Abstract. We observed slow frictional slip occurring at a constant shear stress below the nominal friction level and compared it with the time-dependent strengthening of the frictional interface, which was also tracked experimentally. It was found that slip velocity decreases as the interface strengthens duc to aging, whil'• it increases with the applied shear stress. These dependencies wcrc both exponential and were of similar magnitudes, as implied by the framework law of rate-and state-dependent friction. In the spirit of the adhesion theory of friction the dependence of slip velocity on interface strength is understood to be the result of the change of the shear stress acting on frictional junctions duc to the change of junction population, though the observed dependence was somewhat stronger than a simple model based on this idea predicts. By correcting the observed slip velocity for the effect of the change of the interface strength, wc could obtain a unique relationship between stress and slip velocity, which may bc readily compared with a standard theological formulation. Thus the obtained relationship between stress and slip velocity showed a reasonable agreement with the absolute rate theory over a temperature range of 25-800øC for the present experimental condition (fmc albitc powder, 20 MPa normal stress, no pore water).

[1] We propose two major revisions on the rate-and state-dependent friction (RSF) law on the basis of rigorous analysis of friction experiments. First, we find that the direct effect coefficient a, a parameter playing a central role in the RSF constitutive law, is much larger than the traditional, consensual estimate of less than about 0.01. We derive a lower bound of 0.035 for a directly from stress-velocity relations measured during carefully designed step tests, without relying on any evolution laws as traditional methods do. After correcting for state changes during the steps, inferred indirectly from observed changes in acoustic transmissivities across the interface, we obtain an estimate of a as large as 0.05. Second, we calculate values of the RSF state variable Φ by feeding the measured shear stress and slip velocity values into the constitutive law. The results showed systematic deviations from predictions of the RSF evolution law of the aging type. This leads us to propose a revised evolution law, which incorporates a previously unknown weakening effect related to the shear stress. We also present additional experiment results to corroborate the presence of this new effect. Forward simulations based on our revised evolution law, combined with the larger, revised value of a, very well explain observed variations in both the shear stress and Φ throughout different phases of experiments, including quasi-static hold, reloading after a hold, and steady state sliding at different velocities, as well as their mutual transitions, all with an identical set of parameter values.Citation: Nagata, K., M. Nakatani, and S. Yoshida (2012), A revised rate-and state-dependent friction law obtained by constraining constitutive and evolution laws separately with laboratory data,

[1] Experiments on the frictional properties of quartz gouge under hydrothermal conditions have revealed a new fault healing mechanism that operates only at elevated temperature in the presence of liquid-phase water. This distinguishes it from the well-known ''Dieterich-type'' healing that also operates at room temperature and depends only on the chemical activity of water and not on its phase. The requirement for liquid water to be present is diagnostic of some form of solution transfer being the underlying process. Furthermore, the new healing mechanism operates when the aqueous pore fluid is in chemical equilibrium with silica, indicating solution transfer with only local mass redistribution, such as pressure solution. This new solution transfer healing mechanism has the same logarithmic form, Dm = b ln (t h /t c + 1), as Dieterich-type healing. Its b value was 0.010-0.014, with no clear temperature dependence in the range 100-200°C, and its magnitude is not very different from that of Dieterich-type healing. It is distinguished by a cutoff time, t c , many orders of magnitude greater than the Dieterich-type mechanism. This parameter decreases exponentially with temperature, indicating an apparent activation energy of 54 kJ/mol. This mechanism also has a critical slip distance $500 mm, much greater than the 10 mm typical of Dieterich-type healing. Comparison of the manifestations of this solution transfer healing in velocity step and slide-hold-slide tests indicates that this mechanism is compatible with the rate-state friction law, but its different parameter values may have new implications for earthquake physics.