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Rock type irrelevant for earthquake lubrication A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions that are typical of earthquake nucleation depths, the authors conclude that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. This study reviews a large set of fault friction experiments and finds that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions typical of earthquake nucleation depths, it is concluded that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. The determination of rock friction at seismic slip rates (about 1 m s −1 ) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip 1 . Given the difficulty in determining friction by seismological methods 1 , elucidating constraints are derived from experimental studies 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s −1 . The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built 4 , 5 , 6 , quartz-built 3 and carbonate-built 7 , 8 ) rocks and non-cohesive rocks (clay-rich 9 , anhydrite, gypsum and dolomite 10 gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger 11 , 12 a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates 13 , 14 suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7–15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.

The frictional properties of faults control the initiation and propagation of earthquakes and the associated hazards. Although the ambient temperature and instantaneous slip velocity controls on friction in isobaric conditions are increasingly well understood, the role of normal stress on steady‐state and transient frictional behaviors remains elusive. The friction coefficient of rocks exhibits a strong dependence on normal stress at typical crustal depths. Furthermore, rapid changes in normal stress cause a direct effect on friction followed by an evolutionary response. Here, we derive a constitutive friction law that consistently explains the yield strength of rocks from atmospheric pressure to gigapascals while capturing the transient behavior following perturbations in normal stress. The model explains the frictional strength of a variety of sedimentary, metamorphic, and igneous rocks and the slip‐dependent response upon normal stress steps of Westerly granite bare contact and synthetic gouges made of quartz and a mixture of quartz and smectite. The nonlinear normal stress dependence of the frictional resistance may originate from the distribution of asperities that control the real area of contact. The direct and transient effects may be important for induced seismicity by hydraulic fracturing or for naturally occurring normal stress perturbations within fault zones in the brittle crust. Plain Language Summary A long‐term research goal in earthquake science is the development of friction laws describing how rocks break under stress and heal over quiescent periods to enable the seismic cycle. The constitutive behavior of rocks remains elusive because of the wide range of hydrothermal, barometric, and lithological conditions found in the brittle crust. A long‐standing enigma in rock mechanics is the nonlinear effect of normal stress on frictional strength across the confining pressures relevant to seismogenic faults. Equally puzzling are the sudden and delayed variations of friction upon rapid changes in normal stress. In this study, we explain the connection between these seemingly unrelated observations. The transient evolution of friction under perturbations of normal stress is due to the competition between normal‐stress‐dependent healing of the frictional interface, which strengthens the fault, and the direct effect of normal stress, which reduces the frictional strength. If the two effects do not compensate exactly, a dependence of steady‐state friction with normal stress ensues. We propose a constitutive model that explains quantitatively the frictional resistance of various rocks types and the mechanical data of bare contact and gouge friction during sliding at different normal stress for Westerly granite, pure quartz, and a mixture of quartz and smectite. The study helps build increasingly realistic representations of fault mechanics during seismic cycles. Key Points For a wide range of lithology, fault strength and fault friction are nonlinear functions of normal stress Normal stress change causes direct and evolutionary effects on the effective friction coefficient, possibly of different amplitudes The direct and transient effects upon normal stress change are controlled by the area of contact and slip‐dependent healing

Because of the operating environment and load, the main fault form of flywheel bearing is the friction fault between the cage and the rolling elements, which often lead to an increase in the friction torque of the bearing and even to the failure of the flywheel. However, due to the complex mechanism of the friction fault, the characteristic frequencies often used to indicate cage failure are not obvious, which makes it difficult to monitor and quantitatively judge such faults. Therefore, this paper studies the mechanism of the friction fault of the flywheel bearing cage and establishes its fault feature identification method. Firstly, the basic dynamic model of the bearing is established in this paper, and the friction between the cage and the rolling elements is simulated by the variable stiffness. The influence law of the bearing vibration response reveals the relationship between the periodic fluctuation of cage-rolling element friction failure and the bearing load. After analyzing the envelope spectrum of the vibration data, it was found that when a friction fault occurred between the cage and the rolling element, the rotation frequency component of the cage modulated the rotational frequency component of the rolling element, that is, the side frequency components appeared on both sides of the characteristic frequency of the rolling element (with the characteristic frequency of the cage as the interval). In addition, the modulation frequency components of the cage and rolling element changed with the severity of the fault. Then, a modulation sideband ratio method based on envelope spectrum was proposed to qualitatively diagnose the severity of the cage-rolling element friction faults. Finally, the effectiveness of the presented method was verified by experiments.

