François Renard

• Aspaas, Andreas Grøvan; Bievre, Grégory; Lacroix, Pascal; Langet, Nadege; Aga, Juditha & Skrede, Ingrid [Vis alle 9 forfattere av denne artikkelen] (2026). Seasonal and inter-annual evolution of the deformation of two Arctic landslides. Earth Surface Dynamics. ISSN 2196-6311. 14(5), s. 729–761. doi: 10.5194/esurf-14-729-2026. Fulltekst i vitenarkiv Vis sammendrag

  Landslides in glacial and periglacial environments are increasingly affected by climate change, with sudden failures reported in high mountain regions and the Arctic. The complex mechanisms behind these events are often poorly understood due to a lack of dense in situ data. We investigate two slow-moving landslides in Arctic Norway (70° N), the Jettan and Gámanjunni landslides, located approximately 10 km apart: Jettan, a complex slide in micaschist and calcite marble situated below the permafrost boundary, and Gámanjunni, a rotational slide in micaschist situated above the permafrost boundary. Using over a decade of multi-physics observations, including geodetic, borehole, seismic, and hydrological data, we examine surface and subsurface deformation. Both landslides display similar seasonal surface velocity patterns, with peaks in spring and autumn, likely influenced by pore-water infiltration. At Jettan, twelve years of inclinometer data in boreholes reveal a transition from steady state to seasonal deformation in two shear zones. Since 2020, spring accelerations have intensified in years coinciding with deeper snowpacks and associated melt. These observations, together with statistical modeling, suggest that the shear-zones are becoming increasingly localized and sensitive to pore-water pressure. Conversely, autumn acceleration is not seen in localized shear zones but manifests as distributed volumetric deformation. Seismic velocity variations within the landslide body also exhibit seasonal patterns that correspond with geodetic velocity, interpreted as changes in landslide rigidity due to water infiltration. This integrated analysis of surface and subsurface data offers new insights into the evolving deformation of Arctic landslides, emphasizing the influence of hydrological forcings on both seasonal and long-term deformation processes.

• Prastyani, Erina; Wang, Lei; McBeck, Jessica; Cordonnier, Benoît; Rybacki, Erik & Dresen, Georg [Vis alle 7 forfattere av denne artikkelen] (2026). Transition From Faulting to Dilatant Cataclastic Flow in Carrara Marble Revealed by Acoustic Monitoring and 4D X‐Ray Imaging. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 131(8). doi: 10.1029/2026jb034160. Fulltekst i vitenarkiv Vis sammendrag

  The brittle–ductile transition, typically located near the base of the seismogenic zone, controls strain localization and the evolution of physical properties, with direct implications for fluid transport and fault mechanics. However, quantitative constraints on strain partitioning and porosity evolution across this transition remain limited, particularly in low‐porosity carbonate rocks. We conducted triaxial compression experiments on dry Carrara marble at confining pressures of 5 and 100 MPa using two complementary techniques: (a) acoustic emission (AE) monitoring coupled with P‐wave velocity measurement, and (b) in situ X‐ray microtomography combined with digital volume correlation (DVC). AE activity remains low at all pressures, with source mechanisms becoming increasingly compaction‐dominated at the highest confinement. P‐wave velocities fall markedly during deformation by up to 60% at 5–10 MPa, ∼50% at 40 MPa, and ∼40% at 100 MPa, reflecting pervasive microcrack development. DVC reveals that the proportion of dilative volumetric strain decreases with confinement from nearly 90% at 5 MPa to 50% at 70–90 MPa, yet dilation remains a major component and is comparable to compaction even at the highest pressures. X‐ray–derived bulk porosity increases from <1% to ≈1.5% at low confinement prior to failure, and to 10% at higher confinements, indicating substantial porosity generation during distributed dilatant cataclastic flow. Together, these results provide the first quantitative link between P‐wave velocity reduction, volumetric strain partitioning, and transient porosity generation across the brittle to semi‐ductile transition, revealing how the interplay of dilation and compaction governs both elastic properties and strain localization in low‐porosity crystalline carbonate.

• Vishnu, A. R.; Doan, Mai‐Linh; Hollingsworth, James; Barras, Fabian; Cordonnier, Benoit & Renard, Francois (2026). Shear Fault Formation in Granite Under High and Transient Co‐Seismic Strain Rates Unveiled by Ultra‐Fast X‐Ray Imaging. Geophysical Research Letters. ISSN 0094-8276. 53(14). doi: 10.1029/2025gl120857. Fulltekst i vitenarkiv Vis sammendrag

  During an earthquake a dynamic rupture propagates, causing fractures and permanent damage that modify the mechanical properties of the fault and its surroundings. Reciprocally, the energy dissipated by the mechanical transformation of the fault rock has a direct feedback on the dynamics of the seismic rupture. We reproduce and image these co‐seismic damage processes with sub‐microsecond time resolution through dynamic loading experiments on confined Westerly granite samples. Using synchrotron X‐ray imaging and ultra‐high‐speed cameras, we observed that dynamic compression with confinement produces shear faults and a localized granulation of the rock, known as gouge, before significant slip occurs. This fragmentation process begins with volumetric damage, followed by shear deformation less than 5 microseconds later. Our results indicate that gouge formation is initiated by the dynamic rupture itself, which reduces the strength of a fault prior to other weakening mechanisms that may occur during the earthquake slip.

• Aiken, John Mark; Barras, Fabian; Renard, Francois; Hirth, Greg; Kelemen, Peter B. & Sohn, Robert A. (2026). Slowly Migrating Fracture Swarms in an Actively Serpentinizing Borehole. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 131(2). doi: 10.1029/2025jb032133. Fulltekst i vitenarkiv Vis sammendrag

  Peridotite rocks are primary targets for engineered geological carbon sequestration efforts because the carbon in ‐bearing fluids is transferred to the rock in the form of carbonate minerals during alteration reactions. Sequestration efforts must necessarily open fractures in the rocks surrounding a pumped borehole, but the current understanding of fracture growth during serpentinization of peridotite is limited to theoretical models and laboratory experiments on small samples. We deployed hydrophone arrays in peridotite boreholes established by the Oman Drilling Program and detected downward migrating fracture swarms that represent the first field observations of fracture growth in a serpentinizing rock. More than 2 years after the boreholes were established, we detected four, downward migrating tensile fracture swarms during an interval of elevated pore pressure following large rain events. All of the swarms occurred within a partially confined section of the local aquifer, beginning at a depth of 170 m and migrating to the bottom of the 400 m‐deep hole at average rates of 6–20 cm.. We demonstrate that pore fluid processes can explain both the triggering of the tensile fracture swarms and their slow migration rates. Our results indicate that the tip of fractures propagating away from the borehole maintained near‐critical states over time such that relatively small increases in fluid pressure triggered tensile fracturing episodes, indicating that carbon sequestration efforts in mafic rocks should be able to open fractures and expose fresh rock to mineralization.

• Beck, Jessica Ann Mc; Cordonnier, Benoit & Renard, Francois (2026). Asperity dynamics and secondary damage development during the closure of a fracture in granite. Tectonophysics. ISSN 0040-1951. 921. doi: 10.1016/j.tecto.2025.231044. Fulltekst i vitenarkiv
• Linga, Gaute; Pierce, Kevin; Moura, Marcel; Mathiesen, Joachim; Renard, Francois & Borgne, Tanguy Le (2026). Dynamic multiphase flow triggers chaotic mixing in porous media. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 123(34). doi: 10.1073/pnas.2612884123. Fulltekst i vitenarkiv
• Pierce, Kevin; Borgne, Tanguy Le; Renard, Francois & Linga, Gaute (2026). How pore-scale disorder controls fluid stretching in porous media. arXiv. doi: 10.48550/ARXIV.2604.02981. Fulltekst i vitenarkiv
• Mirzaei, Fazel; Jaiswal, Mukul; Zeman, Jessica Ann; Tekseth, Kim Robert Bjørk; Cordonnier, Benoit & Agofack, Nicolaine [Vis alle 10 forfattere av denne artikkelen] (2026). Time-Resolved X-ray and Neutron Imaging of Brine Percolation and Liquefaction in an Ultra-Soft Sandstone. Transport in Porous Media. ISSN 0169-3913. 153(3), s. 1–19. doi: 10.1007/s11242-025-02284-z. Fulltekst i vitenarkiv Vis sammendrag

  The inherent instability and risk of liquefaction associated with unstable glaciomarine clay are both scientifically intriguing and societally important. Stabilizing sensitive soils is increasingly necessary with the unfolding climate crisis, which leads to wilder and wetter weather. The relevant length scales extend from nanometer-sized pores to kilometer-sized geological features. Laboratory experiments are important for gaining a better understanding of these phenomena. Here, we present a study of the ultra-soft sandstone Saltwash South, which essentially consists of quartz particles glued together by clay, as a proxy for nanoscopically fine-grained sensitive soil to facilitate time-resolved imaging of the onset of activation and liquefaction. By employing in situ time-resolved combined X-ray and neutron computed tomography (CT), we visualize the structural deformation caused by the presence of water and salts. While neutron imaging was sensitive to the presence of normal and heavy water, simultaneous X-ray imaging was used to measure the porous structure, swelling, and initial liquefaction response of the consolidated rock. Uniform expansion was observed in regions exposed to water, reflecting clay swelling and disaggregation. In tightly confined samples, the swelling and disaggregation were suppressed. Finally, we discuss future perspectives of this promising approach to studying liquefaction phenomena in porous media.

• Beck, Jessica Ann Mc; Cordonnier, Benoit; Zhu, Wenlu & Renard, Francois (2026). Transition From Fracture Propagation to Coalescence in Westerly Granite During the Preparation Process of Macroscopic Failure. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 131(1). doi: 10.1029/2025JB032676. Fulltekst i vitenarkiv
• Miao, Shuting; Zhang, Xiujun; Renard, Francois; Zang, Arno & Pan, Pengzhi (2025). Strain Localization in Granite via Optical Fiber Sensing: Effects of Crack Heterogeneity and Stress Relaxation. Rock Mechanics and Rock Engineering. ISSN 0723-2632. 58(9), s. 10043–10062. doi: 10.1007/s00603-025-04698-z. Fulltekst i vitenarkiv Vis sammendrag

  Monotonic triaxial compression and multi-stage relaxation tests were conducted on both pristine and thermally fractured granite core samples to investigate the effect of crack heterogeneity and stress relaxation on strain localization. Full-field strain distributions across the sample surfaces were mapped using Distributed Optical Fiber Sensing in combination with a compact fiber configuration. Experimental results indicate that increased crack heterogeneity enhances both the strength and total strain accumulation of granite sample prior to macroscopic failure under confined conditions. Granite with greater heterogeneity exhibits an earlier onset, more prolonged, and slower progression of strain localization compared to less heterogeneous granite, ultimately resulting in a higher degree of strain localization prior to macroscopic failure. In highly heterogeneous granite, high-strain events are redistributed across the sample, preventing excessive strain localization in specific volumes, whereas in less heterogeneous granite, the strain localization zone remains fixed before system-size failure. Optical microscopic observations confirm that higher crack heterogeneity promotes the further development of pre-existing compact and distributed microcracks during mechanical loading, whereas lower heterogeneity promotes the formation of isolated, long microcracks. Additionally, stress relaxation facilitates subcritical crack growth, promoting strain accumulation in pre-damaged zones and enhancing strain heterogeneity within the granite samples. These findings highlight the significant role of stress state, pre-existing damage zones, and initial crack heterogeneities in governing strain localization and macroscopic failure behavior in the upper crust.

• Jacob, Jean-Baptiste; Cordonnier, Benoit; Zhu, Wenlu; Vishnu, A. R.; Wright, Jonathan & Renard, Francois (2025). Tracking Intragranular Stress Evolution in Deforming Sandstone Using X-Rays. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 130(7). doi: 10.1029/2025JB031614. Fulltekst i vitenarkiv
• Beck, Jessica Ann Mc & Renard, Francois (2025). Predicting the location of the process zone in triaxial compression experiments on Darley Dale sandstone. Geophysical Journal International. ISSN 0956-540X. 242(3). doi: 10.1093/gji/ggaf241. Fulltekst i vitenarkiv
• Nanni, Ugo; Bouchayer, Coline Lili Mathy; Åkesson, Henning; Lefeuvre, Pierre-Marie; Mannerfelt, Erik Schytt & Köhler, Andreas [Vis alle 12 forfattere av denne artikkelen] (2025). Observed positive feedback between surface ablation and crevasse formation drives glacier acceleration and potential surge. Nature Communications. 16(1). doi: 10.1038/s41467-025-66349-9. Fulltekst i vitenarkiv Vis sammendrag

  Sudden glacier acceleration and instability, e.g. surges, strongly influence glacier ice loss. However, lack of in-situ observations of the involved processes hampers our ability to understand, quantify and model such a role. We present an analysis of the initiation of a surge (Kongsvegen glacier, Svalbard), focusing on the interplay between climatic and glacier-specific drivers. We integrate two decades of in-situ observations (GNSS, borehole and surface seismometers) with runoff simulations, and remotely sensed surface-elevation changes. We show that initial glacier thinning led to localized acceleration and crevassing. Then, we show that stronger surface melt enabled meltwater to reach the glacier bed. This input promotes high basal water pressure and glacier sliding, and in turn further surface crevassing. Our observations suggest that this positive feedback leads to the expansion of the initially localized instability. Our findings highlight mechanisms that could trigger glacier instabilities under a warming atmosphere beyond the High Arctic.

