Frictional property of metagabbro gouge on a meter-scale laboratory fault

Abstract:

Although various laboratory experiments have provided significant insights related to rock frictional properties, there is a great scale gap between nature and laboratory. To bridge the gap, Yamashita et al. (2015) conducted friction experiments with meter-scale rock specimens and reported that the work rate at which the meter-scale rock friction starts to decrease is one order of magnitude smaller work rate than that of the centimeter-scale one. However, their experiments were conducted under the rock-on-rock configuration, whereas a natural fault generally has small particles of rock, so-called gouge, in it. To investigate whether the scale dependence of gouge friction exists or not, we conducted meter-scale gouge friction experiments. As the simulated gouge, we ground metagabbro blocks from India by a jet mill, so that the average diameter of the gouge particle is approximately 10 μm. We distributed the gouge as flat as possible on a lower driver block and compacted by hands. The thickness of the gouge layer before the compaction was 3 mm. The contacting area was 1.5 m long and 0.1 m wide. We conducted experiments with two different types of shear loading. One is step-change-velocity loading to investigate parameters of the rate- and state-dependent friction (RSF) law of the gouge. We repeatedly changed the loading velocity between 0.01 mm/s and 1 mm/s and found that the metagabbro gouge has a property of velocity weakening. The other is constant-velocity loading to investigate the frictional property of the gouge under stable sliding. The experimental results showed the absence of the great reduction of friction coefficient at a high work rate, different from the rock-on-rock configuration. However, friction coefficients at some experiments with fast loading started to decrease at the last stage, which suggests that a larger slip amount is needed for local shear stress in the gouge layer to concentrate and then cause the great reduction of macroscopic friction. In other words, even under the situation with a gouge rich fault in nature, large stress drop can occur due to a stress heterogeneity generated after a large amount of slip accumulation.