Determining the maximum possible magnitude of fluid-induced earthquakes requires to understand rupture arrest within or outside a fluid-pressurized patch. Recent studies have highlighted the importance of incorporating rupture physics into the study of injection-induced earthquakes. We perform 3D dynamic simulations of spontaneous ruptures propagating across a pressurized fault, stimulated by fluid injection within the nucleation zone. Our simulations unveil two end-member models describing a fluid-induced micro-earthquake: a self-arresting rupture that decelerates spontaneously and a run-away rupture that terminates abruptly at the fault edge. We compute synthetic waveforms radiated from both models and invert them using a probabilistic spectral inversion approach to identify characteristics that distinguish between the two rupture types. We find that self-arresting ruptures radiate less high-frequency waves (with gamma> 3) and lack the typical P/S corner frequency shift. In contrast, run-away ruptures conform to the omega(2) model (gamma similar to 2, f (p)(c)/f (S)(c) similar to 1.3). We interpret these differences as primarily arising from the rupture arrest mechanism, smooth arrest results in gradual variations in the moment-rate function, whereas abrupt arrest at a barrier causes a sharp changes in the moment-rate function. This abrupt arrest generates high-frequency radiation and a back-propagating stopping phase, playing a critical role in controlling the rupture duration and the radiated seismic waves. Our results demonstrate that spectral features such as high-frequency decay and P/S corner frequency shift may provide observational diagnostics to distinguish rupture arrest mechanisms, even in the absence of direct evidence from rupture kinematics.

Understanding the faulting dynamics of natural earthquakes is fundamentally limited by the scarcity of near-source observations and the incompleteness of knowledge about in situ conditions at depth. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland addresses these limitations through controlled hydraulic stimulation experiments that generate seismicity beneath more than 1 km of rock overburden, thereby bridging the scale gap between laboratory studies and observed natural earthquakes. A key advantage of the BedrettoLab is the ability to characterize in situ conditions prior to seismicity induction. This includes imaging the geometry of the target fault, estimating the local stress state and pore fluid pressure, and examining host and fault rock properties.Seismicity induced by controlled hydraulic stimulation is recorded by a comprehensive suite of near-source instrumentation, including strong-motion seismometers, borehole geophones and accelerometers, high-frequency acoustic emission sensors, and fibre-optic cables enabling Distributed Acoustic Sensing (DAS) and Fibre Bragg Grating (FBG) measurements. Past experiments have successfully generated seismicity sequences with mainshock magnitudes between Mw −0.5 and 0.0. We construct dynamic rupture models for one such mainshock, constrained by the available near-source observations, to image slip distribution, rupture directivity, and rupture velocity at meter-scale resolution. We find that rupture directivity has a substantially stronger impact on spectral amplitudes than average stress drop. The inferred stress and friction drops are interpreted in terms of the maximum possible confining pressure, providing insights into dynamic weakening processes during earthquake rupture.The next experiment aims to induce Mw 1.0 earthquakes along a selected fault zone. Using constraints from hydraulic fracture tests, fault geometry imaging, and injection protocols, we seek to forecast the potential rupture dynamics of the induced mainshock by generating a suite of dynamic rupture models representing plausible rupture scenarios, against which the observed mainshock dynamics can be evaluated. In particular, we assess how reliably pre-experiment slip tendency analyses translate into the actual rupture behavior under these controlled conditions. Ultimately, this research will advance our understanding of earthquake source physics and contribute to improved forecasting and mitigation of worst-case scenarios associated with hydraulic stimulation.

Structured metadata defined in a machine-readable format are the foundation of FAIR data and open research. To describe rock deformation laboratory experiments, standardized metadata formats are currently missing. This limits the annotation of metadata describing the experiments, as well as the data and metadata produced during the experiments. To address this issue, we propose a structured metadata model, a configuration for OpenBIS data platform, and a web interface to annotate the metadata of experiments and data files. We developed a structured metadata model based on our user experience on performing rock deformation experiments. Then, we fully configured OpenBIS data platform using a docker container stack. Finally, we modified an existing Streamlit application allowing users to annotate metadata according to our structure, upload and annotate data files automatically. This approach allows us to annotate data and metadata produced in rock deformation laboratory experiments in a systematic and structured way to foster the adoption of FAIR principles and open science across rock deformation laboratories and hopefully across geosciences laboratories in general.

