A theoretical account is given of the microscopic basis of the rate- and state-dependent friction (RSF) law. The RSF law describes rock friction quantitatively and therefore it is commonly used to model earthquakes and the related phenomena. But the RSF law is rather empirical and the theoretical basis has not been very clear. Here we derive the RSF law starting from constitutive laws for asperities, and give the atomistic expressions for the empirical RSF parameters. In particular, we show that both the length constant and the state variable are given as the 0th weighted power means of the corresponding microscopic quantities: a linear dimension and the contact duration of each asperity. As a result, evolution laws for the state variable can be derived systematically. We demonstrate that the aging and the slip laws can be derived and clarify the approximations behind these two major evolution laws. Additionally, the scaling properties of the length constant are clarified for fractal distribution of asperities.
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