Fault friction is central to understanding earthquakes, yet laboratory rock mechanics experiments are restricted to, at most, meter scale. Questions thus remain as to the applicability of measured frictional properties to faulting in situ. In particular, the slip-weakening distance dc strongly influences precursory slip during earthquake nucleation, but scales with fault roughness and is challenging to extrapolate to nature. The 2018 eruption of K¯ılauea volcano, Hawaii, caused 62 repeatable collapse events in which the summit caldera dropped several meters, accompanied by MW 4.7 to 5.4 very long period (VLP) earthquakes. Collapses were exceptionally well recorded by global positioning system (GPS) and tilt instruments and represent unique natural kilometerscale friction experiments. We model a piston collapsing into a magma reservoir. Pressure at the piston base and shear stress on its margin, governed by rate and state friction, balance its weight. Downward motion of the piston compresses the underlying magma, driving flow to the eruption. Monte Carlo estimation of unknowns validates laboratory friction parameters at the kilometer scale, including the magnitude of steady-state velocity weakening. The absence of accelerating precollapse deformation constrains dc to be ≤10 mm, potentially much less. These results support the use of laboratory friction laws and parameters for modeling earthquakes. We identify initial conditions and material and magma-system parameters that lead to episodic caldera collapse, revealing that small differences in eruptive vent elevation can lead to major differences in eruption volume and duration. Most historical basaltic caldera collapses were, at least partly, episodic, implying that the conditions for stick–slip derived here are commonly met in nature.

Seismic rupture in carbonate rocks influences fault friction behavior through thermal evolution and mineral reactions. Focusing on the 1959 Mw 7.2 Hebgen Lake event in western Yellowstone, Montana, the largest earthquake on a normal fault in the United States, we analyze fault rock microstructures and mineralogical changes to constrain frictional heating on the fault plane. We combine thermal maturity of organic matter, magnetic fabric, and thermomagnetic methods with scanning electron microscopy to unravel variations in peak frictional temperature along the fault slip surface. The mineral changes caused by coseismic heating (e.g., nanocalcite formation or goethite to hematite reaction) occur in patches along the fault mirror, hence reflecting considerable differences in frictional heat. While coseismic thermal heterogeneities have been reported in other rock types, this is the first time they are documented and quantified specifically in carbonates. Furthermore, these results provide new mineralogical criteria to quantify coseismic frictional heat in natural faults at temperatures lower than that of decarbonation and highlight the need to consider coseismic friction processes at a scale larger than most deformation experiments. For example, we document the critical role played by fault plane attitude (dip) at the scale of a few tens of centimeters in production of frictional heat. Our results emphasize that while coseismic decarbonation dynamically weakens carbonate-hosted faults, it may generally not occur along an entire fault plane.