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

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

Aseismic slip is increasingly recognized as a fundamental driver of earthquake nucleation, affecting the spatio-temporal evolution of seismicity, yet its direct observation remains rare due to limited strain measurements close to a natural fault system. Here, we present results from the 'FEAR1' experiment conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies (Switzerland), where we used fluid injections to activate a natural fault and fracture network in crystalline rock under in-situ stress conditions at ~1 km depth. This experimental setting is particularly well suited to investigate induced seismicity and the role of aseismic processes in fault activation, thanks to dense near- and on-fault strain, pressure, and seismic monitoring.During several injections performed in FEAR1, we observed the activation of a steeply dipping, highly permeable fracture zone, which intersects a densely instrumented borehole. Hydraulic stimulations triggered seismicity (−4.9 < Mw < −2.3) that organized along a plane whose orientation is consistent with geological observations in boreholes cores, logs and on the laboratory tunnel wall. Simultaneously, high-resolution Fiber Bragg Grating strain measurements revealed progressive, irreversible tensile deformation localized near the fracture intersection with the monitoring borehole, reaching nearly 1000 µε over the course of the experiment.Static elastic modeling demonstrates that the cumulative strain produced by the recorded earthquakes accounts for less than 1% of the observed deformation, indicating that fault slip was dominantly aseismic. The spatial and temporal evolution of seismicity shows systematic up-dip migration toward the strain concentration zone and the emergence of families of repeating earthquakes. The recurrence rate and cumulative slip of these repeaters correlate with the measured strain rate and strain, suggesting a scenario where seismic asperities are embedded within a creeping fault segment sustained by pore pressure stress perturbations.Inversions of irreversible strain for simplified slip sources indicate a predominantly strike-slip mechanism consistent with the estimated local stress field, although trade-offs between source location, source dimension and slip direction highlight the limits of 1D strain observations. Our results provide direct experimental evidence for fluid-driven aseismic slip on a natural fault and demonstrate how microseismicity and repeaters can serve as indirect proxies for underlying slow deformation.

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

Our understanding of earthquake rupture processes is generally limited by the resolution of available observations. In all but exceptional cases, earthquake observations are made at comparatively large distances from the rupture itself, which puts a limit on what spatial scales can be resolved. At the same time, it is clear that small scale processes may play a crucial, if not dominant, role for various seismogenic processes, including rupture nucleation, co-seismic weakening and stress re-distribution. The Fault Activation and Earthquake Rupture ('FEAR') project aims at collecting and interpreting a multitude of earthquake-relevant observations from directly on and around the process zone of an induced earthquake. To this end, we attempt to activate a natural granitic fault zone in the BedrettoLab, at a depth of ~1km, after instrumenting the fault zone with a multi-domain and multi-scale monitoring system. The goal is to observe and study earthquake rupture phenomena in a natural setting, from unusually close distance. In this talk, we outline the project status, the science goals, and the plans for the main experiments, which are scheduled for the years 2024 - 2026. Notable milestones we report on include the identification and detailed characterisation of the target fault zone the beginning of niche and tunnel excavations laboratory experiments that characterise the frictional and mechanical behaviour of both gauge material and host rock of the target fault zone development of numerical models for 2D and 3D dynamic rupture propagation development of tailored monitoring methods for seismicity, strain, temperature, pressure, bio-geo-chemistry and other relevant observables development of remote experiment control methods test stimulations in a nearby rock volume of similar geology, with an already existing monitoring system, where we tested the influence of pre-conditioning injection protocols similar test stimulations in the same volume where we aim at triggering a larger event (target Mw~0) active seismic experiments in an underground salt mine, to calibrate the very- to ultra-high frequency (1k Hz - 500k Hz) acoustic emission sensors Together, these and other efforts constitute the necessary ingredients we need for interpreting the near-source observations that we will collect during the fault activation experiments.

