Fault strength, healing and stability in the Nankai Trough accretionary prism off Kii Peninsula, Japan, as illustrated by friction experiments on gouge of a cored sample

In order to investigate fault strength, healing and stability in the Nankai Trough accretionary prism off Kii Peninsula, Japan, we conducted two series of triaxial friction experiments on gouge of a silty-claystone sample cored from 2183.6 mbsf (meters below seafloor) at IODP Site C0002, at confining pressure (P_c), pore-water pressure ($P_{H_{2}O}$) and temperature (T) conditions simulating those in situ at 1000–6000 mbsf there; rate-stepping tests at axial displacement rates (V_axial) changed stepwise among 0.1, 1 and 10 μm/s, and slide-hold-slide tests at V_axial = 1 μm/s with hold time (t_h) ranging from 10 to 10⁴ s.

Experimentally determined steady-state and static friction coefficients, μ_ss and μ_s, respectively, and the log-linear t_h dependence of frictional healing, β, exhibit a decrease with simulated depth down to 3000 mbsf at which condition T was 100 °C, followed by an increase toward 6000 mbsf. On the other hand, the rate dependence of μ_ss, a – b, gradually decreases with simulated depth, changing from positive at ≤4000 mbsf through ∼0 at 5000 mbsf to negative at 6000 mbsf at which condition stick slips were observed.

Our experimental results suggest the presence of a low fault-strength and weak fault-healing zone at ∼3000 mbsf beneath IODP Site C0002, possibly due to elevated pore pressure induced by smectite dehydration. This zone correlates well with the previously reported low seismic-velocity zone and the source area of very low-frequency earthquakes to the south. Our experimental results also suggest that faulting beneath IODP Site C0002 is stable and aseismic at ≤4000 mbsf, transitional at 5000 mbsf, and potentially unstable and seismic at 6000 mbsf. In fact, stick slips corresponding to seismic faulting were observed at the 6000 mbsf condition. This implies that faulting along the plate-boundary thrust located at ∼5200 mbsf beneath IODP Site C0002 is potentially seismogenic.