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[SCG44-08] Rheological meaning of the direct effect in the rate- and state-dependent friction law: Insight from friction experiments
Keywords:coefficient of friction, fluorite, friction experiment, thermally activated process
Rate- and state-dependent friction (RSF) law describes the stability of fault slips and recovery of fault strength and is thus widely used for earthquake cycle simulations. Laboratory experimental works have shown that the RSF parameters are not only dependent on the rock and mineral types, but also on temperature and pressure conditions. The variation in frictional properties plays essential roles in the earthquake occurrences in subduction zones. To constrain frictional parameters at deep seismogenic zones, better understandings on the physics of friction that govern these parameters is required. Herein we focus on the velocity dependence of friction, which is represented by parameter a of the direct effect in the RSF law. The velocity dependence suggests that thermally activated processes are involved in the solid-solid contact processes. In this study, we examine the theoretical models of the direct effect based on the experimental results obtained for fluorite (CaF2). Fluorite has a cubic structure and is much weaker than ordinary rock forming minerals. For comparison, experiments were also conducted on quartz and albite.
We performed large displacement friction tests using a newly developed rotary shear apparatus (KURAMA). Steady-state friction tests and velocity step tests were conducted at normal stresses up to 200 MPa and slip velocities of 1–1000 μm/s, at room temperature under dry or water-saturated conditions. The samples were natural fluorite and natural albitite powder samples (grain size < 120 μm) and commercially available quartz sand (grain size ~70 μm).
Steady-state friction tests of fluorite, as well as quartz and albitite, yielded the coefficient of friction (μ) of ca. 0.6, in consistent with the Byerlee’s law. These values are close to μ of single-crystal samples of fluorite and quartz obtained in our previous experiments (Tsuge et al., 2023, JpGU). Although the yield strength of fluorite is one-order smaller than that of quartz, there is no significant difference in the friction coefficients. It is suggest that the coefficient of friction is determined by the ratio of the shear strength and yield strength in the real area of contact, and that the adhesion friction mechanism was also dominant in friction of powdered samples.
RSF parameters were determined from the velocity step tests. The value of a for fluorite was found to be 0.007–0.01, which are comparable to 0.008–0.02 of albitite. The value of a for albitite was consistent with the value obtained by the model for the thermal activation process. However, the experimental value for fluorite was smaller than that predicted by the model for the thermal activation process. It is suggested that the reason for this is that in soft fluorite, shear deformation due to plastic flow occurs in the interior of the asperities other than the adhesion zone. This deformation reduces the shear stress, and reduced i a. Considering that ordinary minerals also soften at high temperatures, this result is important for understanding frictional behavior in the brittle-ductile transition zone.
We performed large displacement friction tests using a newly developed rotary shear apparatus (KURAMA). Steady-state friction tests and velocity step tests were conducted at normal stresses up to 200 MPa and slip velocities of 1–1000 μm/s, at room temperature under dry or water-saturated conditions. The samples were natural fluorite and natural albitite powder samples (grain size < 120 μm) and commercially available quartz sand (grain size ~70 μm).
Steady-state friction tests of fluorite, as well as quartz and albitite, yielded the coefficient of friction (μ) of ca. 0.6, in consistent with the Byerlee’s law. These values are close to μ of single-crystal samples of fluorite and quartz obtained in our previous experiments (Tsuge et al., 2023, JpGU). Although the yield strength of fluorite is one-order smaller than that of quartz, there is no significant difference in the friction coefficients. It is suggest that the coefficient of friction is determined by the ratio of the shear strength and yield strength in the real area of contact, and that the adhesion friction mechanism was also dominant in friction of powdered samples.
RSF parameters were determined from the velocity step tests. The value of a for fluorite was found to be 0.007–0.01, which are comparable to 0.008–0.02 of albitite. The value of a for albitite was consistent with the value obtained by the model for the thermal activation process. However, the experimental value for fluorite was smaller than that predicted by the model for the thermal activation process. It is suggested that the reason for this is that in soft fluorite, shear deformation due to plastic flow occurs in the interior of the asperities other than the adhesion zone. This deformation reduces the shear stress, and reduced i a. Considering that ordinary minerals also soften at high temperatures, this result is important for understanding frictional behavior in the brittle-ductile transition zone.