The conventional understanding of tectonic faults primarily categorizes them based on frictional behavior: stable due to velocity-strengthening (VS) behavior, or unstable owing to velocity-weakening (VW) that lead to seismic ruptures. This classification has traditionally led to the assumption that VS faults are unlikely candidates for earthquake nucleation. However, emerging evidence from recent laboratory experiments and field studies is challenging this simplistic view, pointing towards a more complex mechanism. In this study, we utilize a hydro-mechanically coupled fault model, which integrates VS friction governed by rate-and-state friction laws with dynamic weakening influenced by poroelastic effects. A key aspect of our findings is the impact of fluid injection on the mechanical state of the fault. This process decreases the effective normal stress and frictional resistance, initially paving the way for the propagation of an aseismic, slow-slip event. The transition from aseismic to seismic slip on VS faults hinges on the balance between shear-induced dilation and compaction. These opposing mechanisms respectively lead to a decrease and an increase in pore-fluid pressure, dictating the balance between fault stability or instability. Our results show that when the effect of compaction-induced pressurization surpasses the initial dilatancy phase, it enables the propagation of dynamic rupture as a solitary pore-pressure wave. Conversely, when dilation predominates over compaction, an aseismic slow-slip event propagates through the fault, maintaining stability and preventing rapid seismic activity. These findings advance our understanding of seismic risk associated with VS faults. They are especially relevant in the context of fluid injection practices in geothermal energy production and CO2 storage, demonstrating how such activities might activate faults that are considered nominally stable. Additionally, our results underscore the critical need for more experimental and theoretical investigations into shear-induced compaction as an efficient mechanism for fault self-pressurization, which plays a key role in leading to seismic instabilities.

Understanding the dynamics of microearthquakes is a timely challenge with the potential to address current paradoxes in earthquake mechanics, and to better understand earthquake ruptures induced by fluid injection. We perform fully 3D dynamic rupture simulations caused by fluid injection on a target fault for Fault Activation and Earthquake Ruptures experiments generating Mw ≤ 1 earthquakes. We investigate the dynamics of rupture propagation with spatially variable stress drop caused by pore pressure changes and assuming different slip‐weakening constitutive parameters. We show that the spontaneous arrest of propagating ruptures is possible by assuming a high fault strength parameter S, that is, a high ratio between strength excess and dynamic stress drop. In faults with high S values (low rupturing potential), even minor variations in Dc (from 0.45 to 0.6 mm) have a substantial effect on the rupture propagation and the ultimate earthquake size. Modest spatial variations of dynamic stress drop determine the rupture mode, distinguishing self‐arresting from run‐away ruptures. Our results suggest that several characteristics inferred for accelerating dynamic ruptures differ from those observed during rupture deceleration of a self‐arresting earthquake. During deceleration, a decrease of peak slip velocity is associated with a nearly constant cohesive zone size. Moreover, the residual slip velocity value (asymptotic value for a crack‐like rupture) decreases to nearly zero. This means that an initially crack‐like rupture becomes a pulse‐like rupture during spontaneous arrest. These findings highlight the complex dynamics of small induced earthquakes, which differ from solutions obtained from conventional crack‐like models of earthquake rupture.

