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(a) Evolution of friction (μ) as a function of normal stress. Squares and circles represent respectively the μ peak and the μ ss . Brown is for room humidity (RH) experiments; blue is for the 100% RH ones (light for μ peak and dark for μ ss ). Empty symbols are experiments at higher initial grain size (63 < g.s. < 125 μm); full symbols are g.s. < 63 μm. (b) Shear strength in a Mohr-Coulomb space.
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The Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10−19 to 10−21 m2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository proje...
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... (at the end of the "run-in" phase). In Fig. 4 we show the evolution of peak (μ peak ) and steady-state (μ ss ) friction as a function of normal stress (σ n ). We observe that μ peak is normal stress dependent and decreases as the normal stress (σ n ) increases for all the boundary conditions of room humidity and grain size (Fig. 4a). Differently, μ ss is weakly normal stress ...
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... (at the end of the "run-in" phase). In Fig. 4 we show the evolution of peak (μ peak ) and steady-state (μ ss ) friction as a function of normal stress (σ n ). We observe that μ peak is normal stress dependent and decreases as the normal stress (σ n ) increases for all the boundary conditions of room humidity and grain size (Fig. 4a). Differently, μ ss is weakly normal stress dependent and is constant for all the boundary conditions. ...
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... that controls the frictional strength of the fault gouge (e.g. Behnsen & Faulkner, 2012;Haines et al. 2013;Saffer & Marone, 2003). From our experiments emerges that the applied normal stress influences the mode of strain localization while the increase of water vapor content causes a marked decrease in the absolute value of frictional strength ( Fig. 4). At low normal stress (< 35 MPa) friction evolves to a marked peak during the early stages of deformation, followed by a gradual strain weakening to steady-state values (Fig. 3). As the normal stress exceeds 35 MPa, sample yielding is followed by a short strain hardening stage that leads to steady-state friction (Fig. 3). This ...
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... powders has already been extensively studied (e.g. Saffer & Marone, 2003;Crawford et al. 2008;Tembe et al. 2010;Moore & Lockner, 2011;Behnsen & Faulkner, 2012;Ruggeri et al. 2021). Our data clearly show that under 100% RH conditions the friction coefficient is systematically lower than that from room humidity experiments (~25% RH), as much as 0.1 (Fig. 4). This water-assisted weakening mechanism is typically observed in clay-rich fault gouges (e.g. Israelachvili et al. 1988;Saffer et al. 2001;Ikari et al. 2007;Morrow et al. 2017;Orellana et al. 2019) whereas, for pure quartz powders, the results by Frye & Marone (2002) show that increasing humidity has no effect on the steady-state ...
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... is interesting to note that the grain size has little effect on the absolute values of peak and steady-state friction (Fig. 4, see also Fig. S1 in Supporting Information). When considering purely granular material, it has been shown that the grain size has a major effect in controlling mechanical parameters and fault stability (e.g. Marone & Kilgore, 1993;Bedford & Faulkner, 2021). Comparing the X-ray semiquantitative analysis of the experimental fault gouges ...
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... the strain is accommodated in a localized but heterogenous boundary shear zone, where larger microliths prevent the formation of a continuous foliation plane (Figs. 9e and f). The preservation of some microliths testifies the lower efficiency of cataclasis due to the low normal stress, which produces the marked µ peak in the initial stages (Fig. 4a). Under these conditions, upon velocity up-step, the competition between granular deformation and sliding along the phyllosilicate foliae in the heterogeneous B-shears causes the fault to dilate, promoting stabilization by the increase of (a-b) (e.g. Marone et al. 1990;Chen & Spiers 2016;Chen et al. 2017;Ikari et al. 2016). When σ n > ...
