1,721,147 research outputs found
Fault zone strength and failure criteria
No full textThis paper discusses Coulomb failure criteria for brittle deformation of intact rock and fault gouge. Data are presented from laboratory experiments designed to identify the critical gouge layer thickness required to effect a transition from the standard Coulomb criterion to a modified failure law (referred to as Coulomb plasticity) appropriate for simple shear of a gouge layer. Experiments were carried out using tension fractures and quartz powder to simulate granular fault gouge. Fractures sheared without gouge obey the standard Coulomb law. A 0.6mm‐thick gouge layer was required to effect the transition to Coulomb plasticity. I test and reject the hypothesis that fault zone strength and apparent coefficient of internal friction can be predicted from fracture of intact rock simply by accounting for differences in the failure laws and without considering variations in the Coulomb parameters. The data presented indicate that the stress state required for Coulomb plasticity is not developed within very thin gouge layers. This work implies that brittle fault zones have lower friction than predictions based on the strength of intact rock. However, the magnitude of this weakening effect is small (for example, a coefficient of sliding friction of 0.75 would be reduced to 0.6) and thus it is not an independent explanation of the apparent weakness of mature faults. Copyright 1995 by the American Geophysical Union
An experimental study of permeability and fluid chemistry in an artificially jointed marble
No full textTo examine the amount of rock dissolution accompanying changes in joint permeability, deionized water was forced through axially split cylindrical samples of Vermont marble, subjected to a confining pressure of 60 MPa. For freshly polished surfaces, permeability decreased and fluid chemical concentrations increased during the first 50 to 100 hours of fluid flow. For the water-etched surfaces, permeability was not time dependent, and a steady state chemical composition was reached after 20 to 40 hours. A calculation of the steady state chemical concentrations for coexisting calcite and dolomite in deionized water, at a fluid pressure of 10 MPa and a confining pressure of 60 MPa, shows that the steady state values reached in our experiments were supersaturated. -from Author
The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle
No full textThe seismic cycle requires that faults strengthen (heal) between earthquakes, and the rate of this healing process plays a key role in determining earthquake stress drop, rupture characteristics and seismic scaling relations. Frictional healing (as evidenced by increasing static friction during quasi-stationary contact between two surfaces is considered the mechanism most likely to be responsible for fault strengthening. Previous studies, however, have shown a large discrepancy between laboratory and seismic (field) estimates of the healing rate; in the laboratory, rock friction changes by only a few per cent per order-of-magnitude change in slip rate, whereas seismic stress drop increases by a factor of 2 to 5 per order- of-magnitude increase in earthquake recurrence interval. But in such comparisons, it is assumed that healing and static friction are independent of loading rate. Here, I summarize laboratory measurements showing that static friction and healing vary with loading rate and time, as expected from friction theory. Applying these results to seismic faulting and accounting for differences in laboratory, seismic and tectonic slip rates, I demonstrate that post-seismic healing is expected to be retarded for a period of several hundred days following an earthquake, in agreement with recent findings from repeating earthquakes
Effects of normal stress perturbations on the frictional properties of simulated faults
No full textWe report on laboratory experiments to investigate the fictional response of creeping faults to sudden changes in normal stress. Experiments were conducted on layers of quartz powder, bare surfaces of Westerly granite, and layers of a 50/50 mixture of quartz powder and smectite clay powder. The tests were carried out at room temperature and controlled humidity using a servo-controlled double-direct shear configuration. Normal stress perturbations, corresponding to loading and unloading oftectonic fault zones, were applied during steady sliding at constant loading rate from 3 to 1000 |mm/s (shear strain rates of 1.5 × 10∼3 to 0.5 s-1). Sudden changes in normal stress resulted in a linear elastic response of shear stress followed by a transient evolution of friction over a characteristic displacement. The transient, inelastic response is quantified as α = (δτασ)/ln(σ/σ0), where δτα is the transient change in shear stress following a step change from initial normal stress σ0 to final normal stress σ. We find that α is independent of sliding velocity and varies with ambient relative humidity and shear loading history. For unloading, we document a transition from stable to unstable behavior as a function of net slip in the range 3 to 30 mm (shear strains of 1.5 to 15). Increased humidity led to higher values of α for pure quartz gouge, but smaller α for the quartz-clay gouge. The effects ofshear displacement and humidity are discussed in the context ofparticle characteristics and gouge fabric development. The extended rate- and state-dependent friction laws, using one state variable and the Ruina evolution law with normal stress variation, describe our observations. Copyright 2005 by the American Geophysical Union