We model ruptures on faults that weaken in response to flash heating of microscopic asperity contacts (within a rate‐and‐state framework) and thermal pressurization of pore fluid. These are arguably the primary weakening mechanisms on mature faults at coseismic slip rates, at least prior to large slip accumulation. Ruptures on strongly rate‐weakening faults take the form of slip pulses or cracks, depending on the background stress. Self‐sustaining slip pulses exist within a narrow range of stresses: below this range, artificially nucleated ruptures arrest; above this range, ruptures are crack‐like. Natural earthquakes will occur as slip pulses if faults operate at the minimum stress required for propagation. Using laboratory‐based flash heating parameters, propagation is permitted when the ratio of shear to effective normal stress on the fault is 0.2–0.3; this is mildly influenced by reasonable choices of hydrothermal properties. The San Andreas and other major faults are thought to operate at such stress levels. While the overall stress level is quite small, the peak stress at the rupture front is consistent with static friction coefficients of 0.6–0.9. Growing slip pulses have stress drops of ∼3 MPa; slip and the length of the slip pulse increase linearly with propagation distance at ∼0.14 and ∼30 m/km, respectively. These values are consistent with seismic and geologic observations. In contrast, cracks on faults of the same rheology have stress drops exceeding 20 MPa, and slip at the hypocenter increases with distance at ∼1 m/km.

Establishing a constitutive law for fault friction is a crucial objective of earthquake science. However, the complex frictional behavior of natural and synthetic gouges in laboratory experiments eludes explanations. Here, we present a constitutive framework that elucidates the rate, state, and temperature dependence of fault friction under the relevant sliding velocities and temperatures of the brittle lithosphere during seismic cycles. The competition between healing mechanisms, such as viscoelastic collapse, pressure‐solution creep, and crack sealing, explains the low‐temperature stability transition from steady‐state velocity‐strengthening to velocity‐weakening as a function of slip‐rate and temperature. In addition, capturing the transition from cataclastic flow to semi‐brittle creep accounts for the stabilization of fault slip at elevated temperatures. We calibrate the model using extensive laboratory data on synthetic albite and granite gouge, and on natural samples from the Alpine Fault and the Mugi Mélange in the Shimanto accretionary complex in Japan. The constitutive model consistently explains the evolving frictional response of fault gouge from room temperature to 600°C for sliding velocities ranging from nanometers to millimeters per second. The frictional response of faults can be uniquely determined by the in situ lithology and the prevailing hydrothermal conditions. The frictional behavior of rocks is essential to understand fault activity and seismic unrest. Despite decades of research, the frictional behavior of rocks remains elusive. Although empirical parameters can be used to characterize the frictional behavior of fault gouge, they cannot consistently capture the evolution of frictional properties with temperature and sliding velocity. This is a theoretical bottleneck to earthquake forecasting. In this article, we present a physical model that explains the complex frictional response of various types of rocks from room temperature to 600°C within five orders of magnitude of sliding velocities. The model captures the dominance of distinct healing mechanisms at different ranges of temperature and the transition to crystal plasticity in the fault zone at elevated temperatures. As a result, a single set of constitutive parameters can explain the frictional response of rocks throughout the seismic cycle from the Earth's surface to the bottom of the lithosphere. Frictional properties evolve with the thermal activation of competing healing and deformation mechanisms at different slip‐rates A constitutive law explains gouge friction from room temperature to 600°C and slip‐rates from nanometers to millimeters per second The frictional response during seismic cycles is controlled by lithology and the prevailing hydrothermal conditions

Due to the high in situ stresses, dynamic disasters occurred frequently in the Huainan mining area, China. Our understanding of the in situ stresses in this area is still insufficient. In this study, the in situ stresses of 18 sections in two boreholes in the Xinji No. 1 coalfield were measured using the hydraulic fracturing method, and the distribution of in situ stresses in the Huainan mining area were investigated. The relationship between in situ stress and geological structure in the Huainan mining area was summarized and the limitation of fault friction strength on in situ stress was discussed. The result showed that the maximum horizontal principal stress (σH) at Xinji No. 1 mine was 13.95–25.23 MPa, the minimum horizontal principal stress (σh) was 12.16–21.17 MPa. The average azimuth of the maximum horizontal principal stress was N83.61°E. The statistical results showed that the in situ stresses in Huainan mining area were characterized by a strike-slip faulting regime. Both the horizontal and vertical principal stresses increased approximately linearly with the increase of burial depth. The orientation of the maximum principal stress in the study area is closely related to the tectonic movement and the ratio of maximum principal stress to minimum principal stress was primarily limited by the friction strength of the faults. The outcomes of this research can provide some reliable engineering parameters and benefit the roadway layout and support design in the Huainan mining area.

