1:45 PM - 2:00 PM
[SCG62-06] Experimental study on the effect of humidity on the frictional behavior of quartz-bearing rocks at intermediate to high velocities
Keywords:Friction, Granite, Slip-weakening
Experimental study on the effect of humidity on the frictional behavior of quartz-bearing rocks at intermediate to high velocities
To understand the mechanism of generating a wide range of fault slip behavior from slow to fast earthquakes, evaluation of steady-state frictional properties of rocks over a wide range of slip rates are fundamentally important. It has been shown that frictional coefficient (µ) of various kind of rocks dramatically reduces at high slip velocities [Di Toro et al., 2011]. Among the various composition of the material tested so far, the frictional weakening of quartz-rocks occurs at relatively low slip velocities (V ≧~1 mm/s), which implies the uniqueness of the frictional weakening mechanism of quartz rocks [Di Toro et al., 2004]. Recently, experiments using quartz samples have shown that steady-state frictional coefficient (µss) at high slip velocities are dependent on relative humidity. Iida and Tsutsumi (2016) conducted frictional experiment on synthetic quartz samples and showed that decrease of µss of the quartz occurs at V = 10 µm/s under dry air conditions, which slip rate is lower than those previously reported in the experiments performed. Onoe and Tsutsumi (2020) conducted similar experiments under humidity-controlled conditions and found that µss of quartz is depended on the tested relative humidity values. Although it has been recognized that evaluation of effect of humidity on frictional properties of quartz is important, humidity-dependence of the frictional properties of quartz-bearing rocks that contains other minerals in addition to quartz (e.g. granite and siliceous sandstone) are not fully investigated yet. In this study, for the purpose of investigating the humidity dependance of the µss of various kind of rocks that contains quartz, we conducted rock-to-rock rotary-shear experiments on Berea sandstones and Inada granites at a constant normal stress of 1.5 MPa and at various slip velocities V ranging from 0.01 to 10 mm/s under humid (5%-30% RH) and nominally dry (< 3% RH) conditions. Relative humidities were controlled by conditioning dry air with a humidifying membrane (HACU-2, Kitz microfilter). Our experimental results shows that the µ value of Inada granite is dependent on slip, slip velocity, and relative humidities. For example, at V = 10 mm/s, µ value decreases with slip (slip weakening) under 0-30%RH, and reaches steady state friction µss at about 1 m slip. Values of µss was found to increase with humidity; µss = 0.40 at 0%RH, 0.43 at 10%RH, and 0.51 at 30%RH. At V = 1 mm/s, µ value does not reach steady-state; slip strengthening follows slip weakening behavior under ≧20%RH. In contrast, µ values under 0%RH and 10%RH decrease with slip and reach steady state µss at about 1.5 m of the slip. Although we observed frictional strengthening behavior following the initial slip weakening behavior at V = 1 mm/s under high humidity conditions, it was observed that overall level of µ values increase with relative humidity RH in all experiments on Inada granite. Experimental results obtained thus far for Berea sandstone do not show systematic dependence of µ values on relative humidities. It was revealed that the humidity dependence of the frictional behavior is different between Berea sandstone and Inada granite, suggesting different effect of the absorbed water on the fault surface and the gouge particles on the measured friction values between different types of rocks. To elucidate the differences of the effect of humidity on rocks, we plan to carry out microscopic observation on specimens before and after experiments.
To understand the mechanism of generating a wide range of fault slip behavior from slow to fast earthquakes, evaluation of steady-state frictional properties of rocks over a wide range of slip rates are fundamentally important. It has been shown that frictional coefficient (µ) of various kind of rocks dramatically reduces at high slip velocities [Di Toro et al., 2011]. Among the various composition of the material tested so far, the frictional weakening of quartz-rocks occurs at relatively low slip velocities (V ≧~1 mm/s), which implies the uniqueness of the frictional weakening mechanism of quartz rocks [Di Toro et al., 2004]. Recently, experiments using quartz samples have shown that steady-state frictional coefficient (µss) at high slip velocities are dependent on relative humidity. Iida and Tsutsumi (2016) conducted frictional experiment on synthetic quartz samples and showed that decrease of µss of the quartz occurs at V = 10 µm/s under dry air conditions, which slip rate is lower than those previously reported in the experiments performed. Onoe and Tsutsumi (2020) conducted similar experiments under humidity-controlled conditions and found that µss of quartz is depended on the tested relative humidity values. Although it has been recognized that evaluation of effect of humidity on frictional properties of quartz is important, humidity-dependence of the frictional properties of quartz-bearing rocks that contains other minerals in addition to quartz (e.g. granite and siliceous sandstone) are not fully investigated yet. In this study, for the purpose of investigating the humidity dependance of the µss of various kind of rocks that contains quartz, we conducted rock-to-rock rotary-shear experiments on Berea sandstones and Inada granites at a constant normal stress of 1.5 MPa and at various slip velocities V ranging from 0.01 to 10 mm/s under humid (5%-30% RH) and nominally dry (< 3% RH) conditions. Relative humidities were controlled by conditioning dry air with a humidifying membrane (HACU-2, Kitz microfilter). Our experimental results shows that the µ value of Inada granite is dependent on slip, slip velocity, and relative humidities. For example, at V = 10 mm/s, µ value decreases with slip (slip weakening) under 0-30%RH, and reaches steady state friction µss at about 1 m slip. Values of µss was found to increase with humidity; µss = 0.40 at 0%RH, 0.43 at 10%RH, and 0.51 at 30%RH. At V = 1 mm/s, µ value does not reach steady-state; slip strengthening follows slip weakening behavior under ≧20%RH. In contrast, µ values under 0%RH and 10%RH decrease with slip and reach steady state µss at about 1.5 m of the slip. Although we observed frictional strengthening behavior following the initial slip weakening behavior at V = 1 mm/s under high humidity conditions, it was observed that overall level of µ values increase with relative humidity RH in all experiments on Inada granite. Experimental results obtained thus far for Berea sandstone do not show systematic dependence of µ values on relative humidities. It was revealed that the humidity dependence of the frictional behavior is different between Berea sandstone and Inada granite, suggesting different effect of the absorbed water on the fault surface and the gouge particles on the measured friction values between different types of rocks. To elucidate the differences of the effect of humidity on rocks, we plan to carry out microscopic observation on specimens before and after experiments.