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Induced seismicity poses a challenge to the development of Enhanced Geothermal Systems (EGS). Improving monitoring and forecasting techniques is essential to mitigate induced seismicity and thereby fostering a positive perception of EGS projects among local authorities and population. Induced seismicity is the result of complex and coupled thermo-hydro-mechanical-chemical mechanisms. Injection flux and pressure are crucial controlling parameters for both hydraulic stimulation and circulation protocols. We develop a methodology combining a hydro-mechanical model with a seismicity rate model to estimate the magnitude and frequency of mainshocks and aftershocks induced by fluid injection. We apply the methodology to the case of the Basel EGS (2006, Switzerland) to compare the effects of progressive, cyclic and constant injections on the mechanical response of discrete faults. Results from the coupled hydro-mechanical models show that the pore pressure diffusion and consequent enhancement of fault permeability are limited to the vicinity of the injection well during cyclic injection. Additionally, constant injection induces seismicity from the start of the injection but enhances the permeability of most of the faults within a shorter duration, inducing less post-injection seismicity. The methodology can be adapted to any numerical model and allows new projects to be developed by anticipating the safest injection protocol.This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'.

SUMMARY: As the high-frequency analogue to field-scale earthquakes, acoustic emissions (AEs) provide a valuable complement to study rock deformation mechanisms. During the load-stepping creep experiments with CO2-saturated water injection into a basaltic sample from Carbfix site in Iceland, 8791 AE events are detected by at least one of the seven piezoelectric sensors. Here, we apply a cross-correlation-based source imaging method, called geometric-mean reverse-time migration (GmRTM) to locate those AE events. Besides the attractive picking-free feature shared with other waveform-based methods (e.g. time-reversal imaging), GmRTM is advantageous in generating high-resolution source images with reduced imaging artefacts, especially for experiments with relatively sparse receivers. In general, the imaged AE locations are found to be scattered across the sample, suggesting a complicated fracture network rather than a well-defined major shear fracture plane, in agreement with X-ray computed tomography imaging results after retrieval of samples from the deformation apparatus. Clustering the events in space and time using the nearest-neighbour approach revealed a group of ‘repeaters’, which are spatially co-located over an elongated period of time and likely indicate crack, or shear band growth. Furthermore, we select 2196 AE events with high signal-to-noise-ratio (SNR) and conduct moment tensor estimation using the adjoint (backpropagated) strain tensor fields at the locations of AE sources. The resulting AE locations and focal mechanisms support our previously assertion that creep of basalt at the experimental conditions is accommodated dominantly by distributed microcracking.

Stiffness degradation of track–bridge systems under earthquake excitations can lead to track dynamic irregularity, impacting the safety of train operations post-earthquake. In this paper, the nonlinear time-history analysis of CRTS II track–bridge system model established by ANSYS finite element analysis software is carried out under the action of transverse random earthquake. The train–track–bridge system model considering the stiffness degradation caused by earthquake is established by MATLAB, and the earthquake-induced track dynamic irregularity sample library is constructed. The probability distribution mode of the power spectral density sample of the earthquake-induced track dynamic irregularity is explored and the calculation method based on the probability guarantee rate is proposed. The influence of structural damping ratio, train running speed and train type on the power spectral density curve of the earthquake-induced track dynamic irregularity is analyzed. The results show that the power spectrum samples of track dynamic irregularity conform to the hypothesis test of a normal distribution. The power spectral density of earthquake-induced dynamic irregularity primarily consists of medium and low-frequency components. When the structural damping ratio increases from 0.03 to 0.04, 0.04 to 0.05, 0.05 to 0.06, and 0.06 to 0.07, the variation gradients are 0.1701, 0.1240, 0.1034 and 0.0999, respectively, which indicates that the structural damping ratio has a significant effect on the power spectral density of earthquake-induced irregularity, and the impacts of train speed and train type on the power spectral density of near-fault earthquake-induced irregularity are minimal. [ABSTRACT FROM AUTHOR]

This paper offers a comprehensive re-analysis of the Beni-Ilmane 2010 seismic sequence, using a data set that is 100 per cent larger than previous studies. This unprecedented sequence in Algeria features three main shocks with magnitudes M w 5.4, 5.1 and 5.1. Our approach involves high-precision relocation, which includes the development of a new 1-D minimum velocity model, followed by a double-difference (DD) procedure and hierarchical clustering. We determined the focal mechanisms (FMs) for 128 key events and identified 21 multiplet groups using an average cross-correlation threshold of 0.8. Our analysis offers new insights into fault geometry and addresses ongoing debates, by proposing a seismotectonic model that reveals the activation of 14 fault segments during the sequence, in contrast to previous oversimplified models that suggested two or three faults. The computed stress field from the inversion of 128 FMs aligns with a tectonic loading force due to the convergence of the African and Eurasian plates. These findings highlight the complexity of the fault network in the study area and shed light on the role of strike-slip faults in shaping the thrust belt. We found a strong link between multiplet groups and fluid movement along the fault network. Analysis of the temporal history of these multiplet groups provides new insights into fluid dynamics timescales, with an estimated hydraulic diffusivity (D) of 0.36 m2 s−1 suggesting a fluid pressure diffusion process. The observed expansion of the aftershock area with the logarithm of time and the existence of repeating earthquakes indicates, for the first time, an aseismic slip mechanism that adds an additional layer to the driven processes. In conclusion, our results suggest that the underlying mechanisms governing the BI-2010 seismic sequence involve a complex interplay of tectonic loading, coseismic stress transfer, fluid dynamics and aseismic slip transients. We attempt to correlate our findings with various studies linking the structure, mechanics and fluid flow properties of fault zones and fault systems. The activation of smaller fault segments potentially averted a larger quake, resulting in three moderate main shocks and numerous aftershocks. This work not only enrich our understanding of seismic phenomena but also provides useful insights for seismic hazard assessment and risk mitigation strategies. [ABSTRACT FROM AUTHOR]

Classifying the source type of small seismic events is a key task in seismology. A common goal is distinguishing tectonic earthquakes from explosions and human induced seismicity. To this end, we applied a spectral modelling workflow to Pg and Sg waves from ∼10   000 seismic events that occurred in or near Utah and were recorded by broadband seismometers in the western United States at distances of 10–300 km. The events were a mixture of tectonic earthquakes (EQ), industrial explosions (EX) and mining-induced seismicity (MIS, primarily collapses) and were mostly small (median magnitude of 1.34 M C). Our spectral modelling was successful for 54 per cent of the events, resulting in a new catalogue of M 0 and f c values. We evaluated 13 physics-based features—including differential magnitudes, Pg / Sg spectral amplitude ratios, long-period/short-period spectral amplitude ratios and spectral misfit—as source classifiers. We found that Φ ≡ log10(M 0) + 3log10(f c) was the most effective individual feature for distinguishing EQ from EX and MIS sources because EQ spectra are relatively enriched in high frequencies. We selected five less correlated features that spanned the feature space and used a naïve Bayes approach to create a three-way classification model. The model had 97.5 per cent accuracy when applied to an independent test data set. Model performance deteriorated when more than six features were combined. We conclude that models developed with a few physics-based waveform features can classify small seismic events with performance comparable to high-dimensional deep-learning models. Simple models that rely on physics-based features require less training data and make more interpretable decisions than deep-learning models, though they may require higher signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

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]

In this paper, we performed microseismicity detection and location using the deep learning method and obtained a high-precision earthquake catalog in the Changning gas field, which is one of the largest shale gas production demonstration areas in China. We found that the spatial and temporal characteristics of seismicity in the region are indicative of its correlation with industrial operations. The distribution of earthquakes at depth reflected variations in reservoir depth and provided valuable constraints on it. The horizontal layered distribution of earthquakes at depth is due to the formation of fracture surfaces from interconnected fractures near the reservoir during hydraulic fracturing (HF) operations, which clearly demonstrates how HF operations impact seismicity. We suggested that the horizontally layered distribution was driven by two fundamental mechanisms: reactivation of pre-existing faults during HF and injection of high-pressure fluids into the reservoir, leading to fracture creation. Several ML ≥4 earthquakes, which did not occur on well-defined seismogenic faults, may have been triggered by pore elastic coupling resulting from regional stress accumulation and fluid injection. Significantly, the ML 4.9 seismic sequence occurring at the basement indicates that fracking has reactivated and promoted pre-existing faults, highlighting the need for further investigation into potential seismic hazards in the region. [ABSTRACT FROM AUTHOR]

