17:15 〜 18:45
[SSS05-P18] Frictional properties of basalt under hydrothermal conditions: implications for the seamount subduction and fault motion
キーワード：海山、沈み込み、摩擦、玄武岩
Subducting seamounts seem to act as seismic asperities due to seamount geometry-related normal stress increase (e.g., Cloos, 1992; Scholz & Small, 1997). On the other hand, recent studies suggested that they appear to act as barriers for rupture propagation or may promote creep or slow slip behavior (e.g., Kodaira et al., 2000; Mochizuki et al., 2008). In this study, we have performed triaxial friction experiments in order to examine the frictional properties of seamount material and determine the friction parameter (a-b) (rate dependence of steady-state friction). Experiments were conducted on gouge of basalt cored from ~200 mbsf (meters below seafloor) of the Takuyo-Daisan Guyot at Site 879 of ODP Leg 144, at a confining pressure of 150 MPa, pore pressures of 50~125 MPa, temperatures of 25~200℃, and axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s.
At an effective normal stress of 50 MPa, the gouges show a friction coefficient of about 0.40~0.52 under temperatures of 25~200℃. At 200oC, the samples exhibit stick-slip behavior. (a-b) values of the basalt gouge tend to decrease from positive to negative with increasing temperature; (a-b) values are positive at temperatures of ≦50℃, neutral at 100℃ and negative at ≧150℃. Although the basalt gouge indicates the temperature dependence, there is not clear effective normal stress dependence. At 50℃, the behavior is velocity strengthening and shows positive (a-b) values with background frictions of 0.44~0.53 over the entire effective pressure range tested. However, the friction parameter Dc decreases with decreasing effective normal stress (i.e., with increasing pore pressure). At 150℃, frictions are 0.41~0.56 and slip is velocity weakening (negative (a-b) value) under all pressure conditions. Our results suggest that a seamount may be a site of earthquake nucleation at depths with temperatures of >100℃ and decreasing the friction parameter Dc with increasing pore pressure may play an important role at temperature of 50℃, i.e. at shallow portion of subduction zone.
At an effective normal stress of 50 MPa, the gouges show a friction coefficient of about 0.40~0.52 under temperatures of 25~200℃. At 200oC, the samples exhibit stick-slip behavior. (a-b) values of the basalt gouge tend to decrease from positive to negative with increasing temperature; (a-b) values are positive at temperatures of ≦50℃, neutral at 100℃ and negative at ≧150℃. Although the basalt gouge indicates the temperature dependence, there is not clear effective normal stress dependence. At 50℃, the behavior is velocity strengthening and shows positive (a-b) values with background frictions of 0.44~0.53 over the entire effective pressure range tested. However, the friction parameter Dc decreases with decreasing effective normal stress (i.e., with increasing pore pressure). At 150℃, frictions are 0.41~0.56 and slip is velocity weakening (negative (a-b) value) under all pressure conditions. Our results suggest that a seamount may be a site of earthquake nucleation at depths with temperatures of >100℃ and decreasing the friction parameter Dc with increasing pore pressure may play an important role at temperature of 50℃, i.e. at shallow portion of subduction zone.