[1] Numerical models of granular shear show lower friction and a greater tendency for stick slip than laboratory studies designed to investigate fault mechanics. Here we report on laboratory experiments designed to reproduce the conditions of numerical models and to test the role that grain characteristics play in controlling frictional behavior. Friction and microstructural data are compared for direct shear experiments on thin layers (2-3 mm) of angular quartz sand and spherical glass beads. We study the effect of grain shape, roughness, size distribution, and comminution. In a nonfracture loading regime, sliding friction for smooth spherical particles (m $ 0.45) is measurably lower than for angular particles (m $ 0.6). A narrow particle size distribution (PSD) of spherical beads (105-149 mm) exhibits unstable stick-slip behavior, whereas a wide PSD of spheres (1-800 mm) and the angular gouge display stable sliding. At higher stress, where grain fracture is promoted, initially spherical particles become stable with accumulated slip, and friction increases to the level observed for angular gouge. We find that frictional strength and stability of a granular shear zone are sensitive to grain shape, PSD, and their evolution. We suggest that a low friction translation mechanism, such as grain rolling, operates in gouge composed of smooth particles. Our results show that the first-order disparities between laboratory and numerical studies of granular shear can be explained by differences in grain characteristics and loading conditions. Since natural faults predominantly contain angular gouge, we find no evidence for a fault-weakening mechanism associated with the presence of gouge.
[1] We report on laboratory experiments designed to investigate the microphysical processes that result in rate-and state-dependent friction behavior. We study the effect of relative humidity (RH) (<5 to 100%) in velocity stepping tests (10-20 mm/s) and slidehold-slide (SHS) tests (3-1000 s) on 3 mm thick layers of quartz and alumina powders sheared at 25 MPa normal stresses. Granular powders are conditioned in situ under controlled RH to create new surface area before shearing. We find a transition from velocity strengthening to velocity-weakening frictional behavior as RH increases. The transition occurs at 30-35% RH for quartz and 55-60% RH for alumina. Frictional healing is negligible at low humidity and increases with increasing RH for both materials. The coefficient of sliding friction is independent of humidity. We use normal stress vibrations in SHS tests to isolate chemically assisted healing mechanisms operative within contact junctions from compaction induced granular strengthening. We find that reorganization of granular particles influences friction but that chemically assisted mechanisms dominate. Our data show that rate-and state-dependent friction behavior for granular materials, including time-dependent healing and steady state velocity dependence, is the result of chemically assisted mechanisms that can be reduced or turned off at low humidity at room temperature in quartz and alumina.INDEX TERMS: 1045 Geochemistry: Low-temperature geochemistry; 5199 Physical Properties of Rocks: General or miscellaneous; 7209 Seismology: Earthquake dynamics and mechanics; KEYWORDS: rate-and state-dependent friction, quartz, humidity Citation: Frye, K. M., and C. Marone, Effect of humidity on granular friction at room temperature,
Abstract. A central problem in explaining the apparent weakness of the San Andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake-like unstable rupture. Our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress (<30 MPa). Our data are consistent with previous work, which has focused on higher normal stress conditions (50 MPa and greater) and found only velocity strengthening. If natural fault zones contain significant smectite, one key implication of our results is that localized zones of high pore pressure, which reduce effective normal stress, could be important in controlling potential sites of earthquake nucleation. Our experiments indicate that friction of smectite is complex, and depends upon both sliding velocity and normal stress. This complexity highlights the need for detailed experiments that reflect in-situ conditions for fault gouges.
[1] To match the boundary conditions of numerical models and to examine the effect of particle dimensionality on granular friction, we conducted laboratory experiments on rods sheared in 1
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