Gravity is a force contributing to the strain energy and the tectonic stress driving faulting and generating earthquakes. This paper discusses the role of gravity in earthquake mechanics for different tectonic settings. Considering the stress state in normal and reverse tectonic settings, including gravity as a direct contribution to lithostatic load, it is possible to show that earthquakes on normal faults do not have a different energy source than elastic rebound and that this explains differences with reverse faulting earthquakes. The paper discusses the implications from dismissing the elastic rebound theory or limiting its validity to reverse or strike‑slip faulting, as suggested to support the graviquakes model, and the consequences on the mechanics of dip‑slip earthquakes. A simple model of tectonic stress relying on Anderson theory of faulting can describe the different stress state of normal and reverse faulting earthquakes, showing higher values of tectonic stress acting on reverse faults than normal faults, for different values of the static friction coefficient. The model shows that the difference between tectonic stress before and after a dip‑slip earthquake increases with the static friction coefficient, emphasizing the effect of the drained conditions on compressional tectonic stress, and the negligible effect for extensional tectonic settings. Slip can occur on normal faults creating horizontal extensional deformation when the minimum stress is compressional, since extension is caused by the deviatoric stress acting on the fault plane. The different stress state can explain numerous seismological observations, likely accounting for non‑Byerlee friction, stress and strength heterogeneity and geometrical complexity. The adoption of elastic rebound does not imply that the energetics of normal and reverse faulting earthquakes is the same. Considering crustal faults as passive subjects accommodating slip caused by volume collapse contradicts geological observations of fault zone structure, laboratory experiments and the spectrum of fault slip behavior. Faults are active geological subjects characterizing the strain localization and the energy release.

Earthquakes are associated with the propagation of a dynamic rupture, which radiates elastic energy through seismic waves. The generation of seismic radiation is related to dynamic weakening of shear stress and stress drop. In modeling dynamic ruptures, shear stress evolution is commonly imposed through a constitutive law, such as the widely adopted slip weakening laws. According to these constitutive laws, shear stress evolves as a function of slip in each point of the rupturing fault, prescribing strength excess, stress drop and dynamic weakening.Here, we compare two well-known slip weakening laws: namely, the classic Ida’s (1972) and the Ohnaka’s (1996) slip weakening laws. The former prescribes that fault stress increases from the initial stress to the peak stress with zero slip and then linearly decreases from the peak value to a residual value over a slip-distance Dc (dynamic weakening). The latter assumes that the initial stress hardening phase occurs over a non-negligible slip-distance Da and that shear stress decrease from the peak value is not linear. The Ohnaka’s law was validated with numerous laboratory experiments. The evolution of shear stress with slip allows the estimate of the breakdown work Wb, i.e. the excess of work above a minimum stress level with slip from 0 to Dc.We collected data from high-velocity friction experiments to quantify yield, peak and residual stresses, Da and Dc distances for bare-rock samples of Carrara Marble and Gabbro deformed under various experimental conditions (room humidity, vacuum, pressurized fluids) and normal stress (from 5 to 40 MPa). The ratio Da/Dc is much lower for Carrara marble (0.015) than for Gabbro (0.12). We implemented the Ohnaka’s constitutive law in a 2D finite difference code for spontaneous dynamic ruptures characterized by a fault in a homogeneous elastic material. We perform simulations using the two different slip weakening laws. We kept constant Dc, and we compared the results of the simulations in terms of rupture style, rupture velocity, breakdown work, and cohesive zone size. As expected both laws yield crack-like ruptures. Moreover, Ohnaka’s law in comparison to the linear slip weakening law produces:rupture velocity ~2 % higher; breakdown work (Wb) up to 60 % lower. Moreover, dividing the breakdown work into the energy dissipated between the yield stress and the peak stress over the slip-distance Da (Wba), we notice that Wba can reach up to the 30% of the total Wb in case of Gabbro (Da/Dc = 0.12). a cohesive zone size (defined as the portion of the fault in which the slip velocity is higher than zero and the stress is higher than its residual value) up to 50% larger. Therefore, Ohnaka’s law generates more energetic ruptures (i.e. faster rupture velocity and peak slip-rate) despite having a larger cohesive zone due to the lower breakdown energy dissipated during rupture propagation. We discuss our results in terms of the difference between breakdown highlighting the implications on dynamic rupture propagation and earthquake energy budget. We emphasize that common interpretations of energy dissipated during rupture propagation are model-dependent.