• Shafabakhsh, Paiman; Cordonnier, Benoit; Borgne, Tanguy Le; Mathiesen, Joachim; Linga, Gaute & Pluymakers, Anne [Vis alle 9 forfattere av denne artikkelen] (2025). Coupling Neutron and X‐Ray Imaging of Fluid Mixing and Precipitation in Rocks: Challenges and Opportunities. Water Resources Research. ISSN 0043-1397. 61(12). doi: 10.1029/2025wr041911. Fulltekst i vitenarkiv Vis sammendrag

  Solute mixing in rocks plays a central role in a wide range of reactive processes. However, how the complex 3D pore structure of rocks governs mixing rates remains largely unknown. Moreover, some mixing‐driven reactions—such as dissolution and precipitation—can modify the pore space, with poorly understood consequences for mixing itself. Recent advances in X‐ray imaging techniques have significantly enhanced our ability to visualize the pore‐scale rock architecture of rocks and a wide range of fluid processes. However, capturing solute mixing and its impact on chemical reactions—such as mineralization—remains a major challenge. Here, we investigated the potential of coupling time‐lapse 3D neutron and X‐ray imaging to characterize reactive fluid mixing and subsequent calcium carbonate mineralization in porous basalt. Two flow‐through experiments were performed with co‐injected CaCl 2 and Na 2 CO 3 , leading to precipitation. Neutron imaging tracked fluid mixing, while X‐ray imaging distinguished the solid matrix from pore space for fluid analysis. The first experiment showed steady transverse mixing, while a second experiment revealed temporal fluctuations due to trapped air, causing multiphase flow. Neutron images indicated significant fluid mixing driven by these fluctuations. A synchrotron X‐ray image post‐experiment indicated additional mineral precipitates from long‐term diffusive mixing. Despite the promising results, several challenges remain, including resolution limits, temporal synchronization between modalities, and accurate fluid phase segmentation. Overall, our findings highlight both the potential and limitations of integrated neutron and X‐ray imaging for studying pore‐scale reactive transport and mineralization processes.

• Jacob, Jean-Baptiste; Lantman, Hugo Willem van Schrojenstein; Cordonnier, Benoît; Menegon, Luca; Wright, Jonathan & Renard, Francois (2025). Exhumation-induced residual stress in undeformed ultra-high-pressure metamorphic rock. Earth and Planetary Science Letters. ISSN 0012-821X. 671. doi: 10.1016/j.epsl.2025.119615. Fulltekst i vitenarkiv Vis sammendrag

  High-pressure metamorphic rocks in subduction zones undergo extreme pressure-temperature changes during exhumation, triggering complex micromechanical responses at the grain-to-sub-grain scale. While mineralogical and thermodynamic aspects of these processes are well studied, the development and preservation of residual stress in exhumed rocks remain poorly understood, besides very localized effects in host-inclusion systems. Here, we apply synchrotron scanning three-dimensional X-ray diffraction (s3DXRD) to an ultra-high-pressure garnet–quartz rock from the Lago di Cignana unit (Western Alps), revealing intra-crystalline misorientation, residual strain, and stress distributions at the ten-micrometer scale. Results show long-range residual stresses reaching several hundred Megapascals (MPa) over 10–100 μm, despite negligible strain at the macroscopic scale and absence of an oriented rock fabric. Quartz displays intra-grain misorientation up to 5∘, reflecting very local crystal plastic deformation resulting from dislocation activity. Garnet shows lower misorientation below 0.8∘, likely reflecting elastic strain gradients. These residual stresses are interpreted to result from anisotropic decompression during exhumation, locally amplified by grain interactions such as indenting tips and multi-anvil geometries. Composition gradients in garnet may also contribute to lattice distortion and stress retention. These long-range stress heterogeneities may influence rock rheology, metamorphic reactions, and affect elastic barometry results, underscoring the importance of rock microstructures in macroscale processes. Finally, our study demonstrates the value of synchrotron-based diffraction microscopy techniques to investigate metamorphic rocks. Besides in situ characterization of residual strain and stress, these methods open new avenues for multi-scale, three-dimensional investigation of intra-crystalline deformation and metamorphic reactions.

• Prastyani, Erina; Cordonnier, Benoit & Renard, Francois (2025). Generation of large transient porosity at the brittle to semi-ductile transition. Earth and Planetary Science Letters. ISSN 0012-821X. doi: 10.1016/j.epsl.2025.119805. Fulltekst i vitenarkiv Vis sammendrag

  Fluid circulation within Earth’s crust is contingent on the presence of connected porosity. Yet, at depths near the brittle to ductile transition regimes, where the base of the seismogenic zone is located, high pressure compacts the rock, reducing porosity and inhibiting fluid circulation. At greater depths, plastic mechanisms during ductile deformation further reduce the likelihood of porosity generation. In this study, we experimentally simulated the brittle to semi-ductile transition in Carrara marble by performing triaxial deformation experiments coupled with time-lapse 3D synchrotron Xray microtomography at five confining pressures, ranging from 5 MPa to 70 MPa at room temperature and tracked the evolution of porosity over time. Contrary to the prevailing belief, our results show that porosity increases with increasing axial stress at stresses corresponding to the brittle to semi-ductile transition, where fluid circulation is thought to be impeded. In both the semi-brittle and semi-ductile regimes, porosity exhibits similar values, close to 18%, at equivalent axial strain. Although the porosity is comparable, the dominant deformation mechanisms differ: localized shear zones and distributed microcracking characterize the semi-brittle regime, whereas distributed microcracking, without localization, becomes more prominent in the semi-ductile regime. Our findings reveal the formation of high porosity within the semi-brittle and semi-ductile regimes and offer new insight into how this porosity, interpreted as transient within the crust, may enable fluid circulation at the base of the seismogenic zone in Earth's crust.

• Barras, Fabian; Aharonov, Einat & Renard, Francois (2025). A Minimal Model Illuminates the Physics of Pulse-Like Seismic Rupture and Oscillatory Slip Rates in Damaged Faults. Geophysical Research Letters. ISSN 0094-8276. 52(4). doi: 10.1029/2024GL111189. Fulltekst i vitenarkiv Vis sammendrag

  Fault zones are often surrounded by a damage zone that exhibits lower seismic velocities than the wall rock, influencing earthquake propagation and arrest. We present a one-dimensional minimal model of frictional rupture that approximates the elastodynamics of a fault embedded within a damage zone. This model predicts two families of steady-state rupture solutions: an overdamped regime, describing a crack-like rupture, and an underdamped regime with oscillating slip-rate in the wake of the rupture, which promotes pulse-like dynamics. The minimal model contains two free parameters: the pre-stress on the fault, and the seismic velocity reduction in the damage zone. We present how the one-dimensional prediction is consistent with previously published two-dimensional simulations and discuss the applicability of our results to natural observations, identifying the preferred rupture style as function of structure of the fault zone, and the geological consequences of oscillatory slip in the wake of pulse-like ruptures.

• Prastyani, Erina; Cordonnier, Benoit; Beck, Jessica Ann Mc; Weiss, Jérôme & Renard, Francois (2025). Mixed-mode deformation in a rock bridge between two fault segments. Tectonophysics. ISSN 0040-1951. 897. doi: 10.1016/j.tecto.2025.230624. Fulltekst i vitenarkiv Vis sammendrag

  Analyzing how fracture networks develop from preexisting isolated fault segments in immature faults may provide knowledge of the preparation process leading to fault growth and dynamic rupture. We reproduce this process experimentally using triaxial compression experiments coupled with time-lapse in-situ synchrotron X-ray tomography in Westerly granite core samples (10 mm diameter, 20 mm height) that contain two parallel notches oriented at 30° with respect to the axis of the cylinder. We conducted the experiments at room temperature with a constant confining pressure of 20 MPa. We image microfracture development, characterize the microphysical processes of damage and fault growth in an intact rock bridge between the notches, and analyze the evolution of fractures oriented at 0°–17° (extensile) and 17°–32° (shear) as the rock approached failure. We also elucidate the strain localization process in the deforming rock using digital volume correlation. Our study offers a detailed comparison of microfracture and strain field development during fault growth. Results indicate that the rock bridge between the notches becomes damaged with a damage rate and a fracture rate diverging as the rock approaches macroscopic failure. Immediately preceding failure, shear fractures dominate the fracture networks. DVC analysis showed that the deformation process is mixed-mode, accommodated by dilation and shear strain, with the shear strains being more localized than the dilation. Furthermore, regions of high dilative strain host both extensile and shear fractures. These findings provide valuable insights into the fault growth process within an intact rock bridge between two fault segments in nature, demonstrating how mixed-mode deformation facilitates fault maturation.

• Mathiesen, Joachim; Beck, Jessica Ann Mc; Cordonnier, Benoit; Ben-Zion, Yehuda & Renard, Francois (2024). Deep‐learning analysis of fracture networks leading to system‐size failure in rocks. Journal of Geophysical Research (JGR): Machine Learning and Computation. 1(3). doi: 10.1029/2023JH000103. Fulltekst i vitenarkiv Vis sammendrag

  Fracture networks in rocks and other geomaterials form by seismic and aseismic damage processes that could lead to system-size failure. Here, we use a multi-view convolutional neural network model to predict the stress proximity to macroscopic failure of experimentally deformed rock samples using two-dimensional images. Models are trained on time series of fractures observed in rock samples through dynamic in situ synchrotron X-ray tomography experiments. The results demonstrate that deep learning models outperform traditional estimates based on fracture density, increasing the accuracy of the predictions. Furthermore, the models provide insights into fundamental characteristics of fracture patterns that may provide precursory information on impending material failure. The trained deep learning models estimate the angle of the fracture plane relative to the principal loading direction, which is a key factor contributing to shear failure. The predicted angle of the fracture plane, in the range of 10°–30° with respect to the direction of maximum compressive stress, is consistent with the established failure criteria used in rock mechanics.

• Jacob, Jean-Baptiste Robert; Wright, Jonathan; Cordonnier, Benoit & Renard, Francois (2024). Exploiting Friedel pairs to interpret scanning 3DXRD data from complex geological materials. Journal of Applied Crystallography. ISSN 0021-8898. 57(6), s. 1823–1840. doi: 10.1107/S1600576724009634. Fulltekst i vitenarkiv Vis sammendrag

  The present study introduces a processing strategy for synchrotron scanning 3D X-ray diffraction (s3DXRD) data, aimed at addressing the challenges posed by large, highly deformed, polyphase materials such as crystalline rocks. Leveraging symmetric Bragg reflections known as Friedel pairs, our method enables diffraction events to be precisely located within the sample volume. This method allows for fitting the phase, crystal structure and unit-cell parameters at the intra-grain scale on a voxel grid. The processing workflow incorporates several new modules, designed to (i) efficiently match Friedel pairs in large s3DXRD datasets containing up to 108 diffraction peaks; (ii) assign phases to each pixel or voxel, resolving potential ambiguities arising from overlap in scattering angles between different crystallographic phases; and (iii) fit the crystal orientation and unit cell locally on a point-by-point basis. We demonstrate the effectiveness of our technique on fractured granite samples, highlighting the ability of the method to characterize complex geological materials and show their internal structure and mineral composition. Additionally, we include the characterization of a metal gasket made of a commercial aluminium alloy, which surrounded the granite sample during experiments. The results show the effectiveness of the technique in recovering information about the internal texture and residual strain of materials that have undergone high levels of plastic deformation.

• Shafabakhsh, Paiman; Cordonnier, Benoit; Pluymakers, Anne Marie Henriette; Borgne, Tanguy Le; Mathiesen, Joachim & Linga, Gaute [Vis alle 9 forfattere av denne artikkelen] (2024). 4D Neutron Imaging of Solute Transport and Fluid Flow in Sandstone Before and After Mineral Precipitation. Water Resources Research. ISSN 0043-1397. 60(3). doi: 10.1029/2023WR036293. Fulltekst i vitenarkiv Vis sammendrag

  In many geological systems, the porosity of rock or soil may evolve during mineral precipitation, a process that controls fluid transport properties. Here, we investigate the use of 4D neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in-situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in-situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl2 and Na2CO3, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co-flow experiment). We use the contrast in neutron attenuation from time-resolved tomograms to derive three-dimensional fluid velocity field by using an inversion technique based on the advection-dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co-flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. The outcome of this study contributes to progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface using the promising technique of neutron imaging.

• Bouchayer, Coline Lili Mathy; Nanni, Ugo; Lefeuvre, Pierre-Marie; Hulth, John; Schmidt, Louise Steffensen & Kohler, Jack [Vis alle 8 forfattere av denne artikkelen] (2024). Multi-scale variations of hydro-mechanical conditions at the base of the surge-type glacier Kongsvegen, Svalbard. The Cryosphere. ISSN 1994-0416. 18(6), s. 2939–2968. doi: 10.5194/tc-18-2939-2024. Fulltekst i vitenarkiv Vis sammendrag

  The flow of glaciers is largely controlled by changes at the ice–bed interface, where basal slip and sediment deformation drive basal glacier motion. Determining subglacial conditions and their responses to hydraulic forcing remains challenging due to the difficulty of accessing the glacier bed. Here, we monitor the interplay between surface runoff and hydro-mechanical conditions at the base of the Kongsvegen glacier in Svalbard. From July 2021 to August 2022, we measured both subglacial water pressure and till strength. Additionally, we derived median values of subglacial hydraulic gradient and radius of channelized subglacial drainage system from seismic power, recorded at the glacier surface. To characterize the variations in the subglacial conditions caused by changes in surface runoff, we investigate the variations of the following hydro-mechanical properties: measured water pressure, measured sediment ploughing forces, and derived hydraulic gradient and radius, over seasonal, multi-day, and diurnal time

• McBeck, Jessica Ann; Cordonnier, Benoit; Ben-Zion, Yehuda & Renard, Francois (2024). Identifying the fracture characteristics that control deformation localization and catastrophic failure in fluid-saturated crystalline rocks in upper crustal conditions. Tectonophysics. ISSN 0040-1951. 871. doi: 10.1016/j.tecto.2023.230191. Fulltekst i vitenarkiv
• Aspaas, Andreas Grøvan; Lacroix, Pascal; Sena, Clara; Kristensen, Lene; Langet, Nadege & Renard, Francois (2024). Creep Bursts on the Åknes Landslide, Norway: Detection, Characterization and Possible Mechanisms. Journal of Geophysical Research (JGR): Earth Surface. ISSN 2169-9003. 129(12). doi: 10.1029/2024JF007733. Fulltekst i vitenarkiv Vis sammendrag

  Creeping landslides may fail catastrophically, posing significant threats to infrastructure and lives. Landslides weaken over time through rock mass damage processes that may occur by steady-state creep or transient accelerations of slip, called creep bursts. Creep bursts may control landslide stability by inducing short-term damage and strain localization. This study focuses on the Åknes landslide in Norway, which moves up to 6 cm per year and could potentially trigger a large tsunami in the fjord below. An 11-year data set is compiled and analyzed, including kinematic, seismic, and hydrogeological data acquired at the landslide surface and in a series of boreholes. An annual average of two creep bursts with millimeter amplitude has been recorded within the shear zone in each borehole, accounting for approximately 11% of the total displacement. Creep bursts detected simultaneously in multiple boreholes are preceded by increased seismic activity and rising water pressure. However, most creep bursts are observed in only one or a few boreholes. These bursts often happen during seasonal high and low groundwater levels in autumn and spring, respectively, correlating with local peaks in water pressure. No such correlation is observed during summer. We propose that creep bursts can have different causes and hypothesize that rock degradation leads to some creep bursts independent of water pressure variations. In contrast, the largest creep bursts are correlated with variations in absolute water pressure or gradients of water pressure within the shear zone. Our findings emphasize the complexity of a dense data set requiring multiple mechanisms to explain creep burst dynamics.

