Rocks can undergo fatigue failure when subjected to cyclic mechanical, hydraulic, or thermal loadings, or a combination of these. Therefore, accounting for possible fatigue damage is important for subsurface engineering projects, such as the cyclic stimulation of geothermal reservoirs. However, existing models do not simultaneously account for degradation of both tensile strength and stiffness under varying-amplitude loading and coupled thermo-hydro-mechanical (THM) conditions. To address this, a new cohesive zone model is developed to account for the effect of fatigue on tensile strength and stiffness. The model is then used within the framework of zero-thickness interface elements to simulate the response of pre-existing or new fractures. Hydraulic and thermal processes are included in both the cohesive interface elements and the continuum elements, allowing the consideration of coupled thermo-hydro-mechanical processes. The fatigue damage variable is set to evolve with the number and magnitude of cycles according to Palmgren-Miner’s rule. The proposed method is validated against three laboratory tests from the literature, including cyclic Brazilian test, cyclic hydraulic fracturing test and cyclic thermal stimulation test. All three validation results show that the fatigue damage or reduced breakdown pressure can be well reproduced. Mesh sensitivity based on the simulation of the Brazilian test, in which interface elements are inserted in-between all the continuum elements, highlights the influence of the mesh orientation and mesh density on the simulation results. In addition, stabilisation of the method is demonstrated by increasing the mechanical viscosity, which must be used with care to avoid predicting a longer fatigue life. The ability of the method to handle varying-amplitude cyclic loading is demonstrated by the simulation of a synthetic cyclic loading scheme based on the Brazilian test. The proposed method can be used to support the design of cyclic thermal stimulation campaigns for geothermal (or other) reservoirs, by being able to simulate the reduction in strength due to fatigue, and thus reducing stimulation pressures needed.

Abstract. Deep geological repositories for high-level radioactive waste (HLW) rely to a large extend on the long-term hydraulic integrity of host rocks to limit fluid flow and radionuclide migration. Low hydraulic conductivity (K < 10-10 m/s) is a key factor for effective long-term barrier performance, and argillaceous formations are promising candidates due to their strong aquitard characteristics. However, predicting their bulk hydraulic behaviour across temporal and spatial scales remains difficult, as it reflects the combined effects of intrinsic material properties and post-depositional evolution. This study compiles 782 hydraulic conductivity measurements from six European argillaceous formations, including laboratory and field scales. By integrating petrophysical, mineralogical, and reconstructed burial history data, we identify systematic links between burial evolution and hydraulic behaviour. Results show that maximum burial depth and associated stress and temperature conditions exert a first-order control on matrix-scale hydraulic conductivity, which is largely retained after uplift. In contrast, bulk hydraulic behaviour at the rock-mass scale reflects interactions between maximum burial depth and present-day depth, defining processes such as decompaction, fracturing, and self-sealing processes. Three evolutionary trends emerge from the compiled data: (1) Shallowly buried (<400 m), poorly indurated formations show limited hydraulic variability and scale independence; (2) Moderately buried (~800 m – 2,000 m), overconsolidated formations retain low matrix hydraulic conductivity after uplift, but exhibit gradually (partly significantly) enhanced hydraulic conductivity at depths <100 m due to the evolution of a pronounced decompaction zone. When devoted to less pronounced uplift and at greater present-day depths (>250 m) matrix and bulk hydraulic conductivities converge and predominantly range within a natural variability between 10-14 to 10-12 m/s, indicating effective self-sealing processes; (3) deeply buried formations (>2,000 m) become increasingly indurated and brittle, with reduced self-sealing capacity due to the loss of swellable clay mineral phases and fracture-dominated bulk hydraulic behaviour. Matrix and rock-mass hydraulic conductivities may diverge by several orders of magnitude. These trends provide predictive insights into the long-term barrier performance of argillaceous host rocks in HLW repositories.