The spatiotemporal pattern of surface displacements from large earthquakes provides crucial insights about the deformation of Earth’s crust at various scales and the interactions among tectonic plates. However, the lack of extensive and large-scale geodetic networks near such seismic events hinders our thorough understanding of the large-scale crustal deformation resulting from earthquakes. Using Türkiye’s extensive and continuous global navigation satellite system (GNSS) network during the moment magnitude 7.8 and 7.6 Kahramanmaraş earthquakes on 6 February 2023, we show that large earthquakes can induce far-field crustal deformations (>700 kilometers), exceeding current predictions from elastic dislocation models. They can lead to the mobilization of tectonic plates and the triggering of far-field earthquakes, which carries profound implications for seismic hazard assessments and necessitates a new perspective on crustal deformation and earthquake mechanics. [ABSTRACT FROM AUTHOR]

• Semi-automatic technique has been utilized using VHR imagery to develop landslides inventory. • Most prone transportation infrastructure has been identified using susceptibility analysis. • Temporal recovery of landslides based on the vegetation change has been evaluated. • Rainfall induced landslides has been identified using timeseries data. In mountainous terrains, the frequent landslides and their associated impacts on human lives and the economy is increasing globally. Development of landslide inventory and afterward landslide susceptibility mapping are the main prerequisites for implementing landslide mitigation measures and protection in mountainous regions. The 2005 Kashmir earthquake induced different small and large landslides and some were active for the long term. So far many studies have used medium and high-resolution data to develop landslide inventory. This study aims to develop a 1st detailed, comprehensive and accurate landslide inventory using a very high-resolution image using a semi-automatic technique. The precise landslide inventory is employed to develop an accurate and comprehensive landslide susceptibility map considering the landslide inventory data using a logistic regression training model. Furthermore, the landslide's temporal recovery from the earthquake and its reactivation due to rainfall in spare vegetation areas have been evaluated. Fine-resolution satellite images of Worldview-2 are applied to develop a detailed landslide inventory using a Support Vector Machine (SVM) classifier. A total of 63,630 landslides were identified using a semi-automatic technique within a study area of 265 km2. From regression modeling, the results show that geology, topography, and road networks have a significant impact on the spatial distribution of landslides. Model performance was evaluated based on the testing data, the model gives an AUC of 0.93 and the kappa value of 0.9353. The spatiotemporal NDVI has been assessed to identify the landslide recovery and its reactivation due to extreme rainfall. The results show that 72.1 % of the landslides occurred in Muzaffarabad formation in the study area. The developed landslide susceptibility map can be further used for land-use planning and implementing mitigation measures for the safety of roads and other infrastructure in the area. [ABSTRACT FROM AUTHOR]

Disposal of industrial wastewater and activities such as CO2 ${\mathrm{C}\mathrm{O}}_{2}$ underground sequestration depend upon pressure conditions within deep geologic reservoirs. Sometimes, injection and storage are associated with induced seismicity, suggested to result from reservoir compartmentalization, leakage into faults, or other mechanisms in the subsurface. To understand subsurface pressure conditions within a major regional disposal reservoir, the Arbuckle Group of Oklahoma, we monitored the water levels in 15 inactive injection wells. The wells were monitored at 30‐s intervals, with eight wells monitored since September 2016, and an additional seven from July 2017. All the wells were monitored until early March 2020. Since 2016, hydraulic head decreased in 13 of the 15 wells, proportional to near‐borehole fluid pressure even considering decreasing regional injection volumes during this period. The well pressures respond to three types of perturbations: (i) gravitational fluctuations (a.k.a. solid‐earth tides) (ii) distal and proximal earthquakes, and (iii) injections into nearby wells. Parameterization of tidal responses illustrates that the near wellbore environments have negative fluid flux (i.e., are leaking). Earthquakes cause differing pressure responses from well to well, with some highly sensitive to proximal events, some to distal events, and some apparently insensitive. Injections have variable impacts in some cases masking tidal and earthquake pressure signals. Collectively, there appears to be a threshold injection rate above which well pressure becomes less sensitive to the volume of injections within 15 km. Multi‐scale geological structure and temporal permeability changes are likely controlling the pressure field, indicating leakage of fluids across the system. Plain Language Summary: A common industry practice is to inject fluids that are produced during oil and gas extraction back into the ground. However, movement and exchange of the fluids in the subsurface can be difficult to monitor, and wastewater injection has led to an increase in earthquakes, also known as induced seismicity. Understanding the flow of these fluids is potentially instructive for geological sequestration of carbon‐dioxide to reduce greenhouse gas emissions, and subsurface storage of cleaner‐burning fuels such as hydrogen. Here, we highlight how pressure monitoring in wells shows how complex the subsurface is, with varying pressure changes over space and time. Moreover, the pressures responded to earthquakes and gravitational shifts from the orbits of the sun and moon which allow for temporal analysis. We investigate these changes in an attempt to better understand flow in the Arbuckle Group, a deep disposal reservoir associated with induced seismicity in the mid‐continental United States. Key Points: The fluid pressure in the Arbuckle Group of North‐Central Oklahoma is decreasing from 2016 to 2020Dynamic responses to stress fluctuations from solid earth tides and seismic waves vary significantly between different measuring wellsThere is a first order phase change in the pressure injection relationship above which nearby injection has a weaker effect on pressure [ABSTRACT FROM AUTHOR]

When designing anti-landslide measures, slope stability assessment is an important factor and is determined by conducting comprehensive engineering and geological studies on a slope with potential landslide risks. The results of correct engineering and geological justification for the design of anti-landslide measures lead to the preservation of the structure and safe operation of infrastructure facilities. Our experience shows that to eliminate the negative effects of landslide processes, it is necessary to accurately determine the engineering and geological conditions of the slope, including the stability of the slope, taking into account natural dynamic phenomena (seismicity of the territory and tectonic processes) and, if possible, to see the complete elimination of the causes that lead to landslide processes when developing anti-landslide measures. To achieve these goals, the geotechnical justification of the design is the only correct solution. Geotechnical justification of protective measures intended to protect the structural elements of the Baku-Russian Federation State Border highway, which is part of the north-south transport corridor and passes through the foot of a potentially landslide-hazardous slope west of the city of Shabran, is one of the most important conditions that increase the effectiveness of these measures. On a slope located 1500 meters southwest of the current research area, the results of our scientific research were not properly utilized and many of our warnings were not heeded, resulting in a landslide during construction, leading to serious damage to the road's structural elements (Fig. 1) and additional costs. This event once again confirms that landslides are the most common natural and man-made processes that pose a threat to the safe operation of infrastructure facilities, civil and industrial construction in mountainous and foothill areas (Shiraliyev et al., 2024). The purpose of this study is to determine the geotechnical conditions of the slope that is the object of research, analyze and assess the stability of the slope and provide geotechnical data for geotechnical justification for the design of landslide protection structures. Thanks to the application of the project solutions developed based on the results of the studies presented in the article, the danger of landslides on the mentioned slope has been completely eliminated and the safe operation of infrastructure facilities has been ensured. [ABSTRACT FROM AUTHOR]

Predicting ground motions due to induced seismicity is a challenging task owing to the scarcity of data and heterogeneity of the uppermost crust. Dealing with this requires a thorough understanding of the underlying physics and consideration of inter-site variability. The most common ground motion model used in practice is the parametric ground motion prediction equation (GMPE), of which hundreds exist in the literature. However, relatively few are developed with a focus on induced seismicity. Developing GMPEs that are specific to an appropriate magnitude-distance range ( R < 30 km; 2 ≤ M ≤ 6 ) is important for induced seismicity applications. This paper proposes a framework for the development of physically-based GMPEs to provide more accurate and reliable estimates of the potential induced-seismicity ground motion hazard, allowing for better risk assessment and management strategies. To demonstrate this approach, a new set of GMPEs for the 2018-2019 induced seismicity sequence at the Preston New Road (PNR) shale gas site near Blackpool, United Kingdom, is presented. The physically-based GMPE was developed based on a pseudo-finite-fault stochastic ground motion simulation, calibrated with parameters derived from the spectral analysis of weak-motion records from induced seismic events. An optimization-based calibration technique using the area metric (AM) was subsequently performed to calibrate optimal parameters for simulating ground motion at the PNR site. Finally, using a suite of forward simulations for events with 1 ≤ M ≤ 6 recorded at distances up to 30 km, combined with empirical data, a location-specific GMPE was derived through adjustment of an existing model. [ABSTRACT FROM AUTHOR]

