐valueb‐value negatively correlates with the shear stress rather than differential stressShear stress is the cause of low b‐value for injection‐induced seismicity case [ABSTRACT FROM AUTHOR]

The offshore Castor Underground Gas Storage (UGS) project had to be halted after gas injection triggered three M4 earthquakes, each larger than any ever induced by UGS. The mechanisms that induced seismicity in the crystalline basement at 5–10 km depth after gas injection at 1.7 km depth remain unknown. Here, we propose a combination of mechanisms to explain the observed seismicity. First, the critically stressed Amposta fault, bounding the storage formation, crept by the superposition of well‐known overpressure effects and buoyancy of the relatively light injected gas. This aseismic slip brought an unmapped critically stressed fault in the hydraulically disconnected crystalline basement to failure. We attribute the delay between induced earthquakes to the pressure drop associated to expansion of areas where earthquakes slips cause further instabilities. Earthquakes occur only after these pressure drops have dissipated. Understanding triggering mechanisms is key to forecast induced seismicity and successfully design deep underground operations. Plain Language Summary: Underground fluid injection for energy‐related activities usually induces microseismicity (not felt earthquakes). But felt earthquakes are sometimes induced, which may cause public concern and project cancellation. This was the case of the offshore Castor underground gas storage (UGS), Spain. Gas injection induced numerous seismic events, including three with magnitudes around 4, larger than any other earthquake ever induced by UGS. These earthquakes occurred after the stop of injection in the crystalline basement, which is hydraulically disconnected from, and significantly deeper than the storage formation. To explain this seismicity, we propose a combination of mechanisms. First, the Amposta fault, bounding the site, crept (failed slowly, aseismically) because of pore pressure buildup induced by injection, which reduces the effective compression stabilizing faults, and stress variations caused by gas buoyancy. Aseismic slip of the Amposta fault provoked underground displacements, which reactivated a critically stressed unmapped fault in the crystalline basement. We attribute the delay between earthquakes of the sequence to rock expansion, and stabilizing pore pressure drop, in areas where slip displacement might cause new earthquakes. Earthquakes occur only after the pressure drop dissipates, which takes some time. Assessing fault stability prior to gas injection would have identified the risk of inducing seismicity. Key Points: Gas injection, through both pore pressure buildup and buoyancy, reactivated mainly aseismically the critically stressed Amposta faultShear slip stress transfer due to creep of the Amposta fault reactivated an unmapped critically stressed fault in the crystalline basementFelt earthquakes did not occur until pore pressure reductions had dissipated where shear slip stress transfer led to worsened stability [ABSTRACT FROM AUTHOR]

We estimate vertical rotation rates for 204 earthquakes that were induced by the 2018 stimulation of the Espoo/Helsinki geothermal reservoir from wavefield gradients across geophone arrays. The array‐derived rotation rates from seismograms recorded at 6–9 km hypocentral distances vary between 10−9 and 10−7 rad s−1, indicating a comparable sensitivity to portable rotational instruments. Using co‐located observations of translational and rotational motion, we estimate the local propagation direction and the apparent phase speed of SH waves, and compare these estimates with those obtained by S wave beamforming. Propagation directions generally align with the earthquake back azimuths, but both techniques show deviations indicative of heterogeneous seismic structure. The rotational method facilitates a station‐by‐station approach that resolves site specific variations that are controlled by the local geology. We measure apparent S wave speeds larger than 5 km s−1, consistent with steep incidence angles and high propagation velocities in the Fennoscandian Shield. Plain Language Summary: Earthquakes generate seismic waves consisting of both translational (back‐and‐forth) and rotational ground motion. Translational motion is routinely measured by standard seismometers, but the observation of the rotational motion requires relatively expensive and rare instruments. In this study we estimate rotational ground motion caused by earthquakes using groups of translational seismometers. The computation of rotational motion from translational seismometers has been demonstrated before, but the novelty of our study is to use high‐quality recordings of earthquakes that were induced by the creation of a geothermal reservoir at 6 km depth in bedrock. We use our measurements of ground rotation to estimate the speed and direction in which the seismic waves are travelling when they reach the seismometers. We find that the direction in which the seismic waves travel usually points back to the earthquake location, but at some seismometers the waves arrive from a different direction. At these locations, it is likely that local geological features are altering the direction of the waves. We expect that our findings will provide access to approaches for determining earthquake characteristics and Earth structure that currently require highly specialized instruments. Key Points: Wavefield gradiometry is applied to arrays of translational seismometers to estimate rotational motions from induced earthquakesHigh‐quality records of M0.0 to M1.8 events obtained within 10 km hypocentral distance facilitate array‐based resolution of rotation ratesThe obtained apparent phase speed and wave propagation directions are linked to subsurface structure [ABSTRACT FROM AUTHOR]

