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We performed a series of 12 hydraulic stimulation experiments in a 20 x 20 x 20 m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: i) ductile ones with intense foliation and ii) brittle-ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydro-mechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in-situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b-values, spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle-ductile shear zones, while injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided. [ABSTRACT FROM AUTHOR]

To characterize the stress field at the Grimsel Test Site (GTS) underground rock laboratory a series of hydrofracturing test and overcoring test were performed. Hydrofracturing was accompanied by seismic monitoring using a network of highly sensitive piezo sensors and accelerometers that were able to record small seismic events associated with decimeter-sized fractures. Due to potential discrepancies between the hydro-fracture orientation and stress field estimates from overcoring, it was essential to obtain high-precision hypocenter locations that reliably illuminate fracture growth. Absolute locations were improved using a transverse isotropic P-wave velocity model and by applying joint hypocenter determination that allowed computation of station corrections. We further exploited the high degree of waveform similarity of events by applying cluster analysis and relative relocation. Resulting clouds of absolute and relative located seismicity showed a consistent east-west strike and 70° dip for all hydro-fractures. The fracture growth direction from microseismicity is consistent with the principal stress orientations from the overcoring stress tests provided an anisotropic elastic model for the rock mass is used in the data inversions. σ1 is significantly larger than the other two principal stresses, and has a reasonably well-defined orientation that is subparallel to the fracture plane. σ2 and σ3 are almost equal in magnitude, and thus lie on a circle defined by the standard errors of the solutions. The poles of the microseismicity planes also lie on this circle towards the north. The trace of the hydraulic fracture imaged at the borehole wall show that they initiated within the foliation plane, which differs in orientation from the microseismicity planes. Thus, fracture initiation was most likely influenced by a foliation-related strength anisotropy. Analysis of P-wave polarizations suggested double-couple focal mechanisms with both thrust and normal faulting mechanisms present, whereas strike-slip and thrust mechanisms would be expected from the overcoring-derived stress solution. The reasons for these discrepancies are not well understood, but may involve stress field rotation around the propagating hydrofracture. Our study demonstrates that microseismicity monitoring along with high-resolution event locations provides valuable information for interpreting stress characterization measurements. [ABSTRACT FROM AUTHOR]

Clay shale specimens were exposed to cyclic relative humidity variations to investigate the response of the material to natural environmental changes. Opalinus Clay, a clay shale chosen as host rock for nuclear waste disposal in Switzerland, was utilized. The specimens were exposed to stepwise relative humidity cycles where they were alternately allowed to equilibrate at 66 and 93% relative humidity. Principal strains were monitored throughout the experiments using strain gauges. After each relative humidity cycle, Brazilian tensile strength tests were performed to identify possible changes in tensile strength due to environmental degradation. Results showed that Opalinus Clay follows a cyclic swelling-shrinkage behaviour with irreversible expansion limited to the direction normal to bedding, suggesting that internal damage is restricted along the bedding planes. The Brazilian tensile strength in direction parallel and normal to bedding as well as the water retention characteristic remained unaffected by the RH variations. [ABSTRACT FROM AUTHOR]

The onset of dilation and uniaxial compressive strength (UCS) of rocks are known to be affected by mineralogical, petrographic and microstructural features. Therefore micro-textural variations are essential for understanding variation in strength and failure behaviour of rocks. In this study volcanic rock samples with different micro-textural characteristics from different locations in Turkey were evaluated. The mechanical tests revealed considerable variations in unconfined uniaxial compressive strength and the crack initiation (σ CI ) threshold. The UCS ranges between 108–289 MPa, and σ CI ranges between 46–130 MPa. Detailed quantitative petrographic and mineralogical analyses were carried out on each tested specimen to understand these strength variations. Additionally, microstructural studies were carried out on thin sections before and after mechanical perturbation to analyze (1) the effect of the main constituents of the rock specimens and mineral heterogeneity on crack initiation and propagation, and (2) the orientation and distribution of micro- and macro-cracks formed during laterally unconfined compression. The synthesis of mechanical, mineralogical, petrographic and microstructural data suggests that both the crack initiation threshold and peak strength are influenced by the ratio of the area of fine grain groundmass (grains smaller than 200 μm) to amphibole and plagioclase phenocrysts. In addition σ CI and UCS decrease with an increasing normalized area of biotite phenocryst. [ABSTRACT FROM AUTHOR]