Abstract. The brittle damage around tunnels in the form of spalling is common in massive crystalline rocks subjected to high in-situ stresses. Understanding the short-term development and long-term evolution of the Excavation Damage Zone (EDZ) is particularly important for deep geological repositories (DGRs) for heat-generating nuclear waste, as the EDZ may provide preferential pathways for radionuclide migration. This paper presents an overview of the PRECODE mine-by experiment at BedrettoLab, a newly developed underground research laboratory (URL) in southern Switzerland designed to investigate brittle fracturing and EDZ evolution in crystalline rocks under repository-relevant conditions. The PRECODE tunnel was excavated in Rotondo Granite using both controlled drill-and-blast (D&B) and line-drilling and rock-breaking (LDRB) methods to evaluate the influence of excavation technique on EDZ characteristics. The experiment combines multidisciplinary in-situ monitoring including acoustic emission, distributed acoustic and strain sensing, hydraulic monitoring, electrical resistivity tomography, and terrestrial laser scanning with laboratory-based fracture mechanics and time-dependent testing under varying environmental conditions. The main objectives are to characterize short-term EDZ formation, investigate long-term crack growth and stress corrosion processes, quantify permeability evolution, and assess the interaction between excavation-induced damage and nearby fault zones. Preliminary results indicate clear differences in AE activity and EDZ development between excavation methods, while observations from the mature EDZ of the more than 40-year-old Bedretto Tunnel suggest ongoing time-dependent brittle fracturing. The coupled in-situ and laboratory datasets provide a unique basis for improving predictive models and long-term safety assessments for nuclear waste repositories in crystalline rocks.

Tropical glacier dynamics are influenced by topography, near-surface thermodynamic conditions, basal processes indirectly reflected in surface motion, and climate variability, yet the regional-scale relationship between radar backscatter and glacier motion remains poorly constrained. We present a regional analysis of ice surface velocity and X-band SAR backscatter across the central and southern Cordillera Blanca, Peru, using 72 high-resolution TerraSAR-X StripMap scenes acquired between December 2016 and May 2021. Ice surface velocities were derived from SAR intensity offset tracking, yielding a continuous 4.42-year record for 195 glaciers at an effective ground resolution of ∼60 m, with mean velocities ranging from 0.01 to 0.69 m d⁻¹. Radiometrically calibrated γ⁰ backscatter shows strong seasonal and elevation-dependent variability, with pronounced annual amplitudes at mid-elevations (∼5000-5500 m a.s.l.) and nearly invariant signals above ∼5800 m a.s.l. While γ⁰ decreases during the wet season, glacier velocities show recurrent dry-season accelerations. Correlations between detrended velocity residuals and precipitation are weak to moderate and spatially heterogeneous, whereas correlations with the Niño 3.4 index are spatially heterogeneous. In contrast, γ⁰-precipitation correlations are predominantly negative, consistent with the sensitivity of X band SAR to near-surface snow/firn moisture conditions. Cumulative displacement is predominantly linear over the 4.42 year observation period, with departures from linearity confined to dynamically complex sectors such as glacier tongues, crevassed basins, and areas adjacent to proglacial lakes. Overall, the results provide spatially distributed evidence for a vertically and seasonally stratified glacier response in the analyzed sectors: radar backscatter primarily reflects elevation- and season-dependent near-surface snow/firn conditions, whereas glacier surface velocity appears to be more strongly associated with glacier geometry and broader glacier-dynamic processes.

Rock masses essentially consist of two components: the intact rock and spatially distributed rock joints of various orientations and persistence. The first step to characterize the behavior of a rock mass is to understand the strength and deformability of the intact rock by performing repetitive destructive laboratory tests on intact rock specimens. The challenge imposed in these tests is the existence of inherent microscopic heterogeneity, which can lead to a substantial variability. 3D printing technologies, specifically the binder jetting technique using sand and furan, has been recently adopted in studies of rock analogs due to its ability to produce synthetic sedimentary rocks with complex geometries and known characteristics, and reduced variability. For a synthetic material to be used as an analog for a natural brittle rock, it is essential to demonstrate that this analog exhibits brittle behavior similar to that of natural rocks. Although there have been several recent advancements in 3D printing applications in the geomechanics field, studies that thoroughly evaluate the 'brittle' behavior aspects of rock analogs are lacking. In this study, a set of requirements is established to evaluate the brittle behavior of the 3D-printed rock analogs, derived from uniaxial and triaxial compression, Brazilian, and fracture toughness laboratory tests, including the tensile and compressive strengths, crack initiation and crack damage thresholds, stiffness, and brittle-ductile transition. Analysis of laboratory test results showed that the analogs behaved similar to moderately-to-strong natural sandstones with a UCS in the range of 28–32 MPa and tensile strength of 5–6 MPa, with an acceptable repeatability among the test results. Under triaxial loading, the failure process was dominated by extensional fracturing at low confinement with a transition to a macroscopic shear failure mode at higher confining pressures (up to 15 MPa). At confining pressure beyond the Mogi line, compaction failure occurred indicating a typical brittle to ductile transition, which is corroborated by microstructural analyses.