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... lowering μ ss values. After a sudden dilation upon velocity up-step the local pore pressure can be released, increasing the effective normal stress and thus, promoting frictional stability by dilatancy hardening (Segall & Rice, 1995;Brantut et al. 2020). In general, the increase of water vapor content produces the frictional strength reduction (Fig. 4) and enhances the velocity strengthening behaviour possibly inhibiting frictional instabilities ( Fig. 5b; see also Tembe et al. 2010;Morrow et al. 2017;Orellana et al. ...
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... before the first hold of the slide-holdslide (SHS) test. This is because, after the first cycle of velocity-steps (at ∼9 mm of displacement and velocity of 10 μm s -1 ), almost all experiments show a steady-stable sliding, which does not always occur after the peak friction and subsequent strain weakening (at the end of the 'run-in' phase). In Fig. 4 we show the evolution of peak (μ peak ) and steady-state (μ ss ) friction as a function of normal stress (σ n ). We observe that μ peak is normal stress dependent and decreases as the normal stress (σ n ) increases for all the boundary conditions of room humidity and grain size (Fig. 4a). Differently, μ ss is weakly normal stress ...
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... strain weakening (at the end of the 'run-in' phase). In Fig. 4 we show the evolution of peak (μ peak ) and steady-state (μ ss ) friction as a function of normal stress (σ n ). We observe that μ peak is normal stress dependent and decreases as the normal stress (σ n ) increases for all the boundary conditions of room humidity and grain size (Fig. 4a). Differently, μ ss is weakly normal stress dependent and is constant for all the boundary ...
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... fault gouge with g.s. < 63 μm (Fig. 4a), shows an average reduction of ∼19 per cent in μ peak and of ∼22 per cent in μ ss , with the increase in relative humidity. The experiments with gouge grain size 63 < g.s. < 125 μm, displayed an average reduction due to RH increase of 17 per cent for μ peak and 18 per cent, for μ ss . The friction coefficient calculated in a ...
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... shows an average reduction of ∼19 per cent in μ peak and of ∼22 per cent in μ ss , with the increase in relative humidity. The experiments with gouge grain size 63 < g.s. < 125 μm, displayed an average reduction due to RH increase of 17 per cent for μ peak and 18 per cent, for μ ss . The friction coefficient calculated in a Mohr-Coulomb space (Fig. 4b) using a linear fit of the shear stress values at steady-state (τ ss ), is 0.41 ± 0.01 for room humidity experiments and 0.35 ± 0.01 for 100 per cent ...
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... (i.e. microliths, Laurich et al. 2017) distributed in a matrix composed of sheet silicates, quartz, and calcite grains (Figs 9b and d). The OPA Clay aggregates have heterogeneous grain size, typically between 20 and 60 μm, and are constituted by undeformed vestigial OPA minerals assemblage, that indicate little deformation in the bulk (see also Fig. S4 in Supporting Information). Increasing humidity has little influence on how deformation is accommodated, being characterized by localized cataclasis and grain size reduction within the B-shears (Figs 9c and d). Compared to the samples at room humidity we observe two B-shears (Fig. 9c), characterized by grain size reduction combined ...
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... that controls the frictional strength of the fault gouge (e.g. Saffer & Marone 2003;Behnsen & Faulkner 2012;Haines et al. 2013). From our experiments emerges that the applied normal stress influences the mode of strain localization while the increase of water vapour content causes a marked decrease in the absolute value of frictional strength (Fig. 4). At low normal stress (<35 MPa) friction evolves to a marked peak during the early stages of deformation, followed by a gradual strain weakening to steadystate values (Fig. 3). As the normal stress exceeds 35 MPa, sample yielding is followed by a short strain hardening stage that leads to steady-state friction (Fig. 3). This behaviour ...
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... been extensively studied (e.g. Saffer & Marone 2003;Crawford et al. 2008;Tembe et al. 2010;Moore & Lockner 2011;Behnsen & Faulkner 2012;Ruggieri et al. 2021). Our data clearly show that under 100 per cent RH conditions the friction coefficient is systematically lower than that from room humidity experiments (∼25 per cent RH), as much as 0.1 (Fig. 4). This water-assisted weakening mechanism is typically observed in clay-rich fault gouges (e.g. Israelachvili et al. 1988;Saffer et al. 2001;Ikari et al. 2007;Morrow et al. 2017;Orellana et al. 2019) whereas, for pure quartz powders, the results by Frye & Marone (2002) show that increasing humidity has no effect on the steady-state ...