• Julia, Maude; Putnis, Christine V.; Plümper, Oliver & Renard, Francois (2024). Porosity and fluid pathway development during cadmium sequestration by calcium carbonate replacement. Environmental Science: Advances. 4, s. 115–124. doi: 10.1039/d4va00316k. Fulltekst i vitenarkiv
• Xue, Chunqi; McBeck, Jessica Ann; Lu, Hongjun; Yan, Changhao; Zhong, Jianhua & Wu, Jianguang [Vis alle 7 forfattere av denne artikkelen] (2024). Classification of shale lithofacies with minimal data: Application to the early Permian shales in the Ordos Basin, China. Journal of Asian Earth Sciences. ISSN 1367-9120. 259. doi: 10.1016/j.jseaes.2023.105901. Fulltekst i vitenarkiv
• Wang, Lei; Kwiatek, Grzegorz; Renard, Francois; Guérin-Marthe, Simon; Rybacki, Erik & Bohnhoff, Marco [Vis alle 8 forfattere av denne artikkelen] (2024). Fault roughness controls injection-induced seismicity. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 121(3). doi: 10.1073/pnas.2310039121. Fulltekst i vitenarkiv Vis sammendrag

  Fluid-induced earthquakes associated with industrial activities, such as wastewater disposal, geothermal energy production, and CO2 storage, have received increasing scientific concerns. Natural faults that host earthquakes typically display nonplanar structures. This study provides straightforward information on how fault structure and heterogeneity control injection-induced fault slip and associated seismicity characteristics using well-designed fluid injection experiments on laboratory faults and relevant numerical modeling. The results indicate that geometrical roughness slows down injection-induced fault slip and affects hypocenter distribution, frequency–magnitude characteristics, and source mechanisms of induced seismicity. We highlight the key roles that fault roughness and stress heterogeneity play in modulating a transition from aseismic to seismic slip when subjected to increased fluid pressure, leading to large induced events localized around highly stressed asperities.

• Shafabakhsh, Paiman; Borgne, Tanguy Le; Renard, Francois & Linga, Gaute (2024). Resolving pore-scale concentration gradients for transverse mixing and reaction in porous media. Advances in Water Resources. ISSN 0309-1708. 192. doi: 10.1016/j.advwatres.2024.104791. Fulltekst i vitenarkiv Vis sammendrag

  Mixing-limited reactions are central to a wide range of processes in natural and engineered porous media. Recent advances have shown that concentration gradients sustained by flow at the pore-scale influence macroscopic reaction rates over a large range of reactive transport regimes. Yet, resolving concentration gradients driven by fluid mixing at the pore-scale is challenging with current simulation methods. Here, we introduce a computational methodology to resolve concentration gradients at the pore scale in mixing-limited reactions. We consider a steady-state reactive transport problem characterized by reactive fluids flowing in parallel in a porous material. Given a mesh representation of the pore space and a steady velocity field, we solve the steady advection-diffusion equation for conservative scalar transport using a stabilized finite-element method combined with mesh refinement adapted to local scalar gradients. Based on this solution and assuming instantaneous reaction kinetics in the fluid, we infer the distribution of species involved in an irreversible bi-molecular reaction. We validate the method by comparing our results for uniform flow with analytical solutions and then apply it to simulate mixing-limited reactions in a three-dimensional random bead pack and Berea sandstone sample. Chaotic flow within the pore space leads to sustained concentration gradients, which are captured by our numerical framework. The results underscore the ability of the methodology to simulate transverse mixing and mixing-limited reactions in complex porous media and to provide bottom-up numerical data to improve the prediction of effective reaction rates at larger scales.

• Madathiparambil, Aldritt Scaria; Mirzaei, Fazel; Tekseth, Kim Robert Bjørk; Cordonnier, Benoit; Agofack, Nicolaine & Cerasi, Pierre [Vis alle 9 forfattere av denne artikkelen] (2024). Mechanical response of cement and shale admixtures under cyclic triaxial loading monitored by in-situ synchrotron micro-computed tomography. Cement and Concrete Research. ISSN 0008-8846. 186. doi: 10.1016/j.cemconres.2024.107682. Fulltekst i vitenarkiv Vis sammendrag

  Understanding the mechanical behaviour of natural soils as mixed with cement for stabilization is crucial for civil engineering developments. The response of cement-soil admixtures when subjected to cyclic loads is a largely unexplored topic, despite the importance of understanding fatigue in these ubiquitous construction materials. We present cyclic loading experiments on Portland cement mixed with fragmented shale fragments using triaxial testing, monitored with synchrotron-based μCT. Through digital volume correlation (DVC), the temporal evolution of the displacement, volumetric, and von Mises equivalent strain fields were obtained. We observed in detail the fatigue damage evolution during cyclic loading and found that following high-strain deformation of the much softer shale fragments, the ultimate failure of the samples occurred in the adjacent cement matrix. The failure mechanism under periodic stress and its relevance for accelerated laboratory testing of slow degradation long-term processes are of key importance to technical infrastructure, including subsea CO2 storage.

• Beck, Jessica Ann Mc; Cordonnier, Benoit; Zhu, Wenlu & Renard, Francois (2024). Spatial Persistence of High Strain Events During Brittle Failure. Geophysical Research Letters. ISSN 0094-8276. 51(21). doi: 10.1029/2024GL110668. Fulltekst i vitenarkiv Vis sammendrag

  The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three-dimensional strain tensor using X-ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system-sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure.

• Linga, Gaute; Mathiesen, Joachim; Renard, Francois & Borgne, Tanguy Le (2023). Fingerprints of Chaos in a Two-Phase Porous Media Flow. I Médici, Ezequiel F. & Otero, Alejandro D. (Red.), Album of Porous Media. Springer. ISSN 9783031238000. doi: 10.1007/978-3-031-23800-0_64. Fulltekst i vitenarkiv
• Wang, Jianfeng; Dziadkowiec, Joanna; Liu, Yuke; Jiang, Wenmin; Zheng, Yijun & Xiong, Yongqiang [Vis alle 8 forfattere av denne artikkelen] (2023). Combining atomic force microscopy and nanoindentation helps characterizing in-situ mechanical properties of organic matter in shale. International Journal of Coal Geology. ISSN 0166-5162. 281. doi: 10.1016/j.coal.2023.104406. Fulltekst i vitenarkiv Vis sammendrag

  The quantification of mechanical properties of organic matter in shale is of significance for the fine prediction and characterization of shale reservoir's mechanical properties. Due to the micron-sized and dispersed distribution of organic matter particles in shale, the accurate evaluation of the actual mechanical response remains challenging. This work focuses on shale from Wufeng-Longmaxi Formation, which is the main shale gas exploration and development formation in China. A method based on atomic force microscopy (AFM) with an optical microscope (i.e., in-situ AFM technique) is presented to locate the organic matter in-situ and then visualize and quantify its mechanical properties using AFM Young's modulus mapping. The merits and limitations for determining the mechanical properties of organic matter in shale between the AFM and the more conventional nanoindentation technique are discussed. Results show that combining in-situ nanoindentation and in-situ AFM mapping provides more accurate description of the mechanical properties of organic matter in shale than traditional grid indentation methods with low spatial resolution. The Young's moduli of organic matter calculated from nanoindentation are around twice smaller than those obtained from AFM measurements mainly because the elasto-plastic deformation zone of organic matter in nanoindentation tests is larger and can be additionally affected by the presence of inorganic particles and/or larger micro-pores in organic matter. The Young's modulus and hardness of graptolite in the shale obtained by nanoindentation are slightly larger than those of solid bitumen at the same thermal maturity. Both in-situ AFM and in-situ nanoindentation results show that the mechanical strength of organic matter increases with increasing maturity. Overall, the presented approach shows a great potential for accurate and in-situ measurement of the mechanical properties of organic matter in shale at the nanoscale, which may be beneficial to the development of rock mechanical models for the accurate evaluation of the actual mechanical properties of shale.

• Barras, Fabian; Thøgersen, Kjetil; Aharonov, Einat & Renard, Francois (2023). How Do Earthquakes Stop? Insights From a Minimal Model of Frictional Rupture. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(8). doi: 10.1029/2022JB026070. Fulltekst i vitenarkiv Vis sammendrag

  The question “what arrests an earthquake rupture?” sits at the heart of any potential prediction of earthquake magnitude. Here, we use a one-dimensional, thin-elastic-strip, minimal model, to illuminate the basic physical parameters that may control the arrest of large ruptures. The generic formulation of the model allows for wrapping various earthquake arrest scenarios into the variations of two dimensionless variables Tk (initial dimensionless stress parameter on the fault) and Gc (dimensionless fracture energy), valid for both in-plane and antiplane shear loading. Our continuum model is equivalent to the standard Burridge-Knopoff model, with an added characteristic length scale, H, that corresponds to either the thickness of the damage zone for strike-slip faults or to the thickness of the downward moving plate for subduction settings. We simulate the propagation and arrest of frictional ruptures and derive closed-form expressions to predict rupture arrest under different conditions. Our generic model illuminates the different energy budget that mediates crack- and pulse-like rupture propagation and arrest. It provides additional predictions such as generic stable pulse-like rupture solutions, stress drop independence of the rupture size, the existence of back-propagating fronts, and predicts that asymmetric slip profiles arise under certain pre-stress conditions. These diverse features occur also in natural earthquakes, and the fact that they can all be predicted by a single minimal framework is encouraging and pave the way for future developments of this model.

• McBeck, Jessica Ann; Cordonnier, Benoit; Ben-Zion, Yehuda & Renard, Francois (2023). The Influence of Confining Pressure and Preexisting Damage on Strain Localization in Fluid-Saturated Crystalline Rocks in the Upper Crust. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(8). doi: 10.1029/2023JB026987. Fulltekst i vitenarkiv Vis sammendrag

  The spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low-magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X-ray tomography experiments at varying confining pressures (5–20 MPa) and fluid pressures (0–10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining pressure promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain.

• Madathiparambil, Aldritt Scaria; Tekseth, Kim Robert Bjørk; Mürer, Fredrik Kristoffer; Cordonnier, Benoit; Agofack, Nicolaine & McBeck, Jessica Ann [Vis alle 10 forfattere av denne artikkelen] (2023). Time-Resolved in Situ Imaging of Strain Localization in Draupne Shale under Triaxial Compression. Physical Review Applied. 20(3). doi: 10.1103/PhysRevApplied.20.034046. Fulltekst i vitenarkiv
• Montes-Hernandez, G.; Feugueur, L.; Vernier, C.; Driessche, Driessche Van & Renard, Francois (2023). Efficient removal of antibiotics from water via aqueous portlandite carbonation. Journal of Water Process Engineering. 51. doi: 10.1016/j.jwpe.2022.103466. Fulltekst i vitenarkiv
• Aslan, Gökhan; Michele, Marcello De; Raucoules, Daniel; Renard, Francois; Dehls, John F. & Penna, Ivanna Marina [Vis alle 8 forfattere av denne artikkelen] (2023). Dynamics of a giant slow landslide complex along the coast of the Aral Sea, Central Asia*. Turkish Journal of Earth Sciences. ISSN 1300-0985. 32(6), s. 819–832. doi: 10.55730/1300-0985.1876. Fulltekst i vitenarkiv Vis sammendrag

  We report here a slow-moving landslide complex of the lateral spreading type revealed by Sentinel-1 interferometric timeseries analysis. Located along the western coast of the Aral Sea, with a >150-km length and 3-km width, a giant active landslide complex, slides with a constant velocity of up to 60 mm/year. Systematic subsidence up to 5 mm/year is also observed along narrow strips of terraces that appear to result from rotations of fault-bounded blocks. The landslide deformation velocity does not correlate with the annual variations of the water level in the southwestern lake of the Aral Sea during the observation period of 2014-2022, indicating a long-term forcing of this landslide that is rather interpreted to be caused by the long-term sea-level drop. The lateral spreading involves the competent limestone beds lying horizontally on plastic clay- and evaporite-rich layers. We propose a conceptual model for the kinematic of landslides that appear to be controlled by the attitude of bedding, lithological sequence, hydrogeology, and low angle faults.