Friction of sheared granular layers: Role of particle dimensionality, surface roughness, and material properties
No full textWe report on laboratory experiments designed to investigate three fundamental deformation mechanisms for frictional shear of granular fault gouge: sliding, rolling, and dilation. Mechanisms were isolated by shearing layers composed of rods in geometric configurations that resulted in one-dimensional, two-dimensional, and rolling-only particle interactions. Results of digital video are presented with measurements of friction and strain to illuminate the distribution of shear and the relationship between particle motions and friction. The double-direct-shear configuration was used with boundary conditions of constant layer normal stress (1 MPa) and controlled shear loading rate (10 μm/s) with initial layer thickness of 6 mm. Layers were sheared in a servo-hydraulic testing machine at room temperature (22°C) and relative humidity (5 to 10%). Three materials were studied: alloy 260 brass, dried semolina pasta, and hardwood dowels, with particle diameters of 1.59 mm, 1.86 mm, and 2.06 mm, respectively. Pasta layers had mean sliding friction coefficients of 0.24, 0.11, and 0.02 in 2-D, 1-D, and rolling configurations, respectively. Layers of brass rods had average friction coefficients of 0.23, 0.15, and 0.01, respectively, in 2-D, 1-D, and rolling configurations; and the wood samples exhibited friction values of 0.18, 0.19, and 0.09, respectively. Evolution of strength during shear correlated strongly with the displacement derivative of layer thickness. SEM images document the role of surface finish on frictional properties. Rapid reorientations of particles correspond to stick-slip stress drops and may be related to the collapse and reformation of granular force chains. We find a systematic relationship between the strength of granular layers and (1) the surface roughness of particles and (2) the number of particle contact dimensions. Our data provide important insights on the mechanics of granular fault gouge and constraints on the fundamental parameters used in numerical models of tectonic faulting. Copyright 2007 by the American Geophysical Union
Transformation shear instability and the seismogenic zone for deep earthquakes
No full textWe use a numerical model for olivine-spinel transformation to study deep earthquake nucleation and to delineate the seismogenic region within a subducting slab. The model includes laboratory-derived flow laws, latent heat release, and phase transformation kinetics. We calculate deformation, transformation state, grain growth, and rheology for several paths within a subducting slab. Strain rate perturbations are imposed to define the necessary conditions for instability. Strain rate perturbations decay for ξ < a critical value ξc, and thus the coldest, interior portion of the metastable wedge deforms stably. For ξ≥ξc, strain rate perturbations grow, shear strength decreases with strain, and the system is potentially unstable. The instability condition is mapped to delineate the seismogenic zone within a subducting slab. The model seismogenic zone is bounded by ξc, and, at larger percent transformations, by coarsening of spinel grains and saturation of the transformation weakening effect. The model predicts a narrow seismogenic region along the outer edges of the metastable wedge and thus a "double seismic" zone, consistent with some seismic observations. Several simplifying assumptions are required due to lack of thermo-kinetic data and incomplete knowledge of constituent mineralogy and rheology. However, the model provides a quantitative definition of the instability condition and a framework for testing the hypothesis of transformation-induced instability. Copyright 1997 by the American Geophysical Union
Friction of simulated fault gouge for a wide range of velocities and normal stresses
