Fault Gouge Research Articles

Mechanical Amorphization of Synthetic Fault Gouges During Rotary‐Shear Friction Experiments at Subseismic to Seismic Slip Velocities

AbstractAlthough the effects of mechanical amorphization by fault motion on fault rocks have been investigated both in nature and experiments, the relationship between slip processes and the amount of amorphous materials produced remains unclear. We performed rotary‐shear friction experiments on synthetic quartz and kaolinite gouges at room temperature, normal stresses of 1 or 3 MPa, slip velocities of 0.001 or 1 m s−1, and displacements of 1–101 m. X‐ray diffraction and microscopic observation data revealed that mechanical amorphization in both materials was accompanied by grain‐size reduction, particle rounding, and the formation of ultrafine particles. The amount of amorphous materials produced was strongly dependent on mineralogy and total frictional work regardless of slip velocity. Therefore, mechanical amorphization can occur during both coseismic and subseismic slip. Amorphization of ~7 wt% of quartz gouge required 30.33 MJ kg−1 of frictional work, whereas for kaolinite gouge, only 0.77 MJ kg−1 was required, which is consistent with observed preferential amorphization of clay minerals in faults. For kaolinite, up to 6% of frictional work can be used for mechanical amorphization, indicating its potential importance for faulting energetics. Because some kinds of amorphous phyllosilicates release water at lower temperature than crystalline, mechanical amorphization of fault rocks may influence thermochemical pressurization. Gradual weakening of quartz gouge at 0.001 m s−1 slip velocity suggested that <4 wt% of amorphous silica can weaken a fault by amorphous silica‐related lubrication. Mechanical amorphization may accelerate pressure solution healing of faults if dissolution can occur under high porous conditions.

Dating recurrent shear zone activity and the transition from ductile to brittle deformation: White mica geochronology applied to the Neoproterozoic Dom Feliciano Belt in South Brazil

Argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation, as temperature ranges for mineral crystallization and isotopic closure of different grain-size fractions overlap between ca. 275–425 °C. This study presents over 40 K-Ar ages of coarse-grained muscovite and multiple clay-sized white mica fractions extracted from mylonites and fault gouges in southernmost Brazil, complemented by detailed structural and XRD characterization. The widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low-grade ductile deformation into brittle faulting. The onset of the Ibaré Shear Zone is constrained in the Tonian, followed by reactivation during the Ediacaran evolution of the adjacent Dom Feliciano Belt, which records a remarkable long-term history of recurrent deformation for over 100 Myr. New data extends the geochronological constraints of the Dorsal do Canguçu Shear Zone into low-temperature conditions, estimating the onset of brittle activity at 525-470 Ma by dating early fault gouges formed shortly after final syn-kinematic white mica crystallization in the hosting mylonites. Dating of recurrent faulting events during the Paleo-Mesozoic complement the regional sedimentary and thermochronological record.

Piezoelectricity as a mechanism on generation of electromagnetic precursors before earthquakes

SUMMARYTo produce the electromagnetic (E&M) precursors of an earthquake, the existence of electric field due to stress-induced charges on the ground surface or in shallow depths of upper crust inside the fault zone is a basic condition. Here, we consider the piezoelectric effect or the elastic–electric coupling as a major mechanism on generating such an electric field. A 1-D model based on the elastic mechanics and electromagnetic Maxwell equations is built up to formulate the relationship between electric field and slip as well as stress on a fault before an earthquake. From the model, we may estimate the low-bound values of stress and slip to yield the critical electric field, Ec, for generation of electromagnetic signals. The normal and shear stresses on the fault planes for three faulting types are constructed. The normal stress is stronger than the shear stress to result in piezoelectricity. The depth ranges for yielding an average normal stress being able to generate Ec are similar for thrust and strike-slip faults and deep for normal faults. The possibility of generating Ec is almost the same for thrust and strike-slip faults and low for normal faults. The pre-earthquake slip could be related to nucleation phases or microfractures. The possible occurrence time of E&M signals may be several 10 min to few hours before impending earthquakes. The major factor in yielding a piezoelectric field to generate the TEC anomalies before an earthquake is the existence of fault gouges composed mainly of clays. A thick gouge layer with low electric resistivity and a piezoelectric coupling coefficient ≥0.67 × 10−12 coul nt–1 is an important condition for yielding piezoelectricity.