• Wang, Jianfeng; Liu, Yuke; Yang, Chao; Zheng, Yangcheng; Jiang, Wenmin & Menegon, Luca [Vis alle 9 forfattere av denne artikkelen] (2023). Upscaling the creep behavior of clay-rich and quartz-rich shales from nanoindentation measurements: Application to the Wufeng-Longmaxi shale, China. International Journal of Rock Mechanics And Mining Sciences. ISSN 1365-1609. 171. doi: 10.1016/j.ijrmms.2023.105580. Fulltekst i vitenarkiv
• Gratier, Jean-Pierre; Menegon, Luca & Renard, Francois (2023). Pressure Solution Grain Boundary Sliding as a Large Strain Mechanism of Superplastic Flow in the Upper Crust. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(4). doi: 10.1029/2022JB026019. Fulltekst i vitenarkiv Vis sammendrag

  Abstract A mechanism accommodating large strain in superplastic flow with elongation of 100%–300%, is described for fine‐grained calcareous shales deformed at temperature of 200–335°C and depths of 5–8 km up to slate schists in the Oisans massif, Western Alps, France. Using electron microscopy techniques on thin sections parallel to the principal finite strain axes, we show that the shape ratios of the slightly elongated grains (1.4–1.6), mainly calcite and quartz, do not match the finite strains recorded by the markers of the deformation (truncated belemnites, folded veins) in the maximum elongation and shortening plane (ratio 6.7) or in the maximum and minimum elongations plane (ratio 2.4). Consequently, a grain boundary sliding mechanism is required to explain the measured large finite strains. The most soluble minerals (quartz, calcite, dolomite, and albite), which represent about 95% of the rock, accommodate deformation by pressure solution grain boundary sliding whereas the least soluble minerals (muscovite, chlorite, Fe‐Ti oxides) act as indenters or are passively reoriented. Pressure solution is especially efficient in polymineralic rocks. Soluble minerals, which have been healed together in veins or fossils, are much more resistant to deformation and act as rigid objects. Models with idealized tessellation of hexagonal grains and creep laws derived from pressure solution indentation experiments provide deformation maps. We discuss the main parameters of this ductile deformation in the upper crust (thermodynamic conditions, strain rate, stress, distance of mass transfer) and show possible drastic decrease of mass transfer efficiency with decrease of stress and strain rate.

• Mathiesen, Joachim; Linga, Gaute; Misztal, Marek; Renard, Francois & Borgne, Tanguy Le (2023). Dynamic Fluid Connectivity Controls Solute Dispersion in Multiphase Porous Media Flow. Geophysical Research Letters. ISSN 0094-8276. 50(16). doi: 10.1029/2023GL105233. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Solute transport in multiphase flow through porous media plays a central role in many natural systems and geoengineering applications. The interplay between fluid flow and capillary forces leads to transient flow dynamics and phase distributions. However, it is not known how such dynamic flow affects the dispersion of transported species. Here, we use highly resolved numerical simulations of immiscible two‐phase flow to investigate dispersion in multiphase flows. We show that repeated activation and deactivation of different flow pathways under the effect of capillary forces accelerates the spreading of solutes compared to single phase flow. We establish the transport laws under dynamic multiphase flows by linking the dispersion coefficient to the Bond number, the ratio of the force driving the flow and the surface tension. Our results determine the controlling factors for solute dispersion in porous media, opening a range of applications for understanding and controlling transport in porous geological systems.

• Julia, Maude; Putnis, Christine V.; King, Helen E. & Renard, Francois (2023). Coupled dissolution-precipitation and growth processes on calcite, aragonite, and Carrara marble exposed to cadmium-rich aqueous solutions. Chemical Geology. ISSN 0009-2541. 621. doi: 10.1016/j.chemgeo.2023.121364. Fulltekst i vitenarkiv Vis sammendrag

  Calcium carbonate and cadmium-rich fluid interactions have been studied at the nano and microscale with fluid flow and static fluid conditions for three forms of CaCO3: calcite in single crystals of Iceland Spar, calcite in a polycrystalline Carrara marble, and aragonite single crystals. Atomic Force Microscopy (AFM) showed the nanoscale effect of cadmium on CaCO3 dissolution and growth under flow-through conditions at ambient temperature, with the modification of calcite dissolution behaviour and simultaneous precipitation of a Cd-rich phase on all the different samples. Hydrothermal experiments at 200 °C revealed that the reactivity of single calcite crystals is passivated by epitaxial growth of the less soluble Cd-rich endmember of the (Ca,Cd)CO3 solid-solution on the sample surface due to the similar crystallographic structures of calcite and otavite (CdCO3). Conversely, the presence of grain boundaries in Carrara marble or the change of crystallographic structure and reaction-induced fracturing in aragonite allowed, to some extent, the pseudomorphic replacement of Carrara marble and aragonite samples by a porous (Ca,Cd)CO3 solid-solution phase of variable composition. These phenomena have been observed in solutions undersaturated with respect to all solid phases and are the result of an interface-coupled dissolution-precipitation mechanism where the dissolving CaCO3 provides ions to supersaturate the mineral-fluid interfacial layer, leading to the precipitation of a Cd-containing phase on the samples' surfaces. This coupled dissolution-precipitation mechanism could potentially be used as a remediation process to sequester cadmium from contaminated effluents.

• McBeck, Jessica Ann; Cordonnier, Benoit; Cooke, Michele; Fattaruso, Laura & Renard, Francois (2023). Deformation evolves from shear to extensile in rocks due to energy optimization. Communications Earth & Environment. 4. doi: 10.1038/s43247-023-01023-w. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Determining how fracture network development leads to macroscopic failure in heterogeneous materials may help estimate the timing of failure in rocks in the upper crust as well as in engineered structures. The proportion of extensile and shear deformation produced by fracture development indicates the appropriate failure criteria to apply, and thus is a key constraint in such an effort. Here, we measure the volume proportion of extensile and shear fractures using the orientation of the fractures that develop in triaxial compression experiments in which fractures are identified using dynamic in situ synchrotron X-ray imaging. The fracture orientations transition from shear to extensile approaching macroscopic, system-size failure. Numerical models suggest that this transition occurs because the fracture networks evolve in order to optimize the total mechanical efficiency of the system. Our results provide a physical interpretation of the empirical internal friction coefficient in rocks.

• McBeck, Jessica Ann; Cordonnier, Benoit & Renard, Francois (2023). The evolving representative elementary volume size in crystalline and granular rocks under triaxial compression approaching macroscopic failure. Geophysical Journal International. ISSN 0956-540X. 232(3), s. 1898–1913. doi: 10.1093/gji/ggac426. Fulltekst i vitenarkiv
• Guren, Marthe Grønlie; Renard, Francois & Noiriel, Catherine (2023). Dissolution rate variability at carbonate surfaces: 4D X-ray micro-tomography and stochastic modeling investigations. Frontiers in water. 5. doi: 10.3389/frwa.2023.1185608. Fulltekst i vitenarkiv Vis sammendrag

  We provide a detailed 3D characterization of the geometry evolution and dissolution rate mapping at the surface of four carbonate samples, namely a calcite spar crystal, two limestone rock fragments, and an aragonite ooid, using time-lapse X-ray micro-tomography during dissolution experiments at pH 4.0. Evaluation of the retreat and mapping of the reaction rates at the whole surface of the samples reveals a large spatial variability in the dissolution rates, reflecting the composition and the specific contributions of the different regions of the samples. While crystal edges and convex topographies record the highest dissolution rates, the retreat is slower for flat surfaces and in topographic lows (i.e., concave areas), suggesting surface-energy related and/or diffusion-limited reactions. Microcrystalline aragonite has the highest rate of dissolution compared to calcite. Surprisingly, rough microcrystalline calcite surface dissolves globally more slowly than the {1014} faces of the calcite spar crystal. The presence of mineral impurities in rocks, through the development of a rough interface that may affect the transport of species across the surface, may explain the slight decrease in reactivity with time. Finally, a macroscopic stochastic model using the set of detachment probabilities at corner, edge, and face (terrace) sites obtained from kinetic Monte Carlo simulations is applied at the spar crystal scale to account for the effect of site coordination onto reactivity. Application of the model to the three other carbonate samples is discussed regarding their geometry and composition. The results suggest that the global dissolution process of carbonate rocks does not reflect only the individual behavior of their forming minerals, but also the geometry of the crystals and the shape of the fluid-mineral interface.

• Michalchuk, Stephen Paul; Zertani, Sascha; Renard, Francois; Fusseis, Florian; Chogani, Alireza & Plümper, Oliver [Vis alle 7 forfattere av denne artikkelen] (2023). Dynamic Evolution of Porosity in Lower-Crustal Faults During the Earthquake Cycle. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(8). doi: 10.1029/2023JB026809. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of &gt;85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.

• Bouchon, Michel; Socquet, Anne; Marsan, David; Guillot, Stéphane; Durand, Virginie & Gardonio, Blandine [Vis alle 11 forfattere av denne artikkelen] (2022). Observation of rapid long-range seismic bursts in the Japan Trench subduction leading to the nucleation of the Tohoku earthquake. Earth and Planetary Science Letters. ISSN 0012-821X. 594. doi: 10.1016/j.epsl.2022.117696. Fulltekst i vitenarkiv
• Stappen, Van F. Van; McBeck, Jessica Ann; Cordonnier, Benoit; Pijnenburg, R.P.J.; Renard, Francois & Spiers, C.J. [Vis alle 7 forfattere av denne artikkelen] (2022). 4D Synchrotron X-ray Imaging of Grain Scale Deformation Mechanisms in a Seismogenic Gas Reservoir Sandstone During Axial Compaction. Rock Mechanics and Rock Engineering. ISSN 0723-2632. 55(8), s. 4697–4715. doi: 10.1007/s00603-022-02842-7. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Understanding the grain-scale processes leading to reservoir compaction during hydrocarbons production is crucial for enabling physics-based predictions of induced surface subsidence and seismicity hazards. However, typical laboratory experiments only allow for pre- and post-experimental microstructural investigation of deformation mechanisms. Using high-resolution time-lapse X-ray micro-tomography imaging (4D µCT) during triaxial deformation, the controlling grain-scale processes can be visualized through time and space at realistic subsurface conditions. We deformed a sample of Slochteren sandstone, the reservoir rock from the seismogenic Groningen gas field in the Netherlands. The sample was deformed beyond its yield point (axial strain &gt; 15%) in triaxial compression at reservoir P–T-stress conditions (100 °C, 10 MPa pore pressure, 40 MPa effective confining pressure). A total of 50 three-dimensional µCT scans were obtained during deformation, at a spatial resolution of 6.5 µm. Time lapse imaging plus digital volume correlation (DVC) enabled identification of the grain-scale deformation mechanisms operating throughout the experiment, for the first time, both at small, reservoir-relevant strains (&lt; 1%), and in the approach to brittle failure at strains &gt; 10%. During small-strain deformation, the sample showed compaction through grain rearrangement accommodated by inter-granular slip and normal displacements across grain boundaries, in particular, by closure of open grain boundaries or compaction of inter-granular clay films. At intermediate and large strains (&gt; 4%), grain fracturing and pore collapse were observed, leading to sample-scale brittle failure. These observations provide key input for developing microphysical models describing compaction of the Groningen and other producing (gas) reservoirs.

• Eijsink, Agathe M.; Kirkpatrick, James D.; Renard, Francois & Ikari, Matt J. (2022). Fault surface morphology as an indicator for earthquake nucleation potential. Geology. ISSN 0091-7613. 50(12), s. 1356–1360. doi: 10.1130/G50258.1. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Laboratory measurements can determine the potential for geologic materials to generate unstable (seismic) slip, but a direct relation between sliding behavior in the laboratory and physical characteristics observable in the field is lacking, especially for the phyllosilicate-rich gouges that are widely observed in natural faults. We integrated laboratory friction experiments with surface topography microscopy and demonstrated a quantitative correlation between frictional slip behavior and fault surface morphology of centimeter-scale samples. Our results show that striated, smooth fault surfaces were formed in experiments that exhibited stable sliding, whereas potentially unstable sliding was associated with rougher, isotropic fault surfaces. We interpret that frictional stability and fault surface morphology are linked via the evolution of asperity contacts on localized slip surfaces. If fault surface roughness obeys a fractal relationship over a large range of length scales, then we infer that the morphological characteristics observed in the laboratory could indicate the earthquake nucleation potential on natural fault surfaces.

• Bouchayer, Coline Lili Mathy; Aiken, John Mark; Thøgersen, Kjetil; Renard, Francois & Schuler, Thomas (2022). A Machine learning framework to automate the classification of surge-type glaciers in Svalbard. Journal of Geophysical Research (JGR): Earth Surface. ISSN 2169-9003. 127(7). doi: 10.1029/2022JF006597. Fulltekst i vitenarkiv Vis sammendrag

  Surge-type glaciers are present in many cold environments in the world. These glaciers experience a dramatic increase in velocity over short time periods, the surge, followed by an extended period of slow movement, the quiescence. This study aims at understanding why only few glaciers exhibit a transient behavior. We develop a machine learning framework to classify surge-type glaciers, based on their location, exposure, geometry, climatic mass balance and runoff. We apply this approach to the Svalbard archipelago, a region with a relatively homogeneous climate. We compare the performance of logistic regression, random forest, and extreme gradient boosting (XGBoost) machine learning models that we apply to a newly combined database of glaciers in Svalbard. Based on the most accurate model, XGBoost, we compute surge probabilities along glacier centerlines and quantify the relative importance of several controlling features. Results show that the surface and bed slopes, ice thickness, glacier width, climatic mass balance, and runoff along glacier centerlines are the most significant features explaining surge probability for glaciers in Svalbard. A thicker and wider glacier with a low surface slope has a higher probability to be classified as surge-type, which is in good agreement with the existing theories of surging. Finally, we build a probability map of surge-type glaciers in Svalbard. The framework shows robustness on classifying surge-type glaciers that were not previously classified as such in existing inventories but have been observed surging. Our methodology could be extended to classify surge-type glaciers in other areas of the world.

• McBeck, Jessica Ann; Ben-Zion, Y.; Zhou, X. & Renard, Francois (2022). Precursory Off-Fault Deformation in Restraining and Releasing Step Overs: Insights From Discrete Element Method Models. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 127(9). doi: 10.1029/2022JB024326. Fulltekst i vitenarkiv Vis sammendrag

  Accelerating geophysical activity is detected preceding some, but not all, large earthquakes. This observation may indicate that no precursors occur before some earthquakes, or that the instrumentation lacks the required sensitivity. To aid crustal monitoring efforts, we use discrete element method models to identify the locations and styles of deformation that may provide useful information about approaching fault reactivation. We model the reactivation of two healed rough faults in a variety of step over configurations, embedded in a host rock with varying amounts of damage subject to shear velocity loading parallel to the faults. Both the fault geometry and ratio of fault to host rock strength control the amount of off-fault deformation. Consistent with field observations, models with larger steps and more preexisting host rock damage produce higher amounts of off-fault deformation. We assess the size of the continuous regions of high velocities and strains to compare the value of the precursory information of each velocity and strain component. Comparing the three components of the velocity vector suggests that the fault-parallel velocity produces the largest and most temporally continuous regions of elevated velocity. The size of these regions increases toward failure, indicating the usefulness of tracking this component. Comparing the volumetric and shear components of the three-dimensional strain tensor suggests that during most of the interseismic period, the shear strain provides more information about approaching fault slip than the volumetric strain. However, in the days and months preceding fault reactivation, both the shear and volumetric strains provide similarly valuable information.

