Scale dependence of rock friction at high work rate

In metre-sized rock specimens, rock friction starts to decrease at a much smaller work rate than in centimetre-sized rock specimens, thus demonstrating that rock friction is scale-dependent. Frictional sliding at faults controls the rupture of earthquakes and much effort has gone into the study of the frictional properties of rocks in laboratory experiments. This study shows that previously used scaling of results obtained in lab studies investigating the Earth's crust didn't accurately take into account the effect of sample size. Futoshi Yamashita et al. used metre-sized rock specimens — not the centimetre-sized samples commonly used — and demonstrate that friction in the metre-size range starts to decrease at a work rate that is an order of magnitude smaller than that of the smaller specimens. The authors propose that stress-concentrated areas exist in which more gouge materials are produced due to frictional slip, resulting in further stress concentrations at these areas. As such heterogeneity is common in nature, the authors conclude that a natural fault could lose its strength faster than that expected from the properties estimated from small rock samples. Determination of the frictional properties of rocks is crucial for an understanding of earthquake mechanics, because most earthquakes are caused by frictional sliding along faults. Prior studies using rotary shear apparatus1,2,3,4,5,6,7,8,9,10,11,12,13 revealed a marked decrease in frictional strength, which can cause a large stress drop and strong shaking, with increasing slip rate and increasing work rate. (The mechanical work rate per unit area equals the product of the shear stress and the slip rate.) However, those important findings were obtained in experiments using rock specimens with dimensions of only several centimetres, which are much smaller than the dimensions of a natural fault (of the order of 1,000 metres). Here we use a large-scale biaxial friction apparatus with metre-sized rock specimens to investigate scale-dependent rock friction. The experiments show that rock friction in metre-sized rock specimens starts to decrease at a work rate that is one order of magnitude smaller than that in centimetre-sized rock specimens. Mechanical, visual and material observations suggest that slip-evolved stress heterogeneity on the fault accounts for the difference. On the basis of these observations, we propose that stress-concentrated areas exist in which frictional slip produces more wear materials (gouge) than in areas outside, resulting in further stress concentrations at these areas. Shear stress on the fault is primarily sustained by stress-concentrated areas that undergo a high work rate, so those areas should weaken rapidly and cause the macroscopic frictional strength to decrease abruptly. To verify this idea, we conducted numerical simulations assuming that local friction follows the frictional properties observed on centimetre-sized rock specimens. The simulations reproduced the macroscopic frictional properties observed on the metre-sized rock specimens. Given that localized stress concentrations commonly occur naturally, our results suggest that a natural fault may lose its strength faster than would be expected from the properties estimated from centimetre-sized rock samples.