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... is interesting to note that the grain size has little effect on the absolute values of peak and steady-state friction (Fig. 4, see also Fig. S1). When considering purely granular material, it has been shown that the grain size has a major effect in controlling mechanical parameters and fault stability (e.g. Marone & Kilgore 1993;Bedford & Faulkner 2021). Comparing the X-ray semiquantitative analysis of the experimental fault gouges at different grain sizes it ...
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... 35 MPa, the strain is accommodated in a localized but heterogenous boundary shear zone, where larger microliths prevent the formation of a continuous foliation plane (Figs 9e and f). The preservation of some microliths testifies the lower efficiency of cataclasis due to the low normal stress, which produces the marked μ peak in the initial stages (Fig. 4a). Under these conditions, upon velocity up-step, the competition between granular deformation and sliding along the phyllosilicate foliae in the heterogeneous B-shears causes the fault to dilate, promoting stabilization by the increase of (a-b) (e.g. Marone et al. 1990;Chen & Spiers 2016;Ikari et al. 2016;Chen & Niemeijer 2017). When σ ...
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... cause for lowering μ ss values. After a sudden dilation upon velocity up-step the local pore pressure can be released, increasing the effective normal stress and thus, promoting frictional stability by dilatancy hardening (Segall & Rice 1995;Brantut 2020). In general, the increase of water vapour content produces the frictional strength reduction (Fig. 4) and enhances the velocity strengthening behaviour possibly inhibiting frictional instabilities ( Fig. 5b; see also Ikari et al. 2009;Tembe et al. 2010;Morrow et al. 2017;Orellana et al. ...
Citations
... In-situ experiments involving decametric fault reactivation have been essential for understanding the relationship between injected fluids, dilatancy and fault slip under controlled conditions (Guglielmi et al., 2015;Duboeuf et al., 2017;Cappa et al., 2019;De Barros et al., 2024;Cappa et al., 2022a). For instance, recent observations at the Mont-Terri underground laboratory have shown that fluid injection in a highly velocity-strengthening fault (Orellana et al., 2018;Bigaroni et al., 2023) with very low initial permeability (k = 10 −17 m 2 ) can induce significant dilatant aseismic slip, which is a key component of the deformation process in an interconnected fault network (Guglielmi et al., 2020;Cappa et al., 2022b). This slip is often associated with an increase in low-frequency tremor signals, linked to fluid-driven slip propagation (De Barros et al., 2023). ...
To better understand the mechanics of injection-induced seismicity, we developed a two-dimensional numerical code to simulate both seismic and aseismic slip on non-planar faults and fault networks driven by fluid diffusion along permeable faults. Our approach integrates a boundary element method to model fault slip governed by rate-and-state friction with a finite volume method for simulating fluid diffusion along fault networks. We demonstrate the method's capabilities with two illustrative examples: (1) fluid injection inducing slow slip on a primary rough, rate-strengthening fault, which subsequently triggers microseismicity on secondary, smaller faults, and (2) fluid injection on a single fault in a network of intersecting faults, leading to fluid diffusion and reactivation of slip throughout the network. In both cases, the simulated slow slip migrates more rapidly than the fluid pressure diffusion front. The observed migration patterns of microseismicity in the first example and slow slip in the second example resemble diffusion processes but involve diffusivity values that differ significantly from the fault hydraulic diffusivity. These results support the conclusion that the microseismicity front is not a direct proxy for the fluid diffusion front and cannot be used to directly infer hydraulic diffusivity, consistently with some decametric scale in-situ experiments of fault activation under controlled conditions. This work highlights the importance of distinguishing between mechanical and hydrological processes in the analysis of induced seismicity, providing a powerful tool for improving our understanding of fault behavior in response to fluid injection, in particular when a network of faults is involved.