• A 3.0 × 106 m3 rock slope accelerated measurably before failure. • Slope stability changed rapidly before failure. • 15 debris flows occurred within 7 days after failure (13 in absence of rainfall). • First debris flow initiated within 30 s after rock avalanche deposition. • Wet sediment entrainment and impact loading enhanced deposit mobility. Catastrophic collapse of large rock slopes ranks as one of the most hazardous natural phenomena in mountain landscapes. The cascade of events, from rock-slope failure, to rock avalanche and the near-immediate release of debris flows has not previously been described from direct observations. We report on the 2017, 3.0 × 106 m3 failure on Pizzo Cengalo in Switzerland, which led to human casualties and significant damage to infrastructure. Based on remote sensing and field investigations, we find a change in critical slope stability prior to failure for which permafrost may have played a destabilizing role. The resulting rock avalanche traveled for 3.2 km and removed over one million m3 of glacier ice and debris deposits from a previous rock avalanche in 2011. Whereas this entrainment did not lead to an unusually large runout distance, it favored debris flow activity from the 2017 rock avalanche deposits: the first debris flow occurred with a delay of 30 s followed by ten debris flows within 9.5 h and two additional events two days later, notably in the absence of rainfall. We hypothesize that entrainment and impact loading of saturated sediments explain the initial mobility of the 2017 rock avalanche deposits leading to a near-immediate initiation of debris flows. This explains why an earlier rock avalanche at the same site in 2011 was not directly followed by debris flows and underlines the importance of considering sediment saturation in a rock avalanche's runout path for Alpine hazard assessments. [ABSTRACT FROM AUTHOR]

Performing stimulation experiments at approximately 1 km depth in the Bedretto Underground Laboratory for Geosciences and Geoenergies necessitates identifying and characterizing the target fault zone for on-fault monitoring of induced fault slip and seismicity, which presents a challenge when attempting to understand seismogenic processes. We discuss the multidisciplinary approach for selecting the target fault zone for experiments planned within the Fault Activation and Earthquake Ruptures (FEAR) project, for which the aim is to induce the fault slip and seismicity for an earthquake magnitude of up to 1.0 while enhancing the monitoring and control of fluid-injection experiments. Structural geological mapping, remote sensing, exploration drilling and borehole logging, ground-penetration radar, and laboratory investigations were employed to identify and characterize the target fault – a ductile–brittle shear zone several meters wide with an intensely fractured volume spanning over 100 m. Its orientation in the in situ stress field favors reactivation in normal to strike-slip regimes. Laboratory tests showed slight velocity strengthening of the fault gouge. The fault's architecture, typical for crystalline environments, poses challenges for fluid flow, necessitating detailed hydraulic and stress characterization before each of the FEAR experiments. This multidisciplinary approach was crucial for managing rock volume heterogeneity and understanding implications for the dense monitoring network. Successfully identifying the fault sets the stage for seismic activation experiments commencing in spring 2024. [ABSTRACT FROM AUTHOR]

The relationship between rock-slope failure and glacier retreat is complex, and paraglacial failures often lack clearly identified triggers. To better understand the role of glacier retreat in rock-slope failures, we analysed the processes that led to the October 1967 Grewingk landslide in Kachemak Bay State Park on the Kenai Peninsula, Southcentral Alaska. The rock material collapsed onto the glacier toe and into its pro-glacial lake and produced a tsunami wave that swept the outwash plain. On the day of the failure, rainfall and snowmelt were well within normal ranges, and seismic records show no significant shaking. Three years prior to the 1967 failure, the slope withstood the second largest earthquake ever recorded (Great Alaskan earthquake, MW 9.2). We reassessed the volume of the failure by differencing pre- and post-digital terrain models and found a value of 20–24 × 106 m3, which is four times smaller than a previous estimate. The back analysis of the Grewingk landslide is based on remote sensing data and field measurements including aerial satellite image analysis, detailed surveying and understanding of the structural geology, a kinematic analysis, and runout modelling. Our research provides an example of a major paraglacial failure that lacks an obvious trigger and points to several geological factors and changing environmental conditions that likely promote such failures. This study further indicates that the Grewingk landslide, pre-conditioned by the geometry of faults and joints, may have reached a critical stability state due to internal processes and the potential combined effects of seismic activity and glacier retreat prior to the collapse. [ABSTRACT FROM AUTHOR]

Table 2 and Eq. 3 of the original article are reported incorrectly. The correct Table 2 and Eq. 3 are shown. [ABSTRACT FROM AUTHOR]

• Mode I cracks in anisotropic rocks tend to kink towards foliation/bedding planes. • Experiment data are compared to the predictions of different growth criteria. • Maximum tangential stress criterion predictions are closest to the experiment data. • The inclusion of the T-stress improves the predictions of the growth criteria. This paper presents the results of 124 pure Mode I fracture toughness tests on two types of anisotropic rocks, the metamorphic Grimsel Granite and the sedimentary Mont Terri Opalinus Clay. The results show that Mode I cracks in anisotropic rocks tend to kink towards the foliation or bedding planes that are weaker in strength. The experiment data for kink angle and apparent fracture toughness are compared to the predictions of three well-known crack growth criteria: the maximum tangential stress (MTS), the maximum energy release rate (MERR), and the minimum (maximum in anisotropic materials) strain energy density (MSED). All these criteria overestimate the kink angle, while underestimating the apparent fracture toughness. The MTS yields predictions that are closest to the experiment data, while the MSED predictions are the furthest. We also show that the inclusion of the T-stress into the MTS criterion improves the predictions of the fracture growth. This indicates that the T-stress can play a significant role in the Mode I fracture growth of anisotropic rocks. [ABSTRACT FROM AUTHOR]

