14:00 〜 14:15
[SCG40-12] Temperature-dependent frictional properties of oceanic basalt and pelagic sediments, and their implications for earthquakes at the shallow Japan Trench subduction zone
キーワード：摩擦特性、海洋玄武岩、遠洋性堆積物、地震、日本海溝沈み込み帯浅部
In order to investigate the temperature dependence of frictional properties of oceanic basalt and pelagic sediments, we conducted triaxial friction experiments on gouges of the following samples at a confining pressure of 150 MPa, a pore-water pressure of 50 MPa, temperatures of 25–200°C, and axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s. We used a basalt sample cored from the oceanic basement of the Philippine Sea plate off Kii Peninsula, a chert sample cored from the footwall of the plate-boundary thrust near the Japan Trench, and pelagic and hemipelagic clay samples cored from the cover sediments on the Pacific plate off Sanriku.
The results show that the steady-state friction coefficient µss decreases with increasing content of clay minerals at a given temperature. Although μss of a given sample gouge does not change much with increasing temperature up to 100°C, it significantly increases from 100°C to 200°C, which is more pronounced for sample gouges with higher contents of amorphous silica. μss of the pelagic clay gouge shows a minimum at 100°C, which was likely due to elevated pore-water pressure induced by smectite dehydration. Although all sample gouges showed velocity-strengthening behavior at ≦50°C, the chert gouge, gouges other than the pelagic clay gouge, and all sample gouges showed velocity-weakening behavior at 100°C, 150°C, and 200°C, respectively. In addition, stick slips were observed during the experiments of all sample gouges at 200°C. Temperature Ttr at which the transition from velocity strengthening to velocity weakening occurs increases with increasing content of clay minerals; 50°C < Ttr ≦ 100°C for the chert gouge, Ttr ≈ 100°C for the basalt gouge, 100°C < Ttr < 150°C for the hemipelagic clay gouge, and 150°C < Ttr < 200°C for the pelagic clay gouge.
At 10–20 km depths of the Japan Trench subduction zone, where temperatures are estimated to be 80–130°C, fast earthquakes occur off Miyagi and Fukushima, while slow earthquakes are observed off Sanriku and Ibaragi. Provided that the plate-boundary thrust is present within basalt or pelagic sediments on the subducting Pacific plate, our experimental results suggest that the fast earthquakes off Miyagi and Fukushima occur in basalt or chert, while the slow earthquakes off Sanriku and Ibaragi occur in pelagic or hemipelagic clay. Those temperatures are also within the range of smectite dehydration. Because slow earthquakes likely occur in regions of high pore pressures, those observed off Sanriku and Ibaragi possibly occur in such regions formed by smectite dehydration in pelagic clay.
The results show that the steady-state friction coefficient µss decreases with increasing content of clay minerals at a given temperature. Although μss of a given sample gouge does not change much with increasing temperature up to 100°C, it significantly increases from 100°C to 200°C, which is more pronounced for sample gouges with higher contents of amorphous silica. μss of the pelagic clay gouge shows a minimum at 100°C, which was likely due to elevated pore-water pressure induced by smectite dehydration. Although all sample gouges showed velocity-strengthening behavior at ≦50°C, the chert gouge, gouges other than the pelagic clay gouge, and all sample gouges showed velocity-weakening behavior at 100°C, 150°C, and 200°C, respectively. In addition, stick slips were observed during the experiments of all sample gouges at 200°C. Temperature Ttr at which the transition from velocity strengthening to velocity weakening occurs increases with increasing content of clay minerals; 50°C < Ttr ≦ 100°C for the chert gouge, Ttr ≈ 100°C for the basalt gouge, 100°C < Ttr < 150°C for the hemipelagic clay gouge, and 150°C < Ttr < 200°C for the pelagic clay gouge.
At 10–20 km depths of the Japan Trench subduction zone, where temperatures are estimated to be 80–130°C, fast earthquakes occur off Miyagi and Fukushima, while slow earthquakes are observed off Sanriku and Ibaragi. Provided that the plate-boundary thrust is present within basalt or pelagic sediments on the subducting Pacific plate, our experimental results suggest that the fast earthquakes off Miyagi and Fukushima occur in basalt or chert, while the slow earthquakes off Sanriku and Ibaragi occur in pelagic or hemipelagic clay. Those temperatures are also within the range of smectite dehydration. Because slow earthquakes likely occur in regions of high pore pressures, those observed off Sanriku and Ibaragi possibly occur in such regions formed by smectite dehydration in pelagic clay.