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Structures developed in fault gouge during stable sliding and stick-slip
~ec~ono~~~s~cs, 44 (1978) 161-171 0 Elsevier Scientific Publishing Company, Amsterdam -Printed
STRUCTURES DEVELOPED SLIDING AND STICK-SLIP
J. BYERLEE
161 in The Netherlands
IN FAULT GOUGE DURING
STABLE
l, V. MJACHKIN 2, R. SUMMERS r and 0. VOEVODA 2
r U.S. Geological Survey, Menlo Park, Calif. (U.S.A.) 2 Institute of Physics of the Earth, Moscow (U.S.S.R.) (Submitted February 24,1977;
accepted April 28,1977)
ABSTRACT Byerlee, J., Mjachkin, V., Summers, R. and Voevoda, O., 1978. Structures developed in fault gouge during stable sliding and stick-slip. Tectonophysics, 44 : 161-17 1. We carried out a detailed study of the structural changes that occurred in a thin layer of quartz gouge sheared between saw cuts in granite cylinders at pressures of 2 and 4.7 kbar. At low pressure the material deformed stably, but at high pressure deformation was unstable. During deformation shear zones were developed oblique and parallel to the plane of the saw cuts. Our results suggest that shearing oblique to the strike of the fault zone precedes sudden slip, which is confined to the margin between the intact rock and gouge. If this is true in the natural situation, then it may be possible by studying the spatial distribution of the microseismic activity and creep in shear zones to determine whether sudden slip is imminent.
INTRODUCTION
In some active tectonic regions sliding between fault blocks takes place suddenly to produce large earthquakes, whereas in other regions the movement on the fault is nearly continuous and earthquakes are very small or absent. A similar phenomenon occurs in frictional-sliding experiments carried out in the laboratory. For example, it has been found that at low pressure the sliding between rocks takes place smoothly, but at high pressure it occurs by violent stick-slip. We have carried out a number of experiments to discover what physical mechanism is responsible for instability by studying the structural changes that occurred in a thin layer of quartz gouge deformed under confining pressure. The physical mechanism responsible for instability is still not clear to us, but in this paper we present current results in the hope that they will stimulate interest in this important problem.
EXPERIMENTAL
METHOD
Cylindrical samples of Westerly Granite, 2.54 cm in diameter and 6.05 cm long with their ends ground parallel, were cut with a diamond saw at an angle of 30” to the long axis of the cylinders. A uniform layer, either 2 mm or 4 mm thick, of grains of ASTM N”C-109 Ottawa sand was placed between the two halves of the cylinders. The samples were assembled wet so that they could be inserted into a 0.01 cm thick copper tube. The copper tube was used to give the samples a slight mechanical strength so that they could be assembled without disturbing the sand. After assembly the samples were dried in a vacuum oven at a temperature of 120°C. The samples were then jacketed in a polyurethane sleeve with a wall thickness of 0.032 cm and sealed at the ends to hardened steel plugs. A schematic diagram of the sample assembly is shown in Fig!. 1. The samples were deformed under a constant confining pressure of either 2 kba.r or 4.7 kbar at a constant rate of 10-5/sec. This strain rate refers to the elastic strain rate in the intact rock and not to the strain rate in the shear zones which can vary from 10-5/sec during stick and 103fsec or higher during slip of the stick-slip cycle. During the experiments we recorded the change in axial force as a function of the change in length of the sample with an xy-recorder. The axial force was converted to axial stress 0 by dividing it by the initial cross-sectional area, and the change in length was converted to axial strain E by dividing it by the initial length of the sample. The shear displacement was calculated by resolving the change in axial length on the inclined plane of the saw cut. After each experiment the specimens were impregnated with epoxy resin, and thin sections were then cut from the samples parallel to the long axis and normal to the plane of the fault gouge and examined with a petrologic microscope.
WIRE
CLAMP
SAND
GOUGE
POLYURETHANE COPPER
JACKEl
GRANITE
STEEL
PLUG
Fig, 1. Schematic diagram of the sample assembly.
163 EXPERIMENTAL
RESULTS
In order to study progressive structural changes in the gouge as it was subjected to increasing strains, samples were deformed by increments (5%, lo%, and so on), then impregnated with epoxy, and cut into thin sections. We would repeat the process deforming a new sample to say 10% strain. In this way we could study the progressive changes in the structure of the gouge as it was deformed to larger and larger strains. In samples strained at a confining pressure of 4.7 kbar deformation was accompanied by violent stick-slip, and at a pressure of 2 kbar most of the deformation was stable. To find out what effect thickness of fault gouge had on the structures developed during stick-slip, we did two series of experiments at a pressure of 4.7 kbar, one in which the thickness of the gouge was 4 mm and the other 2 mm. Typical stress-strain curves for the three series of experiments are shown in Figs. 2, 3 and 4. The dashed lines on the figures represent sudden stick-slip sliding. On each figure the numbered circles are drawn to show the point on the stress-strain curves at which each experiment was terminated. Particularly at a pressure of 4.7 kbar, a large part of the initial loading of the samples was linear with strain, but the slopes were different for different thicknesses and also for successive cycles of stick-slip.
