Experimental Study On Velocity-dependence Transition Of Friction For Simulated Fault Gouges

On the basis of rate and state friction law, velocity weakening is the necessary condition for fault instability and the transition from velocity weakening to velocity strengthening controls the transition from unstable slip to stable sliding. Therefore, study on velocity dependence of frictional sliding is a significant topic related to seismicity and earthquake nucleation condition in fault zone. Moreover, research indicates that slow earthquake may result from the transition from velocity weakening to velocity strengthening of frictional sliding or the transition from brittle deformation to plastic deformation. Thus, study on velocity-dependence transition of friction is very important not only for understanding mechanical behavior and earthquake activity of fault zone but also for studying the mechanism of slow earthquakes.In order to better understand the velocity-dependence of friction and its mechanism, several kinds of materials including anhydrite, halite and muscovite-bearing halite which may show velocity-dependence transition at room temperature were chosen as simulated gouge, and the velocity-dependence transition of the gouges were studied by using biaxial friction configuration. Acoustic emission during frictional sliding was recorded, and the microstructure of deformed gouges was observed. The main results are as follows.(1)Frictional experiments on anhydrite gouge show thatσ₂ and loading point velocity have significant effects on the fault stability. Atσ₂=5MPa, anhydrite gouge behaves unstable slip, and the deformation is predominated by localized brittle fracturing and frictional sliding. Asσ₂ increases, transition from unstable slip to stable sliding occurs, and the deformation mode gradually changes into distributed fracturing. Anhydrite gouge shows stable sliding and velocity strengthening at velocity of 0.01μm/s and velocity weakening accompanied by quasi-periodic stick-slip at velocity of 0.05⁴.5μm/s. At velocity of 13.5μm/s, it tends to turn into velocity strengthening again. With increasing ofσ₂, the scope of velocity weakening gradually decreases and frictional sliding becomes stable. Moreover, the characteristics of acoustic emission (AE) corresponding to the two transitions are different. At higher velocities, fault sliding is accompanied by AE activity with small energy but high frequency when friction transforms from velocity weakening to velocity strengthening. While at lower velocities, fault sliding is accompanied by episodic AE events when friction transforms from velocity weakening to velocity strengthening. This coincides with the characteristics of the microstructure of gouge zone, indicating that the mechanism of the two transitions is different. The episodic AE events are similar to episodic tremble recorded along natural faults. Therefore, study on velocity-dependence transition at lower velocities, especially on the transition mechanism and the characteristics of accompanying micro-fracturing and displacement during the transition can provide physical basis for mechanism of slow earthquake(2)Frictional experiments on halite gouge show that dry halite gouge behaves stick-slip and velocity weakening at velocity of 0.1¹00μm/s, and increasingσ₂ can enhance stable sliding and the transition to velocity strengthening but fault strength does not obey Byerlee's law. Wet halite gouge behaves stick-slip and velocity weakening at higher velocity of 1¹00μm/s and velocity strengthening at lower velocity of 0.1⁰.01μm/s, and the velocity-dependence transition occurs at velocity of 0.1¹Î¼m/s and fault behaves oscillation or stick-slip with much longer time than that in the velocity weakening region. Increasingσ₂ has no effect on sliding mode and velocity-dependence. Dry halite gouge shows strong AE activity and each stick-slip event corresponds to a cluster of AE events, while each stick-slip event is accompanied by only one AE event for wet halite gouge. The microscope observation indicates that localized brittle deformation is predominant in the velocity weakening region, and the distributed cataclastic flow is the mechanism in the velocity strengthening region for dry halite gouge, while plastic deformation including grain boundary migration and pressure solution is predominant in the velocity strengthening region for wet halite gouge. Additionally, both brittle deformation and plastic deformation exist in the transition region. The existence of water enhances the plastic deformation of halite gouge and thus causes the transition from velocity weakening to velocity strengthening. Since halite can be taken as an analog of silicate rocks, the results are significant for further study on strength and stability transition and thus for understanding seismicity in fault zone. The oscillation and stick-slip with much longer time are obviously unstable events with low frequency, which should be significant for studying mechanism of slow earthquakes.(3)Frictional experiments on muscovite-bearing halite gouge show that dry gouge behaves stick-slip and velocity weakening at velocity of 0.1¹00μm/s, and increasingσ₂ can enhance the transition to velocity strengthening, and the velocity-dependence transition occurs at velocity of 0.1μm/s and fault behaves either stable sliding or stick-slip with much longer time than that in the velocity weakening region. Wet gouge behaves velocity strengthening at velocity of 0.05 ⁰.01μm/s, velocity weakening at velocity of 0.1¹0μm/s, and velocity strengthening again at velocity of 50¹00μm/s. Each stick-slip event corresponds to one or a cluster of AE events for dry gouge, while there is no any AE event corresponding to stable sliding for wet gouge. The microscope observation indicates that brittle fracturing and localized slip are predominant in the velocity weakening region and the velocity strengthening is controlled by distributed fracturing of halite under dry condition. While under wet condition, the two velocity strengthening corresponding to different mechanisms, at lower velocities, the velocity strengthening may be controlled by frictional sliding on the network developed by muscovite and distributed fracturing of halite and at higher velocities, the deformation of fault is also controlled by pressure solution of halite. Comparing to the results of halite gouge, it can be seen that the existence of muscovite has no effect on sliding mode and velocity-dependence for dry halite gouge, but it enhances the transition to stable sliding for wet halite gouge. The results provide basis for analyzing seismicity in phyllosilicate-bearing faults. The stick-slip with longer time at transition region confirms what observed in halite gouge, which is significant for understanding mechanism of slow earthquakes.