The geological storage of carbon dioxide (CO2) represents a promising strategy for mitigating climate change by securely sequestering CO2 emissions. This review article aims to provide a comprehensive overview of the current state of research and development in the field of geological carbon dioxide (CO2) sequestration. We systematically examined a wide range of recent literature, focusing on advancements in numerical simulations, experimental studies, risk assessments, and monitoring techniques related to CO2 sequestration. Literature was selected based on relevance, recency, and contribution to the understanding of key challenges and solutions in CO2 storage, with sources spanning peer‐reviewed journals, conference proceedings, and significant technical reports. Our review highlights several key themes: the integration of machine learning and advanced numerical models in predicting CO2 behavior in subsurface formations; innovative experimental approaches to understanding the physicochemical interactions between CO2, brine, and geological substrates; and the development of robust risk assessment frameworks to address potential leakage and induced seismicity. We also explore recent advancements in monitoring technologies, emphasizing their critical role in ensuring the long‐term integrity and effectiveness of CO2 storage sites. Overall, this review synthesizes the latest findings and identifies gaps in current knowledge, providing a roadmap for future research directions. Our aim is to enhance the understanding of CO2 sequestration processes, support the development of safer and more efficient storage methods, and contribute to the global effort in mitigating climate change through effective carbon management strategies. © 2024 Society of Chemical Industry and John Wiley & Sons, Ltd. [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]

This article focuses on the analysis of academic literature on technogenic seismicity from a bibliometric perspective. Specifically, it examines the period from 1980 to 2022, inclusive. Over 10,000 scientific documents on this topic were sourced from the SCOPUS database. These documents were analyzed using VOSviewer software to evaluate the most productive journals, books, articles and their keyword co-occurrences, as well as to identify co-authorship among the most prolific countries. Ordinary Least Square Regression was then employed to establish a relationship between scientific production and its driving factors. Based on the results, three main subject areas were identified: (1) case studies on the primary causes of induced seismicity; (2) in-depth knowledge of man-made seismicity from geological and geophysical perspectives; and (3) numerical modeling of induced seismicity. A high correlation was found between the number of published documents and the key driving factors of induced seismicity. As a novel study using bibliometric analysis methods in the field of technogenic and induced seismicity, this research provides a reliable scientific tool for quickly identifying major research areas, current research boundaries, and general knowledge on the topic of interest. Bibliometric analysis can be used by scientists and researchers involved in the study of man-made seismicity to better understand the current intellectual structure, analyze scientific progress, and detect future research directions in this field. [ABSTRACT FROM AUTHOR]

After impounding the Baihetan Water Reservoir in a historically seismically active area, numerous earthquakes occurred, but it is uncertain if they are connected to the impoundment. Based on the dense seismic network, this study used local earthquake tomography to construct high‐resolution VP (P‐wave velocity) and VP/VS models (P‐ to S‐wave velocity ratio) and update earthquake locations after the impoundment of Baihetan reservoir. Our study revealed that the reservoir water spreads from the dam site to the Qiaojia Basin and the Heishuihe branch, and pore pressure diffusion along faults induces many earthquakes. Reservoir water migrates through hidden faults and fractures beneath Hulukou, spreading to both sides and saturating some rocks below 7 km beneath the dam site‐Lianhuatang section, causing multiple magnitude 3.5+ earthquakes. This study reveals that reservoir water migration drives earthquakes in the reservoir area, offering new insights into seismogenesis following the Baihetan reservoir's impoundment, potentially applicable to understanding reservoir‐induced earthquakes in other reservoirs. Plain Language Summary: Numerous earthquakes occurred after the Baihetan reservoir impoundment. It is worth considering whether the significant seismicity was caused by fluid injection. We used the local earthquake tomography to construct high‐precision earthquake locations and high‐resolution 3D VP and VP/VS models of Baihetan reservoir after the impoundment. The 3D VP and VP/VS models reveal the penetration and diffusion range of reservoir water. The relationship between seismic locations and velocity models reveals that microearthquakes are induced by pore pressure diffusion caused by fluid migration, and some of the water‐saturated areas induced bigger earthquakes. Key Points: The highest resolution 3D VP and VP/VS models of the Baihetan reservoir are obtained by using a dense seismic arrayThe relationship between the change of medium structure and seismicity caused by the impoundment of Baihetan reservoir is revealedA new understanding of the seismogenesis of a large number of earthquakes occurred after the Baihetan reservoir impoundment are obtained [ABSTRACT FROM AUTHOR]

Hydraulic fracturing in shale gas production can induce felt earthquakes, making it crucial to understand and mitigate induced earthquakes. The Cen'gong shale gas block in South China offers extensive data—3D seismic, geological structure, microseismic data, and detailed stimulation operations—allowing a comprehensive investigation into induced earthquakes by hydraulic fracturing. Using a dense temporary seismic array and deep‐learning workflows, we build a high precision earthquake catalog and determine their focal mechanisms. Pre‐existing fractures are identified through the Ant Tracking attribute derived from the 3D seismic data. We analyze the distribution, frequency, magnitude, and focal mechanisms of induced earthquakes, compare them spatially with the distribution of the pre‐existing fractures, and track their temporal changes during and after hydraulic fracturing. Most induced earthquakes occurred along pre‐existing fractures, exhibiting relatively larger magnitudes and persistent trailing seismicity. The number of trailing seismicity is proportional to the response time of stimulation earthquakes. The focal mechanism solutions suggest that the rupture mechanism of the trailing seismicity remained unchanged. By analyzing four clusters of earthquakes, we found that in two of these clusters, the induced earthquakes initiated from the far side of the fractures, then linearly migrated along the pre‐existing fractures. This directional migration pattern is explained by stress rotation along the fractures. Our analysis suggests that both pre‐existing fractures and stimulation operations significantly influence induced earthquake occurrences. Therefore, this work may enhance our understanding of pre‐existing fractures, and optimizing stimulation operations can mitigate earthquake hazards in shale gas production. Plain Language Summary: Felt hydraulic fracturing‐induced earthquakes have been reported worldwide. Understanding the triggering mechanisms of induced earthquakes is essential to mitigate earthquake hazards. This study utilizes extensive data recorded in the Cen'gong shale gas block in South China to comprehensively investigate and understand the spatial and temporal distribution, magnitudes, focal mechanisms, initiation, and migration of induced earthquakes. Our results suggest that both pre‐existing fractures and stimulation operations significantly influence induced earthquakes. Thus, induced earthquakes can be mitigated by understanding the interaction of the pre‐existing fractures and the stimulation operations. Key Points: We construct an earthquake catalog and determine focal mechanisms for the Cen'gong hydraulic fracturing through machine‐learning seamless workflowsThe distribution and intensity of induced earthquakes are influenced by both pre‐existing fractures and stimulation operationsThe linear directional migration of induced earthquakes is observed and attributed to stress rotation along the fracture [ABSTRACT FROM AUTHOR]

Under critical conditions where experimental fault slip exhibits self‐sustained oscillation, effects of normal stress oscillation (NSO) on fault strength and stability remain poorly understood, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct‐shear testing to investigate the frictional behavior of a synthetic, slightly velocity‐weakening (SVW) fault gouge (characterized by self‐sustained oscillation under quasi‐static shear loading), when subjected to NSO at different amplitudes (5%–20% of 5 MPa) and frequencies (0.001–1 Hz). During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be triggered at NSO frequencies ranging from 0.03 to 0.1 Hz and amplitudes exceeding 5%. Interestingly, an amplified shear stress drop and weakening effect were observed when the NSO frequency fell in 0.05–0.1 Hz. Analysis of transmitted ultrasonic waves in tests on the SVW gouge revealed fault dilation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the "Chen‐Niemeijer‐Spiers [CNS]" model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiments on the SVW gouge was reproduced, suggesting an approach for modeling fault instability under upper crustal (SVW) conditions where normal stress is perturbed by subsurface operations, such as periodic gas storage stimulation of reservoir formations. Plain Language Summary: To mitigate induced fault slip and seismicity associated with subsurface industrial activities that create oscillating stress, it is crucial to comprehend the effects of oscillation on fault friction. We used an experimental fault to mimic a potentially unstable fault at shallow depth and tested the effects of normal stress oscillation (NSO) on its shear strength. Various amplitudes and frequencies of NSO were investigated. An active ultrasonic source was employed to probe the microstructure of the gouge layer. Our results suggest that unstable slip can be easily triggered by applying NSO. Fault weakening and shear stress fluctuations are amplified within a limited range of oscillation frequency. Increasing amplitude causes greater fault weakening and stress fluctuations. The transmitted ultrasonic waves are sensitive to the change in mechanical properties of fault zone, reflecting that fault dilation is the main mechanism associated with fault weakening and instability. We present a microphysical model that reproduces and explains the mechanical and ultrasonic results, offering a potential route to extrapolate laboratory data to field conditions in the future. Key Points: Fault weakening and unstable events can be readily triggered by normal stress oscillations when the gouge is slightly velocity‐weakeningFault resonance occurs at oscillation frequencies in the range 0.05–0.1 HzAn extended microphysical model was built to quantify the mechanical behavior and tested using active ultrasonic technology [ABSTRACT FROM AUTHOR]

