Search USGS⌕ Search

Geology topics

R. Summers

Publications and source records attributed to R. Summers.

15 recordsLinked to original sources
USGS1997

Strengths of serpentinite gouges at elevated temperatures

Serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones. To test these hypotheses, we have measured the strength of chrysotile-, lizardite-, and antigorite-rich serpentinite gouges under hydrothermal conditions, with emphasis on chrysotile, which has thus far received little attention. At 25°C, the coefficient of friction, μ, of chrysotile gouge is roughly 0.2, whereas the lizardite- and antigorite-rich gouges are at least twice as strong. The very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content. When the adsorbed water is removed, chrysotile is as strong as pure antigorite gouge at room temperature. Heating to ∼200°C causes the frictional strengths of all three gouges to increase. Limited data suggest that different polytypes of a given serpentine mineral have similar strengths; thus deformation-induced changes in polytype should not affect fault strength. At 25°C, the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities, consistent with previous studies. At temperatures up to ∼200°C, however, chrysotile strength is essentially independent of velocity at low velocities. Overall, chrysotile has a restricted range of velocity-strengthening behavior that migrates to higher velocities with increasing temperature. Less information on velocity dependence is available for the lizardite and antigorite gouges, but their behavior is consistent with that outlined for chrysotile. The marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth. The velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip, but in the presence of a hydrostatic fluid pressure gradient, all varieties of serpentine are too strong to explain the apparent weakness of faults such as the San Andreas.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Frictional strength and velocity-dependence of serpentine gouges under hydrothermal conditions and their seismogeological implications

The velocity - dependence of frictional strength of serpentine gouges has been measured at constant normal stress of 110 MPa, pore pressure of 10 MPa, temperature 25, 100 and 200??C , and at sliding rate ranging from 0. 001 to 10??m/s. At 25??C, the coefficient of friction of chrysotile gouge is very low (?????0. 2-0. 25), while lizardite and antigorite gouge are much stronger, with ?????0. 39 and 0. 45, respectively. The frictional strengths of chrysotile and antigorite gouges change little with a temperature increase to 200??C, whereas the strength of lizardite gouge increases substantially with increasing temperature. At 25??C, all three gouges show a transition from weak velocity weakening at high slip rates to velocity strengthening at low slip rates. With increasing temperature, the velocity dependence of each gouge shifts towards more positive values, especially at high slip rates. Based on this study and previous results, we suggest that the presence of serpentine in the fault zone may contribute to the occurrence of stable creep rather than earthquakes, but this effect may be limited to shallow depths. Although chrysotile is one of the weakest rock - forming minerals, it is still too strong to explain the weakness of the San Andreas fault deduced from heat flow data.

Dizhen Dizhi↗
USGS1996

Strength of chrysotile-serpentinite gouge under hydrothermal conditions: Can it explain a weak San Andreas fault?

Chrysotile-bearing serpentinite is a constituent of the San Andreas fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction (μ ≈ 0.2), raising the possibility that under hydrothermal conditions μ might be reduced sufficiently (to ≤0.1) to explain the apparent weakness of the fault. To test this hypothesis, we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 °C and axial-shortening velocities as low as 0.001 μm/s. As temperature increases to ≈ 100 °C, the strength of the chrysotile gouge decreases slightly at low velocities, but at temperatures ≥200 °C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas fault. However, chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures.

Geology↗
USGS1989

Sliding behavior and deformation textures of heated illite gouge

The run products of a series of triaxial friction experiments on an illite-rich gouge have been examined petrographically to study the relationship between textural development and sliding mode. The samples show a complete range of textures, from ones in which the entire gouge layer is deformed to ones in which the deformation is concentrated along narrow subsidiary shears and the rest of the gouge layer is massive. The samples with a pervasively developed deformation fabric slide stably, whereas the samples containing shear bands show stick-slip motion if the intersection angles between boundary-parallel and cross-cutting (Riedel) shears are also relatively high. These textural differences suggest that localization of shear combined with higher-angle Riedel shears are somehow involved in stick-slip motion. The orientation of Riedel-type shears in natural fault zones may also have potential as a paleoseismological tool.

Journal of Structural Geology↗
USGS1986

The effects of sliding velocity on the frictional and physical properties of heated fault gouge

The frictional properties of a crushed granite gouge and of gouges rich in montmorillonite, illite, and serpentine minerals have been investigated at temperatures as high as 600??C, confining pressures as high as 2.5 kbar, a pore pressure of 30 bar, and sliding velocities of 4.8 and 4.8??10-2 ??m/sec. The gouges showed nearly identical strength behaviors at the two sliding velocities; all four gouges, however, showed a greater tendency to stick-slip movement and somewhat higher stress drops in the experiments at 4.8??10-2 ??m/sec. Varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges. The principal mineralogical difference was that at 400??C and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4.8??10-2 ??m/sec but not in those at 4.8 ??m/sec. The textures developed in the gouge layers were in part functions of the gouge type and the temperature, but changes in the sliding velocity affected, among other features, the degree of mineral deformation and the orientation of some fractures. ?? 1986 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1986

Effects of temperature and sliding rate on frictional strength of granite

Layers of artificial granite gouge have been deformed on saw-cut granite surfaces inclined 30?? to the sample axes. Samples were deformed at a constant confining pressure of 250 MPa and temperatures of 22 to 845??C. The velocity dependence of the steady-state coefficient of friction (??ss) was determined by comparing sliding strengths at different sliding rates. The results of these measurements are consistent with those reported by Solberg and Byerlee (1984) at room temperature and Stesky (1975) between 300 and 400??C. Stesky found that the slip-rate dependence of (??ss) increased above 400??C. In the present study, however, the velocity dependence of (??ss) was nearly independent of temperature. ?? 1986 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1982

Laboratory measurements of reservoir rock from the Geysers geothermal field, California

Rock samples taken from two outcrops, as well as rare cores from three well bores at the Geysers geothermal field, California, were tested at temperatures and pressures similar to those found in the geothermal field. Both intact and 30?? sawcut cylinders were deformed at confining pressures of 200-1000 bars, pore pressure of 30 bars and temperatures of 150?? and 240??C. Thin-section and X-ray analysis revealed that some borehole samples had undergone extensive alteration and recrystallization. Constant strain rate tests of 10-4 and 10-6 per sec gave a coefficient of friction of 0.68. Due to the highly fractured nature of the rocks taken from the production zone, intact samples were rarely 50% stronger than the frictional strength. This result suggests that the Geysers reservoir can support shear stresses only as large as its frictional shear strength. Velocity of p-waves (6.2 km/sec) was measured on one sample. Acoustic emission and sliding on a sawcut were related to changes in pore pressure. b-values computed from the acoustic emissions generated during fluid injection were typically about 0.55. An unusually high b-value (approximately 1.3) observed during sudden injection of water into the sample may have been related to thermal cracking. ?? 1982.

International Journal of Rock Mechanics and Mining↗
USGS1978

Structures developed in fault gouge during stable sliding and stick-slip

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 spacial distribution of the microseismic activity and creep in shear zones to determine whether sudden slip is imminent.

Tectonophysics↗
USGS1977

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

The frictional properties of fault gouge have been studied at confining pressures to 6 kbars. If 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, montmorillonite 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. ?? 1977.

International Journal of Rock Mechanics and Mining↗
USGS1977

Summary of results of frictional sliding studies, at confining pressures up to 6.98 kb, in selected rock materials

This report is a collection of stress-strain charts which were produced by deforming selected simuiated fault gouge materials. Several sets of samples consisted of intact cylinders, 1.000 inch in diameter and 2.500 inches long. The majority of the samples consisted of thin layers of the selected sample material, inserted within a diagonal sawcut in a 1.000-inch by 2.500-inch Westerly Granite cylinder. Two sorts of inserts were used. The first consisted of thin wafers cut from 1.000-inch-diameter cores of the rock being tested. The other consisted of thin layers of crushed material packed onto the sawcut surface. In several groups of tests using various thicknesses (0.010 inch to 0.160 inch) of a given type material there were variations in the stress level and/or stability of sliding as a function of the fault zone width. Because of this we elected to use a standard 0.025-inch width fault zone to compare the frictional properties of many of the different types of rock materials. This 0.025-inch thickness was chosen partially because this thickness of crushed granite behaves approximately the same as a fractured sample of initially intact granite, and also because this is near the lower limit at which we could cut intact wafers for those samples that were prepared from thin slices of rock. One series of tests was done with saw cut granite cylinders without fault gouge inserts. All of these tests were done in a hydraulically operated triaxial testing machine. The confining pressure (δ 1 , least principal stress) was applied by pumping petroleum ether into a pressure vessel. The differential stress (δ 3 -δ 1 ) was applied by a hydraulically operated ram that could be advanced into the pressure vessel at any of several strain rates (10- 4 sec- 1 , 10- 5 sec- 1 , 10- 6 sec- 1, 10- 7 sec- 1 , or 10- 8 sec- 1 ). All samples were jacketed in polyurethane tubing to exclude the confining pressure medium from the samples. The majority of the samples, with the exception of some of the initially intact rocks, also had thin copper jackets. These served to hold the saw cut parts of the granite sample holders in alignment while the samples were handled and pushed into the polyurethane jackets.

Open-File Report↗
USGS1976

A note on the effect of fault gouge thickness on fault stability

At low confining pressure, sliding on saw cuts in granite is stable but at high pressure it is unstable. The pressure at which the transition takes place increases if the thickness of the crushed material between the sliding surfaces is increased. This experimental result suggests that on natural faults the stability of sliding may be affected by the width of the fault zone. ?? 1976.

International Journal of Rock Mechanics and Mining↗
USGS1975

Stable sliding preceding stick-slip on fault surfaces in granite at high pressure

The distance of stable sliding before sudden slip on fault surfaces in granite decreases rapidly as the confining pressure is increased. At a pressure of 6 kb the amount of stable creep is very small or absent. Two orders of magnitude change in strain rate has no effect on the distance of stable sliding. Our results suggest that in the earth, fault creep should predominate in the shallow crust but in the deep crustal layer most of the stresses are probably relieved by sudden earthquake type of motion. Below the crust high temperature would promote stable-slip so in this region creep would once more predominate.

Pure and Applied Geophysics↗