... In clay, on which the current paper focuses, the thickness of the localised shear zones in the fault gouge is considered to be approximately 10 ÷ 100 times the clay platelet diameter, which is of the order of 1 µm (Morgenstern & Tchalenko, 1967;Rice, 2006;Haines, Kaproth, Marone, Saffer, & Van der Pluijm, 2013;Bigaroni et al., 2023;Volpe, Collettini, Taddeucci, Marone, & Pozzi, 2024). Although studies have explored the effects of earthquake nucleation and seismic events on the layer thickness using both laboratory samples and natural fault gouges (e.g., Byerlee & Summers, 1976;Scott et al., 1994;Ikari et al., 2011;Evans, 1990;Marone et al., 1990;Scuderi et al., 2014;Lyu et al., 2019;Bedford & Faulkner, 2021, among others), the quantification of this relationship remains elusive. ...
The development of rate- and state-dependent friction laws offered important insights into the key physical mechanisms of the frictional behaviour of fault gouges and their seismic cycle. However, past approaches were specifically tailored to address the problem of fault shearing, leaving questions about their ability to comprehensively represent the gouge material under general loading conditions. This work establishes an alternative approach for developing a physical friction law for fault gouges that is grounded on the rigour of the hydrodynamic procedure with two-scale temperatures through Terracotta, a thoroughly robust constitutive model for clay in triaxial loading conditions. By specifying the model for direct shearing, the approach yields an alternative friction law that readily captures the frictional dynamics of fault gouges, including explicit dependencies on gouge layer thickness, normal stress, and solid fraction. Validated against available laboratory experiments, the friction law retains the original predictive capabilities of Terracotta in triaxial conditions and explains the rate- and state-dependent, dilatational behaviour of fault gouges in direct shear conditions. Finally, when the Terracotta friction law is connected to a spring-dashpot representation of the host rock, the combined model predicts an elastic buildup precursor to the onset of and subsequent seismicity, with results closely reflecting experimental evidence and field observations. While this study focuses on clay-rich gouges, the approach and findings are expected to offer much wider implications to a variety of materials.
... at 30-MPa normal stress (Fig. 8b). Previous studies have reported a similar phenomenon (Saffer & Marone 2003;Giacomel et al. 2021;He et al. 2022;Bigaroni et al. 2023) (Fig. 8c). ...
Basalt is a major component of the lunar soil and crust. The frictional properties, and thus the potential for seismicity and associated hazards on basaltic faults are significantly influenced by particle size distribution and normal stress conditions. We conducted velocity-stepping and slide-hold-slide shear experiments on simulated basalt gouges at gouge particle sizes of 50–250 μm, and normal stresses of 5–30 MPa under wet/dry conditions with different gouge structures to investigate the frictional stability and healing of basaltic faults. We observe a transition from velocity-strengthening (a – b = 0.52) to velocity-weakening behavior (a – b = -0.0005) and an enhanced healing rate (from 0.0075 to 0.0044), as particle size decreases. Elevated normal stress is typically associated with reduced grain size, resulting in unstable slip behavior (a – b = -0.0006), though accompanied by a diminished healing rate (from 0.0062 to 0.0050). The presence of fluid contributes to stable slip behaviors and fault compaction. These observations contribute to a better understanding of the potential and nucleation mechanism of lunar seismicity within basaltic faults.
HighlightsSmall grain size gouges facilitate fault instability and strength recovery in basaltic faults.
High normal stress on basalt gouges assists grain crushing and weakens frictional strength recovery.
Fluid enhances fault stability and fault compaction of basaltic faults at room temperature.
Deeper crust with higher confining stress and smaller grains suggests higher seismic potential.