No full textDuring earthquake rupture, faults slip at velocities of cm/s to m/s. Fault friction at these velocities strongly influences dynamic rupture but is at present poorly constrained. We study friction of simulated fault gouge as a function of normal stress (a = 25 to 70 MPa) and load point velocity (V= 0.001 to 10 mm/s). Layers of granular quartz (3 mm thick) are sheared between rough surfaces in a direct shear apparatus at ambient conditions. For a constant a, we impose regular step changes in V throughout 20 mm net slip and monitor the factional response. A striking observation at high velocity is a dramatic reduction in the instantaneous change in frictional strength for a step change in velocity (friction direct effect) with accumulated slip. Gouge layers dilate for a step increase in velocity, and the amount of dilation decreases with slip and is systematically greater at higher velocity. The steady state friction velocity dependence (a-b) evolves from strengthening to weakening with slip but is not significantly influenced by For a, Measurements of dilation imply that an additional mechanism, such as grain rolling, operates at high velocity and that the active shear zone narrows with slip. Data from slow (nm/s) and fast (mm/s) tests indicate a similar displacement dependent textural evolution and comparable comminution rates. Our experiments produce a distinct shear localization fabric and velocity weakening behavior despite limited net displacements and negligible shear heating. Under these conditions we find no evidence for the strong velocity weakening or low friction values predicted by some theoretical models of dynamic rupture. Thus certain mechanisms for strong factional weakening, such as grain rolling, can likely be ruled out for the conditions of our study. Copyright 1999 by the American Geophysical Union
Shear heating in granular layers
No full textHeat-flow measurements imply that the San Andreas Fault operates at lower shear stresses than generally predicted from laboratory friction data. This suggests that a dramatic weakening effect or reduced heat production occur during dynamic slip. Numerical studies intimate that grain rolling or localization may cause weakening or reduced heating, however laboratory evidence for these effects are sparse. We directly measure frictional resistance (μ), shear heating and microstructural evolution with accumulated strain in layers of quartz powder sheared at a range of effective stresses (Gn = 5-70 MPa) and sliding velocities (V = 0.01 - 10 mm/s). Tests conducted at Gn ≥ 25 MPa show strong evidence for shear localization due to intense grain fracture. In contrast, tests conducted at low effective stress (Gn = 5 MPa) show no preferential fabric development and minimal grain fracture hence we conclude that non-destructive processes such as grain rolling/sliding, distributed throughout the layer, dominate deformation. Temperature measured close to the fault increases systematically with Gn and V, consistent with a one-dimensional heat-flow solution for frictional heating in a finite width layer. Mechanical results indicate stable sliding (μ ∼ 0.6) for all tests, irrespective of deformation regime, and show no evidence for reduced frictional resistance at rapid slip or high effective stresses. Our measurements verify that the heat production equation (q = μσnV) holds regardless of localization state or fracture regime. Thus, for quasistatic velocities(V ≤10 mm/s) and effective stresses relevant to earthquake rupture, neither grain rolling/sliding or shear localization appear to be a viable mechanism for the dramatic weakening or reduced heating required to explain the heat flow paradox
The apparent friction of granular fault gouge in sheared layers
No full textData are presented from a series of experiments on layers of granular quartz gouge in the double-direct-shear geometry at a normal stress of 25 MPa. The apparent friction of a layer shows considerable variability depending on the thickness of the layer and the particle size distribution of the gouge. Measurements of layer thickness during the experiments also show that the layers thin as shearing proceeds. When densification is also admitted, a simple flow law with one adjustable parameter is required to relate the volumetric and shear strain rates. -from Author
On the mechanics of earthquake afterslip
No full textProposes a model for earthquake afterslip based on rate and state variable friction laws. In the model, afterslip is attributed to the interaction of a velocity-weakening region at depth (within which earthquakes nucleate) with an upper region of velocity-strengthening frictional behaviour. Afterslip is the result of relaxation of a stress perturbation within the velocity-strengthening region, which arises when an earthquake propagates into that region from below. The afterslip-time histories of the 1966 Parkfield and 1987 Superstition Hills earthquakes are modeled and the model parameters are related to physical parameters which may govern the rheologic behaviour of the faults. In accord with field observations, the model predicts 1) that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth. Combining results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip. -from Author
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