Faults accommodate shear motion in the upper crust through brittle deformation such as cataclastic flow. Plenty of field evidence suggests that the width of the embrittled volume, the lateral extent of the fault plane, and the grain size distribution within the fault core scale with shear displacement, indicating fairly solid scaling laws. However, at the onset of brittle failure, faults commonly exploit pre-existing anisotropic structures that facilitate slip localization and affect the early fault geometry as well as the fabric of the cataclastic products. The Bedretto Underground Laboratory represents a unique chance to study the structure of immature faults, whose pristine brittle structures are hardly preserved elsewhere during exhumation and exposure to weathering. We present the case study of the Waterfault (WF), a brittle shear zone hosted in the Bedretto tunnel within the Rotondo granite, which exploits pre-existing Alpine mylonites.On the tunnel wall, the brittle products of the WF are confined within a 50 cm thick volume, comprised between two boundaries defined by the local mylonitic foliation. The WF is characterized by a large water outflow, suggesting that the fault behaves as a major permeable conduit inside the granite host. Such high permeability is commonly related to the occurrence of intense but localised brittle damage. However, careful sampling across the fault and detailed microstructural investigations revealed that the brittle damage introduced by shear displacement is much less volumetrically widespread than expected. Cataclasis is in fact confined to a thin (5 mm) fine-grained gouge shear band developed at the boundary of the fault zone. The damaged rock surrounding this layer presents intense fracturing, dominated by oriented fractures at high angle to the shear plane, but no sensitive displacement down to the grain-scale. Damage distribution and geometry is further controlled by the anisotropic mylonitic fabric and its mineralogy, showing grain boundary cracking and shard-like fragmentation of quartz and feldspar, as well as micro-boudinage-like cracking of mica-grains. More than 50% in weight of the material recovered both within and in proximity of the gouge layer is finer than 125 µm. These observations are evidence of dynamic rock shattering, pointing to a seismic origin of the embrittlement. The seismic damage preferentially fractured along the grain boundaries of the mylonite, producing a fine-grained material that eased the onset of localised brittle shear.Our microstructural observations suggest that the WF might represent an optimal model for the early onset of faulting in anisotropic rocks driven by initial seismic damage, which unlocked the cohesive shear zone to favour slip localization. The “shallow” estimated depths of this event (< 5 km) might also open an interpretative window for the small-magnitude natural and induced seismicity (M

Faults accommodate most of the brittle deformation that occurs in the lithosphere through a spectrum of fault slip behaviours including, but not limited to, seismic and aseismic slip. The rocks deforming inside the core of the faults are the main actors that control the modality of slip and thus their mechanical properties are a key subject of study that is carried out through experimental investigation. The most relevant characterization is that of friction, a property that commensurates the resistance to shear motion of the rocks. Nevertheless, friction is not an intrinsic constant feature of the investigated materials. It is instead modulated by several attributes and external factors. For instance, the rate and state constitutive framework describes the sensitivity of friction to the sliding velocity, proving a successful theory to quantify the potential of the onset of dynamic instabilities and seismic slip in natural faults. Several works have also demonstrated that the frictional properties of the same material can dramatically change as function of the fabric (textural, geometrical attributes of the deforming rock). It is therefore evident that brittle deformation of rocks cannot be assessed in isolation of the conditions at which the phenomenon is measured. To fully understand the complex bulk behaviour of a deforming fault zone material we must investigate the interaction of several scale-dependent mechanisms that are active from the grain-scale up to the entire fault zone thickness.In this work we present the results of several case-studies that cover relevant lithotypes: anhydrite-dolomite, quartz-calcite-mica, lizardite-magnetite mixtures. These studies collect more than 60 friction experiments performed on BRAVA biaxial apparatus (INGV, Italy), presented here by associating the analysis of mechanical data with the analysis of rock microstructures. This joined investigation highlights the mechanisms that control rock friction: cataclasis, crystal plasticity, pressure-solution, grain-boundary sliding, cementation, and indentation. We also show the emergence of complex slip behaviours (experimental fault stability) as function of the coexistence of processes with different timescales and explained by the spatial arrangement of the mineral phases in the fault core.Our results shed light on the origin of the macroscopic frictional properties of fault rocks, stressing the fact that they are not a characterising property but rather the observable of a complex, dynamic, and highly non-linear system.