Fluid injection into subsurface reservoirs may cause existing faults/fractures to slip seismically. To study the effect of temperature on injection-induced fault slip, at a constant confining pressure of 10 MPa, we performed a series of injection-induced shear slip experiments on critically stressed sandstone samples containing saw-cut fractures (laboratory-simulated faults) under varying fluid pressurization rates (0.1 and 0.5 MPa/min, respectively) and temperatures (25, 80, and 140 °C, respectively). At 25 °C, slow fault slip events with a peak slip velocity of about 0.13 μm/s were observed on a tested sample in response to a low fluid pressurization rate of 0.1 MPa/min. In contrast, fluid injection with a high pressurization rate of 0.5 MPa/min caused fault slip events with a peak slip rate up to about 0.38 μm/s. In response to a given fluid pressurization rate, several episodes of slip events with a higher slip velocity were induced at an elevated temperature of 140 °C, indicating an appreciable weakening effect at elevated temperatures. We also experimentally constrained the rate-and-state frictional (RSF) parameters at varying effective normal stresses and temperatures by performing velocity-stepping tests. The obtained RSF parameters demonstrate that for a relatively high normal stress, increasing temperature tends to destabilize fault slip. Post-mortem microstructural observations reveal that elevated temperatures promote the generation of abundant fine-grained gouge particles associated with injection-induced shear slip. Our experiments highlight that injection-induced fault slip is affected by temperature-related wear production over the fault surface. [ABSTRACT FROM AUTHOR]

This study provides a comprehensive exploration of ground motions associated with micro-earthquakes induced by geothermal power plants (GPP) in Southern Germany and proposes corresponding ground motion prediction equations (GMPE). Initiating with a statistical analysis of recorded seismic data from the GPP in Insheim, the study is extended to the greater Munich area. For the latter, the scarce recorded data are merged with physics-based simulation data. The recorded data in Insheim, Poing, Unterhaching and the simulated data in Munich are compared to existing GMPEs for GPP-induced events, highlighting the need of new region-specific prediction equations. The proposed GMPEs are expressed in terms of peak quantities, spectral accelerations and velocities, separating the horizontal and vertical direction. The regression curves exhibit a good alignment with both recorded and simulated data, within an acceptable range. Notably, the results reveal higher spectral quantities at shorter periods ( < 0.1 s), underscoring the importance of this characteristic in seismic assessment. The article shows an exemplary application for a low-rise residential building, located at a hypocentral distance of 3 km. While the building meets serviceability standards for an M W up to 2.5, the verification fails at M W = 3 , emphasizing the need for robust risk assessment. These findings contribute to the understanding of ground motions of GPP-induced events, offering practical implications for serviceability verifications and aiding informed decision-making in geothermal energy projects. [ABSTRACT FROM AUTHOR]

This paper presents a modified evaluation approach for the structural-capacity-reserve (SCR) based on the static stability behavior of single-layer spherical lattice shells to achieve more accurate assessment results of seismic damage endurance. The SCR index in this paper is calculated by integrating the redefined residual structural capacity (RSC) ratio based on post-earthquake static stability behavior within the range of damage degrees caused by strong earthquakes. Compared with the assessment work for seismic damage endurance in the existing literature, the proposed RSC ratio in this paper could take into account both ultimate load-bearing strength and residual deformation. Additionally, the specific quantitative indicator in the proposed assessment method should also be determined according to whether the residual nodal displacement of the lattice shell after an earthquake exceeds the limit value state specified by the design code. The numerical results in the case study indicate that the residual load-bearing strength of the lattice shell remains relatively constant under the same damage state induced by different earthquakes, with residual deformation significantly impacting the assessment of seismic damage endurance. Thereby, the required RSC can be controlled by reducing the residual nodal displacements of the lattice shell under strong earthquakes to achieve a well-balanced design scheme. [ABSTRACT FROM AUTHOR]

The fractal formalisms are well known for providing new understandings regarding the geometrical, spatial, and temporal behaviour of seismicity. Particularly, the fractal dimensions give information about the seismic events self‐organization and self‐similarity. On the other hand, the Gutenberg–Richter value, known as the b‐value, has shown through the years to give handy information regarding the statistical distribution of earthquakes, on‐site physical parameters, and geomechanical inputs. The Gutenberg–Richter value (b) and the capacity and correlation fractal dimensions, (D0 and D2), of the spatial distribution of earthquake hypocentres interact mathematically for micro‐ and macro‐events. From this interaction, it is possible to obtain new insights into the fracture network development and the microseismicity source characterization in terms of single fractures, fault planes, or densely fractured volumetric spaces. Here we show this interaction for the open‐source Decatur CO2 project seismicity catalogue, comparing it with the results obtained for a natural earthquake catalogue of Illinois, in the United States. The fractal dimension D0 is calculated using two different methodologies: box‐counting and correlation integral partitioning. This last method is also used to calculate D2. The results presented in this study allow us to describe how the fracture network geometry influences the earthquake complexity. Together with the calculation of the b‐value, we present clear indications which show that seismicity recorded in the Illinois tectonic environment partially follows the Aki relationship D0 ∼ 2b, which is not the case for induced events. In addition, the induced earthquake dataset shows that D2 > D0, an anomalous behaviour in terms of the fractal formalisms. All these facts might be used to establish spatial fracture network control techniques and seismicity‐type distinctions in CO2 injection sites located in highly active tectonic areas, respectively. [ABSTRACT FROM AUTHOR]

We adopt extreme value theory to estimate the upper limit of the next record‐breaking magnitudes of induced seismic events. The methodology is based on order statistics and does not rely on knowledge of the state of the subsurface reservoir or injection strategy. The estimation depends on the history of record‐breaking events produced by the anthropogenic activities. We apply the methodology to three different types of industrial operations: natural gas production, saltwater disposal and hydraulic fracturing. We show that the upper limit estimate provides a reliable and realistic upper bound for magnitudes of the record‐breaking events in investigated datasets including 15 publicly available datasets. The predicted magnitudes do not overestimate the observed magnitudes by more than 1.0 magnitude unit and underestimation is rare, probably resulting from insufficient sampling of the statistical distribution of the induced seismicity. The richest dataset, sourced from downhole and surface monitoring of the Preston New Road hydraulic fracturing, provides reliable estimates of the magnitudes over three orders of magnitudes with only slight underprediction of the largest observed event. While the detection of weaker events improves the performance of the method, we show that it can be applied even with a few observed record‐breaking events to provide reliable estimates of magnitudes. However, care must be taken to ensure that event catalogues are estimated consistently across a range of magnitudes. [ABSTRACT FROM AUTHOR]