The fractographic analysis of plumose axes of exfoliation fracture surfaces from a preceding study in the Grimsel region of the Swiss Alps suggested complex directional trends of near-surface (i.e., within ~100 m below ground surface) maximum principal stress ( σ 1 ) trajectories within the investigated inner-trough valleys (i.e., U-shaped valleys). The stress trajectories describe a pattern governed by local topographic variations. In situ stress measurements from the region are scarce, locally scattered, based on different methods, and, thus, difficult to interpret at regional scale. In this study, we inferred that plumose structure axes form parallel to compressive σ 1 , and improve our knowledge of the near-surface three-dimensional stress field in alpine settings with complex topography. We investigated near-surface stress tensors utilising three-dimensional, elastic numerical models. Our models account for morphological details of the Grimsel area at the decametre scale. We used two models with vertical boundaries aligned N–S/W–E (0°-model) and NW–SE/SW–NE (45°-model). These models allowed investigating gravitational stresses and superimposed isotropic and anisotropic compressive stresses arising from (active) tectonic shortening and/or stresses induced by exhumation (remnant stresses). Our model results illustrate that the superposition of gravitational stresses and realistic horizontal strains reveals complex near-surface stress trajectories that widely follow the patterns of exfoliation fracture plumose axes. Our models demonstrate large variations of stress orientations within the shallow subsurface, including depth levels where exfoliation fractures formed. These variations cannot be captured by a small number of local stress measurements. Our study reveals that directional data from exfoliation fracture plumose axes of Middle to Upper Pleistocene ages can be used to constrain geologically recent and current maximum principal stress directions of the shallow subsurface of up to a few hundred metres below ground. [ABSTRACT FROM AUTHOR]

In this study, organic-lean mudstones that have undergone variable maximum burial were investigated to assess potential effects of diagenesis on permeability, pore space and unconfined compressive strength. This is particularly relevant for mudstone formations, such as encountered in the Lower Saxony Basin, where local subsidence and uplift periods caused a strongly heterogeneous basin development. The studied sample set provides a unique opportunity for such a study insofar that over a relatively short distance of 50 km (Hils area) and with a relatively homogeneous mineral composition, thermal maturities increase from 0.48 to 1.45% vitrinite reflectance. This corresponds roughly to temperatures and maximum burial depths of 75 to 165 °C and 1,300 m to 3,600 m, respectively. The results demonstrate that permeabilities and porosities decrease strongly up to a maximum burial depth of approximately 2,500 m, from 2.7 to 0.21 × 10-21 m2 and approximately 14 to 8%, respectively. Reduction of porosity occurs predominantly in the macro- and mesopore range as evidenced by lowpressure nitrogen adsorption measurements. Further burial to a maximum depth of approximately 3,600 m leads only to negligible change of these properties. Initial reduction is due to mechanical compaction, whereas further mechanical compaction is possibly counteracted by the generation of secondary porosity via cracking of solid bitumen and/or kerogen, similar to findings reported for various organic-rich shales but much less pronounced. Specific surface areas (approximately 15 to 33 m2/g) gradually decrease with increasing maximum burial depth. Unconfined compressive strength (UCS) values increase from 25 MPa at a maximum burial depth of 1,300 m to 40 (Liassic) and 50 MPa (Doggerian) at 2,600 m. Decreasing UCS values were observed for specimen that experienced larger burial depths up to 3,600 m (31 MPa for Liassic and 36.5 for Doggerian specimen). Whether the properties obtained correspond to those encountered at the proposed maximum burial depths is questionable, since severe uplift, storage of the cores (for over 30 years in wooden boxes at ambient conditions), and sample preparation may have resulted in changes in these properties. However, data on mineralogy and comparison of petrophysical properties obtained in this study with those from mudstones that have not been uplifted indicate that at least the trends observed and to some extent the absolute values observed are representative of maximum burial conditions. [ABSTRACT FROM AUTHOR]

