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Rock type irrelevant for earthquake lubrication A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate.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 Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s −1 .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.

\"Simply describes the forces of friction and other physics concepts using common toys such as bicycles and yo-yos. Includes experiments\"--Provided by publisher.

To understand physico‐chemical processes at real contacts (asperities) on fault surfaces, we conducted pin‐on‐disk friction experiments at room temperature, using single crystalline quartz disks and quartz pins. Velocity weakening from friction coefficientμ ∼ 0.6 to 0.4 was observed under apparent normal stresses of 8–19 (18 > 19), when the slip rate was increased from 0.003 to 2.6 m/s. Frictional surfaces revealed ductile deformation of wear materials. The Raman spectra of frictional tracks showed blue shifts and broadening of quartz main bands, and appearance of new peaks at 490–520 and 610 cm−1. All these features are indicative of pressure‐ and strain‐induced amorphization of quartz. The mapping analyses of Fourier transform infrared (FT‐IR) spectroscopy at room dry conditions suggest selective hydration of wear materials. It is possible that the strained Si‐O‐Si bridges in amorphous silica preferentially react with water to form silica‐gel. In natural fault systems, amorphous materials would be produced at real fault contacts and accumulate over the fault surfaces with displacements. Subsequent hydration would lead to significant reduction of fault strength during slip. Key Points Frictional strength of quartz was reduced by amorphization and hydration Velocity‐weakening occurred at aseismic slip rates under low normal stresses Detailed amorphization process was clarified by Raman spectroscopic imaging

Discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite‐rich, argillite‐derived slip zones during subduction earthquakes. We conducted high‐velocity friction experiments on argillite at a slip rate of 1.13 m/s and normal stresses of 2.67–13.33 MPa. Experiments show slip weakening followed by slip strengthening. Slip weakening is associated with the formation and shearing of low‐viscosity melt patches. The subsequent slip strengthening occurred despite the reduction in shear strain rate due to the growth (thickening) of melt layer, suggesting that the viscosity of melt layer increased with slip. Microstructural and chemical analyses suggest that the viscosity increase during the slip strengthening is not due to an increase in the volume fraction of solid grains and bubbles in the melt layer but could be caused primarily by dehydration of the melt layer. Our experimental results suggest that viscous braking can be efficient at shallow depths of subduction‐accretion complexes if substantial melt dehydration occurs on a timescale of seismic slip. Melt lubrication can possibly occur at greater depths within subduction‐accretion complexes because the ratio of viscous shear to normal stress decreases with depth. Argillite‐derived natural pseudotachylytes formed at seismogenic depths in subduction‐accretion complexes are more hydrous than the experimentally generated pseudotachylytes and may be evidence of nearly complete stress drop.

We investigate the interaction between two rupture fronts as they propagate toward each other and ultimately collide. This phenomenon was observed during laboratory experiments conducted on poly methyl methacrylate. Subsequently, we used numerical simulations to elucidate key aspects of these observations and draw broader conclusions. Our findings indicate that the collision of the rupture fronts generates interface waves that propagate along the sliding interface at the Rayleigh wave speed. Additionally, the rupture fronts interact with the starting and stopping S‐wave phases radiated by the opposite rupture fronts, which can locally change their velocity and generate additional interface waves. We discuss the implications of these results for understanding earthquake source phenomena. Plain Language Summary Earthquakes are caused by sudden and rapid sliding along tectonic faults. Sliding generally begins at a specific location, the hypocenter, and then expands over the fault. The manner in which this expansion occurs determines the properties and severity of the shaking generated by the earthquake. The edge of the slipping zone is called the rupture front. If the rupture front becomes very distorted, it might arrive in an area of the fault from two different sides and coalesce. In this study, we conducted experiments and numerical simulations to understand what happens when two rupture fronts propagate toward each other and collide. We show that the two rupture fronts disappear upon collision, and produce a specific type of wave that propagates along the sliding surface, called interface waves. Both the rupture front and the waves emitted by the opposing rupture front interact, which can alter the rupture front's speed and create additional interface waves. If similar waves were observed during real earthquakes, they could provide valuable information about the friction between fault rocks. Key Points We present a unique experimental observation of the collision of two mode II rupture fronts The collision radiates interface waves that propagate at the Rayleigh wave speed along the sliding interface Interface waves are also generated when stopping waves enter the sliding area of the opposite rupture

Recent laboratory friction experiments on large rock samples revealed that dynamic weakening, a remarkable reduction in the friction coefficient at elevated slip rates, occurs at lower slip rates in larger samples. There is a large difference between the sizes of natural faults and those in laboratory experiments. Therefore, it is crucial to understand the effect of size on rock friction. In the field of tribology, the interaction between frictional heating and thermoelastic effect has long been investigated. It was shown that higher slip rates than the critical value Vcr causes growth of temperature and normal stress heterogeneity (thermoelastic instability), and Vcr is proportional to the wavenumber of the heterogeneity. Severely heterogeneous normal stress may cause concentration of frictional power, thus locally activating dynamic weakening and leading to macroscopic weakening. Because a larger sample hosts a perturbation of a smaller wavenumber, it is expected to weaken at a lower slip rate than a smaller sample. In this study, a new numerical method was developed for analysis of thermoelastic instability based on the definition of memory variables and numerical approximation to the integration kernel, for the 2-dimensional problem of a planar fault embedded in an infinite medium. This method was advantageous over the standard integral equation method in terms of numerical costs. Numerical solutions with the new method on sinusoidal perturbations in the normal stress were compared with previously derived steady-state solution and its stability for validation. The typical thermoelastic properties of gabbro yield Vcr in a range of experimentally adopted slip rates, indicating that the thermoelastic effect may play an important role in high-velocity friction experiments. Because the temperature rise and the resulting normal stress change smear out after the friction experiments, measurement of the temperature distribution in a sample during a friction experiment is important for further understanding the dynamic weakening and scale effect of rock friction.