Slope instabilities pose serious risks to infrastructure and communities in mountainous regions. Understanding their internal structure and time-dependent dynamics is vital for effective hazard assessment and mitigation. The Cuolm da Vi instability in central Switzerland, one of the largest slow-moving instabilities in the Alps, offers an ideal setting for field-based slope instability research. We present the motivation, design, and implementation of a novel large-scale multi-sensor seismic network to study the subsurface structure and deformation dynamics of Cuolm da Vi across an unprecedented range of spatial and temporal scales: from decimetres to kilometres and milliseconds to years. The sensor network includes a hexagonal grid of more than 1,000 seismic nodes primarily deployed for high-resolution 3D characterization. This temporary nodal array was complemented with a trenched 6.5km fibre-optic configuration, which covers the most unstable parts of Cuolm da Vi using a multi-directional cable layout, suited for Distributed Acoustic and Strain Sensing measurements (DAS & DSS). Data acquisition spanned two years so far, including controlled-source experiments and continuous seismic and strain sensing campaigns. Initial data screening demonstrates the network's potential to facilitate imaging of the internal structure and monitoring of seasonal subsurface instability processes. Our study shows the feasibility of dense long-term seismic monitoring in challenging Alpine terrain using nodal and distributed fibre-optic sensing techniques, opening new opportunities for slope instability research and hazard assessment.

Freeze-Thaw (FT) cycles can affect rock slope stability in periglacial environments, where temperature fluctuations near 0 degrees C may promote progressive damage in fractured rock masses. It is hypothesized that FT may trigger crack-tip propagation and the failure of rock bridges, thereby reducing rock slope stability. Although FT effects in porous rock have been extensively studied, the response of very low-porosity rocks remains poorly constrained, particularly with respect to fracture toughness. This study investigates the influence of FT cycling on Mode I fracture toughness KIc and fracture processes in two lithologies with contrasting microstructure: a low-porosity, fine-grained quartzite of an alpine periglacial environment and a higher-porosity, coarser-grained sandstone. Semi-Circular Bending (SCB) tests were conducted on specimens in both their natural state and after 60 FT cycles, performed in an environmental chamber between-12 degrees C and room temperature. The results show that quartzite exhibits a limited, though not negligible (only 1.3%), reduction in KIc after 60 FT cycles, whereas for the sandstone a 7.2% reduction was observed. Digital Image Correlation (DIC) was used to analyze the Fracture Process Zone (FPZ) development in the quartzite specimens, and post-failure thin sections were examined to characterize fracture paths and microstructural damage. Only minor changes are observed in quartzite, whereas sandstone exhibits increased crack tortuosity and intragranular cracking. These differences highlight the role of porosity, grain size, and microstructural arrangement in controlling FT-induced damage. A simplified linear damage model was applied to estimate the potential long-term evolution of the quartzite fracture toughness after the FT cycles. Despite the limited experimental damage, the result of the model suggests that even low-porosity rocks may experience a significant degradation in years, with potential implications for the long-term stability of rock slopes in periglacial environments.

