HR: 0800h
AN: T21B-0476    [Abstracts]
TI: Effects of Normal Stress on the Sliding Behavior of Gabbro during Frictional Melting
AU: * Tsutsumi, A
EM: tsutsumi@kueps.kyoto-u.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, Kyoto, 606-8502 Japan
AB: Recent friction experiments at high slip rates to about 2 m/sec and at low normal stresses to about 1.5 MPa have shown that frictional properties of rocks at high slip rates change dramatically due to the initiation of frictional melting. For example, shear strength of the rock specimen increased rapidly to reach an ultimate strength at which visible frictional melting started. With the onset of visible frictional melting, shear resistance decreased gradually to a steady-state level, and the level of steady state friction during frictional melting decreased markedly with the increase of slip rate (velocity weakening in high slip-rate range, Tsutsumi and Shimamoto, 1997). Shear strength thus decreases dramatically with increasing displacement in the velocity-weakening regime at high-velocities, and this creates a large critical slip weakening distance, Dc. In the previous experiments, data at higher normal stresses are quite limited due primarily to a difficulty of applying a higher normal stress to un-jacketed and frictionally heated rock specimens. In this study, we performed a series of frictional melting experiments on gabbro at normal stresses to 6.5MPa using a rotary-shear high-speed friction-testing machine to investigate the effects of normal stress on the frictional properties of rocks during frictional melting. Experiments were performed on a pair of hollow-cylindrical specimens of gabbro, same in sizes to the previous experiments, with outer and inner diameters of 25 and 16 mm, respectively, initially at room temperature and at constant slip rates to 1.6 m/sec. In the experiments, we slid specimens at a constant normal stress at about 1MPa firstly and the normal stress was changed gradually at an almost constant increasing rate when steady state of the friction during frictional melting was attained, while the slip rate was maintained at a constant rate during a run. Experimental results revealed that frictional behavior changed at a critical normal stress toward higher normal stress conditions. Within low normal stress range, shear strength of the fault increased only slightly with the increase of normal stress. In another word, ratio of shear stress to normal stress decreased with the increase of normal stress. In the higher normal stress range, however, shear strength became obviously dependent on the applied normal stress, and the ratio of shear stress to the applied normal stress became almost constant. For example, in the case of the result at a constant slip rate of 1.24 m/s, change of the frictional behavior appeared at about 2.5MPa, and after this point the ratio became constant at 0.45. Preliminary results showed that the constant value of the shear- to normal stress ratio were independent to the rate of normal stress increase. At low normal stresses, a molten layer separates sliding surfaces of steady state and the viscosity and the strain rate of the molten layer determine the shear strength. As normal stress increases, it is expected that the sliding surfaces become to be closer and solid-solid contacts between the two surfaces will contribute to the bulk shear strength level. Observation of the texture of the fault tested at higher normal stress under the microscope showed evidences of the contacting surfaces. Maximum time duration of the presented melting experiments were limited by the total length of the paired specimens, because the total length of the specimens became short during frictional melting due to drop of the melt from the sliding surfaces. In order to examine whether a steady state friction was attained in the presented results, a series of experiments on a longer specimens is desired in the near future.
DE: 8010 Fractures and faults
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005