The northern Hikurangi subduction margin hosts slow slip events (SSEs), which are exceptionally shallow (<15 km). The sedimentary sequence on the incoming plate is therefore representative of the shallow fault material where the SSEs will take place once they enter the subduction zone. Knowledge about the frictional behavior of these sediments is required to know which lithologies are capable of hosting SSEs, and what mechanisms are causing them. Frictional behavior is material specific and depends on sliding velocity, but it is unknown how these natural sediments behave at plate‐rate velocities. We performed laboratory shearing experiments testing the major lithologies sampled during International Ocean Discovery Program (IODP) Expedition 375, at velocities ranging from the plate convergence rate at the Hikurangi margin (5 cm/year), up to those characteristics of the shallow SSEs (160 and 530 cm/year), under simulated in‐situ as well as standardized conditions. We find that the calcite‐rich pelagic sediments are relatively strong and display the velocity‐weakening frictional behavior required for slip events, whereas other lithologies are weaker and show velocity‐neutral to velocity‐strengthening friction. We observe spontaneous laboratory SSEs in the calcareous pelagic sediments, which show partial locking in between sliding events, consistent with the interpretation of SSEs within the spectrum of slow to fast earthquakes. For the Hikurangi margin, our results suggest that SSE occurrence requires the stronger carbonate‐rich unit to be incorporated into the plate‐boundary fault zone, which we suggest occurs because the rough incoming plate introduces geometrical complexity into the fault zone. Plain Language Summary In the Hikurangi subduction zone, located offshore the east coast of the North Island of New Zealand, the movement between the downgoing and overriding tectonic plates can occur as slow slip events (SSEs). During SSEs the two plates move relative to each other, a process in many aspects similar to ordinary earthquakes, except SSEs take weeks instead of seconds and no ground‐shaking can be felt on the surface. SSEs occur in many subduction zones worldwide, but at the Hikurangi margin they occur shallow, relatively often and close to many mostly land‐based GPS stations that track the movement of the two plates, which makes them ideal to study. A recent research expedition (IODP Expedition 375) drilled the stack of sediments going into the subduction zone to learn more about the circumstances that control where and when SSEs occur. In this paper, we test all the major different sediment types going into the subduction fault zone to find out which one of them is responsible for the SSEs. Using a laboratory device that mimics the subduction zone, we find that sediments containing the mineral calcite cause slip events that are similar to the SSEs observed in the Hikurangi margin. Key Points Spontaneous laboratory slow slip events (SSEs) suggest the shallow Hikurangi SSEs are favored in carbonate‐rich sediments Rate‐and‐state friction and critical stiffness theory can partly explain SSEs, but they can occur under velocity‐strengthening conditions SSE generation in Hikurangi is probably related to heterogeneity in the plate‐boundary fault zone

The contact behavior greatly influences the damping performance of frictional interfaces. Numerous experimental studies on friction and fretting wear have investigated the evolution of contact parameters. An in-house friction and wear test rig has been developed to obtain hysteresis loops at certain normal forces. However, the test rig lacks load control and is thus unable to ensure precise stabilization at a preset normal force, which affected the hysteresis behavior. In this paper, we developed a frequency-domain PID controller to ensure the stable application of a target normal force with constant (0–300 N) and harmonic (0–50 N) components. Compared to the commonly used time-domain strategy, the control signal error is reduced from 6.30% to 0.54% at 50 Hz. With a 3% error as the standard, the controller enables stabilized control of signals with frequencies up to 300 Hz. Friction experiments on various typical materials are conducted using this improved test rig. The results indicate a general tendency for contact stiffness to increase with a rising normal force, while the relationship between the friction coefficient and the normal force does not exhibit a clear pattern. The contact stiffness is not sensitive to the relative displacement or vibration frequency.

We developed a humidity control system in a biaxial friction testing machine to investigate the effect of relative humidity and interlayer cations on the frictional strength of montmorillonite. We carried out the frictional experiments on Na- and Ca-montmorillonite under controlled relative humidities (ca. 10, 30, 50, 70, and 90%) and at a constant temperature (95 °C). Our experimental results show that frictional strengths of both Na- and Ca-montmorillonite decrease systematically with increasing relative humidity. The friction coefficients of Na-montmorillonite decrease from 0.33 (at relative humidity of 10%) to 0.06 (at relative humidity of 93%) and those of Ca-montmorillonite decrease from 0.22 (at relative humidity of 11%) to 0.04 (at relative humidity of 91%). Our results also show that the frictional strength of Na-montmorillonite is higher than that of Ca-montmorillonite at a given relative humidity. These results reveal that the frictional strength of montmorillonite is sensitive to hydration state and interlayer cation species, suggesting that the strength of faults containing these clay minerals depends on the physical and chemical environment.