Depth-dependent static frictional properties of mud gouge in the Nankai Trough accretionary prism

抄録

In order to investigate the depth-dependent static frictional properties of mud gouge in the Nankai Trough accretionary prism, we conducted triaxial slide-hold-slide friction experiments on gouge of a mud sample cored from 2183.6 mbsf (meters below seafloor) at IODP Site C0002, at pressure, pore-water pressure and temperature conditions supposed at depths of 1000–6000 mbsf there, and an axial displacement rate of 1 μm/s during which sliding was held for periods ranging from 10 to 10⁴ s.

The results show that the coefficient of static friction μ_s decreases from 0.37–0.38 at the 1000 mbsf condition to 0.30–0.31 at the 3000 mbsf condition, while it increases to 0.40–0.45 at the 6000 mbsf condition where stick slips were also observed. Healing Δμ (increase in friction coefficient upon resuming sliding after a holding time t_h) increases logarithmically with t_h. We then fitted the Δμ and t_h data by the equation Δμ = b ln(t_h/t_c + 1), where t_c is a cutoff time, beyond which healing shows a log-linear increase with a slope b. b is also equivalent with a friction parameter relevant to the time-dependent evolutionary effect of the rate- and state-dependent friction constitutive law. Such fitting revealed that b value decreases from ≈0.0040 at the 1000 mbsf condition to ≈0.0014 at the 3000 mbsf condition, while it increases to ≈0.0057 at the 6000 mbsf condition.

Dehydration of smectite at the 3000 mbsf condition where temperature was 100°C possibly increased pore pressure in the impermeable gouge layer, which was responsible not only for the minimum μ_s but also for the minimum b value, because increased pore pressure likely reduced the area of grain contacts and suppressed healing. This suggests the presence of a high pore-pressure and low fault-strength zone at ≈3000 mbsf of IODP Site C0002. While increasing μ_s as well as b value with increasing supposed depth at ≧3000 mbsf suggest that fault strength increases downward from ≈3000 mbsf of IODP Site C0002 due to time-dependent healing, which also promotes seismic faulting as illustrated by stick slips at the 6000 mbsf condition.