• McBeck, Jessica Ann; Ben-Zion, Y. & Renard, Francois (2022). Predicting fault reactivation and macroscopic failure in discrete element method simulations of restraining and releasing step overs. Earth and Planetary Science Letters. ISSN 0012-821X. 593. doi: 10.1016/j.epsl.2022.117667. Fulltekst i vitenarkiv
• Guren, Marthe Grønlie; Sveinsson, Henrik Andersen; Malthe-Sørenssen, Anders & Renard, Francois (2022). Nanoscale Damage Production by Dynamic Tensile Rupture in α-Quartz. Geophysical Research Letters. ISSN 0094-8276. 49(20). doi: 10.1029/2022GL100468. Fulltekst i vitenarkiv Vis sammendrag

  The creation of new fractures during an earthquake produces rock damage and contributes to the dissipation of strain energy. During dynamic rupture propagation, tensile microfractures can form in the earthquake process zone and in the domains around a fault that host large transient tensile stress. These microfractures can produce rock fragments with a wide range of sizes. Using molecular dynamics simulations, we model tensile rupture propagation in α-quartz under conditions of stress that occur during earthquake propagation. Our results show that for rupture speeds below 15% of the Rayleigh wave speed, fractures propagate linearly. At higher speeds, fracture propagation undergoes path instabilities with crack oscillations and microbranching leading to the formation of nanoscale roughness and fragments. This nanoscale damage can form in and around the earthquake process zone before any significant slip has occurred on the fault. The produced nanoparticles may control further energy dissipation during frictional slip.

• Johnson, James Ronald; Kobchenko, Maya; Johnson, Andrew Charles; Mondol, Nazmul Haque & Renard, Francois (2022). Experimental modelling of primary migration in a layered, brittle analogue system. Tectonophysics. ISSN 0040-1951. 840. doi: 10.1016/j.tecto.2022.229575. Fulltekst i vitenarkiv Vis sammendrag

  A 2D Hele-Shaw cell was built to study microfracture nucleation, growth, and network formation during internal fluid production. Fluid is slowly produced into a low permeability solid, which leads to a local fluid pressure increase that controls the nucleation of microfractures that grow and then connect to create flow pathways. This process occurs during the primary migration of hydrocarbons in source rocks, which is the main topic of our study. It may also occur in other geological systems, such as the expulsion of water during dehydration of clay-rich sediments in sedimentary basins or serpentinite rocks in subduction zones and the transport of magmatic melts. Our system consists of a transparent, brittle gelatin material mixed with yeast and sugar. The consumption of sugar by yeast leads to CO2 formation, resulting in microfracture nucleation and growth. We varied three parameters, (1) anisotropy (i.e., number of layers), (2) lateral sealing, and (3) rate of fluid production. We tracked fluid movement through the opening and closing of microfractures within the system. Microfracture nucleation density is similar in a layered system to previous studies (0.45 microfracture per cm2). However, we observed that lateral confinement (0.31 microfracture per cm2) and rate of expulsion (0.99 microfracture per cm2) affect nucleation density and the geometrical characteristics of the microfracture network. The size, extent, and geometry of the microfracture network are dependent on all three parameters investigated, where lateral confinement and a higher rate of expulsion result in greater microfracture network connectivity. Layers control the angle of intersection between microfractures. Furthermore, layering and sealing have an impact on fracture topology. Results also show that the microfracture pattern significantly influences the fluid expulsion rate. Our results have direct applications to understanding how fluid migration occurs in low-permeability rocks through the development of a connected microfracture network produced by internal fluid generation.

• Johnson, James Ronald; Hansen, Jørgen André; Rahman, Md Jamilur; Renard, Francois & Mondol, Nazmul Haque (2022). Mapping the maturity of organic-rich shale with combined geochemical and geophysical data, Draupne Formation, Norwegian Continental Shelf. Marine and Petroleum Geology. ISSN 0264-8172. 138. doi: 10.1016/j.marpetgeo.2022.105525. Fulltekst i vitenarkiv Vis sammendrag

  The evaluation of the interplay between kerogen maturation and total organic carbon (TOC) content within an organic-rich shale is critical to characterizing source rock shale's elastic behavior. We analysed a comprehensive database of twenty-nine well logs, two 3D seismic surveys, and geochemical data of eighteen wells within the Draupne Formation in the Norwegian North Sea. The Upper Jurassic (Kimmeridgian) Draupne Formation Shale is found throughout the Norwegian Continental Shelf and is the primary source, carrier, and seal rock in several oil and gas fields. The dataset explores a broad range of depths (2–5 km), total organic contents (0–20 wt%), and various maturation states ranging from immature to overmature. We study the relationship between rock brittleness and organic content as shale matures. We use calculated values of Young's modulus and Poisson's ratio. Academic and industry-standard cutoffs are applied to both the TOC content and maturation level. We introduce here the Organic Maturation Product (OMP) rock physics template, which incorporates a relationship between organic content and any subsequent maturation. The degree of brittleness or ductility is impacted directly by the kerogen maturation process, with lower TOC content having a dampening effect on the transition towards increasing brittleness. This template allows one to classify rock maturation in nine categories, from low to high maturation, and link these categories to rock geomechanical properties. Seismic inversion of the 3D surveys and mapping utilizing the OMP classification reveal how large-scale depositional environment and subsequent diagenetic events influence both placement and quality of source rock broadly within the basin context of the North Sea.

• Noiriel, Catherine & Renard, Francois (2022). Four-dimensional X-ray micro-tomography imaging of dynamic processes in geosciences. Comptes rendus Geoscience. ISSN 1631-0713. 354, s. 255–280. doi: 10.5802/crgeos.137. Fulltekst i vitenarkiv Vis sammendrag

  The dynamic response of rocks to thermal, hydrodynamical, mechanical, and geochemical solicitations is of fundamental interest in several disciplines of geosciences, including geo-engineering, geophysics, rock physics, hydrology, mineralogy, and environmental and soil sciences. From crystal shape to rock microstructure or pore space and fluid distribution, parameters characterizing the rock physico-chemical properties evolve at different time and spatial scales. X-ray micro-tomography (XMT), as a non-invasive and non-destructive imaging technique, offers an unprecedented opportunity to add the fourth dimension, i.e. time, to the three-dimensional spatial visualization of rock and mineral microstructures. The technique is increasingly used to explore dynamic processes in porous and fractured rocks, thanks to synchrotron sources and laboratory XMT scanners, new generations of detectors, and increasing computational power. Image processing allows for tracking the evolution of the fluid–fluid or fluid–mineral interfaces as well as measuring incremental deformations, as rocks deform and react through time under in situ conditions of the sub-surface. Here, we review recent advances in 4D X-ray micro-tomography applied to thermo-hydro-mechano-chemical (THMC) sub-surface processes where fluids, porosity, minerals, and rock microstructures evolve together.

• Johnson, James Ronald; Kobchenko, Maya; Mondol, Nazmul Haque & Renard, Francois (2022). Multiscale synchrotron microtomography imaging of kerogen lenses in organic-rich shales from the Norwegian Continental Shelf. International Journal of Coal Geology. ISSN 0166-5162. 253. doi: 10.1016/j.coal.2022.103954. Fulltekst i vitenarkiv Vis sammendrag

  Kerogen lens shape and size distribution control how organic-rich shales may behave as either source or seal rocks. Prior to thermal conversion, kerogen is a brittle, load-bearing constituent of the shale matrix. During thermal maturation, kerogen lenses become more ductile, and hydrocarbon expulsion may lead to the creation of microfractures, a process controlled not only by temperature and pressure but also by the size and shape of kerogen lenses and their total content in the rock. Here, we use high-resolution multiscale synchrotron microtomography imaging of centimeter-scale shale rock samples collected in two boreholes at different depths in the North Sea and the Barents Sea, respectively. From these three-dimensional microtomography data, we quantify the various shapes of kerogen lenses and discuss how each step of a kerogen lens's life-cycle (i.e. original biological structure, deposition, degradation, and diagenesis) impacted its shape before catagenesis. We quantify the relationship between kerogen volume and the number of kerogen lenses in a given rock volume. The relationship between total organic carbon (TOC) content and the average kerogen lens volume is also measured. For a given rock volume, results show that organic content increases with the number of kerogen lenses up to a point (~8–12 wt% TOC) above which TOC continues to increase, but the number of kerogen lenses decreases. These results combined with kerogen lens orientation may control microfracturing during kerogen maturation.

• Aiken, John Mark; Sohn, Robert A.; Renard, Francois; Matter, Juerg; Kelemen, Peter & Jamtveit, Bjørn (2022). Gas Migration Episodes Observed During Peridotite Alteration in the Samail Ophiolite, Oman. Geophysical Research Letters. ISSN 0094-8276. 49(21). doi: 10.1029/2022GL100395. Fulltekst i vitenarkiv Vis sammendrag

  Serpentinization and carbonation of mantle rocks (peridotite alteration) are fundamentally important processes for a spectrum of geoscience topics, including arc volcanism, earthquake processes, chemosynthetic biological communities, and carbon sequestration. Data from a hydrophone array deployed in the Multi-Borehole Observatory (MBO) of the Oman Drilling Project demonstrates that free gas generated by peridotite alteration and/or microbial activity migrates through the formation in discrete bursts of activity. We detected several, minutes-long, swarms of gas discharge into Hole BA1B of the MBO over the course of a 9 month observation interval. The episodic nature of the migration events indicates that free gas accumulates in the permeable flow network, is pressurized, and discharges rapidly into the borehole when a critical pressure, likely associated with a capillary barrier at a flow constriction, is reached. Our observations reveal a dynamic mode of fluid migration during serpentinization, and highlight the important role that free gas can play in modulating pore pressure, fluid flow, and alteration kinetics during peridotite weathering.

• McBeck, Jessica Ann; Ben-Zion, Yehuda & Renard, Francois (2022). Volumetric and shear strain localization throughout triaxial compression experiments on rocks. Tectonophysics. ISSN 0040-1951. 822. doi: 10.1016/j.tecto.2021.229181. Fulltekst i vitenarkiv
• McBeck, Jessica Ann & Renard, Francois (2022). Deriving three-dimensional properties of fracture networks from two-dimensional observations in rocks approaching failure under triaxial compression: Implications for fluid flow. Water Resources Research. ISSN 0043-1397. 58(11). doi: 10.1029/2022WR032783. Fulltekst i vitenarkiv
• McBeck, Jessica Ann; Ben-Zion, Yehuda & Renard, Francois (2021). How the force and fracture architectures develop within and around healed fault zones during biaxial loading toward macroscopic failure. Journal of Structural Geology. ISSN 0191-8141. 147. doi: 10.1016/j.jsg.2021.104329. Fulltekst i vitenarkiv
• Thøgersen, Kjetil; Aharonov, E.; Barras, Fabian & Renard, Francois (2021). Minimal model for the onset of slip pulses in frictional rupture. Physical Review E (PRE). ISSN 2470-0045. 103(5). doi: 10.1103/PhysRevE.103.052802. Fulltekst i vitenarkiv
• McBeck, Jessica Ann; Mair, Karen & Renard, Francois Marie Paul L (2021). Decrypting healed fault zones: How gouge production reduces the influence of fault roughness. Geophysical Journal International. ISSN 0956-540X. 225(2), s. 759–774. doi: 10.1093/gji/ggab003. Fulltekst i vitenarkiv
• Guren, Marthe Grønlie; Sveinsson, Henrik Andersen; Hafreager, Anders; Jamtveit, Bjørn; Malthe-Sørenssen, Anders & Renard, Francois (2021). Molecular dynamics study of confined water in the periclase-brucite system under conditions of reaction-induced fracturing. Geochimica et Cosmochimica Acta. ISSN 0016-7037. 294, s. 13–27. doi: 10.1016/j.gca.2020.11.016. Fulltekst i vitenarkiv Vis sammendrag

  The volume-increase associated with hydration reactions in rocks may lead to reaction-induced fracturing, but requires a stable water film to be present at reactive grain boundaries even when subject to compressive stress. Hydration of periclase to brucite is associated with a solid volume increase of ca. 110%. Recent experiments on the periclase-brucite system observed that when the effective mean stress exceeds 30 MPa, the reaction rate slows down dramatically. We hypothesize that for the brucite forming reaction to progress, the fluid film between grains must remain stable. If the applied pressure becomes larger than the hydration force, the fluid film will collapse and be squeezed out of the grain contacts. To quantify this effect, we study the behavior of a water film confined between periclase or brucite surfaces subject to compressive stress, by performing molecular dynamics simulations. The simulations are carried out using the ClayFF force field and the single point charge (SPC) water model in the molecular dynamics simulations program LAMMPS. The setup consists of two interfaces of either periclase or brucite surrounded by water. Our simulations show that when the pressure reaches a few tens of MPa, the water film collapses and reduces the water film to one or two water layers, while the self-diffusion coefficient of water molecules by a factor of eight. A water film thickness below two water layers is thinner than the size of the hydration shell around Mg2+-ions, which will limit ion-transport. The observed collapse of the water film to a single layer at a normal pressure of 25–30 MPa might explain the observed slow-down of reaction-induced fracturing in the periclase-brucite system.