SHEAR I
’
I
DISPLACEMENT 4
2
0
’
I
AXIAL
6 ’
I
(MM) 10
6 ’
SHORTENING
I
’
I
12 ’
I
( %)
Fig. 2. Differential stress plotted as a function of axial strain and shear displacement for samples containing 2 mm of sand gouge and deformed at a confining pressure of 4.75 kbar. The numbered circles are the points of permanent strain in the samples at the termination of each experiment.
SHEAR 3
2
4 5-“---_-
DISPLACEMEN? (MM: i. i :; 8 .-._/ _~. ...‘..-~~--_..--_‘.._.’
;5
:
AXIAL
SHORTENING
i % 1
Fig. 3. Differential stress plotted as a function of axial strain and shear displacement for samples containing 4 mm of gouge and deformed at a confining pressure of 4.7 kbar. The numbered circles are the points of permanent strain in the samples at the termination of each experiment.
DISPLACEMENT
SHEAR 0 i
2 j
’
(MM
1
4 ’
i
Fig. 4. Differential stress plotted as a function of axial strain and shear displacement for samples containing 4 mm of gouge and deformed at a confining pressure of 2 kbar. The numbered circles are the points of termination of each experiment.
To find out how much of this was due to hnear compaction of the gouge and how much due to elastic deformation of the granite, we deformed intact samples of granite at pressures of 4.7 kbar and 2 kbar, The stress-strain curves for these experiments are numbered 302 on Figs, 2, 3 and 4. The slope of these curves does not exactly represent the elastic modulus of the rock because both the axial force and the axial displacement were measured outside the pressure vessel so that some of the elastic strain is contributed by the piston, end plugs, and pressure vessel. However, a comparison of these curves with the curves from the cut samples gives a true measure of the deformation of the gouge. For samples containing saw cuts the stress increases almost linearly with strain (Figs. 2, 3 and 4), but the slope Kr, for loading the saw cuts is much less than the slope Ku for loading of the intact samples. The deformation is not elastic because the slope K,, for unloading is much greater than Ii,,. The unloading KU, is ~pprox~mately equal to I&> the unloading modulus for the intact samples. At a given stress the axial displacement of the ends of the sample is the sum of the axial displacement caused by strain in the intact rock plus the axial displacement caused by the strain in the fault gouge, so that if KU, equals KUi, then the fault gouge must have the same modulus as the dense rock. Figures 2 and 3 also show that after the first stick-slip event K,, is constant and is approximately equal to Kn. Thus the gouge reaches its maximum density during the first stick-slip cycle, and for ah subsequent cycles its elastic modulus is approximately equal to the modulus of the intact sample. Our me~urements of the elastic moduli are not very accurate, and because the fault zone is narrow, the slope of the stress-strain curves is not strongly dependent on the porosity of the gouge. Thus even though our results would suggest that the density of the gouge during stable sliding and stick-slip is the same, small differences in porosity could exist. The average stress drop for the stick-slip events in the experiments with a gouge thickness of 4 mm and deformed at a pressure of 4.7 kbar was 7.5 kbar, and for the samples deformed at the same pressure but containing 2 mm of gouge it was 8.0 kbar. Although the difference in the average stress drop between the two series of experiments is small, these results are similar to the result reported by Byerlee and Summers (1976) for sliding with crushed granite between saw cuts, that showed that the wider the gouge the smaller the stress drop. There is a small amount of nonlinear strain before each violent stick-slip event (Figs. 2 and 3). If the material between the sliding surfaces becomes fully dense during the fist stick-slip cycle, then the nonelastic strain preceding all subsequent cycles must be due to permanent deformation of shearing within the gouge layer. We measured the nonlinear deformation parallel to the plane of the saw cuts for all but the first cycle for all the experiments carried out at a pressure of 4.7 kbar. For the samples with 4 mm of gouge, the average distance of stable slip was 0.39 mm, and for samples with 2 mm of gouge, it was 0.52 mm, Byerfee and Summers (1976) suggested that the
166
distance of stable sliding preceding sudden slip may increase with an increase in thickness of the gouge, but the results from the experiments reported here do not support this idea. Study of the thin sections showed that sand grains in the gouge were crushed and that the amount of crushing depended on the amount of strain, the confining pressure, and the thickness of the sand layer. The cracking occurred at grain contacts, and the cracks did not appear to have a preferred o~en~tion. There was, at least in the early stages of defo~ation, only a minor amount of displacement on the newly formed cracks so that the outlines of the well-rounded original grains could be identified. To describe the degree of cracking of the original grams, we define the factor Q by the equation:
Q
=!=nn,x
100%
where n is the total number of both cracked and untracked grams in which the outlines of the original grains could be identified and a0 is the number of untracked grains. The results are shown in Fig. 5 which plots Q for all the experiments as a function of the permanent strain. A number of the thin sections were studied independently by two investigators and the difference between their calculations of Q is shown by the error bar in Fig. 5. The results show that for the samples that had undergone a permanent axial
50 0
2
4
----+c-k’
6
AXIAL
J
8
SHORTENING
14
I6
(% 1
Fig. 5. Percentage of cracked grains plotted as a function of permanent strain for all of the experiments. The data points plotted as open circles are from samples that were run with a 4 mm thick quartz sand layer, under a confining pressure of 4.7 kbar. The points plotted as solid circles are from samples that were run with a 2 mm quartz sand layer, under a confining pressure of 4.7 kbar. The points plotted as open squares are from samples that were run with a 4 mm quartz sand layer under a confining pressure of 2.0 kbar. See text for a description of the method used to obtain the data.
