Geology topics
J.D. Byerlee
Publications and source records attributed to J.D. Byerlee.
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.
The fracture strength and frictional strength of Weber Sandstone
The fracture strength and frictional strength of Weber Sandstone have been measured as a function of confining pressure and pore pressure. Both the fracture strength and the frictional strength obey the law of effective stress, that is, the strength is determined not by the confining pressure alone but by the difference between the confining pressure and the pore pressure. The fracture strength of the rock varies by as much as 20 per cent depending on the cement between the grains, but the frictional strength is independent of lithology. Over the range 0 < σ n < 2 kb "> 0 < σn< 2kb , the frictional strength follows the relationship τ=0·85 σ n where τ "> τ=0·85 σnwhereτ is the shear stress and σ n "> σn is the normal stress, and for σ n >2 kb , τ=0·5 + 0·6σ n "> σn>2kb, τ=0·5 + 0·6σn . This relationship also holds for other rocks such as gabbro, dunite, serpentinite, granite and limestone.
The mechanics of stick-slip
Physical mechanisms that have been proposed to explain the occurrence of stick-slip motion during frictional sliding have been examined in the light of results obtained from experiments with rocks and brittle minerals. An instability caused by sudden brittle fracture of locked regions on surfaces in contact is the most likely explanation for stick-slip during dry frictional sliding of brittle rocks at room temperature. Areas requiring further study and the uncertainties in applying the results of laboratory experiments to earthquake studies are emphasized.
Static and kinetic friction of granite at high normal stress
Frictional sliding on ground surfaces of granite, angle of sliding planes 30° and 45°, was investigated as a function of confining pressure. Over the normal stress range of 2–12 kb, the static frictional shear stress τ s follows the relationship τ s = 0·5 + 0· σ n and the kinetic frictional shear stress τ k was calculated to be τ k = 0·25 + 0·47 σ n .
California earthquakes: Why only shallow focus?
Frictional sliding on sawcuts and faults in laboratory samples of granite and gabbro is markedly temperature-dependent. At pressures from 1 to 5 kilobars, stick-slip gave way to stable sliding as temperature was increased from 200 to 500 degrees Celsius. Increased temperature with depth could thus cause the abrupt disappearance of earthquakes noted at shallow depths in California.
High-pressure mechanical instability in rocks
At a confining pressure of a few kilobars, deformation of many sedimentary rocks, altered mafic rocks, porous volcanic rocks, and sand is ductile, in that instabilities leading to audible elastic shocks are absent. At pressures of 7 to 10 kilobars, however, unstable faulting and stick-slip in certain of these rocks was observed. This high pressure-low temperature instability might be responsible for earthquakes in deeply buried sedimentary or volcanic sequences.
Theory of friction based on brittle fracture
A theory of friction is presented that may be more applicable to geologic materials than the classic Bowden and Tabor theory. In the model, surfaces touch at the peaks of asperities and sliding occurs when the asperities fail by brittle fracture. The coefficient of friction, μ, was calculated from the strength of asperities of certain ideal shapes; for cone‐shaped asperities, μ is about 0.1 and for wedge‐shaped asperities, μ is about 0.15. For actual situations which seem close to the ideal model, observed μ was found to be very close to 0.1, even for materials such as quartz and calcite with widely differing strengths. If surface forces are present, the theory predicts that μ should decrease with load and that it should be higher in a vacuum than in air. In the presence of a fluid film between sliding surfaces, μ should depend on the area of the surfaces in contact. Both effects are observed. The character of wear particles produced during sliding and the way in which μ depends on normal load, roughness, and environment lend further support to the model of friction presented here.