A note on the effect of fault gouge composition on the stability of frictional sliding

Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 14. pp. 155-160. Pergamon Press 1977. Printed in Great Britain

A Note on the Effect of Fault Gouge Composition on the Stability of Frictional Sliding R. SUMMERS* J. BYERLEE* The frictional properties of fault gouge have been studied at confining pressures to 6 kbars. I f the gouge is composed of strong materials such as crushed granite or quartz sand, the frictional strength is high, and violent stick-slip occurs at confining pressures above approximately 1.5 kbars. If the gouge is composed of minerals such as illite, kaolinite, chlorite, or antigorite, which have weak bonding forces between the structural layers, the frictional strength is slightly lower, but violent stick-slip still occurs under high confining pressure. The expanding clays, montmoriilonite and vermiculite, which have free water between their structural layers, slide stably at confining pressures as high as 6.25 kbars and exhibit low friction. A similar stable behavior with lowered strength is observed in water-saturated quartz sand when the water is confined within the fault zone during deformation. The results of this series of experiments support water being the stabilizing influence when it is either (1) trapped within or between rocks of low permeability and can provide a high pore pressure when the rocks are deformed, or (2) loosely bonded in a mineral structure, as in the hydrated clays, where it can produce a pseudo-pore pressure when the clay is compressed. In both these cases, the effective stress can be reduced and the deformation stabilized.

INTRODUCTION Faults may release accumulated strain by creep or by infrequent but large sudden slip events which produce earthquakes. In regions where the displacement is by fault creep, there is frequent movement within the fault zone---mostly aseismic creep though often there are very minor earthquakes. It is usually the large violent events that are responsible for most earthquake damage to man-made structures. These two types of movement, creep and sudden violent displacement are regularly observed when modeling faulting processes in the laboratory. Creep is known as stable sliding, and the violent deformation is known as stick-slip. Since most strain release occurs in relatively narrow fault zones, which contain large amounts of gouge, it would be helpful in understanding the mechanics of faulting to know as much as possible about the behavior of gouge in a fault zone under high stress conditions. Byerlee and Brace [1] observed that in a dunite that contained three % serpentine, deformation was significantly more stable than in a similar dunite containing no serpentine. Byerlee[2"1 has suggested that weak alteration minerals such as serpentine might act as solid lubricants which could allow the more competent grains in a rock to slide over one another stably rather than deform by brittle fracture. Raleigh and Pater* U.S. Geological Survey, Menlo Park, CA 94025, U.S.A.

.son [3] found that with serpentinite the width of the fault zone, which formed in intact samples, increased as a function of confining pressure. It has been shown [4] that unstable behavior in fault gouge composed of crushed Westerly granite can be partially suppressed by increasing the thickness of the gouge zone. Quartz fault gouge has been observed to reduce the stress drops in saw cut sandstone samples when compared to similar bare Saw cuts [5]. Small amounts of gouge on sliding surfaces of granite samples have been observed to significantly reduce the stress drop [6]. It has been pointed out by Wu et al. [71 that fine grained gouge can be easily altered to various clay minerals. They also have discussed the importance of gouge zones, especially clay gouge, and how these zones may possibly allow the low stress drops observed with earthquakes, as compared with relatively large stress drops observed in many laboratory stick-slip tests. In view of the importance of the possible effect of weak alteration minerals on the stability of frictional sliding we have carried out a series of experiments using materials that may occur in natural fault zones. In these experiments we kept the thickness of the artificial gouge constant in order to eliminate the effect that the thickness of the gouge has on the stability of sliding. EXPERIMENTAL PROCEDURES For the series of experiments presented in this paper, we used the sample configuration illustrated in Fig. 1

