14:00 〜 14:15
[SSS07-02] 三軸摩擦実験に基づく紀伊半島沖南海トラフ付加体中の断層の強度・安定性・癒着度の深度変化
★招待講演
キーワード：三軸摩擦実験、南海トラフ付加体、断層強度、断層運動安定性、断層癒着度
In order to investigate the depth dependence of fault strength, stability and healing in the Nankai Trough accretionary prism off Kii Peninsula, we conducted two series of triaxial friction experiments on gouge of a mud sample cored from 2183.6 mbsf (meters below seafloor) at IODP Site C0002, at confining pressure, pore-water pressure and temperature conditions supposed at depths of 1000–6000 mbsf there; rate-stepping tests at axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s, and slide-hold-slide tests at an axial displacement rate of 1 μm/s with hold time ranging from 10 to 104 s.
Steady-state and static frictional strengths, and log-linear time dependence of frictional healing decrease from the 1000 mbsf condition to the 3000 mbsf condition, while they increase from the 3000 mbsf condition to the 6000 mbsf condition. Such changes are largely attributable to dehydration of smectite, but also to thermally activated pressure solution. The former occurring at 50–150°C increased pore pressure within the impermeable gouge layer, reduced frictional strength, and suppressed frictional healing, while the latter promoted gouge densification and frictional healing which increased frictional strength with increasing temperature. The rate dependence of steady-state frictional strength gradually decreases from positive at ≦4000 mbsf conditions where temperatures were ≦130°C, through ≈0 at the 5000 mbsf condition where temperature was 150°C, to negative at the 6000 mbsf condition where temperature was 170°C.
Our experimental results suggest the presence of a high pore-pressure, low fault-strength and weak fault-healing zone around ≈3000 mbsf beneath IODP Site C0002, which is likely the northern extension of the low P-wave velocity zone reported at 2000–4000 mbsf southward of the outer ridge. 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 during both rate-stepping and slide-hold-slide tests at the 6000 mbsf condition. Such change in faulting stability with depth is applicable not only to the accretionary prism beneath IODP Site C0002, but also to the plate-boundary thrust of the Nankai Trough subduction zone. At the Nankai Trough subduction zone off Kii Peninsula, the plate-boundary thrust is located at ≈5000 mbsf beneath the outer ridge. Therefore, the updip limit of seismogenic faulting along the plate-boundary thrust is supposed to be around there.
Steady-state and static frictional strengths, and log-linear time dependence of frictional healing decrease from the 1000 mbsf condition to the 3000 mbsf condition, while they increase from the 3000 mbsf condition to the 6000 mbsf condition. Such changes are largely attributable to dehydration of smectite, but also to thermally activated pressure solution. The former occurring at 50–150°C increased pore pressure within the impermeable gouge layer, reduced frictional strength, and suppressed frictional healing, while the latter promoted gouge densification and frictional healing which increased frictional strength with increasing temperature. The rate dependence of steady-state frictional strength gradually decreases from positive at ≦4000 mbsf conditions where temperatures were ≦130°C, through ≈0 at the 5000 mbsf condition where temperature was 150°C, to negative at the 6000 mbsf condition where temperature was 170°C.
Our experimental results suggest the presence of a high pore-pressure, low fault-strength and weak fault-healing zone around ≈3000 mbsf beneath IODP Site C0002, which is likely the northern extension of the low P-wave velocity zone reported at 2000–4000 mbsf southward of the outer ridge. 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 during both rate-stepping and slide-hold-slide tests at the 6000 mbsf condition. Such change in faulting stability with depth is applicable not only to the accretionary prism beneath IODP Site C0002, but also to the plate-boundary thrust of the Nankai Trough subduction zone. At the Nankai Trough subduction zone off Kii Peninsula, the plate-boundary thrust is located at ≈5000 mbsf beneath the outer ridge. Therefore, the updip limit of seismogenic faulting along the plate-boundary thrust is supposed to be around there.