Precambrian faulting episodes and insights into the tectonothermal history of north Australia: microstructural evidence and K–Ar, <sup>40</sup>Ar–<sup>39</sup>Ar, and Rb–Sr dating of syntectonic illite from the intracratonic Millungera Basin

Abstract. Australian terranes concealed beneath Mesozoic cover record complex Precambrian tectonic histories involving a successive development of several Proterozoic to Palaeozoic orogenic systems. This study presents an integrated approach combining K–Ar, 40Ar–39Ar, and Rb–Sr geochronologies of Precambrian authigenic illites from the recently discovered Millungera Basin in north-central Australia. Brittle deformation and repeated fault activity are evident from the sampled cores and their microstructures, probably associated with the large-scale faults inferred from interpretations of seismic surveys. Rb–Sr isochron, 40Ar–39Ar total gas, and K–Ar ages are largely consistent in indicating late Mesoproterozoic and early Proterozoic episodes (∼1115±26, ∼ 1070±25, ∼1040±24, ∼1000±23, and ∼905±21 Ma) of active tectonics in north-central Australia. K–Ar results show that illites from fault gouges and authigenic matrix illites in undeformed adjacent sandstones precipitated contemporaneously, indicating that advection of tectonically mobilized fluids extended into the undeformed wall rocks above or below the fracture and shear (fault gouge) zones. Isotopic age data clearly indicate a Mesoproterozoic minimum age for the Millungera Basin and thus previously unrecorded late Mesoproterozoic–early Neoproterozoic tectonic events in north-central Australia. This study provides insight into the enigmatic time–space distribution of Precambrian tectonic zones in central Australia, which are responsible for the formation of a number of sedimentary basins with significant energy and mineral resources.

The Cenozoic Evolution of Crustal Shortening and Left‐Lateral Shear in the Central East Kunlun Shan: Implications for the Uplift History of the Tibetan Plateau

AbstractThe timing of crustal shortening and strike‐slip faulting along the East Kunlun Shan provides insight into the history of surface uplift and may constrain the time at which the Tibetan Plateau reached high elevations. We investigate a series of extensional basins and restraining bends along the Xidatan strand of the Kunlun strike‐slip fault, which provide an ideal setting to unravel the tectonic history of the northern plateau margin. We present new apatite (U‐Th)/He, apatite fission track, and zircon (U‐Th)/He ages and QTQt thermal modeling, 40Ar/39Ar fault gouge dating, and structural mapping from the central East Kunlun Shan. Our data suggest that the East Kunlun Shan experienced slow to negligible exhumation until late Cretaceous time, followed by an increase in rate by 65–50 Ma. Along with a ~47 Ma fault gouge age, we posit that the Paleocene–early Eocene was a time of crustal shortening along the northern plateau. Rapid exhumation along transpressional portions of the Xidatan fault initiated by 23–20 Ma, which we interpret as the local onset of strike‐slip faulting. An early Miocene transition from north‐south crustal shortening to left‐lateral shear along the East Kunlun Shan, the onset of normal and strike‐slip faulting in central and southern Tibet by 18 Ma, and lower crustal flow in eastern Tibet by 13 Ma suggest the establishment of orogen‐wide east‐west oriented extension and extrusion by the middle Miocene. The plateau‐wide shift in stress accommodation implies that high gravitational potential energy, and likely high elevation, was attained by the middle Miocene.

Timescales of porosity and permeability loss by solid-state sintering

The physical properties of granular geologic materials are transient – a variety of densification processes operate throughout Earth's crust. One such process is solid-state sintering, which causes crystalline clasts to coalesce in the absence of fluids or melt. Solid-state sintering operates wherever unconsolidated granular materials are subjected to elevated pressures and temperatures for protracted periods of time. There are, however, few studies that constrain the conditions and timescales for densification and lithification of crystalline geologic materials by solid-state sintering. Here, we present the results of hot-pressing experiments designed to cause a natural glass-free volcanic fault gouge to undergo solid-state sintering. Unconsolidated starting materials subjected to volcanic temperatures (700-900°C) and pressures (20-70 MPa) are transformed by solid-state sintering into more-coherent porous composites over a period of 4-60 hours. The relative density and competence of experimental products increase and porosity and permeability decrease as sintering pressure, temperature and time increase. We use the experimental results to develop a robust densification model that predicts time-dependent porosity and permeability loss at a pressure-temperature range that includes volcanic and some upper-crustal environments. In these environments, solid-state sintering causes reductions in porosity and permeability of ∼0.35 and ∼104 m2, respectively, over days to months depending on pressure-temperature conditions. Applied to volcanic environments, the short timescales of solid-state sintering-driven permeability loss and lithification can dictate the efficiency of outgassing and therefore modulate eruption style (i.e., explosive vs. effusive).

