14:15 〜 14:30
[SCG51-03] 南海トラフ付加体泥質ガウジの摩擦特性の深度変化
キーワード：泥質ガウジ、摩擦特性、南海トラフ付加体
In order to investigate the depth-dependent frictional properties of mud gouge in the Nankai Trough accretionary prism, we conducted triaxial 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 axial displacement rates Vaxial changed stepwise among 0.1, 1 and 10 μm/s.
The results show that the steady-state friction coefficient μss decreases from 0.32−0.35 at the 1000 mbsf condition to 0.29−0.31 at the 3000 mbsf condition, while it increases from those values at that condition to 0.34−0.37 at the 6000 mbsf condition. Fitting the friction data for each step change in Vaxial by the rate-and state-dependent friction constitutive law reveals that (a − b) value (rate dependence of μss) monotonously decreases from positive values at the 1000 mbsf condition through ≈0 at the 5000 mbsf condition to negative values at the 6000 mbsf condition.
Dehydration of smectite at the 3000 mbsf condition where temperature was 100℃ possibly increased pore pressure in the impermeable gouge layer, which was likely responsible for the minimum μss at this condition. This suggests the presence of a high pore-pressure zone at ≈3000 mbsf of IODP Site C0002 due to dehydration of smectite in impermeable mud sediments. Our experimental results also suggest a transition from aseismic faulting with a − b >0 to potentially seismic faulting with a − b <0 at ≈5000 mbsf of IODP Site C0002. In fact, stick-slip corresponding to seismic faulting was observed at the 6000 mbsf condition.
The results show that the steady-state friction coefficient μss decreases from 0.32−0.35 at the 1000 mbsf condition to 0.29−0.31 at the 3000 mbsf condition, while it increases from those values at that condition to 0.34−0.37 at the 6000 mbsf condition. Fitting the friction data for each step change in Vaxial by the rate-and state-dependent friction constitutive law reveals that (a − b) value (rate dependence of μss) monotonously decreases from positive values at the 1000 mbsf condition through ≈0 at the 5000 mbsf condition to negative values at the 6000 mbsf condition.
Dehydration of smectite at the 3000 mbsf condition where temperature was 100℃ possibly increased pore pressure in the impermeable gouge layer, which was likely responsible for the minimum μss at this condition. This suggests the presence of a high pore-pressure zone at ≈3000 mbsf of IODP Site C0002 due to dehydration of smectite in impermeable mud sediments. Our experimental results also suggest a transition from aseismic faulting with a − b >0 to potentially seismic faulting with a − b <0 at ≈5000 mbsf of IODP Site C0002. In fact, stick-slip corresponding to seismic faulting was observed at the 6000 mbsf condition.