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) is a deep underground laboratory in the southern Swiss alps that allows to induce and study rock deformation processes in situ, from exceptionally short distances, at 1 km depth. In the context of the Fault Activation and Earthquake Rupture (FEAR) project, we have conducted a series of fluid injection experiments, with the goal of inducing an ML~0.0 earthquake. In this 'M-zero' experiment series, we have used a custom-built remote control system to implement fluid injection protocols that were informed by 2D continuum hydro-mechanical earthquake cycle models, with the target of increasing the likelihood of inducing a larger event. In the first of two major experiments, after 16 hours of high pressure (20 MPa) fluid injection, we have induced a Mw~ -0.4 "main shock", which triggered an aftershock sequence with >100 events in the first 3 seconds alone. The fluid injection was stopped 10 minutes after this main shock.The experiment and earthquake sequence was recorded on an extensive multi-domain and multi-scale 3D monitoring array, which combines various types of seismic sensors with distributed acoustic, strain and temperature sensing, Fibre Bragg Grating strain sensors, as well as pressure and flow monitoring. Although the main shock magnitude is smaller than the target magnitude (which was unclear at the time of its occurrence), the wide range of observations made, including high-resolution micro-seismicity catalogues covering 5 units of magnitude, offer a detailed look at seismological and rock mechanic processes before, during and after the main shock. In this talk we summarize some of the key observations and insights including i) the foreshock sequence, which shows signatures of a triggering cascade but which did not accelerate before the main shock; ii) a distinct Gutenberg-Richter b-value anomaly where the main shock occurred, and which may reflect the structural inventory and stress state of the activated fracture network; iii) the first-order consistency between early aftershock distributions and kinematic rupture models from spectral fitting, and a pronounced aftershock gap across a 2-by-2 metre patch where the main shock occurred, iv) static deformation patterns resolved on the strain sensor network, and their consistency with the focal mechanisms inferred from seismic data. We also discuss the implications for the planned fault activation experiments in the coming three years at the BedrettoLab.

Open Science is the paradigm driving the sharing of research data worldwide. It includes the ambition to make FAIR (Findable, Accessible, Interoperable and Re-usable) data sharing the default. FAIR guiding principles for research data have been recently proposed to scientific communities as the new horizon for sharing data. The FAIR principles create the conditions to foster data sharing and improve data stewardship, provided that several legislative, organizational, and ethical issues are addressed. In this paper, we aim to discuss the ethical dimension of sharing solid Earth science data. Earth scientists have a long-lasting tradition in data acquisition, quality control, and standardization, being the key actors in feeding and implementing metadata and services for qualification, storage, and accessibility. Pan-European Research infrastructures like EPOS (European Plate Observing System) involve scientific communities and research organizations federating facilities and resources to ensure data management and interoperability through e-science innovation. After introducing the ethical issues associated with the protection of personal data, intellectual property rights, and data misuse, we will focus on the impartiality for public good. This opens a new horizon to the ethical dimension of open access to research data, going well beyond research integrity. This assumes an outstanding relevance when referring to solid Earth science data since they also concern natural and anthropogenic hazards and risk communication relying on sharing scientific information with different stakeholders. Although we present a specific perspective for solid Earth science, we believe that the addressed ethical dimension is relevant for environmental science in general.