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An increasing number of geological hazards along high-speed railways on the Qinghai–Tibetan Plateau have occurred and have resulted in a profound influence on old infrastructure, which has attracted increasing attention. The landslide event that occurred on September 15, 2022, in Jiujiawan village, Xining city, Qinghai Province, is a typical case. Based on field investigations and remote sensing interpretations, a comprehensive analysis was conducted on the landslide. Additionally, the potential secondary failure of the current Jiujiawan landslide was assessed using Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D). Based on the application of the small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) technique to SAR images from February 24, 2017 to September 14, 2022, a significant westward horizontal deformation was found to have been formed prior to the occurrence of the landslide. The maximum annual average deformation rate in the line of sight (LOS) direction reached −45 mm/yr, with a maximum cumulative deformation of −178 mm. This value was consistent with the continual increase in annual precipitation (2.51 mm/yr) prior to the occurrence of the landslide. The accumulated precipitation before the landslide was 279.8 mm, accounting for 54.2% of the total annual precipitation, with a particularly notable surge in monthly precipitation observed during August (250.3 mm). Additionally, the occurrence of a seismic event with a magnitude of Ms 6.9 in Menyuan County, 80 km away from Xining, could be a potential triggering factor to the landslide, as evidenced by an abrupt subsidence alteration observed prior to and following the earthquake. The maximum subsidence in the line of sight (LOS) direction exceeded 11 mm, exhibiting a highly consistent spatial distribution with the occurrence range of landslides. These results suggest that the Jiujiawan landslide was likely induced by earthquake events in the early stage and heavy rainfall in the later stage. The FLAC3D numerical simulation show that after the landslide, the slope remained marginally stable under natural conditions; however, it is susceptible to reactivation with heavy rainfall. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we performed microseismicity detection and location using the deep learning method and obtained a high-precision earthquake catalog in the Changning gas field, which is one of the largest shale gas production demonstration areas in China. We found that the spatial and temporal characteristics of seismicity in the region are indicative of its correlation with industrial operations. The distribution of earthquakes at depth reflected variations in reservoir depth and provided valuable constraints on it. The horizontal layered distribution of earthquakes at depth is due to the formation of fracture surfaces from interconnected fractures near the reservoir during hydraulic fracturing (HF) operations, which clearly demonstrates how HF operations impact seismicity. We suggested that the horizontally layered distribution was driven by two fundamental mechanisms: reactivation of pre-existing faults during HF and injection of high-pressure fluids into the reservoir, leading to fracture creation. Several M L ≥4 earthquakes, which did not occur on well-defined seismogenic faults, may have been triggered by pore elastic coupling resulting from regional stress accumulation and fluid injection. Significantly, the M L 4.9 seismic sequence occurring at the basement indicates that fracking has reactivated and promoted pre-existing faults, highlighting the need for further investigation into potential seismic hazards in the region.

Abstract A key element in the assessment of seismic hazard is estimation of the maximum possible earthquake magnitude, Mmax. A great deal of effort has been invested in developing approaches to estimate Mmax for natural (tectonic) earthquakes, especially in regions of relatively low seismicity where it is difficult to associate observed seismicity with known geological faults. In probabilistic seismic hazard analysis, there has been a tendency to assign a narrow range of large values to Mmax. This results in the impression that hazard results are insensitive to this parameter, which is not the case when the Mmax distribution captures the full range of possible values. For induced seismicity, Mmax estimates can have far-reaching implications both in terms of quantitative assessments of the resulting seismic hazard and risk, and in terms of the public and regulatory perception of this risk. Estimates of Mmax for induced seismicity need to distinguish between driven earthquakes, for which magnitudes are largely controlled by operational parameters, and triggered tectonic earthquakes, together with estimates of the likelihood of such triggering. Distributions of triggered Mmax may be limited to smaller magnitudes than distributions for natural seismicity due to the shallow depth of most injection/extraction wells. For the management of induced seismic risk, the expected largest event magnitude (which may be influenced by a Traffic Light Scheme in operation) may be more relevant than any physical upper bound truncating the recurrence relationship.

The subsurface fluid injection can cause pressure increase within faults, leading to earthquake occurrences. However, the factors controlling earthquake rupture due to pressure perturbation remain poorly understood. To resolve this problem, we simulate the physical processes of earthquake nucleation and rupture on strike-slip faults perturbated by pressure migration based on the slip-weakening law. Multiple kinds of factors, including background stress, fluid injection rates, the area of the pressurized region, fault geometry, and fault friction coefficients, are considered in our simulations. Our simulation results reveal that the ratio of shear stress to normal stress rather than their absolute values controls the rupture behavior. With the large stress ratios, high injection rates, and large pressurized areas, earthquakes are prone to propagate as runaway ruptures. Additionally, faults with large aspect ratios of length to width are also favorable for causing runaway ruptures. In contrast, the factors of fault strike, dip angles and friction coefficients have minor influence on rupture behavior.

The energy transition towards a more sustainable and renewable future is a pivotal global endeavor. Central to this shift for the United States is the critical role of domestically sourced lithium, a key mineral in the production of high-performance batteries essential for electric vehicles and renewable energy storage systems. This has driven the United States to invest heavily in a domestic supply chain for battery-grade lithium to enhance energy security, reduce supply chain vulnerabilities, and foster economic growth by tapping into local resources. A notable example is the Biden Administration’s “American Battery Materials Initiative,” which was included in the $2.8-billion Bipartisan Infrastructure Law (The White House, 2022). The “Salton Sea Known Geothermal Resource Area” in Imperial County, California has been identified as a potential domestic U.S. resource of lithium due to the brine-hosted lithium in the deep subsurface geothermal reservoir. An analysis funded by the U.S. Department of Energy provides an overview of opportunities and challenges associated with developing the lithium resource in the Salton Sea geothermal reservoir, as well as potential environmental and societal impacts to the county and surrounding region. The geologic history of the region suggests that lithium in the subsurface brines could have come from multiple sources, including water and sediments from the Colorado River, which have been periodically deposited over the past several million years; rocks from the mountain ranges surrounding the Imperial Valley; and lithium-bearing volcanic rocks and igneous intrusions from past geologic events. Further, several processes may have concentrated lithium in the brine over time, including evaporative concentration of lithium-bearing water that flowed into the basin and leaching of lithium from the sediments and rocks by the circulating geothermal brines. Geothermal brine production at the Salton Sea Geothermal Field, the area with existing geothermal power plants, has averaged just over 120 million metric tons per year since 2004. Using an approximate lithium brine concentration of 198 parts per million (ppm), the amount of dissolved lithium contained in these produced brines is estimated to be 127,000 metric tons of lithium carbonate equivalent (LCE) per year. The total dissolved lithium content in the well-characterized portion of the Salton Sea Geothermal Reservoir is estimated at 4.1 million metric tons of LCE, and the estimated total resource increases to 18 million metric tons of LCE if assumptions for porosity and total reservoir size are increased to reflect the probable resource extent. Analysts measured lithium concentrations in the reservoir rocks, which were shown to vary with depth and mineralogy. These data were used to help refine conceptual and computer models of the reservoir; specifically, two complementary computer models of the reservoir were developed. Analysts used the first model to simulate the approximate 30-year history of geothermal power production in the area using historical production and reinjection data, then used that model to simulate a 30-year forecasting period. This forecast assumed continued production and reinjection rates at current levels but removes 95% of the lithium from the produced geothermal brine starting January 1, 2024. The model found that lithium recovery declines by more than half, from 0.8 to 0.3 kilograms per second (kg/s). Forecast scenarios that are optimized to both recover lithium and harness geothermal energy are expected to sustain lithium production rates much more effectively.The second model included more detailed simulations of the movement of brine and chemical reactivity of lithium within the reservoir. It showed that the reactions of relatively stable lithium-bearing minerals are slow, and that the primary replenishment mechanism for lithium in the brines is the upward flux of convecting lithium-rich brine from below the producing reservoir. However, these replenishment rates are not fast enough to produce significant increases in lithium, which could limit the long-term sustainability of the lithium resource. It is important to note that these models are preliminary and are based on current understanding of fluid replenishment rates, the minerals present in the geothermal system, and their chemical properties and reactivity. Further work should be undertaken to improve them and the associated predictions. The report also considered potential impacts on regional water resources, air quality, chemical use, and solid waste disposal needs, as well as the seismic risk associated with geothermal power production and lithium extraction activity. These investigations highlighted the need to proceed with good monitoring and verification systems and with appropriate mitigation technologies. However, the analysis illustrates that if these things are done properly, lithium development is not likely to create significant negative environmental impacts.Specifically, expanding geothermal energy production and lithium extraction will have a modest impact on water availability in the region. Initial estimates suggested that ~3% of historically available water supply for the region would be needed for currently proposed geothermal energy and lithium recovery operations; the majority of current water usage is for agriculture. It is not anticipated that expanding geothermal capacity or lithium production would impact the availability or quality of water used for human consumption and will not directly affect the water quality of the Salton Sea. However, the long-term drought conditions in the western United States may restrict future availability of water to the region, which is sourced from the Colorado River.In terms of regional air emissions of all pollutants identified in the analysis (particulate matter, hydrogen sulfide, ammonia, and benzene, expanding geothermal energy and adding lithium extraction overall have a small impact. Chemical use involved in geothermal power production and lithium extraction is consistent with chemical use in industrial settings, and the analysis did not identify any persistent organic pollutants or acutely toxic chemicals among those currently being used. Moving fluids within the subsurface can impact subsurface pressures and stresses, potentially triggering seismic activity. Early in geothermal energy production, increasing seismicity rates in the Salton Sea Geothermal Field correlated strongly with energy production activity; however, that correlation weakened after 1996. Even following the onset of geothermal energy production, seismic hazard in the Salton Sea Geothermal Field has not increased beyond that of the surrounding region.In addition to technical outcomes from the analysis, the report describes an initial effort to incorporate community engagement into lithium research by understanding the local context and priorities and identifying how to effectively communicate to share information and gather feedback. The report includes information about the social and historical context of the region to enable a more holistic understanding of the resource and its potential impact, and identifies key community questions by observing public meetings, visiting the region, and consulting with local organizations. The report provides recommendations about how future research efforts can address community concerns and implement more community-engaged practices. These include developing formal partnerships with local organizations and establishing a community advisory board to facilitate ongoing dialogue and opportunities for feedback. The future work will build on and further refine the models and scenarios presented in the report and strive to deepen engagement with local communities.

