10:00 〜 10:15
[SSS08-11] 緑色岩の摩擦特性：海山の沈み込みと地震発生への示唆
キーワード：海山、沈み込み、摩擦特性、緑色岩
Subducted seamounts are thought to generate large earthquakes by seamount geometry-related normal stress increase and to act as asperities (e.g., Cloos, 1992). There is also a possibility that they act as barriers for rupture propagation due to the large resistance on the subduction interface (e.g., Kodaira et al., 2000). On the other hand, since a seamount near the southern end of the Japan Trench has been mostly aseismic over the 80 years while many earthquakes have occurred in its adjacent areas, Mochizuki et al. (2008) suggested that the interplate coupling is weak there. In spite of many studies on subducting seamounts, 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 conducted friction experiments on gouges of a greenstone from the Akiyoshi terrane in Yamaguchi, where a huge seamount with Carboniferous to Permian limestone accreted during the Permian time (e.g., Sano, 2006). Thin layer of greenstone underlying the limestone is distributed over 10 km along a fault. Experiments were conducted at a confining pressure of 150 MPa, a pore-water pressure of 100 MPa, temperatures of 25–200℃, 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 temperatures ≦100℃ (a – b) value of the greenstone gouge is positive and does not change noticeably, while at temperatures ≧100℃ it decreases with increasing temperature, becoming ≈0 at 150℃ and negative at 200℃ where stick-slip was observed at 0.1 μm/s. Thus (a – b) value of the greenstone gouge changes from positive to negative with increasing temperature, i.e. with increasing depth. Our results suggest that a seamount may be a site of earthquake nucleation at depths where temperatures are higher than 150℃.
We conducted friction experiments on gouges of a greenstone from the Akiyoshi terrane in Yamaguchi, where a huge seamount with Carboniferous to Permian limestone accreted during the Permian time (e.g., Sano, 2006). Thin layer of greenstone underlying the limestone is distributed over 10 km along a fault. Experiments were conducted at a confining pressure of 150 MPa, a pore-water pressure of 100 MPa, temperatures of 25–200℃, 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 temperatures ≦100℃ (a – b) value of the greenstone gouge is positive and does not change noticeably, while at temperatures ≧100℃ it decreases with increasing temperature, becoming ≈0 at 150℃ and negative at 200℃ where stick-slip was observed at 0.1 μm/s. Thus (a – b) value of the greenstone gouge changes from positive to negative with increasing temperature, i.e. with increasing depth. Our results suggest that a seamount may be a site of earthquake nucleation at depths where temperatures are higher than 150℃.