The M5 Mentone earthquake that occurred on March 26, 2020, was the largest event recorded over the last 2 decades in West Texas within the Delaware Basin, a U.S. major petroleum‐producing area. Also, numerous hydrofracturing and wastewater disposal wells are spread across this region. Within a 30 km distance to mainshock, eight class‐II injection wells for industrial wastewater disposal target the deep porous Ellenburger aquifer at an average rate of 1.36 × 106 barrel (BBL) per month during 2012–2020. Poroelastic models of fluid diffusion show these nearby injectors collectively imparted up to 80.5 kPa of Coulomb stress at the mainshock location, capable of triggering this M5 event. Assuming the Mentone event occurs when pore‐pressure increase is maximum, the time delay between peak injection and the M5 occurrence corresponds with an optimal permeability of 6.76 × 10‐14 m2 for the Ellenburger aquifer layer, in agreement with independent estimates. Plain Language Summary: On March 26, 2020, a M5 earthquake took place near the town Mentone in West Texas, located within one of the largest petroleum‐producing areas in the United States. This area also contains injection sites designated for disposing of wastewater coproduced from unconventional hydrocarbon production processes. Our study found that this earthquake could be triggered by fluid diffusion from the nearby disposal wells. Seismicity's timing and location are consistent with injection occurring in an intermediately permeable aquifer and are indicators for individual wells' operation. These provide essential information to help industrial practitioners and regulatory agencies to assess the associated seismic hazard, implement mitigation measures, and draft policies or guidelines for disposal well management. Key Points: Fluid diffusion from nearby injection wells is likely the primary triggering mechanism for the M5 Mentone earthquakeTwo nearby injectors could individually impart adequate Coulomb stress to induce the M5 eventTime delay between injection and seismicity is used to constrain the mechanical properties of the aquifer unit [ABSTRACT FROM AUTHOR]

Geothermal resources offer a stable low‐carbon energy source. However, geothermal sites can collocate with the hypocenters of large‐magnitude seismic events. Large seismic events pose a risk to public safety and are therefore a liability for efforts to develop geothermal resources. Here, we propose "fracture caging" to limit induced seismic event magnitudes and present evidence from numerical model predictions, laboratory experiments, and field observations. Fracture caging involves drilling tactical production wells around a geothermal injection zone to contain fluids in fracture‐dominated flow systems. Prior to our work, the effect of small wells on the growth of large fractures and on flow through fractures was subject to debate. Our work shows that production wells can impede fracture growth and contain high‐pressure fluids in fracture‐dominated rocks. This containment offers a mechanism to limit induced seismicity. Plain Language Summary: Below the ground's surface, fluid can lubricate rocks which enables them to more easily slip and generate earthquakes. This is a problem for developing geothermal energy resources that require long‐term injection of water. If water could be injected without risk of earthquakes, geothermal energy production could be expanded to supply more than 60 GW of electricity in the United States alone. To confront the earthquake risk, we propose that tactical arrangements of production wells around geothermal injection wells can be used to prevent large seismic events. We refer to this arrangement as "fracture caging" because it contains injected fluid in a limited volume of fractured rock in order to prevent uncontrolled stimulation of the larger fractures that generate earthquakes. Prior to this work, we incorrectly assumed that small wells (e.g., 0.3 m diameter) could not inhibit the growth of large fractures (e.g., greater than 100 m diameter). We also incorrectly assumed that these wells could not contain fluids in the dynamic and uncertain fracture networks that are common in natural rocks. However, our models, experiments, and field data now show that fracture caging can work to contain injected fluids and thereby also limit induced seismicity, even in complex natural rock. Key Points: A production well cage can enable high‐pressure fluid‐injection simultaneously with closed‐loop flow‐containment in fractured rockModel, laboratory, and field evidence indicate that fracture caging can reduce induced seismic magnitudes despite natural complexityA cage of production wells improves subsurface flow control and adaptability to transient and dynamic fracture networks [ABSTRACT FROM AUTHOR]