167
strain of greater than 6.5%, the degree of grain crushing is consistently greater in the samples deformed at a pressure of 4.7 kbar than in the samples deformed at 2 kbar. It is tempting to suggest that the greater the amount of grain crushing, the lower the porosity and the more unstable the deformation. But this is not universally true because with sample 227, for instance, the percentage of cracked grains was only 80 and the sample had undergone one violent stick-slip event. With sample 300, however, most of the deformation was stable even though the percentage of cracked grains was about 95. This does not establish that the gouge in sample 300 was more dense than in sample 227. Compaction in brittle materials occurs by crushing and rearranging of grains so that the small particles become lodged in the spaces between the larger particles. The grain-size distribution of the particles is therefore very important, but we did not attempt to study this factor because many of the particles were extremely small and approached the resolving power of our microscope. The degree of grain cracking was about the same when deformation was stopped either before sudden-slip sample, 230, or just after a violent stickslip event, sample 227. The degree of grain cracking became much greater if deformation is allowed to continue after the violent stick-slip, sample 231. These results suggest that cracking of the grains within the bulk of the gouge zone occurs on the ascending part of the stress-strain curve, but during violent stick-slip the crushing is probably confined to the very narrow but intensely sheared zone between the granite and gouge. Two orientations of intensely sheared material are developed within the gouge, one parallel to the sliding direction at the boundaries between the granite and gouge and the other an oblique shear at an angle of about 20” to the plane of the saw cut (for a schematic diagram of these shear zones see Fig. 7). In addition there are places along the boundaries where the granite has been eroded and crushed minerals from the granite have become mixed with the finely crushed particles of quartz sand. The damage to the granite, however, is only slight compared to the intense shearing that occurs in the sand gouge. A photomicrograph of a thin section with well-developed shear zones is shown in Fig. 6. All of the structures shown in this example are observed in varying degrees of development in all of the samples that we deformed, but differences between the three series of experiments do exist. For example, samples with the same thickness of gouge exhibit differences related to confining pressure. The oblique shear zones developed in the samples deformed at low pressure are less numerous, wider, and more diffuse than those developed in the experiment at high pressure. In samples deformed at the same pressure but with different thicknesses of gouge, the narrower the gouge, the more numerous are the shear zones and they have sharper boundaries with the surrounding material. Figure 7 is a schematic diagram of the development of the structures that we have observed. First the zone of shear at the granite boundary grows and develops a stepped form, with the sharp face of the step facing opposite to
Fig. 6. Photomicrograph of the thin section cut from sample 263 after deformation. The maximum principal stress was applied parallel to the long axis, Details of the fine structure in the gouge can only be observed with a petroiogical microscope, but in this photomicrograph a number of well-developed oblique shears can be identified.
169
12 Fig. 7. Schematic diagram of the development of structure in the gouge. The progress of deformation was from A to B to C.