... Hence, the hydro-mechanical properties of the OPA have been widely investigated. This includes, among others, studies concerning mechanical creep (Naumann et al., 2007;Schulze, 2011), compressive strength (Minardi et al., 2021;Schuster et al., 2021;Winhausen et al., 2022), dry and wet frictional properties (Bigaroni et al., 2023;Orellana et al., 2018bOrellana et al., , 2019Schuster et al., 2023), swelling strain, stress, and its anisotropy (Bossart & Thury, 2008;Minardi et al., 2016;Thury & Bossart, 1999;C. L. Zhang et al., 2010), and the closure of cracks by self-sealing processes (Bock et al., 2010;Fang et al., 2017;Voltolini & Ajo-Franklin, 2020;Wenning, Madonna, Kurotori, et al., 2021;C. ...
... In the exponential model, the friction coefficient μ f and inherent shear strength S 0 provide an approximation of the initial slope and intercept of the failure envelope (i.e., at σ N = 0), respectively, and the exponential fitting parameter β controls the extent of slope decrease (weakening) with increasing normal stress. The linear model revealed a friction coefficient of 0.35 (Figure 9b and Table 2), which is consistent with previous studies using OPA gouge (Bigaroni et al., 2023;Orellana et al., 2018b) and intact OPA samples . Roman numbers refer to white markings on the photos: (i) imprints of BER surface, (ii) intersection between saw cut and foliation, (iii) slickenlines, (iv) brighter patch, (v) darker patch, (vi) steps from material accumulation, (vii) slickenlines, (viii) steps from material accumulation, (ix) intersection between saw cut and foliation. ...
... 3.1 and 3.2, the stress state at initial H 2 O contact was below the wet frictional slip envelope, suggesting that the alteration of the frictional strength cannot be the only explanation for initial slip and that the effective stress at the saw-cut interface cannot be fully described by the measured mechanical data (Figures 10b and 10f). Possibly, the build-up of swelling stress has to be considered in order to represent the stress conditions at the OPA-BER interface (Bigaroni et al., 2023;Paterson & Wong, 2005; C. L. Zhang, 2017Zhang, , 2018. ...
Clay‐rich rocks are integral to subduction zone dynamics and of practical importance, for example, as barriers in nuclear waste and CO2 repositories. While the effects of swelling strain on the self‐sealing capabilities of these rocks are relatively well‐established, the implications of polar fluids interacting with charged clay particles on the frictional behavior, and the role of swelling stress in initiating slip in critically stressed faults, remain ambiguous. To address these uncertainties, we conducted triaxial friction experiments using saw‐cut samples, with the upper half composed of Opalinus claystone (OPA) and the lower half of Berea sandstone (BER). The frictional strength of the non‐wetted OPA‐BER interface was estimated based on experiments at confining pressures of 4–25 MPa and constant axial loading rate (0.1 mm/min). Fluid injection friction experiments were performed using decane (non‐polar fluid) or deionized water (polar fluid) at 10 and 25 MPa confining pressures and constant piston displacement control. Macroscopic mechanical data were complemented by distributed strain sensing on the sample surface. Compared to decane, the frictional strength of the OPA‐BER interface tended to decrease when injecting water, which is attributed to phyllosilicate lubrication and the transition of the OPA from a solid rock to an incohesive mud near the saw‐cut surface. When injecting water, slip was initiated during initial hydration of the OPA‐BER interface, although the apparent stress state was below the yield stress. To explain this behavior, we propose that the swelling stress is a crucial factor that should be integrated into the effective stress model.
... We note that our experiments were conducted at room humidity on rock surfaces that progressively developed gouge. In contrast, most of the comparative experiments were conducted on unconsolidated gouge (except Kilgore et al., 1993, andsome experiments from Colletini et al., 2011), and many were deformed at water-saturated conditions (e.g., one experiment from Bigaroni et al. (2023), and experiments from Carpenter et al. (2012Carpenter et al. ( , 2016 and Fagereng and Ikari (2020)). For a given material, wet and dry bare surfaces, and wet and dry gouge yield μ values that can differ by 0.4 over a range of normal stresses (e.g., Colletini et al., 2011;Collettini et al., 2009;Mitchell et al., 2016). ...
