1:45 PM - 3:15 PM
[SSS06-P03] Frictional properties of basalt collected from a seamount: Implications for subduction-zone seismicity
The roughness of subducting seafloor is thought to affect subduction earthquakes. For example, 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). Although there are many studies describing the effects of seamount subduction, frictional properties of seamount materials are still poorly understood. In this study, we conducted friction experiments to determine the friction parameter (a-b) (rate dependence of steady-state friction) and investigated how this parameter changes with temperature.
We performed triaxial friction experiments on powders of basalt cored from the Takuyo-Daisan Guyot at Site 879 of ODP Leg 144. For each experiment, basalt gouge of about 1.0 mm thickness was sandwiched between two porous blocks pre-cut at an angle of 30 degrees to the axial direction. Simulated fault gouges were sheared at a confining pressure of 150 MPa, pore pressures of 100 MPa, temperatures of 25-200°C, and axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s, by using a gas-medium, triaxial apparatus installed at Chiba University.
At 25°C, basalt gouges show a friction coefficient of about 0.47-0.50 and positive (a-b) values which decrease with increasing temperature. At 100°C, friction is about 0.40-0.43 and the gouges show neutral values of (a-b). At 150°C, the behavior is velocity weakening and shows negative (a-b) values with a background friction of about 0.44-0.52. Unstable stick-slips were observed at 200°C with a friction of ≧0.47. Our results suggest that a seamount may be a site of earthquake nucleation at depths with temperatures of ≧150°C.
We performed triaxial friction experiments on powders of basalt cored from the Takuyo-Daisan Guyot at Site 879 of ODP Leg 144. For each experiment, basalt gouge of about 1.0 mm thickness was sandwiched between two porous blocks pre-cut at an angle of 30 degrees to the axial direction. Simulated fault gouges were sheared at a confining pressure of 150 MPa, pore pressures of 100 MPa, temperatures of 25-200°C, and axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s, by using a gas-medium, triaxial apparatus installed at Chiba University.
At 25°C, basalt gouges show a friction coefficient of about 0.47-0.50 and positive (a-b) values which decrease with increasing temperature. At 100°C, friction is about 0.40-0.43 and the gouges show neutral values of (a-b). At 150°C, the behavior is velocity weakening and shows negative (a-b) values with a background friction of about 0.44-0.52. Unstable stick-slips were observed at 200°C with a friction of ≧0.47. Our results suggest that a seamount may be a site of earthquake nucleation at depths with temperatures of ≧150°C.