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in order to compare the behavior of various fault gouge material under controlled conditi~s. For each sample, a cylinder of Westerly granite was prepared by grinding a rough drilled core to a diameter of 2.54 cm and grinding the ends parallel, with a length of 6.35 cm. Each cylinder was further prepared by making a smooth surface saw cut at an angle of 30 ° to the long axis. The final preparation consisted of washing the saw cut faces with acetone. In every case samples were assembled using granite cylinders that were initially dry (air dried at 20 C). These sample holders provided boundary conditions which were uniform for all samples. The sample materials were prepared as discussed below, using either granular materials or thin, solid wafers, and placed between the two halves of the granite cylinder. Those samples which were prepared utilizing layers of granular materials were assembled wet so that the loose material could be handled during the sample assembly process. The thickness of the gouge layer (0.064 or 0.046 cm) was controlled by holding one half of the granite cylinder in a jig so that a uniform layer of predetermined thickness could be packed onto the 30° face of the cylinder. After packing the gouge material into the jig, the half cylinder with the gouge layer was carefully removed from the jig and slipped into a 0.13 mm copper sleeve. This sleeve served to hold the two halves of the granite cylinder parallel and allowed the samples to be handled with the gouge in place. The copper sleeve has very little strength of its own to interfere with the sample and in addition, it was soon sheared due to the large displacements to which the samples were subjected. The samples were finally jacketed in a polyurethane sleeve with a wall thickness of 3 mm. The polyurethane jacket was secured to steel end plugs with wire clamps. For the samples of Westerly granite, serpentinite, and Ottawa quartz sand, the assembly of all parts into the copper sleeve was completed while

the gouge was still wet. The Ottawa quartz sand samples were run while water-saturated. The Westerly granite and serpentenite samples were thoroughly dried in a vacuum oven at 120°C and then allowed to equilibrate at room temperature and humidity before each experiment. The clay samples also were prepared wet. Because clay structure can be altered by excessive drying (with the expanding clays, montmorillimite, and vermiculite being especially susceptible), all clay samples--illite, kaolinite, halloysite, montmorillite, and vermiculite-were air dried, (20°C for one week) rather than oven dried prior to jacketing in the copper sleeve. The chlorite was used as a solid slice cut from a cylindrical sample of the material with the plane of the slice parallel to the 001 cleavage of the mineral. After cutting the chlorite, it was rinsed with acetone and stored at laboratory temperature and humidity until needed for the tests. All of our samples were deformed in a hydraulically operated triaxial testing machine, using petroleum ether as the confining pressure medium and a 7 × l0 s kg ram to apply the differential stress. In our experiments the interfaces between the sample and the steel end plugs were not lubricated. It has been found by Byerlee (unpublished results) that lubrication of sample ends with molycote or graphite has no apparent effect on the results of frictional sliding at high confining pressures such as were used in this present study. Apparently the force required to cause lateral movement between the finely ground surfaces at the sample and steel end plugs is a very small fraction of the force required to cause sliding on the saw cuts. In the experiments the confining pressure was held constant to _+ 5 bars. All samples were deformed in compression parallel to their long axis at a constant rate. In all of the experiments with the exception of those carried out with the watersaturated Ottawa quartz sand, the ram was advanced at an axial compression rate of 6 x 10 -3 mm/s. For the quartz sand, rates of 6 × 10 -4 and 6 x 10 - 6 mm/s were used. The stress history of the quartz sand samples indicated strong time-dependent control of the stress. To further investigate the time-dependent effect two additional samples were run in which the deformation was halted at selected times to observe the resulting effect on the stress. In all other samples, the deformation was constant and without interruption. An external strain gauge load cell to measure force in the moveable piston, and a displacement transducer to measure movement of the piston, were used to generate a continuous stress-strain history of each sample during deformation. A correction was made in the stress-strain charts for the confining pressure and frictional force at the O-ring seal where the moving piston enters the pressure vessel. This correction was done in the manner described by Byerlee [81. Other corrections were not considered to be necessary for making comparisons between the different types of materials. Factors such as jacket strength affected all samples equally. The charts have been presented without axial stress correc-

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Westerly granite (Fig. 2) was used as a fault gouge material because the frictional properties of this rock type have been studied extensively. It was used in a crushed form prepared by mechanical grinding until all grains would pass a sieve with 0.088 mm mesh. The serpentinite (Fig. 3) used was an antigorite collected from a sheared zone at New Almaden, California. Like the Westerly granite, it was used in crushed form of grain size less than 0.088 mm. The chlorite (Fig. 4) was a ripidolite from Flagstaff Hill, El Dorado County, California. These samples were 0.064-cm wafers cut parallel to the 001 plane. The kaolinite (Fig. 5) was from the Ione Formation near Lincoln, California. The illite was the A.P.I. No. 35 (American Petroleum Institute, Clay mineral standard project) from Fithian, Illinois. The halloysite was the A.P.I. No. 13 from Eureka, Utah. The montmorillonite was the A.P.I. No. 23 from Chambers, Arizona. The vermiculite was from Libby, Montana. The quartz sand (Fig. 6) was the Ottawa quartz sand, A.S.T.M. Designation C-109. It was furnished by the Ottawa Silica Company, Ottawa, Illinois.

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Fig. 2. Frictional sliding in Westerly granite fault gouge. Differential stress supported (kbars) shown as a function of shear displacement (ram) for confining pressures of 0.80, 1.58, 3.17 and 6.27 kbars. Gouge layer thickness, 0.064cm; layer composed of crushed Westerly granite, particle diameter < 0.088ram. Axial compression rate, 6 x 10 -3 mm/s.

tions for the changing contact area. Making this correction would not alter the conclusions drawn from the data or interfere with making comparisons between the different materials, and it was felt to be preferable to present the data in its raw form. With samples of the configuration used in this study there are alignment problems at the larger displacements. However, this does not appear to have caused unstable behavior at larger displacements. For example, the expanding clays, (montmorillonite and vermiculite in Fig. 5) exhibited low strength stable behavior at high confining pressure. Also the water-saturated quartz sand was stable both at high stress (slow compression rate with low pore pressure) and at low stress (fast compression rate with resulting high pore pressure). The materials which exhibited unstable behavior appear to be truly unstable with the gouge thickness and confining pressure used for this study. AXIAL

EXPERIMENTAL RESULTS For most of the simulated fault-gouge materials examined in this study, two samples were run at each confining pressure, the reproducibility was found to be quite good. In this paper, we present representative stress-strain curves. The results of all the experiments are contained in the report by Summers and Byerlee I-9]. The object of this study was to compare the relative sliding behavior of the different types of gouge, and only the stress-strain result~ will be presented. Other results of the shearing such as grain re-orientation and crushing and the development of shear structure will not be discussed. The test results obtained with the fault gouge of crushed Westerly granite (Fig. 2) provides a good

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example of the behavior which can be expected when deforming a 0.064-cm thick layer of a high strength fault gouge material. The lowest line on the chart was generated by a sample deformed under a confining pressure of 0.80 kbar. As can be seen on the chart, after a small amount of initial elastic compression, the remainder of the deformation took place solely by stable sliding. Using a new identical sample and raising the confining pressure to 1.58 kbars also resulted in an initial elastic compression followed by stable sliding, but two small unstable stick-slip events occurred during the deformation. For the next sample, run at 3.17 kbars confining pressure, there was elastic compression followed by stable sliding, but after 4 mm of stable shear there was a violent stick-slip stress drop. From this point on, there were regular major stick-slip events until the deformation was halted. From the top trace, the sample run at 6.27 kbars, it can be seen that

stress rises were now mainly elastic with only a small amount of stable sliding preceding the sudden violent stick-slip events. In the results obtained by deforming 0.064-cm layers of crushed serpentinite, the maximum stress levels supported were lower than was found using the granite at comparable confining pressures. At confining pressures of 0.73 and 1.53 kbars, all of the deformation in the serpentenite was by stable sliding. The sample run at 3.12 kbars underwent a period of prolonged stable sliding before the onset of regular stick-slip, very much the same as the granite sample that was run at a comparable confining pressure. During the period of regular stock-slip, an average of approx 1 mm of stable shear preceded the stick-slip events. This is more than was observed with the crushed granite. At the highest confining pressure tested (6.26 kbars) for the serpenfinite, a period oi" stable shear of approx 3 mm preceded AXIAL

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Fig. 5. Frictional sliding in clay fault gouge. Differential stress (in kbars) shown as a function of displacement (in mm). Layer thickness, 0.064cm, confining pressure, 6.27kbars. Axial compression rate 6 x 10 .3 mm/s.

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Fig. 6. Pore pressure-dependent frictional strength in water-saturated quartz sand. Breaks are pauses during which compression was halted and water allowed to drain from the 0.406 cm thick sand layer. Confining pressure, 4.70kbars. Axial compression rates, 6 x 10-" and 6 × 10- 6 mm/s.

Fault Gouge Composition the onset of stick-slip. This is considerably more shear than was observed in the granite sample at 6.27 kbars. The oriented chlorite sample (Fig. 4), in which a 0.064-cm thick wafer of chlorite was sheared parallel to the 001 plane, supported stress levels very close to those supported by the crushed serpentinite at comparable confining pressures. In the chlorite samples, which were run at 0.73 and 1.53 kbars confining pressure, all the observed shear took place by stable sliding. For the sample run at 3.12kbars confining pressure, the shear took place by a combination of stable sliding and stick-slip. The onset of the regular stick-slip occurred considerably later than was observed in either the serpentinite or the granite. At a confining pressure of 6.27 kbars, there was only slightly more stable sliding in the chlorite, preceding the onset of regular stick-slip, than was observed with serpentinite. The clay minerals for which tests were conducted (Fig. 5) exhibited a wide range of behavior. All of these clays were run using the same thickness of fault gouge (0.064 cm) and the same confining pressure (6.27 kbars). The vermiculite and montmorillonite supported significantly lower stress than the other clays, and in addition sheared only by stable sliding. The other three clays, kaolinite, halloysite, and illite, all supported high stress, and after an extended initial period of stable sliding began to exhibit regular stick-slip. The stress level in the illite would rise to a maximum then slowly decrease. The stick-slip in this material consistently occurred during the prolonged stable stress decrease. The final set of samples that will be described are those run with the water-saturated quartz sand (Fig. 6). These samples all supported increasing stress levels as the deformation progressed. The sample represented by the lowest stress level trace was deformed at the fastest strain rate. This sample was held at 4.70 kbars confining pressure for 2 h before deformation, then deformed continuously at an axial compression rate of 6 x 10 -4mm/s. The continuous trace at the highest stress levels is from an identical sample that was held at 4.70 kbars confining pressure for 2 h, then deformed continuously at an axial compression rate of 6 x l0 -6 mm/s. To further investigate the time-dependent effect shown by these samples two additional samples were run. The middle two traces which appear as broken lines were generated by samples run at a rate of 6 x l0 -4 mm/s. For each of these samples, the deformation was halted several times and then resumed. In the lower broken trace there was an initial wait of 2h, and then pauses of 2, 2, 16 and 2h. For the upper broken trace, the initial wait and all of the pauses were each 8 h. It can readily be seen that when the deformation was resumed after each pause, the stress level rose higher than it would have had deformation been continued uninterrupted. This strong timedependent effect appears to be due to the pore water, which is trapped within the fault zone, slowly diffusing into the surrounding Westerly granite. The resulting pressure drop does provide an explanation for the observed time-dependent stress increase. These pore

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pressure changes were not measured directly but were inferred from their resulting effect on the strength of the quartz gouge. In identical dry samples of this same quartz sand, run at the same confining pressure, regular stick-slip occurred [9]. DISCUSSION As the confining pressure is increased, fault zones in many different types of rocks can change their behavior from the low-pressure type of deformation, where all of the displacement is by stable sliding, to the high-pressure type, which includes violent stick-slip failure. Since it has been shown that fault gouge thickness can effect the stability of sliding in crushed Westerly granite [4], we felt that for this study we should use fault zones of uniform thickness in order to compare the frictional properties of different rock materials. By using this method, we found that dry samples of widely different rock materials, including crushed granite, crushed serpentinite, and non-expanding clays, exhibited the unstable stick-slip type of behavior when the confining pressure was greater than 1.5-3 kbars. This appears to be the point at which there is sufficient force applied by the confining pressure to close cracks and lock grains together, thus forcing the deformation to take place by fracturing through grains, rather than allowing the grains to lift over each other as they could during deformation at lower pressures. As far as faulting processes are concerned it should be possible for this locking and fracturing process to continue almost indefinitely. The fault gouge thickness of 0.064cm (0.025 in.) we used was chosen partly because with crushed granite it behaves approximately the same as fractured samples of originally intact Westerly granite. Raleigh and Paterson [3] have observed high stress levels during the deformation of a serpentinite. In their experiments, however, repeated stick-slip such as we observe in our restricted-width fault zone did not occur. Significantly. Raleigh and Paterson observed that as they incr,. ~ed the confining pressure, the fault zone changed from one of single shear fractures to systems of conjugate shears, until at 5 kbars in their samples the deformation was distributed through a broad band. Although composition may be reflected in the maximum stress level supported (it can be readily seen, for example, that the crushed Westerly granite consistently supports a higher differential stress than the serpentine, chlorite, or clays), the 0.064 cm layers of many so-called weak minerals support surprisingly high stress. The high strength behavior is expected with the minerals such as quartz and feldspar, the major constituents of Westerly granite. However, finding this same high frictional strength behavior in phyllosilicate minerals such as serpentine, chlorite, and non-expanding clays, which are considered relatively weak, is important when considering rock failure at high pressure. Low frictional strength behavior was shown, significantly, by the expanding lattice clays montmillonite and vermiculite. These stable expanding clays differ sig-