Understanding the dynamics of microearthquakes is a timely challenge to solve current paradoxes in earthquake mechanics, such as the stress drop and fracture energy scaling with seismic moment. Dynamic modelling of microearthquakes induced by fluid injection is also relevant for studying rupture propagation following a stimulated nucleation. The ERC-Synergy project FEAR (Fault Activation and Earthquake Ruptures) in the Bedretto Underground Laboratory (Swiss Alps) at approximately 1500m depth offers a unique opportunity to investigate fluid-induced micro-events on broadband seismic arrays. In this study, we leverage this opportunity to perform dynamic ruptures caused by fluid injection on a target pre-existing fault (50m x 50m), generating a Mw ≤ 1 seismic event. We conduct fully dynamic rupture simulations coupled with seismic wave propagation in 3D using a linear slip-weakening constitutive law, implemented on the supercomputer Leonardo (CINECA) with a multi-GPU distributed system. Stress field and fault geometry are constrained by in-situ characterization, allowing us to minimize the a priori imposed parameters. We investigate the dynamics of rupture propagation and its arrest for a target Mw < 1 induced earthquake with spatially heterogeneous stress drops caused by pore pressure changes and different constitutive parameters (i.e., critical slip-weakening distance, Dc, dynamic friction). We explore different homogenous conditions of frictional parameters, and we show that the spontaneous arrest of a propagating rupture following a dynamic instability is possible in the modeled stress regime by assuming a high fault strength parameter S, that is high ratio between strength excess and dynamic stress drop characterizing the fault before injection. The arrest of rupture propagation in our modeled induced earthquakes depends on the heterogeneity of dynamic parameters caused by the spatially variable effective normal stress, which controls the on-fault spatial increment of fracture energy Gc. Furthermore, in faults with high S values (i.e., low rupturing potential), we find that even minor variations in Dc (from0.45 to 0.6 mm) have a substantial effect on the rupture propagation and on the ultimate size of the earthquakes. Our results show that modest variations of dynamic stress drop determine the rupture mode, distinguishing self-arresting from run-away ruptures. Studying dynamic interactions (stress transfer) among slipping points on the rupturing fault provides insights on the dynamic load and shear stress evolution at the crack tip. The inferred spatial dimension of the cohesive zone in our crack models is roughly ~0.3-0.4m, with a maximum slip of ~0.6cm. Finally, analyzing the radiated synthetic waveforms, we examine the differences in the high-frequency content of simulated waveforms between self-arresting and run-away earthquakes and provide an estimation of the source parameters obtained through the spectral inversion. This estimation is then compared with source parameters of the dynamic forward models. Our results suggest that several features inferred for accelerating dynamic ruptures differ from those observed during rupture deceleration in a self-arresting earthquake caused by the spatial gradients of normal stress and pore-pressure. These results related to rupture arrest integrate those obtained with spatial variations of the initial stress, highlighting the role of the heterogeneities of stress drop and Gc.

Fault zone geological and geometrical complexities are prime parameters playing a fundamental role in controlling the characteristics of both natural and induced seismicity. In the Bedretto tunnel (Switzerland), the Fault Activation and Earthquake Rupture (FEAR) project aims at triggering a Mw = 1 seismic event through fluid injection and stimulation of a natural fault zone situated in a large-scale (> 106 m3) fractured granite reservoir. The limited exposure of the fault zone in the tunnel, however, restricts the possibility to constrain in detail the geometrical and geological characteristics of the experimental target. Therefore, in order to constrain the geological and geometrical characteristics of the target fault zone, we have integrated structural analyses, borehole and core logging, and borehole ground penetrating radar (GPR). Preliminary field investigations in the tunnel allowed to identify the complex fault structure characteristics, fault rock properties, and slip tendency in the current stress field of the selected fault zone. These results were compared to the structural observations obtained from field surveys and remote sensing, constraining the slip history, and lateral extent of the set of natural fault zones occurring on the surface above the Bedretto tunnel. Indeed, the lateral extent of the selected fault has been confirmed through the logging (optical/acoustic televiewer, fracture intensity, fracture typology) of exploration boreholes and the analyses of the related cores. The comparison between the geological characteristics of fault zones in the cores and the characteristics of the selected fault zone exposed in the tunnel allowed to confirm the occurrence of the same typology of fault zone further away from the exposure in the tunnel. In addition, GPR logging of the exploratory boreholes provided fundamental insights on the lateral continuity of the identified fault zones on the tunnel wall, as well as those identified in the borehole/core logging. All geological and geometrical information have been integrated into a preliminary 3D geometrical model (in Leapfrog Geo), representing the overall geometry of the selected fault zone. This preliminary geometrical model has been validated against synthetic GPR profiles, computed through GPR forward modelling along the exploration boreholes. The integrated results define the selected fault zone as a 3-7 m wide zone of higher density (up to 5/m), of variably oriented secondary fractures, and bounded by two main slip surfaces. The slip surfaces are irregularly decorated by phyllosilicate-rich gouge patches, filling the roughness of the fault surface. The lateral extension of each discrete fracture does not exceed 30 m in length, but the overall lateral continuity of the fault zone exceeds several hundreds of meters. The presented integrated characterization approach allowed us to constrain a geologically-sound, first-order 3D geometrical model of a complex natural fault zone, validated against geophysical forward modelling. These preliminary results have fundamental implications for the expected experimental planning and outcomes, modelling and injection strategies, project logistics, as well as the design and deployment of the monitoring network around the stimulated fault zone.