Evidence for late Quaternary brittle deformation and back thrusting within the Indus Suture Zone, Ladakh Himalaya

Although the tectonic evolution of the southern Himalaya has been extensively studied, relatively few studies have focused on potential neotectonic activity in the most hinterland part of Himalaya, north of the region that separates the southern edge of the Tibetan plateau from the peaks of the Higher Himalayan Range. Here we use a combination of new structural and geomorphic field data, Optically Stimulated Luminescence (OSL) chronology, as well as previously published seismicity and denudation rate dataset, to suggest that the Trans-Himalaya within the region of the Indus Suture Zone (ISZ) is neotectonically active. Based on fault gouge, tilted paleolake deposits, and the chronology of deformed fluvial gravels constrained by OSL dating, the present study provides evidence for the tectonic activity along the ISZ between 78 and 58 ka. The occurrence of two earthquakes of Mw ≥ 4.0 and the focal mechanism solution of a shallow focus earthquake (Mw: 3.0, depth ~ 8 km) near the town of Upshi, shows a pure thrust mechanism. We suggest that this earthquake was likely associated with the Upshi-Bazgo thrust, a major structure which separates the mid-Miocene Indus Molasse from the Cretaceous Ladakh Batholith. It is envisaged that the fault and fold systems in the Ladakh region such as the Upshi-Bazgo thrust originated due to (1) acceleration in the northward movement of Indian plate relative to the Eurasian plate, and (2) post-collisional shortening parallel to plate convergence. On account of this, the Eocene to Miocene post-collisional sedimentary sequences, such as the Indus Molasse, rode over the now-extinct Ladakh Arc, as it acted as a rigid body, and formed a system of backthrusts, which are neotectonically active.

The Spatiotemporal Evolution of Granular Microslip Precursors to Laboratory Earthquakes

AbstractLaboratory earthquake experiments provide important observational constraints for our understanding of earthquake physics. Here we leverage continuous waveform data from a network of piezoceramic sensors to study the spatial and temporal evolution of microslip activity during a shear experiment with synthetic fault gouge. We combine machine learning techniques with ray theoretical seismology to detect, associate, and locate tens of thousands of microslip events within the gouge layer. Microslip activity is concentrated near the center of the system but is highly variable in space and time. While microslip activity rate increases as failure approaches, the spatiotemporal evolution can differ substantially between stick‐slip cycles. These results illustrate that even within a single, well‐constrained laboratory experiment, the dynamics of earthquake nucleation can be highly complex.

Estimation of the Structural and Geomechanical Anisotropy in Fault Gouges Using 3D Micro-Computed Tomography (μ-CT).

Fault gouges play an important role in the shear deformation of fault zones, by causing weakness and frictional instability in structures. Previous studies have investigated the evolution of shear deformation of fault zones by observing experiments using remolded and synthetic gouge specimens at a micro-scale. However, how the spatial configuration of the rock constituents accounts for the 3D anisotropy of intact structures of fault gouges, particularly at the core-scale, is not well understood. We obtained 3D μ-CT images of directionally cored gouge specimens and performed statistical analysis to quantify the major orientation of the internal structures. Direct shear tests were conducted to investigate the relationship between the distribution of the internal structures and geomechanical behavior. The results show that the undisturbed fault gouge has a clear anisotropy parallel to the fault plane even at the core-scale. Moreover, the direct shear test results show that the frictional resistance of a fault gouge has anisotropy related to the fault plane. The simple, yet robust method proposed in this study confirms that the core-scale structural anisotropy is correlated to the anisotropic shear resistance.