Understanding how fluid injection perturbs stressed faults and triggers induced seismicity has become an urgent challenge in geophysics and hazard mitigation. Observations from subsurface fluid injection associated with geoenergy exploitation show that variations in injection pressure and rate and injected volume can strongly modulate seismicity rates and magnitudes. Yet, comparable injection operations may result in stable creep, slow slip, or dynamic rupture, highlighting persistent gaps in our understanding of the physical processes governing fluid-driven fault reactivation.Here, we investigate fluid-induced fault reactivation through decimetric-scale laboratory experiments on granite samples containing a 45° precut fault. Experiments are conducted in a biaxial apparatus under critically stressed conditions at 3 MPa normal stress, with independent control of normal and shear stresses. Fluids are injected directly into the fault surface while fault slip is measured using fibre-optic sensors (mini-SIMFIP) installed across the fault. Seismic activity is monitored through passive acoustic emission recordings, and repeated active ultrasonic surveys are performed throughout the experiments to track wave velocity changes and map fluid diffusion along the fault.By systematically varying the injection rate, we observe a clear transition from aseismic creep to slow slip and dynamic rupture. In all cases, fault slip nucleates at the injection point and subsequently propagates within the pressurized region of the fault. High injection rates generate localized overpressure near the injection point, triggering abrupt and seismic fault reactivation. During high-rate injection, we observe a pronounced drop in P-wave velocity, indicating strong mechanical perturbation of the fault zone, followed by a progressive velocity increase as fluids diffuse along the fault. In contrast, low injection rates lead to stable, aseismic slip confined to the pressurized zone, while intermediate rates produce a progressive reactivation sequence in which slip initiates aseismically, evolves into slow slip, and eventually transitions to dynamic rupture as the pressurized region expands.Our results show that injection rate governs fault slip behavior by controlling where slip nucleates and whether it remains confined to, or propagates beyond, the pressurized zone and accelerates dynamically.

A rapid laboratory method is presented for estimating gas permeability in tight, fine-grained rocks based on transient gas uptake following controlled pressure expansion between two connected cells. The technique operates under unconfined, isothermal conditions and enforces radial transport through an axially sealed core plug. Pressure-dependent apparent permeability and Klinkenberg-corrected permeability are derived directly from the pressure decay using analytical solutions and established relationships. Analysis was performed on a synthetic ceramic material and a suite of claystones. Method reliability is demonstrated by: (i) resolving fluid dynamic effects; (ii) reproducing published permeability and slip-flow behaviour in the rigid ceramic, serving as an internal benchmark; and (iii) capturing established maturity-related permeability trends in the claystones, supporting geological sensitivity. Klinkenberg-corrected permeability ranged between 10–19 and 10–21 m2. Slip factors ranged between 1.4 and 9.3 MPa, corresponding to average transport radii of 2–20 nm, smaller in the claystones than in the ceramic and consistent with pore-size measurements. For the ceramic, gas-specific Klinkenberg slopes followed the expected ordering based on kinetic diametre, consistent with classical slip-flow theory. In contrast, the claystones exhibited more complex gas-dependent behaviour: helium and hydrogen showed stronger slip enhancement, whereas methane, nitrogen, and argon deviated from simple slip flow. These deviations are primarily attributed to gas–solid interactions consistent with momentum accommodation behaviour, with methane exhibiting enhanced apparent transport that may reflect additional sorption-related processes. While not intended to replace high-precision permeability measurements under confinement, the proposed method provides a reproducible and physically consistent means of rapidly estimating matrix-scale transport properties in tight rocks. The method confers numerous advantages including simplicity, minimal hardware requirements, and the ability to resolve pressure and gas dependence, making it well suited for early-stage screening, analogue comparison, and resource-limited applications.

Accurately characterizing fault zones in crystalline basement rocks is essential for understanding fluid migration in the Earth's crust and how this influences fault stability and seismicity. While it is known that fault zones exhibit strong heterogeneity in structure and hydraulic properties, quantifying these variations across scales remains a challenge. The study presented investigates a deeply buried fault zone intersected by two inclined boreholes within a high overburden underground research laboratory (URL). As part of the FEAR (Fault Activation and Earthquake Rupture) project, this work provides key hydraulic and structural constraints needed to select and prepare experimental injection sites. These findings pose a necessary foundation for developing controlled fluid injection experiments and emphasize the importance of understanding scale-related effects during multi-scale observations. Through a combination of field-scale hydraulic testing, geophysical logging, and petrophysical analyses of core samples, we evaluate permeability, porosity, wave velocities, and fracture characteristics across multiple structural facies and on varying scales. The study finds that permeability varies over several orders of magnitude, largely controlled by the presence and connectivity of open fractures. Comparisons between lab and field data reveal pronounced scale effects, with lab tests underestimating the in-situ permeability due to the exclusion of large fractures and structural discontinuities. The fault zone shows a combination of localized and distributed flow behaviours, with no evidence of a continuous low-permeability fault core.

