Abstract
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.
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Acknowledgements
We thank A. Schubnel and F. X. Passelègue for help with some of the experiments. We thank G. C. McLaskey for comments. Reviews by Z. Reches improved the manuscript. This study was supported by the NIED research project entitled “Development of monitoring and forecasting technology for crustal activity” and JSPS KAKENHI grant number 23340131.
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E.F., F.Y., K.M. and H.K. designed and developed the large-scale friction experiments. F.Y. performed the analysis of the experimental data and numerical modelling. S.T. analysed the collected gouge material. F.Y. wrote the manuscript, with contributions from E.F. and S.X. All authors contributed to the performance of the large-scale friction experiments.
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This file contains Supplementary Methods, Text and Data, Supplementary Tables 1-10, Supplementary Figures 1-12 and additional references. (PDF 15695 kb)
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Yamashita, F., Fukuyama, E., Mizoguchi, K. et al. Scale dependence of rock friction at high work rate. Nature 528, 254–257 (2015). https://doi.org/10.1038/nature16138
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DOI: https://doi.org/10.1038/nature16138
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