K-Ar fault-gouge dating in the Lower Buller gorge constrains the formation of the Paparoa Trough, West Coast, New Zealand

ABSTRACT K-Ar dating of fault gouge from the intersection of the WNW-striking Ohika Detachment of the Paparoa Metamorphic Core Complex and a NNE-striking high-angle normal fault (Ohikaiti Fault) yielded an age of 103 ± 3.5 Ma (1σ uncertainty) for metamorphic white mica and an age of 35.2 ± 2.3 Ma for fault-gouge formation. The K-Ar four-point isochron is well defined and the upper-intercept age of 103 ± 3.5 Ma is inferred to reflect growth of metamorphic white mica in the footwall of the Paparoa Metamorphic Core Complex. We relate the fault gouge at the Ohikaiti Fault to the formation of the Paparoa Trough in the latest Eocene and the deposition of the Rapahoe Group. The NNE-striking Ohikaiti Fault either reactivated a favourably oriented segment of the Ohika Detachment or cut the detachment and displaced it when the Paparoa Trough formed. We discuss a model of relatively modest deformation in the Paparoa Trough as part of the Challenger Rift System because the northern West Coast region was close to the pole of plate rotation in the late Eocene and Oligocene.

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

Abstract The timing of slip on brittle faults in Earth’s upper crust is difficult to constrain, and direct radiometric dating of fault-generated materials is the most explicit approach. Here we make a direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibers and veins from the same fault. We have dated fault generated materials from the Big Creek fault, a northwest-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ca. 73 Ma and ca. 60–57 Ma, representing at least two periods of fault slip that form part of a complex fault and fluid-flow history. The Cretaceous result lies within previous indirect estimates for major slip on the fault. The Paleocene–Eocene result coincides with the estimated timing of slip of the nearby Tintina and Denali faults, which are crustal-scale, northwest-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model, where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for distinct fault-generated materials using different decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.

Absolute Dating of Past Seismic Events Using the OSL Technique on Fault Gouge Material—A Case Study of the Nojima Fault Zone, SW Japan

AbstractThis study is the first part of a comprehensive paleoseismic study in the Nojima Fault Zone (NFZ), the seismogenic fault of the 1995 Mw 6.9 Kobe (Japan) earthquake, which explores the use of the luminescence dating method for understanding past faulting time and assessing the activity of faults developed in basement rocks. Our approach is focused on methodological aspects of the optically stimulated luminescence (OSL) technique and reports on a series of ages of fault gouge samples. Our analysis revealed that (1) quartz OSL signals have been reset (at least partially) during past seismic faulting events, (2) fault gouge layers bounded by sharp fault planes are younger than gouge layers far from the main fault planes, and (3) the most recent seismic faulting, affecting the different gouge layers on the Nojima fault took place from 62.8 ± 4.3 ka to 18.5 ± 1.3 ka, while on the Asano fault from 139.3 ± 8.9 ka to 45.8 ± 3 ka, with these ages representing maximum possible ages. In this regard, this is the first successful absolute dating attempt on the NFZ using quartz grains and the only luminescence study, as yet known, producing a series of ages representing neotectonic activity of the Nojima and Asano faults during the late Pleistocene and middle to late Pleistocene, respectively. The present work has indicated that OSL is potentially a promising technique for dating fault gouges and assessing the activity of faults, although more work is needed for further refinement.

The Effect of a Liquid Phase on Force Distribution During Deformation in a Granular System

AbstractTwo‐phase systems, where one phase is solid and the other is fluid, are widespread in nature. Examples include reservoir rocks holding vital fluids like water or petroleum, slurries of partially crystallized magmas, fluids migrating along faults filled with fault gouge, and even the semibrittle crust. Previous studies of two‐phase systems have shown that the fluid phase plays an important role in deformation localization and dynamics (e.g., Higashi & Sumita, 2009, https://doi.org/10.1029/2008JB005999; Reber et al., 2014, https://doi.org/10.1002/2014GL059832). Here, we present results from experiments investigating the influence of a fluid phase on force distribution in a granular media during simple shear. We use photoelastic polyurethane discs as the granular or solid phase and a linear viscous silicone as the fluid phase. The photoelastic property of the discs allows for direct observation and measurement of force magnitude and distribution. We compare the two‐phase experiments to granular experiments without silicone. The addition and percentage of the fluid phase has a strong impact on the force distribution and the overall force chain orientations. In the two‐phase system, the force chains form parallel to the shear plane and only rotate to the principal stress direction with an increase in strain. Locally, the fluid phase can support forces and terminate force chains over an extended period of time. Our results are in line with findings from numerical studies investigating the formation of slow slip events, proposing that these events are the result of dynamic interactions between solid and viscous phases.