One major hurdle for understanding earthquake mechanics are observational limitations. Important phenomena like strain localisation, fault dilation, and fault healing are readily studied in rock mechanical laboratory experiments and with numerical models. At the scale of natural earthquakes, however, these phenomena are often unresolvable, even by state-of-the-art observatories. To overcome this limitation, we are currently building the Earthquake Physics Testbed at the Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab), an experimental testbed where we can activate an extensively instrumented natural fault zone via hydraulic stimulation. The goal of the Fault Activation and Earthquake Rupture (FEAR) project is to induce earthquakes of up to Mw~1.0 on this exceptionally well characterised and instrumented fault zone. Here we summarize the main scientific goals and current FEAR project status, and present first results from conducted experiments. We discuss how this large-scale experimental approach may allow us to tackle both fundamental science as well as practical questions on earthquake physics, induced seismicity and seismic hazard.

The fusion of UAV-based LiDAR and RGB surveys with geotechnical, geophysical, and hydrogeological field investigations enables a detailed characterization of the Cuejdel Lake landslide dam and its host landslide. To identify the main landslide features and to map the morphology of the study area, a 3D point cloud was generated from a UAV-based LiDAR survey covering an area of 126 ha on the western slope of the Muncelu Peak. Based on this dataset, a 3D surface model was constructed and textured using RGB imagery from a separate UAV-based photogrammetric survey, revealing the spatial distribution and characteristics of sedimentary facies within the eroded spillway outcrop.A two-dimensional plane representing the results of an electrical resistivity tomography (ERT) survey across the landslide dam was integrated into the 3D model, allowing sedimentary facies to be linked to distinct resistivity zones. The orientation of intact stratigraphy measured in the field was incorporated and extrapolated until intersecting the ERT plane. In addition, representative facies were sampled for grain-size analysis. Thirteen infiltration tests conducted parallel to the ERT profile provided proxy permeability values that were also integrated into the model.The investigations reveal that the feature previously interpreted as a single landslide actually consists of two distinct landslides, of which the northern landslide impounded Cuejdel Lake. Facies mapping shows a highly heterogeneous structure composed of large intact flysch blocks embedded in a low-permeability matrix of sand and clayey silt. Despite this heterogeneity, infiltration measurements indicate a relatively uniform permeability within the saturated phreatic zone, with values between 1 × 10⁻⁷ and 1 × 10⁻⁸ m s⁻¹. While facies distributions, laboratory analyses, and resistivity patterns indicate strong internal heterogeneity, the hydraulic behavior of the dam is controlled by the mixture of sand, silt and clay.This comparatively impermeable structure facilitated rapid lake-level rise and temporary overtopping during the early stage of dam formation. However, geomorphic evidence, water marks on tree trunks, and historical records indicate that this initial overtopping phase was halted by winter-induced lake-level lowering, after which erosion shifted to progressive spillway incision at the landslide toe during the following season.

The search for a suitable host rock for the deep geological disposal of high-level radioactive waste is a major societal challenge of our time. In Germany, clay-bearing formations are under investigation to potentially host a repository for high-level radioactive waste (alongside rock salt and crystalline rock). Their intrinsic properties such as low permeability, self-sealing efficiency with respect to fractures, and sorption capacity provide promising conditions for longterm waste containment. However, these properties are dependent on numerous factors such as mineralogical composition, temperature and stress conditions, and water content. Among these factors, the burial history and thus compaction affect mineralogy, porosity, permeability, and mechanical properties. Within the framework of the MATURITY project, the impact of the burial history on these properties is investigated based on a combination of different laboratory and field methods. For this purpose, a Lower Jurassic claystone formation (the Amaltheenton-Formation, Fm) which was subjected to variable maximum depth and subsequent uplift during its burial history was chosen as target formation. At five locations, in the margin area of the Lower Saxony Basin (Germany) shallow boreholes were drilled through the formation where varying degrees of maturation indicate substantial differences in maximum burial depth. In this contribution, we present the first results of initial project steps that show (a) a similar clay-dominated min eralogical composition of the Amaltheenton-Fm across the borehole locations, (b) an increase of max. burial temperatures (83-169 degrees C) over a lateral distance of similar to 50 km within the investigation area, (c) a gradual increase in bulk density accompanied by a reduction in porosity and permeability for normally-compacted Amaltheenton-Fm sequences along increasing max. burial temperatures, (d) a reverse trend of those parameters for a potentially undercompacted Amaltheenton-Fm sequence, and (e) hydraulic conductivity determined from in-situ hydraulic tests that significantly differs from laboratory derived equivalents and span two orders of magnitude (10(-5) to 10(-7)ms(-1)).