Natural faults when subjected to stimulation by fluid injection may result in slip acceleration because pore pressure (Pf) increases in the rock volumes inside and surrounding the fault zone leading to reduction of effective normal stress (σn’). Slip mode ranges from aseismic creep to seismic ruptures defining a spectrum of fault-slip behavior. Fault stimulation experiments will be conducted in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Switzerland) to understand fault reactivation processes on a target well-identified fault zone, fully instrumented to monitor deformation and seismicity during both fluid injection and fault reactivation. This is envisioned in the ERC-Synergy FEAR (Fault Activation and Earthquake Rupture) project. In BULGG, the target fault zone has both a sub-centimetric fault core containing fault gouge and granite asperities in contact and other fractures in the surrounding rock volume. Therefore, it becomes important to define the frictional properties and slip mode of both gouges and bare rock surfaces taking advantage of a laboratory controlled experimental environment.Fault stimulation by increasing Pf was simulated in laboratory following an injection protocol suitable for the BULGG fluid stimulation. Experiments were performed on the target fault gouge and on bare rock surfaces made of nearby Rotondo Granite. We employed a rotary shear apparatus (SHIVA) allowing the fluid injection under a controlled shear stress. First, we imposed the stresses measured at depth in the underground laboratory, halved due to apparatus limitations: 7.5 MPa σn’, 7.5 MPa confining pressure and 2.5 MPa Pf. Second, we imposed a slip rate of 10-5 m/s for 0.01 m to have a reference texture. Third, we applied a shear stress so that an equivalent slip tendency of 0.35 (equal to the one measured in the target fault) is achieved (ca. 2.7 MPa) keeping it constant. We then increased stepwise the pore fluid pressure by 0.1 MPa every 150 s. After fault slip initiation, the maximum allowed slip velocity was 0.1 m/s. Between each of the experimental stages, permeability and transmissivity were measured with the gradient or Pf oscillations methods.We show that reactivation occurs at lower Pf in bare rock surfaces (4.7 MPa) with respect to MC fault gouge (6.5 MPa), suggesting that the effective coefficient of friction, the ratio of shear stress and σn’, is larger in gouge (0.58) than in bare rock surfaces (0.49). Moreover, upon the application of last Pf step, reactivation is slower in fault gouge (150 s delay) with respect to bare rock surfaces (10 s delay), consistently with the lower hydraulic transmissivity measured for target fault gouge with respect to bare rock surfaces (i.e., 10-19 vs 10-17 m3). Our experiments also show that creep and dilatancy precede reactivation in fault gouge, whereas reactivation is sudden and not preceded by dilatancy in bare rock surfaces.We suggest that well-oriented and smooth bare rock surfaces might be easily reactivated similarly to what observed for fault gouge during fluid stimulation. Our data and observations will contribute to shed light on the mechanics of faults and induced earthquakes by fluid stimulation experiments.

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