the direction of movement on the interface. Oblique shears are then developed and spread until they intersect the opposite boundary. Where they meet the boundaries, they spread out and blend gradually into the shear zone parallel to the granite margin. We carried out a few exploratory experiments with 45” saw cuts and found that the oblique shears were still oriented at an angle of 20” to the plane of the saw cut. DISCUSSION
Byerlee (1970) postulated that at high pressure the gouge becomes densely packed together so that when failure occurs in one region, it leads to a large increase in stress on the adjacent regions and the process becomes catastrophic. There is evidence, such as the increased intensity of grain fracturing at the higher pressure, that tends to support this concept, but the evidence is by no means conclusive, and to judge from the complexity of the structures produced in the gouge, the processes involved in its deformation are far more complex than this simple concept suggests. Engelder et al. (1975) suggested that during stable sliding, movement occurs by shearing along bands within the gouge but during sudden slip the intact rock slips along the contact with the gouge layer. Our experimental results are consistent with this idea. Shear bands are also formed during the ascending part of the stress-strain curve at high pressure in which violent stick-slip occurs. In fact, at high pressure the shear bands are more distinct than those developed at low pressure. The structures developed in the gouge in our experiments bear a remarkable resemblance to the “Riedel shears” that are produced during shear of particulate materials at low pressure (Cloos, 1928; Riedel, 1929). One could postulate that the shears produced during our high-pressure experiments are Riedel shears, but the exact structural evolution of Riedel shear systems is
170
not clear (Tchalenko, 1970). It has been suggested (~orgenste~ and Tchalenko, 1967) that the Riedel shears are produced by failure of the material along planes inclined at an angle 0 to the maximum principal stress where 0 Z 45 - (4/Z) and Q, is the angle of internal friction of the material. In our experiments 0 is approximately 10” so that Q,is approximately 70”, which is an unreasonably high value. In addition we found that the angle of the oblique shears to the plane of the saw cut was indepe~ldent of the angle that the saw cut made with the direction of m~imum principal stress. Thus in our experiments the shears are controlled by the boundaries of the gouge and not by the material properties or the direction of the applied stress. One could postulate that slip on the oblique shears would lead to a large stress concentration at the point where the shears meet the granite boundaries and that failure of the gouge at these points could lead to a sudden drop in shear resistance and sudden slip would then occur along the boundary between the intact rock and the gouge layer. If this were true, then the sharp lending edges where the shear bands intersect the boundary would become rounded off, but in our experiments, this did not happen. in addition the oblique shears were also produced at low pressure and the material deformed stably, but at low pressure the shear bands are diffuse and the stress concentrations may not be as large under these conditions. In the early stages of deformation the boundary between the rock and gouge took on a stepped form similar to the structure of the chatter marks that are produced during sliding on fault surfaces (Paterson, 1958). One could postulate that the two structures are produced in much the same way. In our experiments it is possible that during sliding along the boundary between the granite and gouge the material became jammed up in the same way that galling occurs during sliding between surfaces of clean, like metal. When rupture of these jammed areas takes place there is a large drop in shear resistance and the surfaces suddenly slip forward (Rabinowicz, 1965). The oblique shears in the gouge could simply be shear bands produced in response to the large stress concentration in the jammed regions between the gouge and intact rock. If this mechanism is responsible for the instability, it is difficult to explain why the inclined shears commonly have a very regular distribution. In many samples the structure at both ends of a shear band was the same, which is difficult to explain if the shear bands grow in response to a large stress concentration at only one end of the shear band. In conclusion we must admit that at present we do not have a completely satisfactory physical explanation for why the material deforms stably at a low pressure but unstably at high pressure. However, all the evidence does suggest that stable sliding on the oblique shears precedes sudden slip, which is confined to the margin between the shear zone and the intact rock. The structures produced in the gouge layer during deformation bear a remarkable resemblance to the structures developed in shear zones in active &tonic regions (Tchalenko, 1970). If stable deformation always occurs on oblique shears in the natural situation, then it may be possible to determine whether
171
sudden slip between the shear zone and the intact country rock is imminent. It may be possible to do this by installing a dense network of seismic stations around a section of a fault that was considered from other evidence to be a likely site for a major earthquake, either by studying the details of the spatial distribution of the microseismic activity it should be possible to determine whether the deformation in the fault zone was parallel or oblique to the strike of the major fault. A change in the orientation of the principal stress axis in the volume surrounding the hypocenter preceding the main event of the Oroville earthquake in California in 1975 (Mantis and Lindh, 1976) is consistent with our experimental observations that the direction of stable shear preceding sudden slip is inclined at an angle of about 20” to the direction of the main slip.
REFERENCES Byerlee, J.D., 1970. The mechanics of stick-slip. Tectonophysics, 9: 475-486. Byerlee, J.D. and Summers, R., 1976. A note on the effect of fault gouge thickness on fault stability. Int. J. Rock Mech. Min. Sci., Geomech. Abstr., 13: 35-36. Cloos, H., 1928. Experimente zur inneren Tektonik, Zentralbl. Mineral. Geol. Palaeontol., 12: 609-621. Engelder, J.T., Logan, J&I. and Handin, J., 1975. The sliding characteristics of sandstone on quartz fault gouge. Pure Appl. Geophys., 1.13: 69-86. Mantis, C. and Lindh, A., 1976. The Oroville foreshocks and an apparent coseismic change in fault-plane orientation with short-term precursor. Trans. Am. Geophys. Union, 57: 956. Morgenstern, M.R. and Tchalenko, J.S., 1967. Microscopic structures in Kaolin subjected to direct shear. Geotechnique, 17: 309-328. Paterson, M.S., 1958. Experimental deformation and faulting in Wombeyan marble. Geol. Sot. Am. Bull., 69: 465-475. Rabinowicz, E., 1965. Friction and Wear in Materials. Wiley, New York, Riedel, W., 1929. Zur Mechanik geologischer Brucherscheinungen. Zentralbl. Mineral. Geol. Palaeontol., 1929B: 354-368. Tchalenko, J.S., 1970. Similarity between shear zones of different magnitude. Geol. Sot. Am. Bull., 81: 1625-1640.