Deformation experiments on hematite characterize its slip‐rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite‐coated faults and present‐day deformation at depth. We used a rotary‐shear apparatus to conduct single‐velocity and velocity‐step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity‐step experiments indicate velocity‐strengthening to velocity‐neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi‐continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U‐Th)/He thermochronometry, which can quantify fault‐related thermal and mechanical processes. Comparison of hematite (U‐Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse‐grained hematite by stable sliding, and that hematite (U‐Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite‐bearing fault rocks based on (U‐Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered.
... Several possibilities may explain why the slip on the main injected faults, directly driven by fluidpressure increase, do not occur as fast-slip earthquakes in this experiment. It might be because of (1) a general rate-strengthening frictional properties of the Mont-Terri shales (Orellana et al., 2018;;Bigaroni et al, 2023), (2) a dilatant shear mechanism leading to an interplay between propagating slip and fluid diffusion Zoback et al., 2012), or (3) an increase of the earthquake nucleation length, inversely proportional to the fluid pressure increase, thus inhibiting seismic rupture in the pressurized area . However, while the first part of the test is totally devoid of seismicity, an increase of noise amplitude is observed when the slip on the main injected faults approaches the monitoring probe. ...
... Indeed, shale frictional properties may vary from rate strengthening, like in the Mont Terri Opalinus clay (Bigaroni et al., 2023), to rate weakening, mainly according to their clay and organic matter ...
Fault slip induced by fluid perturbation in shale formations may only lead to as sparse seismicity. However, fault slip may strongly impact the integrity of shale formations that serve as caprocks for geological reservoirs holding buoyant fluids such as CO2, natural gas, or hydrogen. A better understanding of the fluid reactivation processes of fault and the seismic triggering process is therefore critical for reservoir monitoring and fault stability. Here we analyze the seismic responses of a shale fault exposed to fluid pressurization during an in situ field-scale injection experiment at ∼340 m depth in the Mont Terri underground research laboratory (Switzerland). Two main types of seismic signals are observed as the fault was activated and started to slowly slip. After an aseismic phase, we observed tremor signatures and an increase in noise amplitude, which were directly associated with the slowly propagating fault slip in response to fluid injection. These signatures were later followed by micro-earthquakes that seem to occur further away from the fluid-pressurized area. We interpret these micro-earthquakes to be triggered by stress perturbations from the main slip growth. These two classes of seismic responses therefore highlight two different processes. Tremors seem to be a more direct observation for the fluid-induced slip propagation than micro-earthquakes. Even hidden in the noise, they precede earthquake failures, thus providing a useful tool for monitoring fluid leakage activated by slow deformation on low permeable shale faults, with applications for sealing integrity of caprocks.
Upon conducting simulated fault friction tests on granite specimens under various normal stresses utilizing CNL and CNS rock joint shear testers (RJST-616), we aimed to comparatively analyze the stick–slip behavior characteristics during fault sliding. Our results reveal that the sliding behavior of granite simulated faults under different normal stresses is primarily marked by stick–slip features. Both the static shear stress drop and the stick–slip time interval exhibit an exponential growth relationship with the normal stress, whereas the spatial frequency of stick–slip events demonstrates a negative correlation with the normal stress. Furthermore, we observed an exponential growth relationship between the critical slip weakening displacement and normal stress, with the growth rate showing a rapid and then slow trajectory. Additionally, we established an intrinsic connection between different seismic source parameters using normal stress as a bridge. Finally, we unveiled the occurrence and recovery mechanisms of fault stick–slip under varying normal stresses from the perspective of the friction cone. These findings not only enhance our understanding of the behavioral characteristics of fault stick–slip but also provide novel insights into the seismic evolution of natural fault systems.