Brittle failure phenomena such as spalling occur around deep underground excavations in crystalline rock, leading to the formation of excavation damage zones (EDZ). The EDZ is characterized by stress-induced micro- and macrocracks that significantly increase the permeability of the rock mass surrounding the underground opening. In the context of deep geological repositories, such permeability increases pose a safety concern by creating preferential pathways for radionuclide migration around the underground excavation. The PRECODE project (“Progressive Excavation Disturbance Zone Evolution during and Post Mine-by Tunneling”) at the Bedretto Underground Laboratory in Switzerland investigates the temporal and spatial evolution of the EDZ in the Rotondo Granite at ~1300 m overburden depth. The tunnel section investigated in this study was excavated using a non-explosive mechanical excavation method to avoid blast-induced damage in the surrounding rock. Continuous pore pressure monitoring before, during, and after excavation, combined with repeated hydraulic and pneumatic testing using modular multiple packer systems (MMPS) in dedicated boreholes, contributes to a better understanding of transient hydro-mechanical processes in the near-field rock. Analysis of pressure responses from hydraulic and pneumatic testing reveals the impact of stress redistribution on the adjacent rock mass. The spatial extent of the EDZ varies locally between 0.6 and 1.1 m from the tunnel’s sidewall into the rock mass, and the hydraulic conductivity of the borehole intervals within the EDZ increased progressively by up to five orders of magnitude with time after excavation. The evolving EDZ interacts with pre-existing fractures, developing combined natural and induced fracture-controlled flow paths. These results offer new insights into the short-term evolution and hydraulic behavior of EDZs in crystalline rocks, supporting improved assessment and modeling of repository stability and long-term safety.

Understanding the internal structure and geometry of large-scale gravitational slope instabilities is crucial for hazard assessment and risk mitigation in mountainous regions. This study presents a high-resolution 2D and 3D seismic first-arrival traveltime tomography analysis of the Cuolm da Vi (CdV) slope instability, one of the largest active mass movements in the Alps. To achieve this, we conducted an extensive seismic survey, deploying over 1000 autonomous nodes across a 0.7 km2 area and acquiring data from 144 controlled-source shots. Our resulting 2D and 3D tomographic models reveal significant subsurface heterogeneities, including extensive low-velocity zones up to depths of 200 metres, indicative of severe rock mass disintegration. Additionally, strong lateral velocity variations persist throughout the unstable zone, further corroborating its structural complexity. Our findings align with previous studies that suggest toppling as the dominant deformation mechanism. The comparison between 2D and 3D velocity models highlights the critical role of out-of-plane effects, such as observed lateral ray bending, emphasizing the importance of 3D imaging for accurate characterization of complex instability structures. The 2D and 3D velocity models provide important constraints for estimating the total unstable rock volume and serve as a foundation for future geotechnical analyses and hazard assessments. This study also demonstrates the feasibility and effectiveness of large-scale nodal seismic deployments in alpine terrains, paving the way for further applications in monitoring and characterizing deep-seated slope instabilities.

Abstract Improving our understanding of induced and natural earthquakes benefits from controlled experiments in insitu laboratories. To investigate the processes during an Mw∼0 earthquake, we performed the “ Mzero ” experiments in a densely instrumented testbed of the Bedretto Underground Laboratory. These multi‐day hydraulic stimulation experiments went the opposite direction to typical induced seismicity research in that they were designed to enhance seismic rupture. In the first experiment, MzeroA, the rock mass was preconditioned by injecting water for 4 days at a pressure just below fracture reactivation pressure followed by a hydraulic stimulation above reactivation pressure. This strategy aimed at increasing the fractures area near critical stress conditions for shear failure, thus facilitating larger ruptures. During MzeroA, an event with Mw–0.54 was induced, followed by a distinct aftershock sequence. This event was one magnitude unit larger than any preceding event. In the second experiment, MzeroB, stimulation was initiated directly, without preconditioning. Compared to MzeroA, MzeroB exhibited higher seismicity rates, a larger seismicity cloud, and a pronounced migrating seismicity front. Combining seismological and hydromechanical observations, we discuss mechanisms that may have influenced the contrasting seismicity responses. In addition to fluid preconditiong, stress transfer from the Mw–0.54 mainshock, and natural seasonal pressure changes, may have contributed to modulating the seismogenic response. However, the relative importance of these processes remains uncertain given experimental limitations. Our results highlight both the potential and challenge of designing hydraulic stimulations to enhance or suppress seismic rupture, with implications for earthquake physics and induced seismicity.