STRUCTURES DEVELOPED SLIDING AND STICK-SLIP
J. BYERLEE
161 in The Netherlands
IN FAULT GOUGE DURING
STABLE
l, V. MJACHKIN 2, R. SUMMERS r and 0. VOEVODA 2
r U.S. Geological Survey, Menlo Park, Calif. (U.S.A.) 2 Institute of Physics of the Earth, Moscow (U.S.S.R.) (Submitted February 24,1977;
accepted April 28,1977)
ABSTRACT Byerlee, J., Mjachkin, V., Summers, R. and Voevoda, O., 1978. Structures developed in fault gouge during stable sliding and stick-slip. Tectonophysics, 44 : 161-17 1. We carried out a detailed study of the structural changes that occurred in a thin layer of quartz gouge sheared between saw cuts in granite cylinders at pressures of 2 and 4.7 kbar. At low pressure the material deformed stably, but at high pressure deformation was unstable. During deformation shear zones were developed oblique and parallel to the plane of the saw cuts. Our results suggest that shearing oblique to the strike of the fault zone precedes sudden slip, which is confined to the margin between the intact rock and gouge. If this is true in the natural situation, then it may be possible by studying the spatial distribution of the microseismic activity and creep in shear zones to determine whether sudden slip is imminent.
INTRODUCTION
In some active tectonic regions sliding between fault blocks takes place suddenly to produce large earthquakes, whereas in other regions the movement on the fault is nearly continuous and earthquakes are very small or absent. A similar phenomenon occurs in frictional-sliding experiments carried out in the laboratory. For example, it has been found that at low pressure the sliding between rocks takes place smoothly, but at high pressure it occurs by violent stick-slip. We have carried out a number of experiments to discover what physical mechanism is responsible for instability by studying the structural changes that occurred in a thin layer of quartz gouge deformed under confining pressure. The physical mechanism responsible for instability is still not clear to us, but in this paper we present current results in the hope that they will stimulate interest in this important problem.
EXPERIMENTAL
METHOD
Cylindrical samples of Westerly Granite, 2.54 cm in diameter and 6.05 cm long with their ends ground parallel, were cut with a diamond saw at an angle of 30” to the long axis of the cylinders. A uniform layer, either 2 mm or 4 mm thick, of grains of ASTM N”C-109 Ottawa sand was placed between the two halves of the cylinders. The samples were assembled wet so that they could be inserted into a 0.01 cm thick copper tube. The copper tube was used to give the samples a slight mechanical strength so that they could be assembled without disturbing the sand. After assembly the samples were dried in a vacuum oven at a temperature of 120°C. The samples were then jacketed in a polyurethane sleeve with a wall thickness of 0.032 cm and sealed at the ends to hardened steel plugs. A schematic diagram of the sample assembly is shown in Fig!. 1. The samples were deformed under a constant confining pressure of either 2 kba.r or 4.7 kbar at a constant rate of 10-5/sec. This strain rate refers to the elastic strain rate in the intact rock and not to the strain rate in the shear zones which can vary from 10-5/sec during stick and 103fsec or higher during slip of the stick-slip cycle. During the experiments we recorded the change in axial force as a function of the change in length of the sample with an xy-recorder. The axial force was converted to axial stress 0 by dividing it by the initial cross-sectional area, and the change in length was converted to axial strain E by dividing it by the initial length of the sample. The shear displacement was calculated by resolving the change in axial length on the inclined plane of the saw cut. After each experiment the specimens were impregnated with epoxy resin, and thin sections were then cut from the samples parallel to the long axis and normal to the plane of the fault gouge and examined with a petrologic microscope.
WIRE
CLAMP
SAND
GOUGE
POLYURETHANE COPPER
JACKEl
GRANITE
STEEL
PLUG
Fig, 1. Schematic diagram of the sample assembly.
163 EXPERIMENTAL
RESULTS
In order to study progressive structural changes in the gouge as it was subjected to increasing strains, samples were deformed by increments (5%, lo%, and so on), then impregnated with epoxy, and cut into thin sections. We would repeat the process deforming a new sample to say 10% strain. In this way we could study the progressive changes in the structure of the gouge as it was deformed to larger and larger strains. In samples strained at a confining pressure of 4.7 kbar deformation was accompanied by violent stick-slip, and at a pressure of 2 kbar most of the deformation was stable. To find out what effect thickness of fault gouge had on the structures developed during stick-slip, we did two series of experiments at a pressure of 4.7 kbar, one in which the thickness of the gouge was 4 mm and the other 2 mm. Typical stress-strain curves for the three series of experiments are shown in Figs. 2, 3 and 4. The dashed lines on the figures represent sudden stick-slip sliding. On each figure the numbered circles are drawn to show the point on the stress-strain curves at which each experiment was terminated. Particularly at a pressure of 4.7 kbar, a large part of the initial loading of the samples was linear with strain, but the slopes were different for different thicknesses and also for successive cycles of stick-slip.