Opalinus Clay, a Mesozoic clay shale, has been chosen as host rock formation for the disposal of radioactive waste in Switzerland. For this study, borehole damage zones were utilized as a proxy for an excavation damage zone that forms around a circular, mechanically excavated tunnel in intact Opalinus Clay. Pilot boreholes were resin-impregnated and over-cored at different times after drilling, to provide insight into the time-dependent formation of fractures on both the micro- and the macro-scale. Observed fractures were characterized in terms of failure mode, their relation to the rock anisotropy, and the in-situ stress tensor. The analyses show that fractures that form in the short term initiate as shear fractures at the pilot-borehole wall and propagate parallel to bedding. Typically, a dominant shear fracture tangential to the pilot borehole wall was observed. Upon propagation of these shear fractures, wing cracks, horsetail splays and second-order shears form sub-parallel and sub-perpendicular to bedding planes, forming a complex fracture network, which extends a quarter pilot-borehole diameter into the rock mass. In the longer term, tangential shear fractures tend to propagate in a direction opposite to the initial propagation direction. In addition, new bedding-parallel fractures deeper in the rock develop, leading to the formation of thin slabs, buckling of the slabs when unsupported and eventually progression of the buckling zone deeper into the rock mass. Buckling is associated with the formation of extensional fractures normal to bedding in the center and lateral to the buckling zone. The zone of buckled rock slabs was found to have an extension of more than one borehole diameter at the time of preservation with resin. In the short term, the axis connecting the maximum failure depth on opposing sides of the borehole is parallel to the minimum stress direction in a plane normal to the borehole axis. In the long term, this axis rotates significantly towards the maximum stress direction, primarily as a consequence of tangential shear fracture propagation, slab formation and buckling. Dissipation of excess pore pressures may be the key process underpinning longer-term fracture propagation and formation. [ABSTRACT FROM AUTHOR]

Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale. [ABSTRACT FROM AUTHOR]

Over the past few decades, Toarcian (Early Jurassic) black shale deposits of NW Europe have been extensively studied, and the possible global and regional mechanisms for their regional variation have been discussed. In this context, the black shales of the Northwest German Basin are still sparsely studied with regard to their palaeo-depositional history. This study aims to understand the connection between regional and global influences on the widespread Early Toarcian oceanic anoxic event by examining two wells covering the Upper Pliensbachian to Upper Toarcian sediments in the Northwestern German Basin. The core intervals were analysed using a multidisciplinary approach, including geochemistry, biostratigraphy and organic petrography. Marine palaeoenvironmental changes were reconstructed, and sediment sequences were stratigraphically classified to allow a supra-regional stratigraphic correlation. The results reveal complex interactions between sea level changes, climate warming, basin confinement, and Tethys–Arctic connectivity resulting in the Toarcian black shale deposition. Upper Pliensbachian sediments were deposited under terrigenous influence, shallow water depths, and predominantly oxic bottom water conditions. The deposition of black shale is characterized by algal organic material input and anoxic bottom water conditions. Strong correlations between water stratification, anoxia, and bioproductivity suggest that global warming and intensification of monsoonal rainfall, continental weathering, and increasing freshwater and nutrient inputs were the main factors controlling the formation of black shales. Prolonged deposition of OM-rich sediments in the NWGB may be related to intensified monsoonal precipitation in northern Europe and enhanced Tethys–Arctic connectivity at the serpentinum–bifrons transition.Graphical Abstract: Over the past few decades, Toarcian (Early Jurassic) black shale deposits of NW Europe have been extensively studied, and the possible global and regional mechanisms for their regional variation have been discussed. In this context, the black shales of the Northwest German Basin are still sparsely studied with regard to their palaeo-depositional history. This study aims to understand the connection between regional and global influences on the widespread Early Toarcian oceanic anoxic event by examining two wells covering the Upper Pliensbachian to Upper Toarcian sediments in the Northwestern German Basin. The core intervals were analysed using a multidisciplinary approach, including geochemistry, biostratigraphy and organic petrography. Marine palaeoenvironmental changes were reconstructed, and sediment sequences were stratigraphically classified to allow a supra-regional stratigraphic correlation. The results reveal complex interactions between sea level changes, climate warming, basin confinement, and Tethys–Arctic connectivity resulting in the Toarcian black shale deposition. Upper Pliensbachian sediments were deposited under terrigenous influence, shallow water depths, and predominantly oxic bottom water conditions. The deposition of black shale is characterized by algal organic material input and anoxic bottom water conditions. Strong correlations between water stratification, anoxia, and bioproductivity suggest that global warming and intensification of monsoonal rainfall, continental weathering, and increasing freshwater and nutrient inputs were the main factors controlling the formation of black shales. Prolonged deposition of OM-rich sediments in the NWGB may be related to intensified monsoonal precipitation in northern Europe and enhanced Tethys–Arctic connectivity at the serpentinum–bifrons transition. [ABSTRACT FROM AUTHOR]

An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18-24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1-14.4 MPa for σ1, 9.2-10.2 MPa for σ2, and 8.6-9.7 MPa for σ3 with σ1 plunging to the east at 30-40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past. [ABSTRACT FROM AUTHOR]