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]

Spectral analysis is an effective tool for processing seismic signals, particularly when time-domain characteristics are challenging to capture. In this study, we developed a method using P-wave signals to calculate the power spectrum, enabling the estimation of two spectral parameters—peak frequency and shape factor—for earthquakes recorded by regional seismic networks in the Xiangjiaba (XJB) reservoir area from 2010 to 2015. The temporal evolution of the two spectral parameters was analyzed, revealing that the mean values of individual spectral parameters remain relatively stable despite variations in reservoir water levels. However, a notable increase in the ratio of the shape factor to the peak frequency is observed when the XJB reservoir reaches its maximum water level, suggesting its potential as a precursor indicator for reservoir-induced seismicity (RIS). Furthermore, we performed spatial interpolation on the spectral parameters, and the results show that reservoir impoundment significantly influences the spatial distribution of these parameters. In addition, several regions between the two faults in the tail section of the XJB reservoir exhibit an elevation in the proposed precursor indicator. This study presents a new approach for monitoring and early warning of RIS. [ABSTRACT FROM AUTHOR]

The spatiotemporal evolution of earthquakes induced by fluid injections into the subsurface can be erratic owing to the complexity of the physical process. To effectively mitigate the associated hazard and to draft appropriate regulatory strategies, a detailed understanding of how induced seismicity may evolve is needed. In this work, we build on the well-established continuous-time random walk (CTRW) theory to develop a purely stochastic framework that can delineate the essential characteristics of this process. We use data from the 2003 and 2012 hydraulic stimulations in the Cooper Basin geothermal field that induced thousands of microearthquakes to test and demonstrate the applicability of the model. Induced seismicity in the Cooper Basin shows all the characteristics of subdiffusion, as indicated by the fractional order power-law growth of the mean square displacement with time and broad waiting-time distributions with algebraic tails. We further use an appropriate master equation and the time-fractional diffusion equation to map the spatiotemporal evolution of seismicity. The results show good agreement between the model and the data regarding the peak earthquake concentration close to the two injection wells and the stretched exponential relaxation of seismicity with distance, suggesting that the CTRW model can be efficiently incorporated into induced seismicity forecasting. [ABSTRACT FROM AUTHOR]

Hydraulic stimulation is a critical process for increasing the permeability of fractured geothermal reservoirs. This technique relies on coupled hydromechanical processes induced through pressurized fluid injection into the rock formation. The injection of fluids causes poromechanical stress changes that can lead to fracture slip and shear dilation, as well as tensile fracture opening and propagation, so-called mixed-mechanism stimulation. The effective permeability of the rock is particularly enhanced when new fractures connect with pre-existing fractures. While hydraulic stimulation can significantly improve the productivity of fractured geothermal reservoirs, the process is also related to induced seismicity. Hence, understanding the coupled physics is central, for both reservoir engineering and seismic risk mitigation. This article presents a modelling approach for simulating the deformation, propagation and coalescence of fractures in porous media under the influence of anisotropic stress and fluid injection. It uses a coupled hydromechanical model for poroelastic, fractured media. Fractures are governed by contact mechanics and a fracture propagation model. For numerical solutions, we employ a two-level approach, combining a finite volume method for poroelasticity with a finite element method for fracture propagation. The study investigates the impact of injection rate, matrix permeability and stress anisotropy on stimulation outcomes. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

The intermittent storage of hydrogen in subsurface porous media such as depleted gas fields could be pivotal to a successful energy transition. Numerical simulations investigate the intermittent storage of hydrogen in a porous, depleted subsurface reservoir. Various parametric studies are performed to assess the effect of mechanical properties of the reservoir (i.e. Young's modulus, Poisson's ratio, Biot coefficient and permeability) on the induced fault slip of a single through-going fault that transverses the entire reservoir. Simulations are run using a three-dimensional, finite element, fully coupled hydromechanical code with explicit representations of layers and faults. The effect of the domain mesh refinement and fault mesh refinement on the fault slip versus operation time solution is investigated. The fault is observed to slip in two distinct events, one during the second injection period and one in the third injection period. The fault is not observed to slip during the storage or withdrawal periods. It is found that in order to minimize seismic risk, a reservoir rock with high Young's modulus (>40 GPa), high Poisson's ratio (>0.30) and high Biot coefficient (>0.65) would be preferable for hydrogen storage. Reservoir rocks of low Young's modulus (10–30 GPa), intermediate Poisson's ratio (0.00–0.30) and low-to-intermediate Biot coefficient (0.25–0.65), at high injection rates, were found to have higher potential of inducing large seismic events. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

Sealing faults are nearly impermeable barriers that can form boundaries between subsurface pore-pressure domains. In hydrocarbon systems, sealing faults commonly form part of a structural trap; they are thus important elements for future storage of CO2 and other gases in depleted reservoirs. The Triassic Montney Formation in western Canada hosts low-permeability gas reservoirs containing sealing faults that have previously been assumed to compartmentalize pressure domains. In this study, we show that the distribution of induced seismicity associated with hydraulic fracturing (HF) exhibits a statistically significant spatial correlation with zones of high lateral gradient in pore pressure. These high-gradient zones are interpreted as sealing fault systems. The largest induced seismicity sequence, including a 4.5 ML mainshock on 30 November 2018, occurred during HF treatments in two horizontal wells, between which there is an exceptionally large contrast (~10 MPa) in measured pore pressure. Numerical simulation of a simplified model of a hydraulic fracture intersecting a nearby vertical fault, followed by fault rupture using rate-and-state friction rheology, generates results that are in good agreement with observed strike-slip faulting near one of the HF wells. Our study demonstrates that sealing faults exhibit previously unrecognized behaviour that may be important for understanding induced seismicity risk. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

Enhanced geothermal systems (EGSs) developed by hydraulic stimulation are promising for exploiting petrothermal heat by improving fluid pathways in low-permeable geothermal reservoir rocks. However, fluid injection into the subsurface can potentially cause large seismic events by reactivating pre-existing faults, which is a significant barrier to EGSs. The management of injection-induced seismicity is, therefore, essential for the success of EGSs. During the hydraulic stimulation of an EGS, fluid can be injected into a fault zone or into the rock matrix containing pre-existing faults adjacent to the injection well. The differences in hydromechanical responses between fluid injection into and adjacent to a fault have not been investigated in detail. Here, we performed triaxial fluid injection experiments involving injecting fluid directly and indirectly into a fault in granite rock samples to analyse the distinct hydromechanical responses and estimate the injection-induced seismicity in both cases. Our results suggest that in addition to directly injecting fluid into a critically stressed fault, injecting into nearly intact granite adjacent to the fault could also cause injection-induced seismic hazards owing to the high fluid pressure required to create new fractures in the granite matrix. It is, therefore, important to carefully identify pre-existing faults within tight reservoirs to avoid injecting fluid adjacent to them. Additionally, once prior unknown faults are delineated during hydraulic stimulation, appropriate shut-in strategies should be implemented immediately to mitigate seismic risks. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

