13:45 〜 15:15
[SSS06-P05] 太平洋プレート被覆層遠洋性粘土の日本海溝沈み込み帯浅部における摩擦特性
キーワード：摩擦特性、遠洋性粘土、日本海溝沈み込み帯浅部
The plate-boundary fault zone near Japan Trench off Miyagi cored during the IODP Exp. 343 was revealed to be present in smectite-rich pelagic clay on the incoming Pacific plate. In order to examine the frictional properties of pelagic clay at shallow conditions of the Japan Trench subduction zone, we conducted triaxial friction experiments on gouge of a pelagic clay sample cored from the cover sediments on the Pacific plate off Sanriku, at a confining pressure of 150 MPa, a pore water pressure of 50 MPa, temperatures of 25–200°C, 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 increases with increasing temperature from 0.25–0.27 at 25°C to 0.28–0.31 at 50°C, then decreases to 0.26–0.28 at 100°C, and increases again to 0.36–0.45 at 200°C. Fitting the friction data for each step change in Vaxial by the rate-and state-dependent friction constitutive law reveals that although the friction parameter a does not change much with increasing temperature, the friction parameter b decreases from 0.001–0.004 at 25°C to 0.000–0.002 at 100°C, then increases to 0.003–0.015 at 200°C. Accordingly, (a – b) value (rate dependence of μss) increases from 0.000–0.001 at 25°C to 0.002–0.004 at 100°C, then significantly decreases to –0.013 – –0.001 at 200°C. (a – b) values are positive at temperatures ≦ 150°C, while they are negative at 200°C, where stick slips were observed except at Vaxial = 10 μm/s.
The pelagic clay gouge contains ≈42 wt% smectite, which is known to dehydrate at temperatures of 50–150°C. The minimum b and µss at 100°C are therefore likely due to smectite dehydration, which released water and presumably raised the pore pressure within the impermeable gouge layer, and resultantly suppressed gouge healing and reduced frictional strength. This suggests the presence of an overpressure zone along the smectite-rich plate-boundary fault at a depth where temperature reaches ≈100°C. Because slow earthquakes are considered to occur at high pore pressures, such overpressure zone is likely the source area of slow earthquakes. Slow earthquakes observed in areas off Sanriku, Miyagi and Ibaragi at the Japan Trench subduction zone possibly occur in such overpressure zones. Our experimental results also suggest that the transition from aseismic faulting with a – b ≧ 0 to potentially seismic faulting with a – b < 0 occurs along the smectite-rich plate-boundary fault at a temperature between 150°C and 200°C.
The results show that the steady-state friction coefficient μss increases with increasing temperature from 0.25–0.27 at 25°C to 0.28–0.31 at 50°C, then decreases to 0.26–0.28 at 100°C, and increases again to 0.36–0.45 at 200°C. Fitting the friction data for each step change in Vaxial by the rate-and state-dependent friction constitutive law reveals that although the friction parameter a does not change much with increasing temperature, the friction parameter b decreases from 0.001–0.004 at 25°C to 0.000–0.002 at 100°C, then increases to 0.003–0.015 at 200°C. Accordingly, (a – b) value (rate dependence of μss) increases from 0.000–0.001 at 25°C to 0.002–0.004 at 100°C, then significantly decreases to –0.013 – –0.001 at 200°C. (a – b) values are positive at temperatures ≦ 150°C, while they are negative at 200°C, where stick slips were observed except at Vaxial = 10 μm/s.
The pelagic clay gouge contains ≈42 wt% smectite, which is known to dehydrate at temperatures of 50–150°C. The minimum b and µss at 100°C are therefore likely due to smectite dehydration, which released water and presumably raised the pore pressure within the impermeable gouge layer, and resultantly suppressed gouge healing and reduced frictional strength. This suggests the presence of an overpressure zone along the smectite-rich plate-boundary fault at a depth where temperature reaches ≈100°C. Because slow earthquakes are considered to occur at high pore pressures, such overpressure zone is likely the source area of slow earthquakes. Slow earthquakes observed in areas off Sanriku, Miyagi and Ibaragi at the Japan Trench subduction zone possibly occur in such overpressure zones. Our experimental results also suggest that the transition from aseismic faulting with a – b ≧ 0 to potentially seismic faulting with a – b < 0 occurs along the smectite-rich plate-boundary fault at a temperature between 150°C and 200°C.