Temperature and Fluid Pressurization Effects on Frictional Stability of Shale Faults Reactivated by Hydraulic Fracturing in the Changning Block, Southwest China

AbstractA shale fault reactivated during multistage hydraulic fracturing in the Changning block in the Sichuan Basin, southwest China, accompanied a cluster of small earthquakes with the largest reaching ML ~ 0.8. We illuminate the underlying mechanisms of fault reactivation through measurements of frictional properties on simulated fault gouge under hydrothermal conditions. Velocity‐stepping experiments were performed at a confining pressure of 60 MPa, temperatures from 30 to 300°C, pore fluid pressures from 10 to 55 MPa, and shear velocities between 0.122 and 1.22 μm/s. Results show that the gouge is frictionally strong with coefficient of friction of 0.6–0.7 across all experimental conditions. At observed in situ pore fluid pressure (30 MPa), the slip stability response is characterized by velocity strengthening at temperatures of 30–200°C and velocity weakening at temperatures of 250–300°C. Increasing the pore fluid pressure can increase values of (a − b) at temperatures ≥200°C, narrowing the temperature range where velocity weakening occurs. At the in situ temperature (90°C), the simulated gouge shows only velocity strengthening behavior and aseismic slip at elevated pore fluid pressures, contrary to the observed seismicity. We postulate that the aseismic slip at elevated pore fluid pressures may trigger seismicity by activating adjacent earthquake‐prone faults.

Plate motion in sheared granular fault system

Plate motion near the fault gouge layer, and the elastic interplay between the gouge layer and the plate under stick-slip conditions, is key to understanding the dynamics of sheared granular fault systems. Here, a two-dimensional implementation of the combined finite-discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular gouge fault system. We focus on investigating the influence of normal load, driving shear velocity and plate stiffness on the velocities and displacements in the direction parallel to the shear direction (x-direction) measured at locations on the upper and lower plates just adjacent to the gouge. The simulations show that during slip phases the magnitudes of the measured velocities on the upper and lower plates are proportional to the normal load and may be inversely proportional to the square root of the plate's shear modulus. Whereas, the driving shear velocity does not show distinct influence on the measured velocities. Additionally, large slip velocities are generally associated with large macroscopic friction coefficient drops. For the models subjected to smaller normal loads, larger shear velocities and with stiffer shear plates, the same magnitude of slip velocity could cause a larger drop of macroscopic friction coefficient. During stick phases, the velocities of the upper and lower plates are respectively slightly greater and slightly smaller than half of the driving shear velocity and are both in the same direction of shear. The shear strain rate of the gouge is calculated from this velocity difference between the upper and lower plate during stick phases and thus the gouge effective shear modulus can be calculated. The results show that the gouge effective shear modulus increases proportionally with normal load, while the influence of shear velocity and plate stiffness on gouge effective shear modulus is minor. The simulations address the dynamics of a laboratory-scale fault gouge system and may aid in revealing the complexities of earthquake frictional dynamics.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Rupture to the trench? Frictional properties and fracture energy of incoming sediments at the Cascadia subduction zone

The mechanical properties of sediment inputs to subduction zones are important for understanding rupture propagation through the accretionary prism during megathrust earthquakes. Clay minerals strongly influence the frictional behavior of fault gouges, and the clay content of subduction input materials varies through a stratigraphic section as well as for subduction margins globally. To establish the frictional properties of the shallow Cascadia subduction zone and place the results in a global context, we conducted high velocity rotary shear experiments on ODP core samples retrieved from Cascadia input sediments (35-45% clay) and a suite of individual clay species. We compared our results to a compilation of published high velocity experiments conducted on samples of wet gouge, dry gouge, and intact rock. For each sample type, three trends were identified with increasing normal stress: 1) the stress drop (τp−τss) increases linearly, 2) the characteristic thermal weakening distance (Dth) decreases as a power law function except for wet clay-rich gouges, and 3) the fracture energy (Wb) shows no dependence. However, fracture energy does vary with sample type. Clay-rich gouges under wet conditions have the lowest fracture energy, and fracture energy for both dry and wet gouges is at least an order of magnitude lower than estimates from intact rocks. Therefore when clay-rich lithologies are present, they may minimize spatial variations in frictional behavior, allowing earthquakes to propagate to the trench. For Cascadia input sediments, there is little variation in the fracture energy between lithologies, but the fracture energy of Cascadia sediments is around an order of magnitude higher than input sediments from other subduction margins. The high fracture energy of Cascadia sediments relative to other subduction margins may inhibit large amounts of shallow earthquake slip and dynamic overshoot.

