澤井 みち代

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.

Since the 2011 Tohoku-Oki earthquake, multi-disciplinary observational studies have promoted our understanding of both the coseismic and long-term behaviour of the Japan Trench subduction zone. We also have suggestions for mechanical properties of the fault from the experimental side. In the present study, numerical models of earthquake sequences are presented, accounting for the experimental outcomes and being consistent with observations of both long-term and coseismic fault behaviour and thermal measurements. Among the constraints, a previous study of friction experiments for samples collected in the Japan Trench Fast Drilling Project (JFAST) showed complex rate dependences: <italic>a</italic> and <italic>a</italic> − <italic>b</italic> values change with the slip rate. In order to express such complexity, we generalize a rate- and state-dependent friction law to a quadratic form in terms of the logarithmic slip rate. The constraints from experiments reduced the degrees of freedom of the model significantly, and we managed to find a plausible model by changing only a few parameters. Although potential scale effects between lab experiments and natural faults are important problems, experimental data may be useful as a guide in exploring the huge model parameter space. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’.