We present estimates of slip rates for active faults in the External Dinarides. This thrust‐and‐fold belt formed in the Adria‐Eurasia collision zone by the progressive formation of NE‐dipping thrusts in the footwalls of older structures. We calculated the long‐term horizontal velocity field, slip rates and related uncertainties for active faults using a thin‐shell finite element method. We incorporated active faults with different effective fault frictions, rheological properties, appropriate geodynamic boundary conditions, laterally varying heat flow and topography. The results were obtained by comparing the modeled maximum compressive horizontal stress orientations with the World Stress Map database. The calculated horizontal velocities decrease from the southeastern External Dinarides to the northwestern parts of the thrust‐and‐fold belt. This spatial pattern is also evident in the long‐term slip rates of active faults. The highest slip rate was obtained for the Montenegro active fault, while the lowest rates were obtained for active faults in northwestern Slovenia. Low slip rates, influenced by local active diapirism, are also characteristic for active faults in the offshore central External Dinarides. These findings are contradictory to the concept of Adria as an internally rigid, aseismic lithospheric block because the faults located in its interior release a part of the regional compressive stress. We merged the modeling results and available slip rate estimates to obtain a composite solution for slip rates. Key Points To study neotectonics and geodynamics of External Dinarides To address uncertainties of lithosphere properties To estimate active fault long‐term slip rates for External Dinarides

The protolith of the hanging wall and footwall of a fault plays a crucial role in influencing the sliding stability of the fault, and different protoliths have different tendencies toward sliding instability. To investigate the influence of protoliths on fault sliding stability, simulated fault friction sliding tests were conducted on five types of rocks: fine sandstone, limestone, marble, basalt, and granite, under various loading conditions. The test results demonstrate that, under the same loading conditions, basalt and granite exhibit a greater inclination toward unstable sliding during fault simulation, primarily displaying regular stick–slip and regular inclusion chaotic stick–slip behaviors. On the other hand, fine sandstone, limestone, and marble are predominantly characterized by stable sliding behaviors. The order of sensitivity for the influencing factors on sliding mode is the type of protolith, followed by initial normal stress, and then displacement loading rate. Based on the type of protolith and loading conditions (initial normal stress and displacement loading rate), the sliding mode can change during the sliding process of the simulated rock faults, transitioning from stable sliding to chaotic stick–slip, and then to regular stick–slip. Alternatively, the sliding mode can shift from regular inclusion chaotic stick–slip to regular stick–slip, or from regular stick–slip to stable sliding. Finally, the complexity of sliding patterns in different types of protoliths is analyzed from the perspectives of mineral composition and microstructure, elucidating the underlying mechanisms behind three sliding patterns: stable sliding, chaotic stick–slip, and regular stick–slip. Furthermore, the degree to which different types of rocks tend toward stick–slip behavior can be ranked as follows: rock mineral composition, mineral particle size, and structure among rock minerals. These research findings contribute to a deeper understanding of fault sliding behavior.HighlightsExperimental studies have shed light on the influence of protolith type on the stability of fault sliding, revealing that different rock types exhibit a preference for stick–slip behavior in the following descending order: rock mineral composition, mineral grain size, and structure among rock minerals.Further investigations have identified that basalt and granite tend to display unstable sliding, whereas fine sandstone, limestone, and marble are predominantly characterized by stable sliding. Intriguingly, a novel fault sliding mode named regular inclusion chaotic stick–slip has been uncovered.By delving into the mineral composition and microstructure, a comprehensive understanding of the underlying causes for the intricate variations in sliding modes across different protolith types has been attained.