[SSS15-12] Weakening of quartz rocks at subseismic slip rates due to frictional heating
Keywords:quartz rocks, frictional heating, weakening, subseismic slip rates
We conducted rotary-shear friction experiments on intact agate and silica-gel gouge at a normal stress of 1.5 MPa and equivalent slip rates (Veq) of 0.1−10 cm/s monitoring temperature (T) adjacent to the slip surface or the gouge layer. It should be noted that the actual slip-surface or gouge temperature during the experiment was much higher than T. Steady-state friction coefficient μss of both intact agate and silica-gel gouge decreased with increasing Veq from 0.6−0.7 at Veq = 0.1 cm/s to 0.03−0.2 at Veq = 10 cm/s, while T increased with increasing Veq from ≈25°C at Veq = 0.1 cm/s to 88−105°C at Veq = 10 cm/s. Spikes of high friction followed by T maxima and subsequent weakening suggest that slip at strong asperity contacts induced frictional heat, which in turn resulted in weakening. These results indicate that the frictional strength of intact agate and silica-gel gouge at slip rates of 0.1−10 cm/s is controlled by temperature, which increases by frictional heating.
Based on the flash-heating model of Rice (2006), temperature increase ΔT (°C) by flash heating at an asperity contact can be described as follows:
ΔT = μp(4Si3V2σA/α2π3)1/4/ρcp
where μp is peak friction coefficient, Si is indentation strength, V is slip rate, σ is normal stress, A is slip surface area, ρ is density, cp is heat capacity, and α is thermal diffusivity. Because μp, ρ, cp and α are not much different among rocks, the above equation implies that at a given condition of V, σ and A, ΔT depends primarily on Si and is proportional to Si3/4. Although only limited Si data are available at present, indentation hardness Hi can be correlated with Si, and Hi value of quartz (12 GPa) is much larger than those values of other common rock-forming minerals, e.g., 6 GPa for feldspars, 3.4−5 GPa for amphiboles, 3.4−6.5 GPa for pyroxenes, 6.5−8.4 GPa for olivine, 1.5 GPa for calcite, and 1−2 GPa for micas (Spray, 2010). Thus at a given condition of V, σ and A, ΔT of quartz rocks would be much higher than those of other rocks so that much more amount of frictional heat would be induced at asperity contacts in quartz rocks than in other rocks, which must be responsible for weakening of quartz rocks at subseisimic slip rates.