SHEAR I
’
I
DISPLACEMENT 4
2
0
’
I
AXIAL
6 ’
I
(MM) 10
6 ’
SHORTENING
I
’
I
12 ’
I
( %)
Fig. 2. Differential stress plotted as a function of axial strain and shear displacement for samples containing 2 mm of sand gouge and deformed at a confining pressure of 4.75 kbar. The numbered circles are the points of permanent strain in the samples at the termination of each experiment.
SHEAR 3
2
4 5-“---_-
DISPLACEMEN? (MM: i. i :; 8 .-._/ _~. ...‘..-~~--_..--_‘.._.’
;5
:
AXIAL
SHORTENING
i % 1
Fig. 3. Differential stress plotted as a function of axial strain and shear displacement for samples containing 4 mm of gouge and deformed at a confining pressure of 4.7 kbar. The numbered circles are the points of permanent strain in the samples at the termination of each experiment.
DISPLACEMENT
SHEAR 0 i
2 j
’
(MM
1
4 ’
i
Fig. 4. Differential stress plotted as a function of axial strain and shear displacement for samples containing 4 mm of gouge and deformed at a confining pressure of 2 kbar. The numbered circles are the points of termination of each experiment.
To find out how much of this was due to hnear compaction of the gouge and how much due to elastic deformation of the granite, we deformed intact samples of granite at pressures of 4.7 kbar and 2 kbar, The stress-strain curves for these experiments are numbered 302 on Figs, 2, 3 and 4. The slope of these curves does not exactly represent the elastic modulus of the rock because both the axial force and the axial displacement were measured outside the pressure vessel so that some of the elastic strain is contributed by the piston, end plugs, and pressure vessel. However, a comparison of these curves with the curves from the cut samples gives a true measure of the deformation of the gouge. For samples containing saw cuts the stress increases almost linearly with strain (Figs. 2, 3 and 4), but the slope Kr, for loading the saw cuts is much less than the slope Ku for loading of the intact samples. The deformation is not elastic because the slope K,, for unloading is much greater than Ii,,. The unloading KU, is ~pprox~mately equal to I&> the unloading modulus for the intact samples. At a given stress the axial displacement of the ends of the sample is the sum of the axial displacement caused by strain in the intact rock plus the axial displacement caused by the strain in the fault gouge, so that if KU, equals KUi, then the fault gouge must have the same modulus as the dense rock. Figures 2 and 3 also show that after the first stick-slip event K,, is constant and is approximately equal to Kn. Thus the gouge reaches its maximum density during the first stick-slip cycle, and for ah subsequent cycles its elastic modulus is approximately equal to the modulus of the intact sample. Our me~urements of the elastic moduli are not very accurate, and because the fault zone is narrow, the slope of the stress-strain curves is not strongly dependent on the porosity of the gouge. Thus even though our results would suggest that the density of the gouge during stable sliding and stick-slip is the same, small differences in porosity could exist. The average stress drop for the stick-slip events in the experiments with a gouge thickness of 4 mm and deformed at a pressure of 4.7 kbar was 7.5 kbar, and for the samples deformed at the same pressure but containing 2 mm of gouge it was 8.0 kbar. Although the difference in the average stress drop between the two series of experiments is small, these results are similar to the result reported by Byerlee and Summers (1976) for sliding with crushed granite between saw cuts, that showed that the wider the gouge the smaller the stress drop. There is a small amount of nonlinear strain before each violent stick-slip event (Figs. 2 and 3). If the material between the sliding surfaces becomes fully dense during the fist stick-slip cycle, then the nonelastic strain preceding all subsequent cycles must be due to permanent deformation of shearing within the gouge layer. We measured the nonlinear deformation parallel to the plane of the saw cuts for all but the first cycle for all the experiments carried out at a pressure of 4.7 kbar. For the samples with 4 mm of gouge, the average distance of stable slip was 0.39 mm, and for samples with 2 mm of gouge, it was 0.52 mm, Byerfee and Summers (1976) suggested that the
166
distance of stable sliding preceding sudden slip may increase with an increase in thickness of the gouge, but the results from the experiments reported here do not support this idea. Study of the thin sections showed that sand grains in the gouge were crushed and that the amount of crushing depended on the amount of strain, the confining pressure, and the thickness of the sand layer. The cracking occurred at grain contacts, and the cracks did not appear to have a preferred o~en~tion. There was, at least in the early stages of defo~ation, only a minor amount of displacement on the newly formed cracks so that the outlines of the well-rounded original grains could be identified. To describe the degree of cracking of the original grams, we define the factor Q by the equation:
Q
=!=nn,x
100%
where n is the total number of both cracked and untracked grams in which the outlines of the original grains could be identified and a0 is the number of untracked grains. The results are shown in Fig. 5 which plots Q for all the experiments as a function of the permanent strain. A number of the thin sections were studied independently by two investigators and the difference between their calculations of Q is shown by the error bar in Fig. 5. The results show that for the samples that had undergone a permanent axial
50 0
2
4
----+c-k’
6
AXIAL
J
8
SHORTENING
14
I6
(% 1
Fig. 5. Percentage of cracked grains plotted as a function of permanent strain for all of the experiments. The data points plotted as open circles are from samples that were run with a 4 mm thick quartz sand layer, under a confining pressure of 4.7 kbar. The points plotted as solid circles are from samples that were run with a 2 mm quartz sand layer, under a confining pressure of 4.7 kbar. The points plotted as open squares are from samples that were run with a 4 mm quartz sand layer under a confining pressure of 2.0 kbar. See text for a description of the method used to obtain the data.