A Comparison of Gas and Water Permeability in Clay‐Bearing Fault and Reservoir Rocks: Experimental Results and Evolution Mechanisms

AbstractTo better understand the discrepancy between gas and water permeability and the mechanism of permeability in response to the presence of water in clay‐bearing rocks, we performed transport property experiments on synthetic quartz‐clay mixtures and natural fault gouge, as well as clay‐bearing sandstones for comparison purpose. The experiments were performed on a fluid flow apparatus with effective pressures (Pe) cycling between 5 and 105 MPa. Each sample was subjected to nine Pe cycles, along which permeability and porosity of either the gas‐saturated (nitrogen) or water‐saturated samples were measured. The results show that the permeability of all samples investigated decreases with increasing Pe. Gas permeability exhibits strong pore pressure dependence, which can be explained by the slippage (Klinkenberg) effect. Permeability results measured by water are commonly lower than the gas results. The decrease in permeability after the addition of water can be generally explained by the evolution of porosity, as can be determined from the bulk volume and solid mineral volume measurement results. The loss of permeability is associated with the hydration expansion of clay minerals that causes the reduction of porosity. A conceptual model for the evolution of porosity that incorporates clay expansion and grain rearrangement is proposed to describe the mechanism for water permeability in clay‐bearing rocks. Based on these results, we discuss the effect of clay hydration on fluid transport and mechanical behaviors in clay‐bearing fault zones and reservoir formations following fluid injection.

Healing Behavior of Simulated Fault Gouges From the Groningen Gas Field and Implications for Induced Fault Reactivation.

We investigated the frictional strength recovery (healing) and subsequent reactivation and slip‐weakening behavior of simulated fault gouges derived from key stratigraphic units in the seismogenic Groningen gas field (N. E. Netherlands). Direct‐shear, slide‐hold‐slide (SHS) experiments were performed at in situ conditions of 100 °C, 40 MPa effective normal stress and 10–15 MPa pore fluid pressure (synthetic formation brine). Sheared gouges were allowed to heal for periods up to 100 days before subsequent reshearing. The initial coefficient of (steady) sliding friction μ was highest in the Basal Zechstein caprock (μ = 0.65 ± 0.02) and Slochteren sandstone reservoir (μ = 0.61 ± 0.02) gouges, and the lowest in the Ten Boer claystone at the reservoir top (μ = 0.38 ± 0.01) and in the Carboniferous shale substrate (μ ≈ 0.45). Healing and subsequent reactivation led to a marked increase (∆μ) in (static) friction coefficient of up to ~0.16 in Basal Zechstein and ~0.07 in Slochteren sandstone gouges for the longest hold periods investigated, followed by a sharp strength drop (up to ~25%) and slip‐weakening trajectory. By contrast, the Ten Boer and Carboniferous gouges showed virtually no healing or strength drop. Healing rates in the Basal Zechstein and Slochteren sandstone gouges were significantly affected by the stiffness of different machines used, in line with the Ruina slip law, and with a microphysical model for gouge healing. Our results point to marked stratigraphic variation in healed frictional strength and healing rate of faults in the Groningen system, and high seismogenic potential of healed faults cutting the reservoir and Basal Zechstein caprock units, upon reactivation.

Dry, damp, or drenched? The effect of water saturation on the frictional properties of clay fault gouges

Clay minerals often constitute a significant proportion of fault cores, yet the physics of clay friction, in particular the effect of free and interstitial bound (interlayer) water, is poorly understood. Understanding the behaviour of this typically frictionally weak component is fundamental to constraining the larger scale behaviour of fault zones. In this study, laboratory experiments were conducted on synthetic clay fault gouges under carefully controlled saturation states. Samples were sheared at room temperature under triaxial pressure at the following conditions: water saturated; room humidity; thermally dried then sheared at room humidity; in a vacuum at room humidity; thermally dried then sheared in a vacuum; in a vacuum and thermally dried within the pressure vessel. Friction coefficient (μ) is shown to increase with ‘dryness’ by a factor of 3 in 2:1 smectite and a factor of 2 in 1:1 sheet silicate, with largest increases in thermally dried samples. Analysis of constitutive frictional parameters (a-b) show that gouges become less stable with ‘dryness’. The amount of displacement or time required to establish a new steady state μ upon a change in sliding velocity (dc) decreases markedly in drier samples. Results suggest that the presence of water is key in promoting time and slip dependent frictional changes, constraining operative grain-scale deformation mechanisms to those that are fluid assisted. They also highlight the optimum laboratory procedures to investigate the frictional properties of clay-bearing materials.