This article discusses the phenomenon of induced seismicity, which refers to the human-caused generation of tremors and seismic events that alter the state of subsurface stresses. The occurrence of induced seismicity during subsurface technologies, such as shale gas development and renewable energy extraction, can lead to project delays and concerns about safety and environmental impact. The article highlights the need to understand the mechanisms of induced seismicity and develop strategies to minimize its effects. It presents a collection of research papers that explore fluid-induced seismicity in various energy production and storage technologies, offering insights into the interplay between fluid injection, fault behavior, and subsurface structures. The findings contribute to evidence-based policymaking and regulatory processes, supporting the development of sustainable subsurface operations for renewable energy and decarbonization technologies. [Extracted from the article]

Injection-induced seismicity and aseismic slip often involve the reactivation of long-dormant faults, which may have extremely low permeability prior to slip. In contrast, most previous models of fluid-driven aseismic slip have assumed linear pressure diffusion in a fault zone of constant permeability and porosity. Slip occurs within a frictional shear crack whose edge can either lag or lead pressure diffusion, depending on the dimensionless stress-injection parameter that quantifies the prestress and injection conditions. Here, we extend this foundational work by accounting for permeability enhancement and dilatancy, assumed to occur instantaneously upon the onset of slip. The fault zone ahead of the crack is assumed to be impermeable, so fluid flow and pressure diffusion are confined to the interior, slipped part of the crack. The confinement of flow increases the pressurization rate and reduction of fault strength, facilitating crack growth even for severely understressed faults. Suctions from dilatancy slow crack growth, preventing propagation beyond the hydraulic diffusion length. Our new two-dimensional and three-dimensional solutions can facilitate the interpretation of induced seismicity data sets. They are especially relevant for faults in initially low permeability formations, such as shale layers serving as caprock seals for geologic carbon storage, or for hydraulic stimulation of geothermal reservoirs. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

The largest magnitude earthquake in a sequence is often used as a proxy for hazard estimates, as consequences are often predominately from this single event (in small seismic zones). In this article, the concept of order statistics is adapted to infer the maximum magnitude (MMAX) of an earthquake catalogue. A suite tools developed here can discern MMAX influences through hypothesis testing, quantify MMAX through maximum likelihood estimation (MLE) or select the best MMAX prediction amongst several models. The efficacy of these tools is benchmarked against synthetic and real-data tests, demonstrating their utility. Ultimately, 13 cases of induced seismicity spanning wastewater disposal, hydraulic fracturing and enhanced geothermal systems are tested for volume-based MMAX. I find that there is no evidence of volume-based processes influencing any of these cases. On the contrary, all these cases are adequately explained by an unbounded magnitude distribution. This is significant because it suggests that induced earthquake hazards should also be treated as unbounded. On the other hand, if bounded cases exist, then the tools developed here will be able to discern them, potentially changing how an operator mitigates these hazards. Overall, this suite of tools will be important for better-understanding earthquakes and managing their risks. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

There is an ongoing discussion about how to forecast the maximum magnitudes of induced earthquakes based on operational parameters, subsurface conditions and physical process understanding. Although the occurrence of damage caused by induced earthquakes is rare, some cases have caused significant economic loss, injuries and even loss of life. We analysed a global compilation of earthquakes induced by hydraulic fracturing, geothermal reservoir stimulation, water disposal, gas storage and reservoir impoundment. Our analysis showed that maximum magnitudes scale with the characteristic length of pressure diffusion in the brittle Earth's crust. We observed an increase in the nucleation potential of larger-magnitude earthquakes with time and explained it by diffusion-controlled growth of the pressure-perturbed part of faults. Numerical and analytical fault size modelling supported our findings. Finally, we derived magnitude scaling laws to manage induced seismic hazard of upcoming energy projects prior to operation. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

We examine the temporal evolution of sequences of induced seismicity caused by long-term fluid injection using a compilation of over 20 case studies where moderate magnitude (M > 3.0) induced events have been recorded. We compare rates of seismicity with injection rates via the seismogenic index and seismic efficiency parameters, computing both cumulative and time-windowed values. We find that cumulative values tend to accelerate steeply as each seismicity sequence initiates—most cases reach a value that is within 0.5 units of their maximum value within 1–3 years. Time-windowed values tend to increase to maximum values within 25%–35% of the overall sequence, before decreasing as levels of seismicity stabilize. We interpret these observations with respect to the pore pressure changes that will be generated in highly porous, high permeability reservoirs. In such situations, the rate of pore pressure change is highest during the early phases of injection and decreases with time. If induced seismicity scales with the rate of deformation, which in turn is controlled by the rate of pore pressure change, then it is to be expected that induced seismicity is highest during the early phases of injection, and then decreases with time. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

Intraplate earthquakes induced by anthropogenic fluid injection present unexpected seismic risk to previously quiescent or low seismicity-rate regions. Despite many studies of induced seismicity, there are relatively few with detailed openly accessible constraints on the interaction between seismic sources and subsurface structures. In this study of the Raton Basin, we refine source observations from a dense nodal array and constrain basin structure using teleseismic receiver functions. The cross-correlation-based relocated hypocentres and a new set of focal mechanisms light up active fault segments and show clear spatiotemporal patterns. The geometric complexity of reactivated fault clusters appears greatest near higher rate injection wells. Simpler normal fault structure is found farther from injection wells and near abrupt structural transitions suggested by receiver functions. While less induced seismicity in the crystalline basement is expected when injection is >1 km from the top of the basement (like Raton), our receiver function analysis identified a basin thickness ~3 km beneath the nodal array and lateral variations in sedimentary structures. Our results explain potential fluid connectivity between the injection depths focused at ~1–1.5 km below the surface and basement fault activity that begins at ~3 km and reaches peak activity at ~4–8 km depths. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

Anthropogenic factors such as over-pumping and natural events such as earthquakes impact coastal aquifers by reducing freshwater recharge, aquifer water budgets, and increasing saltwater intrusion (SWI). This study investigates the impact of hydrodynamic forces induced by earthquakes on SWI in one hypothetical case, namely, the Henry problem, and a real case of the Biscayne aquifer located in Florida, USA. The analysis was carried out using the analytical solution of estimating the earthquake's induced hydrodynamic pressure and applying the SEAWAT code to investigate the SWI for the base case and three scenarios, namely for the horizontal acceleration (αh) by 0.10 g, 0.20 g, and 0.30 g. The results show that earthquakes might considerably increase the SWI in coastal aquifers. Moreover, the rise in salinity across expansive land areas significantly threatens agricultural productivity and jeopardizes food security. Namely, in the case of Biscayne aquifer, salinity was increased by 12.10%, 21.90%, and 45.70% for the horizontal seismic acceleration of 0.1 g, 0.20 g, and 0.30 g, respectively. Hence, the conclusions drawn from this study underscore the need for carefull consideration of earthquake impacts in future planning and water management strategies for coastal regions. This proactive approach is crucial to preemptively address and mitigate the groundwater salinization hazard associated with SWI fluctuations due to earthquakes. [ABSTRACT FROM AUTHOR]

Coupled thermo-hydro-mechanical (THM) processes in fractured rocks have been a topic of intense scientific research for more than 30 years. The present paper takes a look into the past and highlights some scientific advances which are of an unusual "out-of-the-box" nature, and then looks forward and discusses possible directions of future research in this interesting field of study. Concerning future research directions, we see a trend from a focus on coupled THM processes in single fractures or a few interacting fractures, to the study of coupled THM behavior in complex fracture network systems where the fractures act collectively giving rise to local stress concentration points and points of large pressure gradients. Three examples of future research directions are presented. First is an effort towards identifying characterizing parameters of a fracture network that play a direct controlling role in major coupled THM phenomena (such as induced seismicity and flow channeling), rather than parameters of stochastic distributions of fractures in the network. The second example of research direction is accounting for the heterogeneity and hierarchy of fractures in a fault or fracture zone which has been associated with major THM events in a number of geo-energy projects. The third example is at the opposite end of the first; here it is recognized that in some cases, the coupled THM processes in fractured rocks may be controlled dominantly by only a few key bridges. Identification, characterization, and evaluation of these key bridges should be one of the important research directions in the coming days. Highlights: The research into coupled thermo-hydro-mechanical processes in fractured rock over the last thirty years is reviewed. Three past scientific advances of an "out-of-box" nature are highlighted. Future research directions are discussed with three examples of potentially fruitful research directions. [ABSTRACT FROM AUTHOR]