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nificantly from the unstable non-expanding clays only by presence of the loosely bonded inter-layer water characteristic of expanding clays. Most likely it is this small amount of water that is the cause of the radically weaker behavior found in the expanding clays. Further support for the stabilizing effect of the water is provided by samples of the expanding clays that were vacuum-oven dried before deforming [9]. They consistently exhibited higher frictional strength than the identical air-dried samples. Other clay samples of both expanding and" non-expanding types which have been deformed while water-saturated without being dried always showed a very marked reduction in their ability to support stress [9]. The small amount of inter-layer water in the expanding clays is apparently able to provide a pseudo-pore pressure. This pressure is sufficient to greatly reduce the frictional strength in these clays when they are deformed. Raleigh and Paterson [3], in their high temperature deformation study of serpentinite, attribute the large loss of strength which they observe at higher temperatures partly to an increased pore pressure resulting from water liberated from the serpentinite during heating. A similar weakening effect has been observed by Edmond and Paterson [10] in talc at high temperature. Graphite is another material that appears to provide additional evidence for interlayer water acting to reduce friction in a manner very similar to that observed with the hydrated clays. It has been found by Savage [11] that the layered structure of graphite is not in itself entirely adequate to account for the lubricative effect normally associated with graphite, that there must be, in addition, another substance adsorbed on the basal planes to allow these planes to move more freely over one another. This adsorption is thought by Pierce and Smith [12] to be in the form of clumps at isolated receptive sites rather than as a uniform layer covering an entire surface. This great affinity for surface adsorption is commonly utilized in activated charcoal filters. These clumps serve as spacers between the graphite layers, reducing both friction and wear. There has been discussion as to whether it is rock type or the presence of water that is responsible for

certain areas along a fault creeping constantly and relatively stably while other areas along the same fault remain locked, and are considered the potential location for large, damaging earthquakes. The results of this series of experiments indicate that water can have a significant influence when it is either (1) trapped within or between rocks of low permeability where it can provide a high pore pressure when the rocks are deformed, or (2) loosely bonded in a mineral structure such as is found in the hydrated clays, where it can provide pseudo-pore pressure when the clay is compressed. In both of these cases, the effective stress can be reduced and the deformation stabilized. Receired 11 Noremher 1976.

REFERENCES 1. Byerlee J. D. & Brace W. F. Stick-slip, stable sliding and earthquakes. J. Geophys. Res. 73, 6031~5037 (1968). 2. Byerlee J. D. The mechanics of stick-slip. Tectonophysics 9. 475 (1970). 3. Raleigh C. B. & Paterson M. S. Experimental deformation of serpentinite and its tectonic implications. J. Geophys. Res. 70, 3965-3985 (1965). 4. Byerlee J. D. & Summers R. The effect of fault gouge on the stability of sliding on saw cuts in granite. Trans. Am. Geophys. Union 54, 1210 (Nov. 1973). 5. Engelder J. T., Logan J. M. & Handin J. The sliding characteristics of sandstone on quartz fault-gouge. Pageoph 113, 69-86 (1975). 6. Scholtz C., Molnar P. & Johnson T. Detailed studies of frictional sliding of granite and implications for the earthquake mechanism. J. Geophys. Res. 77, 6392-6406 (1972). 7. Wu F. T., Blatter L. & Roberson H. Clay gouges in the San Andreas fault system and their possible implications. Pageoph 113, 87-95 (1975). 8. Byerlee J. D. Brittle~luctile transition in rocks. J. Geophys. Res. 73, 4741--4750 (1968). 9. Summers R. & Byerlee J. Summary of results of frictional sliding studies, at confining pressures up to 6.98 kbar, in selected rock materials. Open file report M-76-965, U.S. Geological Survey (1976). 10. Edmond J. M. & Paterson M. S. Strength of solid pressure media and implications for high pressure apparatus. Contr. Mineral Petrol. 30. 141-160 (1971). l l. Savage R. H. Graphite lubrication. J. appl. Phys. 19. 1 (1948). 12. Pierce C. & Nelson Smith R. Adsorption-desorption hysteresis in relation to capillarity of adsorbents. J. Phys. Colloid Chem. 54. 748 (1950).