Geothermal energy is considered a sustainable energy source for the transition to a carbon-neutral economy. In Central Europe, sufficiently hot source rocks are buried deep underground and comprise tight crystalline basement formations. To extract their thermal energy, hydraulic stimulation is used to create efficient heat exchangers in the context of Enhanced Geothermal Systems (EGS). Successful geothermal reservoir initiation requires a broad understanding of the hydro-mechanical coupling in fractured rock masses. For this reason, a decimeter-scale true-triaxial setup has been developed to conduct injection-driven shear tests under various stress conditions. To gain a deeper insight into the hydro-mechanical processes involved in hydraulic stimulation, a true triaxial compressive apparatus at the decimeter scale is employed. The experimental setup consists of 30 x 30 x 45 cm cuboidal granite specimens, each containing an oblique saw-cut laboratory fracture with different surface properties. The fracture is crossed by two boreholes equipped with packers to isolate a fracture interval. Fluid injection into the isolated intervals follows the typical HTPF (hydraulic testing of pre-existing fractures) scheme, including stepwise pressure increases and decreases. Stress boundary conditions are introduced by three sets of oil-filled flatjacks, contained within a steel frame which allows a more realistic and accurate replication of the stress conditions experienced by geological formations during hydraulic stimulation experiments. Stresses for individual tests are manipulated from hydrostatic to strike-slip conditions to test for different end member states of slip tendency. Fracture and rock deformations are recorded by 16 linear variable differential transformer (LVDT) sensors mounted externally along the edges of the specimen, volume changes in the flatjacks and a newly developed borehole deformation probe (mini-SIMFIP).

For the design of deep geological repositories for radioactive waste it is mandatory to understand the long-term deformation mechanisms under in-situ conditions, including time-dependent rock behavior at elevated temperatures, which are expected in the host rock after waste emplacement. Continuous convergence of tunnels in low permeability clay shales are associated with phenomena such as consolidation (a hydro-mechanical response) and creep (a rheological property). However, they may occur superimposed and targeted laboratory approaches are necessary for an accurate characterization. In the context of radioactive waste repositories, increased temperature may enhance time-dependent processes and affect the long-term safety assessment. In this context, multi-stage creep tests on pre-consolidated specimens were performed in a triaxial setup using sample material from the shaly facies of the Opalinus Clay formation. The tests were conducted on fully saturated specimens under temperature conditions of 30 and 60 °C. First results of our systematic study indicate that elevated temperature (60 °C) may lead to (1) greater strain accumulation during primary consolidation and primary creep phases, and (2) increased secondary creep rates compared to those observed at 30 °C under similar effective stress conditions.

This study presents a thermo-hydro-mechanical framework to model hydrothermal systems within a simplified faulted synthetic reservoir, replicating current production scenarios in The Netherlands and Germany. The reservoir, composed of porous and permeable sandstone, and the confining layer, made of porous but less permeable shale, undergoes a process where cold water is injected and hot water is extracted. A fault, situated 750 meters from the injection well, is investigated to examine the conditions when fault slip could occur. Various fault and formation stiffnesses are modeled to assess their impact on fault stability. Our analysis reveals that stress changes induced by hydrothermal operations can lead to fault reactivation, with the stiffness contrast between the reservoir and confining layers playing a significant role in when and where fault reactivation can occur. Stiffer confining layers lead to reactivation occurring more closely associated with the passage of the cooling front. In contrast, a stiffer reservoir results in greater and more gradual stress changes, making reactivation more closely related to the total volume of cooled rock.