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strain of greater than 6.5%, the degree of grain crushing is consistently greater in the samples deformed at a pressure of 4.7 kbar than in the samples deformed at 2 kbar. It is tempting to suggest that the greater the amount of grain crushing, the lower the porosity and the more unstable the deformation. But this is not universally true because with sample 227, for instance, the percentage of cracked grains was only 80 and the sample had undergone one violent stick-slip event. With sample 300, however, most of the deformation was stable even though the percentage of cracked grains was about 95. This does not establish that the gouge in sample 300 was more dense than in sample 227. Compaction in brittle materials occurs by crushing and rearranging of grains so that the small particles become lodged in the spaces between the larger particles. The grain-size distribution of the particles is therefore very important, but we did not attempt to study this factor because many of the particles were extremely small and approached the resolving power of our microscope. The degree of grain cracking was about the same when deformation was stopped either before sudden-slip sample, 230, or just after a violent stickslip event, sample 227. The degree of grain cracking became much greater if deformation is allowed to continue after the violent stick-slip, sample 231. These results suggest that cracking of the grains within the bulk of the gouge zone occurs on the ascending part of the stress-strain curve, but during violent stick-slip the crushing is probably confined to the very narrow but intensely sheared zone between the granite and gouge. Two orientations of intensely sheared material are developed within the gouge, one parallel to the sliding direction at the boundaries between the granite and gouge and the other an oblique shear at an angle of about 20” to the plane of the saw cut (for a schematic diagram of these shear zones see Fig. 7). In addition there are places along the boundaries where the granite has been eroded and crushed minerals from the granite have become mixed with the finely crushed particles of quartz sand. The damage to the granite, however, is only slight compared to the intense shearing that occurs in the sand gouge. A photomicrograph of a thin section with well-developed shear zones is shown in Fig. 6. All of the structures shown in this example are observed in varying degrees of development in all of the samples that we deformed, but differences between the three series of experiments do exist. For example, samples with the same thickness of gouge exhibit differences related to confining pressure. The oblique shear zones developed in the samples deformed at low pressure are less numerous, wider, and more diffuse than those developed in the experiment at high pressure. In samples deformed at the same pressure but with different thicknesses of gouge, the narrower the gouge, the more numerous are the shear zones and they have sharper boundaries with the surrounding material. Figure 7 is a schematic diagram of the development of the structures that we have observed. First the zone of shear at the granite boundary grows and develops a stepped form, with the sharp face of the step facing opposite to
Fig. 6. Photomicrograph of the thin section cut from sample 263 after deformation. The maximum principal stress was applied parallel to the long axis, Details of the fine structure in the gouge can only be observed with a petroiogical microscope, but in this photomicrograph a number of well-developed oblique shears can be identified.
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12 Fig. 7. Schematic diagram of the development of structure in the gouge. The progress of deformation was from A to B to C.