• McBeck, Jessica Ann; Aiken, John; Cordonnier, B.; Ben-Zion, Yehuda & Renard, Francois (2021). Predicting Fracture Network Development in Crystalline Rocks. Pure and Applied Geophysics (PAGEOPH). ISSN 0033-4553. 179, s. 275–299. doi: 10.1007/s00024-021-02908-7. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The geometric properties of fractures influence whether they propagate, arrest, or coalesce with other fractures. Thus, quantifying the relationship between fracture network characteristics may help predict fracture network development, and perhaps precursors to catastrophic failure. To constrain the relationship and predictability of fracture characteristics, we deform eight one centimeter tall rock cores under triaxial compression while acquiring in situ X-ray tomograms. The tomograms reveal precise measurements of the fracture network characteristics above the spatial resolution of 6.5 µm. We develop machine learning models to predict the value of each characteristic using the other characteristics, and excluding the macroscopic stress or strain imposed on the rock. The models predict fracture development more accurately in the experiments performed on granite and monzonite, than the experiments on marble. Fracture network development may be more predictable in these igneous rocks because their microstructure is more mechanically homogeneous than the marble, producing more systematic fracture development that is not strongly impeded by grain contacts and cleavage planes. The varying performance of the models suggest that fracture volume, length, and aperture are the most predictable of the characteristics, while fracture orientation is the least predictable. Orientation does not correlate with length, as suggested by the idea that the orientation evolves with increasing differential stress and thus fracture length. This difference between the observed and expected relationship between orientation and length highlights the influence of mechanical heterogeneities and local stress perturbations on fracture growth as fractures propagate, link, and coalesce.

• McBeck, Jessica Ann; Ben-Zion, Yehuda; Zhou, Xiaoyu & Renard, François (2021). The influence of preexisting host rock damage on fault network localization. Journal of Structural Geology. ISSN 0191-8141. 153. doi: 10.1016/j.jsg.2021.104471. Fulltekst i vitenarkiv
• Kandula, Neelima; McBeck, Jessica Ann; Cordonnier, Benoît; Weiss, Jérôme; Dysthe, Dag Kristian & Renard, François (2021). Synchrotron 4D X-Ray Imaging Reveals Strain Localization at the Onset of System-Size Failure in Porous Reservoir Rocks. Pure and Applied Geophysics (PAGEOPH). ISSN 0033-4553. 179, s. 325–350. doi: 10.1007/s00024-021-02902-z. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Understanding the mechanisms of strain localization leading to brittle failure in reservoir rocks can shed light on geomechanical processes such as porosity and permeability evolution during rock deformation, induced seismicity, fracturing, and subsidence in geological reservoirs. We perform triaxial compression tests on three types of porous reservoir rocks to reveal the local deformation mechanisms that control system-size failure. We deformed cylindrical samples of Adamswiller sandstone (23% porosity), Bentheim sandstone (23% porosity), and Anstrude limestone (20% porosity), using an X-ray transparent triaxial deformation apparatus. This apparatus enables the acquisition of three-dimensional synchrotron X-ray images, under in situ stress conditions. Analysis of the tomograms provide 3D distributions of the microfractures and dilatant pores from which we calculated the evolving macroporosity. Digital volume correlation analysis reveals the dominant strain localization mechanisms by providing the incremental strain components of pairs of tomograms. In the three rock types, damage localized as a single shear band or by the formation of conjugate bands at failure. The porosity evolution closely matches the evolution of the incremental strain components of dilation, contraction, and shear. With increasing confinement, the dominant strain in the sandstones shifts from dilative strain (Bentheim sandstone) to contractive strain (Adamswiller sandstone). Our study also links the formation of compactive shear bands with porosity variations in Anstrude limestone, which is characterized by a complex pore geometry. Scanning electron microscopy images indicate that the microscale mechanisms guiding strain localization are pore collapse and grain crushing in sandstones, and pore collapse, pore-emanated fractures and cataclasis in limestones. Our dynamic X-ray microtomography data brings unique insights on the correlation between the evolutions of rock microstructure, porosity evolution, and macroscopic strain during the approach to brittle failure in reservoir rocks.

• McBeck, Jessica Ann; Ben-Zion, Yehuda & Renard, Francois (2021). Fracture Network Localization Preceding Catastrophic Failure in Triaxial Compression Experiments on Rocks. Frontiers in Earth Science. 9. doi: 10.3389/feart.2021.778811. Fulltekst i vitenarkiv Vis sammendrag

  We quantify the spatial distribution of fracture networks throughout six in situ X-ray tomography triaxial compression experiments on crystalline rocks at confining stresses of 5–35 MPa in order to quantify how fracture development controls the final macroscopic failure of the rock, a process analogous to those that control geohazards such as earthquakes and landslides. Tracking the proportion of the cumulative volume of fractures with volumes >90th percentile to the total fracture volume, ∑v90/vtot indicates that the fracture networks tend to increase in localization toward these largest fractures for up to 80% of the applied differential stress. The evolution of this metric also matches the evolution of the Gini coefficient, which measures the deviation of a population from uniformity. These results are consistent with observations of localizing low magnitude seismicity before large earthquakes in southern California. In both this analysis and the present work, phases of delocalization interrupt the general increase in localization preceding catastrophic failure, indicating that delocalization does not necessarily indicate a reduction of seismic hazard. However, the proportion of the maximum fracture volume to the total fracture volume does not increase monotonically. Experiments with higher confining stress tend to experience greater localization. To further quantify localization, we compare the geometry of the largest fractures, with volumes >90th percentile, to the best fit plane through these fractures immediately preceding failure. The r2 scores and the mean distance of the fractures to the plane indicate greater localization in monzonite than in granite. The smaller mean mineral diameter and lower confining stress in the granite experiments may contribute to this result. Tracking these various metrics of localization reveals a close association between macroscopic yielding and the acceleration of fracture network localization. Near yielding, ∑v90/vtot and the Gini coefficient increase while the mean distance to the final failure plane decreases. Macroscopic yielding thus occurs when the rate of fracture network localization increases.

• Vincent-Dospital, Tom; Steyer, Alain; Renard, Francois & Toussaint, Renaud (2021). Frictional anisotropy of 3D-printed fault surfaces. Frontiers in Earth Science. doi: 10.3389/feart.2021.627823. Fulltekst i vitenarkiv Vis sammendrag

  The surface morphology of faults controls the spatial anisotropy of their frictional properties, and hence their mechanical stability. Such anisotropy is only rarely studied in seismology models of fault slip, although it might be paramount to understand the seismic rupture in particular areas, notably where slip occurs in a direction different from that of the main striations of the fault. To quantify how the anisotropy of fault surfaces affects the friction coefficient during sliding, we sheared synthetic fault planes made of plaster of Paris. These fault planes were produced by 3D-printing real striated fault surfaces whose 3D roughness was measured in the field at spatial scales from millimeters to meters. Here, we show how the 3D-printing technology can help for the study of frictional slip. Results show that fault anisotropy controls the coefficient of static friction, with the friction coefficient along the striations being three to four times smaller than the friction coefficient along the orientation perpendicular to the striations. This is true both at the meter and the millimeter scales. The anisotropy in friction and the average coefficient of static friction are also shown to decrease with the normal stress applied to the faults, as a result of the increased surface wear under increased loading.

• Montes-Hernandez, German; Findling, Nathaniel & Renard, Francois (2021). Direct and Indirect Nucleation of Magnetite Nanoparticles from Solution Revealed by Time-Resolved Raman Spectroscopy. Crystal Growth & Design. ISSN 1528-7483. 21(6), s. 3500–3510. doi: 10.1021/acs.cgd.1c00282. Fulltekst i vitenarkiv
• McBeck, Jessica Ann; Cordonnier, B. & Renard, F. (2021). The influence of spatial resolution and noise on fracture network properties calculated from X-ray microtomography data. International Journal of Rock Mechanics And Mining Sciences. ISSN 1365-1609. 147. doi: 10.1016/j.ijrmms.2021.104922. Fulltekst i vitenarkiv Vis sammendrag

  Rock deformation experiments performed at X-ray synchrotrons provide unique insights into the nature of fracture network development. However, these insights depend on the limitations of the X-ray tomography data. Here, we examine how spatial resolution and noise influence the calculated fracture network properties. To assess the influence of spatial resolution, we acquire two overlapping X-ray tomograms with spatial resolution that differ by an order of magnitude. To assess the influence of noise, we produce sets of synthetic tomograms with varying degrees of noise, including point-source noise and blurring noise. In the absence of noise, the differing spatial resolutions produce calculated porosities that differ by 0.05%, or 30% of the porosity measured in the high-resolution data. The fracture property that changes the most in the datasets of varying resolution is the fracture surface area, rather than the volume, length, or aperture. The two types of noise influence the porosity and fracture characteristics in opposite ways. In the synthetic tomograms in which higher values indicate fractures, added point noise increases the porosity while blurring noise decreases the porosity. In volumes with a mapping of gray values in which fractures have lower values, this trend would be reversed. This study is the first to quantify differences in fracture network properties extracted from X-ray tomograms due to spatial resolution and noise.

• McBeck, Jessica Ann; Zhu, Wenlu & Renard, Francois (2021). The competition between fracture nucleation, propagation and coalescence in crystalline rock. Solid Earth (SE). ISSN 1869-9510. 12(2), s. 375–387. doi: 10.5194/se-12-375-2021. Fulltekst i vitenarkiv
• Guren, Marthe Grønlie; Putnis, Christine V; Montes-Hernandez, German; King, Helen E. & Renard, Francois (2020). Direct imaging of coupled dissolution-precipitation and growth processes on calcite exposed to chromium-rich fluids. Chemical Geology. ISSN 0009-2541. 552. doi: 10.1016/j.chemgeo.2020.119770. Fulltekst i vitenarkiv Vis sammendrag

  Recent increases in industrial activity have resulted in elevated chromium (Cr) pollution in the natural environment, from the degradation of concrete and the leakage of water from landfills or mine waste. The release of toxic chromate ions into the environment could have harmful consequences. Methods for Cr removal are therefore becoming increasingly important to control the release of this highly toxic metal into the environment. This study examines how chromate (CrO42−) can be incorporated into a new, stable solid phase at a dissolving calcite surface. To study the relationship between solutions containing chromate and calcite, we performed two series of time-lapse atomic force microscopy (AFM) experiments to identify calcite dissolution and growth rates in the presence of chromate. In addition, we conducted complementary experiments in a stirred flow-through reactor to determine the amount of Cr removal from solution. All experiments were performed at room temperature and under a constant initial pH in the range 6–12. During the AFM experiments, we observed calcite dissolution via etch pit formation and propagation. In the presence of chromate, nanoparticles of a new phase nucleated and grew at the calcite surface by a coupled dissolution-precipitation process. In experiments with concentrations above 10 ppm Cr or at pH 10 and higher, we observed many nanoparticle precipitates, while precipitates were rarely observed at lower concentrations of Cr. At pH 10.5, the precipitates covered the calcite surface. This partly passivated further calcite dissolution, an observation confirmed by the low amounts of Cr removal measured in the flow-through experiments. Scanning electron microscopy analysis demonstrated that Cr was associated with the precipitates, indicating that Cr could be captured from solution and trapped in solid nanoparticles in the presence of calcite. However, the passivation of the calcite surface by a new precipitate may eventually slow down Cr sequestration.

• Montes-Hernandez, German & Renard, Francois (2020). Nucleation of Brushite and Hydroxyapatite from Amorphous Calcium Phosphate Phases Revealed by Dynamic in Situ Raman Spectroscopy. Journal of Physical Chemistry C. ISSN 1932-7447. 124(28), s. 15302–15311. doi: 10.1021/acs.jpcc.0c04028. Fulltekst i vitenarkiv Vis sammendrag

  Since the 1970s, it has been shown that calcium phosphate crystals nucleate from one or several early amorphous calcium phosphate phases into several in vivo and in vitro systems. However, the precise chemical composition, structure, and transformation mechanism of these amorphous phases remain controversial. Here, we characterize the reaction mechanism and kinetics of formation of two phosphate crystals, brushite and hydroxyapatite, by using in situ Raman spectroscopy in batch reactors at 25 °C. We investigate three pH regimes to control the phosphate speciation in solution and used solutions with or without citric acid, a complexing agent that may stabilize the amorphous phases. As expected, brushite (CaHPO4·2H2O) forms at pH < 9.8. Amorphous calcium phosphate (ACP: CaHPO4·nH2O) with a short lifetime (<2 min) and octocalcium phosphate (OCP: Ca8(HPO4)2(PO4)4·5H2O) are the main transient phases prior to brushite nucleation that occurs after ∼8 min. At pH > 11, hydroxyapatite (HAp: Ca10(PO4)6(OH)2) nucleates after >35 min, depending on the experimental conditions. The reaction mechanism steps for hydroxyapatite are more complex compared to brushite. For hydroxyapatite formation, amorphous calcium phosphate phases with different chemical composition (Ca(HPO4)1–x(PO4)(2/3)x·nH2O, with x in the range 0.2–1, and with different lifetime may form. Amorphous tricalcium phosphate (ATCP: Ca(PO4)2/3·nH2O or Ca3(PO4)2·nH2O when x = 1) is the most persistent phase which can either transform into OCP and then hydroxyapatite or directly evolve into hydroxyapatite at pH > 12.2. The presence of citric acid retards the transformation kinetics by increasing the nucleation times of brushite and hydroxyapatite but has little effect on the reaction mechanism steps. Finally, this study identifies new reactive pathways that characterize the formation of amorphous calcium phosphate phases and their transformation into brushite microcrystals or hydroxyapatite nanocrystals under abiotic conditions, relevant to a wide range of technological applications and natural environments.