SUMMARY: The determination of seismic event locations with sparse networks or single-borehole systems remains a significant challenge in observational seismology. Leveraging the advantages of the location approach HADES (eartHquake locAtion via Distance gEometry Solvers), which was initially developed for locating clustered seismicity recorded at two stations, through the solution of a Distance Geometry Problem, we present here an improved version of the methodology: HADES-R (HADES-Relative). Where HADES previously needed a minimum of four absolutely located master events, HADES-R solves a least-squares problem to find the relative inter-event distances in the cluster, and uses only a single master event to find the locations of all events and subsequently applies rotational optimizer to find the cluster orientation. It can leverage iterative station combinations if multiple receivers are available, to describe the cluster shape and orientation uncertainty with a bootstrap approach. The improved method requires P- and S-phase arrival picks, a homogeneous velocity model, a single master event with a known location, and an estimate of the cluster width. The approach is benchmarked on the 2019 Ridgecrest sequence recorded at two stations, and applied to two seismic clusters at the FORGE geothermal test site in Utah, USA, with a microseismic monitoring scenario with a Distributed Acoustic Sensing in a vertical borehole. Traditional procedures struggle in these settings due to the ill-posed network configuration. The azimuthal ambiguity in such a scenario is partially overcome by the assumption that all events belong to the same cluster around the master event and a cluster width estimate. We are able to find the cluster shape in both cases, although the orientation remains uncertain. HADES-R contributes to an efficient way to locate multiple events simultaneously with minimal prior information. The method's ability to constrain the cluster shape and location with only one well-located event offers promising implications, especially for environments where limited or specialized instrumentation is in use. [ABSTRACT FROM AUTHOR]

SUMMARY: Microseismic monitoring is an important technique to obtain detailed knowledge of in-situ fracture size and orientation during stimulation to maximize fluid flow throughout the rock volume and optimize production. Furthermore, considering that the frequency of earthquake magnitudes empirically follows a power law (i.e. Gutenberg–Richter), the accuracy of microseismic event magnitude distributions is potentially crucial for seismic risk management. In this study, we analyse microseismicity observed during four hydraulic fracture treatments of the legacy Cotton Valley experiment in 1997 at the Carthage gas field of East Texas, where fractures were activated at the base of the sand-shale Upper Cotton Valley formation. We perform waveform cross-correlation to detect similar event clusters, measure relative amplitude from aligned waveform pairs with a principal component analysis, then measure precise relative magnitudes. The new magnitudes significantly reduce the deviations between magnitude differences and relative amplitudes of event pairs. This subsequently reduces the magnitude differences between clusters located at different depths. Reduction in magnitude differences between clusters suggests that some attenuation-related biases could be effectively mitigated with relative magnitude measurements. The maximum likelihood method is applied to understand the magnitude frequency distributions and quantify the seismogenic index of the clusters. Statistical analyses with new magnitudes suggest that fractures that are more favourably oriented for shear failure have lower b-value and higher seismogenic index, suggesting higher potential for relatively larger earthquakes, rather than fractures subparallel to maximum horizontal principal stress orientation. [ABSTRACT FROM AUTHOR]

In regions where strong earthquakes occurred before the deployment of dense seismic and accelerometric networks, intensity datasets can help select appropriate ground motion prediction equations (GMPEs) for seismic hazard studies. This is the case for the Hainaut seismic zone, which was one of the most seismically active zones in and around Belgium during the twentieth century. A recent reassessment of the intensity dataset of the area showed that intensities in this region attenuate much faster with distance than in other parts of northwestern Europe. Unfortunately, this characteristic has not yet been taken into account in current hazard maps for Belgium and northern France. Based on this dataset, we evaluate the goodness of fit of published GMPEs with intensities in Hainaut by means of a ground-motion-to-intensity conversion equation (GMICE) and according to different metrics (Likelihood, Log-likelihood and Euclidean-based Distance Ranking) published in literature. We also introduce a new measure to specifically evaluate the distance trend. Our results show that none of the tested GMPEs convincingly fits the intensity dataset, in particular the fast attenuation with distance. Nevertheless, applying the few GMPEs that show a reasonable fit in seismic hazard computations, we observe a decrease of the influence of the Hainaut seismicity on hazard maps for Belgium and northern France. This result is compatible with the earthquake intensity observations for the last 350 years in this part of Europe. [ABSTRACT FROM AUTHOR]

The discovery of gas in Groningen in 1959 has been a massive boon to the Dutch economy. From the 1990s onwards, though, gas production has led to induced seismicity. In this paper, we carry out a comprehensive exploratory analysis of the spatio-temporal earthquake catalogue. We develop a non-parametric adaptive kernel smoothing technique to estimate the spatio-temporal hazard map and to interpolate monthly well-based gas production statistics. Second- and higher-order inhomogeneous summary statistics are used to show that the state-of-the-art rate-and-state models for the prediction of seismic hazard fail to capture inter-event interaction in the earthquake catalogue. Based on these findings, we suggest a modified rate-and-state model that also takes into account changes in gas production volumes and uncertainty in the pore pressure field. [ABSTRACT FROM AUTHOR]

The conventional theory of elastic-wave propagation is based on classical elastodynamics, assuming linear small deformations of particles. However, recent observations of seismic rotation have revealed significant disparities between actual rotational motions induced by earthquakes in focal areas and near fields compared to theoretical calculations and simulations. Considering the nonlinearity may be the main cause of the discrepancies and based on classical elastodynamic principle, we derive seismic elastic-wave equations with Green strain tensor without the linear small deformation assumption, a different way from using complex nonlinear constitutive relation and try to interpret the mechanism of seismic rotation. By simulating and analyzing translational and rotational components subjected to the three basic and typical vibrating sources, namely, isotropic (ISO), double couple (DC), and compensated linear vector dipole (CLVD), represented by moment tensors, we investigate the wavefield differences between elastic-wave equations based on linear and nonlinear geometric relations and quantify the differences in homogeneous elastic full-space model. Subsequently, we simulate two observed six-component Taiwan earthquakes and compare their differences caused by nonlinear simulations. The results indicate that linear approximation errors are more pronounced in seismic ISO and CLVD sources. And the nonlinearity of small deformation has a more pronounced effect on rotational motions deduced by strong earthquakes. Also, the nonlinear mechanics of seismic rotation can attribute to the complex propagation paths and source mechanisms simultaneously. [ABSTRACT FROM AUTHOR]

The Upper Silesian coal basin (USCB) in Poland faces significant ground deformation issues resulting from mining activities conducted without backfill, which can persist for years. These activities can cause damage to surface structures and phenomena such as induced seismicity. Ground deformations can be monitored using differential synthetic aperture radar interferometry (DInSAR). However, various DInSAR approaches have their own advantages and limitations, particularly regarding accuracy and atmospheric filtering. This is especially important for high-frequency displacement signals associated with seismic activity, which can be filtered out. Therefore, this study aims to assess the detectability of mining-induced seismic events using interferometric techniques, focusing on the USCB area. In this experiment, we tested two InSAR approaches: conventional DInSAR without atmospheric filtering and the small baseline subset (SBAS) approach, where the atmospheric phase screen was estimated and removed using high-pass and low-pass filtering. The results indicate that, in most cases, post-seismic ground displacement is not detectable using both methods. This suggests that mining-related seismic events typically do not cause significant post-seismic ground displacement. Out of the 17 selected seismic events, only two were clearly visible in the DInSAR estimated deformation, while for four other events, some displacement signals could neither be definitively confirmed nor negated. Conversely, only one seismic event was clearly detectable in the SBAS displacement time series, with no evidence of induced tremors found for the other events. DInSAR proved to be more effective in capturing displacement signals compared to SBAS. This could be attributed to the small magnitude of the tremors and, consequently, the small size of the seismic sources. Throughout the investigated period, all registered events had magnitudes less than 4.0. This highlights the challenge of identifying any significant influence of low-magnitude tremors on ground deformation, necessitating further investigations. Moreover, SBAS techniques tend to underestimate mining displacement rates, leading to smoothed deformation estimates, which may render post-seismic effects invisible for events with low magnitudes. However, after an in-depth analysis of the 17 seismic events in the USCB, DInSAR was found to be more effective in capturing displacement signals compared to SBAS. This indicates the need for significant caution when applying atmospheric filtering to high-frequency displacement signals. [ABSTRACT FROM AUTHOR]