the direction of movement on the interface. Oblique shears are then developed and spread until they intersect the opposite boundary. Where they meet the boundaries, they spread out and blend gradually into the shear zone parallel to the granite margin. We carried out a few exploratory experiments with 45” saw cuts and found that the oblique shears were still oriented at an angle of 20” to the plane of the saw cut. DISCUSSION
Byerlee (1970) postulated that at high pressure the gouge becomes densely packed together so that when failure occurs in one region, it leads to a large increase in stress on the adjacent regions and the process becomes catastrophic. There is evidence, such as the increased intensity of grain fracturing at the higher pressure, that tends to support this concept, but the evidence is by no means conclusive, and to judge from the complexity of the structures produced in the gouge, the processes involved in its deformation are far more complex than this simple concept suggests. Engelder et al. (1975) suggested that during stable sliding, movement occurs by shearing along bands within the gouge but during sudden slip the intact rock slips along the contact with the gouge layer. Our experimental results are consistent with this idea. Shear bands are also formed during the ascending part of the stress-strain curve at high pressure in which violent stick-slip occurs. In fact, at high pressure the shear bands are more distinct than those developed at low pressure. The structures developed in the gouge in our experiments bear a remarkable resemblance to the “Riedel shears” that are produced during shear of particulate materials at low pressure (Cloos, 1928; Riedel, 1929). One could postulate that the shears produced during our high-pressure experiments are Riedel shears, but the exact structural evolution of Riedel shear systems is
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not clear (Tchalenko, 1970). It has been suggested (~orgenste~ and Tchalenko, 1967) that the Riedel shears are produced by failure of the material along planes inclined at an angle 0 to the maximum principal stress where 0 Z 45 - (4/Z) and Q, is the angle of internal friction of the material. In our experiments 0 is approximately 10” so that Q,is approximately 70”, which is an unreasonably high value. In addition we found that the angle of the oblique shears to the plane of the saw cut was indepe~ldent of the angle that the saw cut made with the direction of m~imum principal stress. Thus in our experiments the shears are controlled by the boundaries of the gouge and not by the material properties or the direction of the applied stress. One could postulate that slip on the oblique shears would lead to a large stress concentration at the point where the shears meet the granite boundaries and that failure of the gouge at these points could lead to a sudden drop in shear resistance and sudden slip would then occur along the boundary between the intact rock and the gouge layer. If this were true, then the sharp lending edges where the shear bands intersect the boundary would become rounded off, but in our experiments, this did not happen. in addition the oblique shears were also produced at low pressure and the material deformed stably, but at low pressure the shear bands are diffuse and the stress concentrations may not be as large under these conditions. In the early stages of deformation the boundary between the rock and gouge took on a stepped form similar to the structure of the chatter marks that are produced during sliding on fault surfaces (Paterson, 1958). One could postulate that the two structures are produced in much the same way. In our experiments it is possible that during sliding along the boundary between the granite and gouge the material became jammed up in the same way that galling occurs during sliding between surfaces of clean, like metal. When rupture of these jammed areas takes place there is a large drop in shear resistance and the surfaces suddenly slip forward (Rabinowicz, 1965). The oblique shears in the gouge could simply be shear bands produced in response to the large stress concentration in the jammed regions between the gouge and intact rock. If this mechanism is responsible for the instability, it is difficult to explain why the inclined shears commonly have a very regular distribution. In many samples the structure at both ends of a shear band was the same, which is difficult to explain if the shear bands grow in response to a large stress concentration at only one end of the shear band. In conclusion we must admit that at present we do not have a completely satisfactory physical explanation for why the material deforms stably at a low pressure but unstably at high pressure. However, all the evidence does suggest that stable sliding on the oblique shears precedes sudden slip, which is confined to the margin between the shear zone and the intact rock. The structures produced in the gouge layer during deformation bear a remarkable resemblance to the structures developed in shear zones in active &tonic regions (Tchalenko, 1970). If stable deformation always occurs on oblique shears in the natural situation, then it may be possible to determine whether
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sudden slip between the shear zone and the intact country rock is imminent. It may be possible to do this by installing a dense network of seismic stations around a section of a fault that was considered from other evidence to be a likely site for a major earthquake, either by studying the details of the spatial distribution of the microseismic activity it should be possible to determine whether the deformation in the fault zone was parallel or oblique to the strike of the major fault. A change in the orientation of the principal stress axis in the volume surrounding the hypocenter preceding the main event of the Oroville earthquake in California in 1975 (Mantis and Lindh, 1976) is consistent with our experimental observations that the direction of stable shear preceding sudden slip is inclined at an angle of about 20” to the direction of the main slip.
REFERENCES Byerlee, J.D., 1970. The mechanics of stick-slip. Tectonophysics, 9: 475-486. Byerlee, J.D. and Summers, R., 1976. A note on the effect of fault gouge thickness on fault stability. Int. J. Rock Mech. Min. Sci., Geomech. Abstr., 13: 35-36. Cloos, H., 1928. Experimente zur inneren Tektonik, Zentralbl. Mineral. Geol. Palaeontol., 12: 609-621. Engelder, J.T., Logan, J&I. and Handin, J., 1975. The sliding characteristics of sandstone on quartz fault gouge. Pure Appl. Geophys., 1.13: 69-86. Mantis, C. and Lindh, A., 1976. The Oroville foreshocks and an apparent coseismic change in fault-plane orientation with short-term precursor. Trans. Am. Geophys. Union, 57: 956. Morgenstern, M.R. and Tchalenko, J.S., 1967. Microscopic structures in Kaolin subjected to direct shear. Geotechnique, 17: 309-328. Paterson, M.S., 1958. Experimental deformation and faulting in Wombeyan marble. Geol. Sot. Am. Bull., 69: 465-475. Rabinowicz, E., 1965. Friction and Wear in Materials. Wiley, New York, Riedel, W., 1929. Zur Mechanik geologischer Brucherscheinungen. Zentralbl. Mineral. Geol. Palaeontol., 1929B: 354-368. Tchalenko, J.S., 1970. Similarity between shear zones of different magnitude. Geol. Sot. Am. Bull., 81: 1625-1640.