• McBeck, Jessica Ann; Ben-Zion, Yehuda & Renard, Francois (2020). The mixology of precursory strain partitioning approaching brittle failure in rocks. Geophysical Journal International. ISSN 0956-540X. 221(3), s. 1856–1872. doi: 10.1093/GJI/GGAA121. Fulltekst i vitenarkiv Vis sammendrag

  SUMMARY We examine the strain accumulation and localization process throughout 12 triaxial compression experiments on six rock types deformed in an X-ray transparent apparatus. In each experiment, we acquire 50–100 tomograms of rock samples at differential stress steps during loading, revealing the evolving 3-D distribution of X-ray absorption contrasts, indicative of density. Using digital volume correlation (DVC) of pairs of tomograms, we build time-series of 3-D incremental strain tensor fields as the rocks are deformed towards failure. The Pearson correlation coefficients between components of the local incremental strain tensor at each stress step indicate that the correlation strength between pairs of local strain components, including dilation, contraction and shear strain, are moderate-strong in 11 of 12 experiments. In addition, changes in the local strain components from one DVC calculation to the next show differences in the correlations between pairs of strain components. In particular, the correlation of the local changes in dilation and shear strain tends to be stronger than the correlation of changes in dilation-contraction and contraction-shear strain. In 11 of 12 experiments, the most volumetrically frequent mode of strain accommodation includes a synchronized increase in multiple strain components. Early in loading, under lower differential stress, the most frequent strain accumulation mode involves the paired increase in dilation and contraction at neighbouring locations. Under higher differential stress, the most frequent mode is the paired increase in dilation and shear strain. This mode is also the first or second most frequent throughout each complete experiment. Tracking the mean values of the strain components in the sample and the volume of rock that each component occupies reveals fundamental differences in the nature of strain accumulation and localization between the volumetric and shear strain modes. As the dilative strain increases in magnitude throughout loading, it tends to occupy larger volumes within the rock sample and thus delocalizes. In contrast, the increasing shear strain components (left- or right-lateral) do not necessarily occupy larger volumes and so involve localization. Consistent with these evolutions, the correlation length of the dilatational strains tends to increase by the largest amounts of the strain components from lower to higher differential stress. In contrast, the correlation length of the shear strains does not consistently increase or decrease with increasing differential stress.

• Kirilova, Martina; Toy, Virginia; Sauer, Katrina; Renard, Francois; Gessner, Klaus & Wirth, Richard [Vis alle 8 forfattere av denne artikkelen] (2020). Micro- And nano-porosity of the active Alpine Fault zone, New Zealand. Solid Earth (SE). ISSN 1869-9510. 11(6), s. 2425–2438. doi: 10.5194/se-11-2425-2020. Fulltekst i vitenarkiv Vis sammendrag

  Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.

• Renard, Francois; Kandula, Neelima; McBeck, Jessica Ann & Cordonnier, Benoit (2020). Creep Burst Coincident With Faulting in Marble Observed in 4-D Synchrotron X-Ray Imaging Triaxial Compression Experiments. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 125. doi: 10.1029/2020JB020354. Fulltekst i vitenarkiv Vis sammendrag

  Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter‐scale cores of Carrara marble at 25°C and imaged the nucleation and growth of faults using dynamic synchrotron X‐ray microtomography. The first sample experienced a constant confinement of 30 MPa and no pore fluid. The second sample experienced confinement in the range 35–23 MPa and water as a pore fluid at 10 MPa pore pressure. We increased the axial stress by steps until creep deformation occurred and imaged deformation in 4‐D. The samples deformed with a quasi‐constant or increasing strain rate when the differential stress was constant, a process called creep. However, for both samples, we also observed transient events that include the acceleration of creep, that is, creep bursts, phenomena similar to slow slip events that occur in continental active faults. During these transient creep events, strain rates increase and correlate in time with strain localization and the slow development of system‐spanning fault networks. In both samples, the acceleration of opening and shearing of microfractures accommodated creep bursts. High‐resolution time‐lapse X‐ray microtomography imaging and digital image correlation during triaxial deformation quantify creep in laboratory faults at subgrain spatial resolution. This work demonstrates that transient creep events, that is, creep bursts or slow slip events, correlate with the nucleation and slow growth of faults and not only with slip on preexisting faults.

• Kartal, Muhammet; Xia, Fang; Ralph, David; Rickard, William D.A.; Renard, Francois & Li, Wei (2020). Enhancing chalcopyrite leaching by tetrachloroethylene-assisted removal of sulphur passivation and the mechanism of jarosite formation. Hydrometallurgy. ISSN 0304-386X. 191. doi: 10.1016/j.hydromet.2019.105192. Fulltekst i vitenarkiv Vis sammendrag

  Chalcopyrite (CuFeS2) is the primary ore mineral for copper, but leaching of this mineral under atmospheric conditions is slow due to the formation of surface passivating phases such as elemental sulphur and jarosite. Here, we studied chalcopyrite leaching in a sulphuric acid solution at 75 °C and 750 mV (relative to the standard hydrogen electrode), and found that after adding 20 vol% of tetrachloroethylene (TCE) into the leaching solution, elemental sulphur was dissolved from chalcopyrite and surface passivation was removed at the early stage of leaching. The removal of surface sulphur significantly enhanced the leaching rate by approximately 600% compared with TCE-free leaching. However, adding dimethyl sulfoxide (DMSO) did not improve the leaching rate. At the later stage of leaching, the increasing concentrations of Fe3+ from the dissolution of chalcopyrite and K+ possibly from the dissolution of minor amount of gangue minerals resulted in the precipitation of a potassium jarosite layer on the surface of chalcopyrite. The jarosite shell did not passivate TCE-free leaching due to its porous structure. However, in the case of leaching with TCE, elemental sulphur filled the pores, and the jarosite shell became nearly impermeable, resulting in passivation after 80% copper extraction. This study demonstrates a way for effective removal of sulphur passivation at the early stage of chalcopyrite leaching by adding sulphur dissolving solvent such as TCE, but to prevent jarosite formation at the later stage of leaching, it is necessary to keep the concentrations of Fe3+ and K+ at low levels.

• Panduro, Elvia Anabela Chavez; Cordonnier, Benoit; Gawel, Kamila; Børve, Ingrid; Iyer, Jaisree & Carroll, Susan [Vis alle 15 forfattere av denne artikkelen] (2020). Real time 3D observations of Portland Cement Carbonation at CO2 storage conditions. Environmental Science and Technology. ISSN 0013-936X. 54(13), s. 8323–8332. doi: 10.1021/acs.est.0c00578. Fulltekst i vitenarkiv Vis sammendrag

  Depleted oil reservoirs are considered a viable solution to the global challenge of CO2 storage. A key concern is whether the wells can be suitably sealed with cement to hinder the escape of CO2. Under reservoir conditions, CO2 is in its supercritical state, and the high pressures and temperatures involved make real-time microscopic observations of cement degradation experimentally challenging. Here, we present an in situ 3D dynamic X-ray micro computed tomography (μ-CT) study of well cement carbonation at realistic reservoir stress, pore-pressure, and temperature conditions. The high-resolution time-lapse 3D images allow monitoring the progress of reaction fronts in Portland cement, including density changes, sample deformation, and mineral precipitation and dissolution. By switching between flow and nonflow conditions of CO2-saturated water through cement, we were able to delineate regimes dominated by calcium carbonate precipitation and dissolution. For the first time, we demonstrate experimentally the impact of the flow history on CO2 leakage risk for cement plugging. In-situ μ-CT experiments combined with geochemical modeling provide unique insight into the interactions between CO2 and cement, potentially helping in assessing the risks of CO2 storage in geological reservoirs.

• McBeck, Jessica Ann; Aiken, John Mark; Mathiesen, Joachim; Ben-Zion, Yehuda & Renard, Francois (2020). Deformation precursors to catastrophic failure in rocks. Geophysical Research Letters. ISSN 0094-8276. 47(24). doi: 10.1029/2020GL090255. Fulltekst i vitenarkiv Vis sammendrag

  Forecasting the timing of catastrophic failure, such as crustal earthquakes, has been a central concern for centuries. Such forecasting requires identifying signals that evolve or accelerate in the precursory phase leading to failure, and the subset of signals that may be detected in the crust. We develop machine learning models to predict the proximity of catastrophic failure in synchrotron X‐ray tomography triaxial compression experiments on rocks using characteristics of evolving fracture networks. We then examine the characteristics that most strongly influence the model results, and thus may be considered the best predictors of the proximity of macroscopic failure. The resulting suite of predictive parameters underscores the importance of dilation in the precursory phase leading to catastrophic failure. The results indicate that the evolution of the strain energy density field may provide more robust predictions of the proximity of failure than other existing metrics of rock deformation.

• Renard, Francois; McBeck, Jessica Ann & Cordonnier, Benoit (2020). Competition between slow slip and damage on and off faults revealed in 4D synchrotron imaging experiments. Tectonophysics. ISSN 0040-1951. 782-783. doi: 10.1016/j.tecto.2020.228437. Fulltekst i vitenarkiv Vis sammendrag

  In the continental crust, faults may accommodate deformation through aseismic creep, slow slip events, or seismic slips that produce dynamic damage, or a combination of these endmembers. A variety of parameters controls the occurrence of these mechanical behaviors. In a series of laboratory experiments, we image centimeter-scale faults during sliding under in situ conditions. We perform four experiments of slip on centimeter-scale crystalline rock samples prepared with a saw-cut interface at 45° from the direction to the maximum compressive stress and at stress conditions of 2–3 km depth. We image fault slip and off-fault fracture development using 4D synchrotron X-ray microtomography. Three faults have an initial rough interface, and deformation occurs with increasing differential stress by a combination of slow slip events and off-fault damage, until catastrophic failure and the formation of new faults. Conversely, the pre-cut fault with a smooth initial surface deforms mainly by slow slip, develops numerous striations along its slip plane, and no microfractures are detected in the wall rock. Our experiments reproduce aseismic and seismic faulting behavior, and demonstrate that the roughness of the fault plane is one of the parameters that control the transition between these two behaviors. A fault with a rougher interface may tend to develop more off-fault damage and seismic behavior. For the rough fault experiments, the secondary faulting occurs along a network of faults oriented at high angles from the pre-existing saw-cut plane, a behavior similar to several earthquake sequences that occurred along orthogonal faults.

• McBeck, Jessica Ann; Aiken, John Mark; Ben-Zion, Yehuda & Renard, Francois (2020). Predicting the proximity to macroscopic failure using local strain populations from dynamic in situ X-ray tomography triaxial compression experiments on rocks. Earth and Planetary Science Letters. ISSN 0012-821X. 543. doi: 10.1016/j.epsl.2020.116344. Fulltekst i vitenarkiv Vis sammendrag

  Predicting the proximity of large-scale dynamic failure is a critical concern in the engineering and geophysical sciences. Here we use evolving contractive, dilatational, and shear strain deformation preceding failure in dynamic X-ray tomography experiments to examine which strain components best predict the proximity to failure. We develop machine learning models to predict the proximity to failure using time series of three-dimensional local incremental strain tensor fields acquired in rock deformation experiments under stress conditions of the upper crust. Three-dimensional scans acquired in situ throughout triaxial compression experiments provide a distribution of density contrasts from which we estimate the three-dimensional incremental strain that accumulates between each scan acquisition. Training machine learning models on multiple experiments of six rock types provides suites of feature importance that indicate the predictive power of each feature. Comparing the average importance of groups of features that include information about each strain component quantifies the ability of the contractive, dilatational and shear strain to predict the proximity of macroscopic failure. A total of 24 models of four machine learning algorithms with six rock types indicate that 1) the dilatational strain provides the best predictive power of the strain components, and 2) the intermediate values (25th-75th percentile) of the strain population provide the best predictive power of the statistics of the strain populations. In addition, the success of the predictions of models trained on one rock type and tested on other rock types quantifies the similarities and differences of the precursory strain accumulation process in the six rock types. These similarities suggest the potential existence of a unified theory of brittle rock deformation for a range of rock types.

• McBeck, Jessica Ann; Cooke, Michele & Renard, Francois (2020). How the energy budget scales from the laboratory to the crust in accretionary wedges. Earth and Planetary Science Letters. ISSN 0012-821X. 541. doi: 10.1016/j.epsl.2020.116276. Fulltekst i vitenarkiv
• Montes-Hernandez, German; Renard, Francois; Auzende, Anne Line & Findling, Nathaniel (2020). Amorphous Calcium–Magnesium Carbonate (ACMC) Accelerates Dolomitization at Room Temperature under Abiotic Conditions. Crystal Growth & Design. ISSN 1528-7483. 20(3), s. 1434–1441. doi: 10.1021/acs.cgd.9b01005. Fulltekst i vitenarkiv Vis sammendrag

  The challenge to produce dolomite CaMg(CO3)2 at low temperature (20–35 °C) over laboratory time scales so far has remained unsuccessful, which has led to long-lasting scientific debates in the last two centuries. This mineral exerts a major control on the natural carbon dioxide sequestration into various sedimentary, basaltic, and mantellic rocks. The present study reports on specific abiotic conditions that allow the precipitation of disordered dolomite, high Mg calcite, and high Ca magnesite at room temperature over time scales of hours to days. Here we show that an amorphous calcium magnesium carbonate (ACMC) phase accelerates dolomitization at room temperature. ACMC is initially precipitated by mixing a carbonate (HCO3–/CO32– = 1; pH ∼10.3 ≈ pKa2) alkaline solution with a Mg-Ca ionic solution (Mg molar fraction between 0 and 1). Then, time-resolved in situ Raman spectroscopy monitored the transformation of ACMC into Mg-rich carbonate minerals. The initial Mg molar fraction controlled both the reaction mechanism (e.g., nature of transient crystalline phases) and the kinetics. Nanosized crystallites with short-range order, called disordered dolomite CaMg(CO3)2, precipitated following a complex reaction pathway. First, nesquehonite (MgCO3·3H2O: nucleation time 2.5 h) and then disordered dolomite (CaMg(CO3)2: nucleation time 3.2 h) followed by monohydrocalcite (CaCO3·H2O: nucleation time 3.4 h) formed from ACMC transformation. Nesquehonite and monohydrocalcite are transient phases that nourish the slow precipitation of disordered dolomite, which reached a spectral equilibrium after 7 days of reaction. The direct transformation of ACMC into disordered dolomite was also measured. Our experimental results demonstrate that disordered dolomite precipitates at room temperature when an ideal Mg/Ca ratio, high carbonate alkalinity, and high ionic concentration are reached in abiotic systems. This result suggests the possibility of a physicochemical rather than biotic control on the formation of disordered dolomite at low temperature in several geosystems.

