オパールガウジの摩擦特性の温度変化と沈み込み帯の断層運動への適用

抄録

We conducted triaxial friction experiments on opal gouge at a confining pressure of 150 MPa, a pore water pressure of 50 MPa, temperatures (T) of 25–200°C, and axial displacement rates (V_axial) changed stepwise among 0.1, 1 and 10 μm/s, in order to investigate its temperature-dependent frictional properties and their controlling factors. Because opal is known as a main component in chert on the Pacific plate subducting at the Japan Trench, its frictional properties would be applicable to those of chert subducting there.

The results showed frictional strength increasing with increasing T, rate-weakening behavior at T ≧100°C, where frictional strength decreases or increases when V_axial is increased or decreased, and stick slips at T ≧150°C. When fitted by the rate- and state-dependent friction constitutive law, steady-state friction coefficient μ_ss increases with increasing T or decreasing V_axial, while a – b (rate dependence of μ_ss implying stable (aseismic) faulting if positive, i.e., rate strengthening, and potentially unstable (seismic) faulting if negative, i.e., rate weakening) decreases with increasing T or decreasing V_axial, and becomes ≈0 at 50°C and negative at T ≧100°C, being larger negative at higher T or lower V_axial in the latter case.

These results indicate that frictional properties of opal gouge are dependent on not only T but also V_axial, suggesting that a thermally activated process is responsible for the observed frictional properties. Gouge layer at higher T contains a smaller fraction of submicron-size particles, which have likely been dissolved away at higher T. In addition, gouge layers at T ≧100°C do not show clear cataclastic microstructures, but are rather dense. These microstructures suggest the activation of thermally activated pressure solution during the experiments, which promoted gouge densification to increase frictional strength with increasing T or decreasing V_axial, thereby decreasing a – b to <0 at T ≧100°C. Our results also show that a – b ≈ 0 at 50°C, suggesting that the transition from aseismic faulting to seismic faulting occurs at ≈50°C along the megathrust at the Japan Trench subduction zone if the megathrust is located in chert on the subducting Pacific plate.