HR: 1340h
AN: T23A-0560    [Abstracts]
TI: Laboratory Experiments of Silica Powder Lubrication Between Rock Faces at Coseismic Velocities
AU: * Lu, K
EM: luey02@ucla.edu
AF: UCLA ESS, 1708 Geology Building 595 Charles Young Drive East, Los Angeles, CA 90095
AU: Kavehpour, P
EM: pirouz@seas.ucla.edu
AF: UCLA ESS, 1708 Geology Building 595 Charles Young Drive East, Los Angeles, CA 90095
AU: Brodsky, E
EM: brodsky@ess.ucla.edu
AF: UCLA MAE, 420 Westwood Plaza Room 46-147A Box 951597, Los Angeles, CA 90095
AB: One of the unresolved problems in earthquake mechanics is the physical process controlling friction on faults during the rupture of large earthquakes. Many studies suggest that coseismic friction is low even at great depths and several mechanisms have been introduced to explain these observations. In these experiments, we attempt to investigate the physics of mechanical lubrication between rock surfaces by using dry powder. To simulate rock friction, we utilize a tribo-rheometer where two novaculite disks, with 1-inch diameter and 5-micron surface roughness, are compressed together with a thin layer of 5-micron silica powder applied in between. The tribo-rheometer is a highly sensitive instrument that measures torque and normal force when a test substance is placed between the rotating plates. The measurements can be used to directly calculate the viscosity and the friction coefficient. These experiments investigate the velocity dependence of friction by rotating the top disk through velocities from 10$^{-3}$ to 10$^{2}$ rad/sec while the normal stress is kept constant on the order of 10$^{4}$ Pa. The preliminary experiments show frictional regimes of boundary, mixed, and hydrodynamic lubrication; together known as the Stribeck curve. At high shear rates of $>$10 rad/sec, hydrodynamic lubrication occurs when fluid-like behavior of granular flow are responsible for the shear stress between the surfaces. In contrast, boundary lubrication has full asperity contact between the top and bottom surfaces during low shear rates of $<$0.01 rad/sec and shear stress arises from physical interactions. Between the two regimes above, the mixed lubrication is where there is a combination of surface asperity and powder lubricant interactions. From the data, we find the friction coefficient drops from a boundary lubrication value of $\sim$0.3 -- 0.4 to a mixed regime minimum of $\sim$0.2 -- 0.3 while transitioning to the hydrodynamic lubrication. The transition corresponds to a change from solid-friction behavior to viscous fluid-like resistance. More experiments are planned to investigate normal stress dependence.
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting