Weakening of quartz rocks at subseismic slip rates due to frictional heating, but not to lubrication by wear materials of hydrated amorphous silica or silica gel

Abstract

In order to clarify the weakening mechanism of quartz rocks at subseismic slip rates of 0.1–10 cm/s, we conducted two series of rotary-shear friction experiments at a normal stress of 1.5 MPa and equivalent slip rates (Veq) of 0.1–10 cm/s; one on intact agate controlling background temperature (TBG) and atmosphere, and the other on intact agate and silica-gel gouge monitoring temperature (T) adjacent to the slip surface or the gouge layer. In the former experiments at Veq = 1 cm/s, agate samples at room TBG showed similar steady-state friction coefficients (μss = 0.62–0.63) irrespective of humid or dry condition in the latter case of which hydration of amorphous wear materials was hampered, while agate sample at TBG = 100 °C exhibited weakening to μss ≈ 0.35. In the latter experiments, 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 0.03–0.16 at Veq = 10 cm/s, while the maximum T increased with increasing Veq from 25–28 °C at Veq = 0.1 cm/s to 88–93 °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 two series of friction experiments indicate that the frictional strength of quartz rocks at subseismic slip rates is controlled by temperature, which increases by frictional heating, but not by wear materials of hydrated amorphous silica or silica gel. Indentation strength of quartz is much higher than that of other common rock-forming minerals so that much more amount of frictional heat would be induced at asperity contacts in quartz rocks than in other rocks, which is likely responsible for weakening of quartz rocks at subseismic slip rates.