• Chauve, Thomas; Scholtes, Luc; Donzé, Frédéric-Victor; Mondol, Nazmul Haque & Renard, Francois (2020). Layering in shales controls microfracturing at the onset of primary migration in source rocks. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 125(5). doi: 10.1029/2020JB019444. Fulltekst i vitenarkiv Vis sammendrag

  The process of primary migration, which controls the transfer of hydrocarbons from source to reservoir rocks, necessitates the existence of fluid pathways in low permeability sedimentary formations. Primary migration starts with the maturation of organic matter that produces fluids which increase the effective stress locally. The interactions between local fluid production, microfracturing, stress conditions, and transport remain difficult to apprehend in shale source rocks. Here, we analyze these interactions using a coupled hydro‐mechanical numerical model based on the discrete element method. The model is used to simulate the effects of fluid production emanating from kerogen patches contained within a shale rock alternating kerogen‐poor and kerogen‐rich layers. We identify two microfracturing mechanisms that control fluid migration: i) propagation of hydraulically driven fractures induced by kerogen maturation in kerogen‐rich layers, and ii) compression induced fracturing in kerogen‐poor layers caused by fluid overpressurization of the surrounding kerogen‐rich layers. The relative importance of these two mechanisms is discussed considering different elastic properties contrasts between the shale layers, as well as various stress conditions encountered in sedimentary basins, from normal to reverse faulting regimes. The layering in shales causes local stress redistribution that controls the prevalence of each mechanism over the other and the onset of microfracturing during kerogen maturation. Results are applied to the Draupne formation, a major source rock in the Norwegian continental shelf in the North Sea.

• Heap, Michael; Baud, Patrick; McBeck, Jessica Ann; Renard, Francois; Carbillet, Lucille & Hall, Stephan A. (2020). Imaging strain localisation in porous andesite using digital volume correlation. Journal of Volcanology and Geothermal Research. ISSN 0377-0273. 404. doi: 10.1016/j.jvolgeores.2020.107038. Fulltekst i vitenarkiv Vis sammendrag

  Strain localisation structures, such as shear fractures and compaction bands, are of importance due to their influence on permeability and therefore outgassing, a factor thought to influence eruptive style. In this study, we aim to develop a better understanding of strain localisation in porous volcanic rocks using X-ray tomographic images of samples of porous andesite (porosity = 0.26) acquired before and after deformation in the brittle and ductile regimes. These 3D images have been first analysed to provide 3D images of the porosity structure within the undeformed andesite, which consists of a large, well-connected porosity backbone alongside many smaller pores that are either isolated or connected to the porosity backbone by thin microstructural elements (e.g., microcracks). Following deformation, porosity profiles of the samples show localised dilation (porosity increase) and compaction (porosity reduction) within the samples deformed in the brittle and ductile regimes, respectively. Digital volume correlation (DVC) of the images before and after triaxial deformation was used to quantify the tensor strain fields, and the incremental divergence (volumetric strain) and curl (used as an indicator of shear strain) of the displacement fields were calculated from the DVC. These fields show that strain localisation in the sample deformed in the brittle regime manifested as a ~ 1 mm-wide, dilatational shear fracture oriented at an angle of 40–45° to the maximum principal stress. Pre- and post-deformation permeability measurements show that permeability of the sample deformed in the brittle regime increased from 3.9 × 10−12 to 4.9 × 10−12 m2, which is presumed to be related to the shear fracture. For the sample deformed in the ductile regime, strain localised into ~1 mm-thick, undulating compaction bands orientated sub-perpendicular to the maximum principal stress with little evidence of shear. Taken together, our data suggest that these bands formed during large stress drops seen in the mechanical data, within high-porosity zones within the sample, and within the large, well-connected porosity backbone. Pre- and post-deformation permeability measurements indicate that inelastic compaction decreased the permeability of the sample by a factor of ~3. The data of this study assist in the understanding of strain localisation in porous volcanic rocks, its influence on permeability (and therefore volcanic outgassing), and highlight an important role for DVC in studying strain localisation in volcanic materials.

• Cordonnier, Benoit; Pluymakers, Anne Marie Henriette; Tengattini, Alessandro; Marti, Sina; Kaestner, Anders & Fusseis, Florian [Vis alle 7 forfattere av denne artikkelen] (2019). Neutron Imaging of Cadmium Sorption and Transport in Porous Rocks. Frontiers in Earth Science. doi: 10.3389/feart.2019.00306. Fulltekst i vitenarkiv
• McBeck, Jessica Ann; Mair, Karen & Renard, Francois (2019). Linking macroscopic failure with micromechanical processes in layered rocks: How layer orientation and roughness control macroscopic behavior. Tectonophysics. ISSN 0040-1951. 750, s. 229–242. doi: 10.1016/j.tecto.2018.11.016. Fulltekst i vitenarkiv Vis sammendrag

  To constrain the impact of preexisting mechanical weaknesses on strain localization culminating in macroscopic shear failure, we simulate triaxial compression of layered sedimentary rock using three-dimensional discrete element method simulations. We develop a novel particle packing technique that builds layered rocks with preexisting weaknesses of varying orientations, roughness, and surface area available for slip. We quantify how the geomechanical behavior, characterized by internal friction coefficient, μ0, and failure strength, σF, vary as a function of layer orientation, θ, interface roughness, and total interface area. Failure of the simulated sedimentary rocks mirrors key observations from laboratory experiments on layered sedimentary rock, including minima σF and μ0 for layers oriented at 30° with respect to the maximum compressive stress, σ1, and maxima σF and μ0 for layers oriented near 0° and 90° to σ1. The largest changes in σF (66%) and μ0 (20%) occur in models with the smoothest interfaces and largest interface area. Within the parameter space tested, layer orientation exerts the most significant impact on σF and μ0. These simulations allow directly linking micromechanical processes observed within the models to macroscopic failure behavior. The spatial distributions of nucleating microfractures, and the rate and degree of strain localization onto preexisting weaknesses, rather than the host rock, are systematically linked to the distribution of failure strengths. Preexisting weakness orientation more strongly controls the degree and rate of strain localization than the imposed confining stress within the explored parameter space. Using the upper and lower limits of μ0 and σF obtained from the models, estimates of the Coulomb shear stress required for failure of intact rock within the upper seismogenic zone (7 km) indicates that a rotation of 30° of σ1 relative to the weakness orientation may reduce the shear stress required for failure by up to 100 MPa.

• Petley-Ragan, Arianne Juliette; Ben-Zion, Yehuda; Austrheim, Håkon Olaf; Ildefonse, Benoit; Renard, Francois & Jamtveit, Bjørn (2019). Dynamic earthquake rupture in the lower crust. Science Advances. 5(7). doi: 10.1126/sciadv.aaw0913. Fulltekst i vitenarkiv Vis sammendrag

  Earthquakes in the continental crust commonly occur in the upper 15 to 20 km. Recent studies demonstrate that earthquakes also occur in the lower crust of collision zones and play a key role in metamorphic processes that modify its physical properties. However, details of the failure process and sequence of events that lead to seismic slip in the lower crust remain uncertain. Here, we present observations of a fault zone from the Bergen Arcs, western Norway, which constrain the deformation processes of lower crustal earthquakes. We show that seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. The succession of deformation processes reported here emphasize brittle failure mechanisms in a portion of the crust that until recently was assumed to be characterized by ductile deformation.

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• Barras, Fabian; Aspaas, Andreas Grøvan; Aharonov, Einat & Renard, Francois (2026). Unveiling the role of seepage forces in the acceleration of frictional creep in fluid-saturated shear zone. doi: 10.5194/egusphere-egu26-17167. Fulltekst i vitenarkiv Vis sammendrag

  How fluid impact frictional slip is a central question in various geological settings, from tectonic faults to friction at the base of glaciers. In this work, we study the impact of fluid infiltration on the creep dynamics of the shear zone located at the base of a densely monitored landslide in Western Norway. In Åknes, approximately 50 million cubic meter of rock mass continuously creeps over a shear zone made of rock fragments, with seasonal accelerations that strongly correlate with rainfall. In this natural laboratory for fluid-induced frictional creep, unprecedented monitoring equipment reveals low fluid pressure across the shear zone, thereby challenging the conventional theory of fluid-driven instability in landslides. Here, we show that a generic micromechanical model can disentangle the effects of fluid flow from those of fluid pressure, and demonstrate that seepage forces applied by channelized flow along the shear zone are the main driver of creep accelerations. We conclude by discussing the significance of seepage forces, the implications for hazard mitigation and the broader applicability of our model to various geological contexts governed by friction across saturated shear zones.

• Dziadkowiec, Joanna; Linga, Gaute; Dunkel, Kristina G.; Valtiner, Markus & Renard, Francois (2026). In-situ, real-time replacement of calcite under geometrical confinement. doi: 10.5194/egusphere-egu26-3925. Fulltekst i vitenarkiv Vis sammendrag

  Mineral replacement by dissolution-precipitation reactions that occur under confinement critically influences subsurface deformation by modifying rock porosity, permeability, and cohesion, and by inducing fracturing. Yet real-time, experimental observations of these phenomena at the nano- to microscale remain insufficient. In this work, we follow the in-situ replacement of confined calcite crystal using a surface force apparatus (SFA) technique. Calcite undergoes dissolution under low pH conditions, followed by replacement into three various Ca-minerals: calcium oxalate, calcium sulfate (gypsum), or calcium phosphate (brushite), depending on the initial composition of the solution. We monitor these reactions in real time, map the spatial distribution of precipitates as a function of confinement gap size, and evaluate how epitaxy between the secondary phases and the parent calcite governs preferred nucleation and growth sites. In addition, we measure forces that act on the confining pore wall during the replacement and estimate the associated crystallization pressures. This work contributes to the understanding of the mineral growth under confinement and its consequences for porous rock integrity, with immediate relevance to subsurface fluid and gas storage operations, where rapid mineralization is common.

• Aspaas, Andreas Grøvan; Renard, Francois; Lacroix, Pascal; Kristensen, Lene & Etzelmüller, Bernd (2022). What causes transient deformations in the Åknes landslide, Norway? Fulltekst i vitenarkiv
• Linga, Gaute; Mathiesen, Joachim; Renard, Francois & Borgne, Tanguy Le (2022). Chaotic mixing in intermittent two-phase porous media flows. Fulltekst i vitenarkiv
• Madathiparambil, Aldritt Scaria; Mürer, Fredrik Kristoffer; Tekseth, Kim Robert Bjørk; Agofack, Nicolaine; Cerasi, Pierre & Cordonnier, Benoit [Vis alle 10 forfattere av denne artikkelen] (2021). In situ computed tomography studies of strain evolution in Draupne shales. Fulltekst i vitenarkiv
• Madathiparambil, Aldritt Scaria; Chattopadhyay, Basab; Tekseth, Kim Robert Bjørk; Agofack, Nicolaine; Cerasi, Pierre & McBeck, Jessica Ann [Vis alle 9 forfattere av denne artikkelen] (2020). Dynamic in situ computed tomography study of strain evolution in Draupne shales under triaxial loading. Fulltekst i vitenarkiv
• Linga, Gaute; Mathiesen, Joachim; Renard, Francois & Borgne, Tanguy Le (2020). Stretching and Folding in Intermittent Two-Phase Porous Media Flows. Fulltekst i vitenarkiv
• Johnson, James Ronald; Hansen, Jørgen André; Mondol, Nazmul Haque & Renard, Francois (2019). Modeling maturation, elastic, and geomechanical properties of the Draupne Formation, Offshore Norway. Fulltekst i vitenarkiv
• Johnson, James Ronald; Hansen, Jørgen André; Renard, Francois & Mondol, Nazmul Haque (2019). Geomechanical Analysis of Maturation for the Draupne Shale, Offshore Norway. Fulltekst i vitenarkiv
• Aspaas, Andreas Grøvan; Krautblatter, Michael; Renard, Francois & Etzelmüller, Bernd (2019). Effects of glimmer rich rocks on the failure criterion of ice-filled permafrost rock joints. Fulltekst i vitenarkiv
• Guren, Marthe Grønlie; Bore, Sigbjørn Løland; Renard, Francois & Sveinsson, Henrik Andersen (2026). Energy dissipation at the atomic scale explains how fracture energy depends on crack velocity in silica glass. arxiv.org. doi: 10.48550/ARXIV.2605.03457. Fulltekst i vitenarkiv Vis sammendrag

  The fracture energy of brittle materials rises with crack velocity, and this effect is typically attributed to surface roughening from path instabilities. Here we show, using molecular dynamics simulations of silica glass with a first-principles machine learned interatomic potential, that the structural fracture energy rises by up to 33 % already below the branching threshold, showing that fracture energy is not a constant material property. This rise in fracture energy is roughly equally partitioned between an increase in the intrinsic surface energy density and nanoscale roughening that increases the real fracture surface area. Results demonstrate that dynamic fracture in silica glass increases the fracture energy not merely by creating more apparent surface, but also by creating a fundamentally different surface at the nanoscale.

• Prastyani, Erina; Renard, Francois; Cordonnier, Benoit & McBeck, Jess (2026). 4D Imaging of Brittle to Semi-ductile Deformation in Crystalline Rock. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  This thesis investigates the fundamental links between strain localization, porosity evolution, and deformation mechanisms across the brittle-to-semi-ductile transition in crystalline rock. Using a multidisciplinary approach that combines triaxial deformation experiments with in-situ 4D X-ray microtomography and acoustic emission monitoring, this work quantifies the seismic and aseismic processes governing deformation. In the brittle regime, this work elucidates how strain partitioning between dilation and shear drives fault growth within intact rock bridges. As deformation reaches the semi-ductile regime, this research captures the evolution of substantial transient porosity and pore connectivity, quantifying the volumetric strain partitioning that might influence fault stability and fluid transport in the middle crust. By challenging the view of the brittle-ductile transition as a static, low-porosity barrier, this thesis provides a dynamic framework for understanding the occurrence of slow slip events, aseismic creep, and deep fluid migration near the base of the seismogenic zone.

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