We show that seismic attenuation (QS−1) along the San Andreas fault (SAF) at Parkfield correlates with the occurrence of moderate‐to‐large earthquakes at local and regional distances. Earthquake‐related QS−1 anomalies are likely caused by changes in permeability from dilatant static stress changes, damage by strong shaking from local sources, and pore unclogging/clogging from mobilization of colloids by dynamic strains. We find that, prior to the 2004 M6 Parkfield earthquake, prefailure conditions for some local events of moderate magnitude correspond to positive anomalies of QS−1 on the Pacific side, with local and regional earthquakes producing sharp attenuation reversals. After the 2004 Parkfield earthquake, we see higher QS−1 anomalies along the SAF, but low sensitivity to local and regional earthquakes, probably because the mainshock significantly altered the permeability state of the rocks adjacent to the SAF, and its sensitivity to earthquake‐induced stress perturbations. Plain Language Summary: We discovered that along the San Andreas fault, the damping of seismic energy through the Earth's crust is modulated by the state of stress in the crust, especially when the fault is close to rupture. The phenomenon depends on the density of cracks that permeate the rock, their interconnection, and the degree that they are filled with fluids. We show examples where attenuation, and thus permeability, is modulated by (i) damage from local earthquake shaking, (ii) dilatation imposed by local earthquakes, and (iii) clogging and unclogging of cracks induced by ground motion from distant earthquakes. These examples correlate with observed water well level changes. We note significant changes to the attenuation signal after the 2004 M6.0 Parkfield earthquake, with less sensitivity to local and distant earthquakes. Key Points: Variations in seismic attenuation can be interpreted in terms of changes in crustal permeabilityPrefailure conditions along the San Andreas fault correspond to clear positive anomalies in seismic attenuationSeismic attenuation may be used as a crustal stress meter [ABSTRACT FROM AUTHOR]

Abstract In the southern Sichuan Basin, China, the recent increase in the seismic activity has been suspected to be related to hydraulic fracturing stimulation for producing the shale gas. In this study, we used the monitoring data from a local seismic network within the shale gas blocks to study the earthquakes near the shale gas production wells that have detailed injection data. Comparison of the timing of earthquakes and stimulation schedule of the studied well pads indicates an apparent correlation between the seismic activity and hydraulic fracturing. The results of seismic velocity tomography reveal that the reactivation of preexisting faults due to fluid diffusion is the primary cause of the observed earthquakes. Focal mechanism analysis combined with geomechanical modeling indicates that the increased pore pressures resulted from hydraulic fracturing are sufficient to trigger seismic slip on the faults.

We analyze data from 6,466 multistage horizontal hydraulic fracturing wells drilled into the Montney Formation over a large region in western Canada to evaluate the impact of geological, geomechanical, and tectonic characteristics on the distribution of hydraulic fracturing‐induced seismicity. Logistic regression was used to obtain a machine learning estimate of the seismogenic activation potential of each well. Our results fit the observed spatial variability, including an enigmatic change in seismicity at 120°W that does not correlate with any change in industrial activity. Feature importance analysis provides insight into data types that have the greatest impact on the results. Based on current data, seismogenic activation potential is most strongly influenced by depth of injection and distance of the well to the Cordilleran thrust belt. Plain Language Summary: A supervised machine learning model is applied within a zone of oil and gas development in western Canada to determine areas where hydraulic fracturing operations are more likely to induce earthquake activity. Geological characteristics are investigated to determine factors that exert the greatest influence on earthquake triggering during or within 3 months after hydraulic fracturing. Our results indicate that an enigmatic change in seismicity patterns at 120°W arises from a combination of parameters, including distance to the northern Canadian Rockies. These inferences may be useful for a general understanding of localization of induced seismicity. Key Points: A supervised learning method reveals seismogenic activation potential for a Canadian shale play in response to hydraulic fracturingThe results match an enigmatic, abrupt change in the distribution of seismicity across a provincial boundaryThe most influential parameters are distance to the Cordilleran deformation front and injection depth [ABSTRACT FROM AUTHOR]