[SSS04-P06] Frictional properties of incoming sediments/rocks at shallow conditions of the Japan Trench subduction zone
キーワード：frictional properties、incoming sediments/rocks、Japan Trench
In order to examine the change in frictional properties of incoming sediments/rocks at shallow conditions of the Japan Trench subduction zone, we conducted triaxial friction experiments on the following samples at effective confining pressures of 50–150 MPa and temperatures (T) of 50−150°C, and at displacement rates (V) changed stepwise among 0.1155, 1.155 and 11.55 μm/s. We used hemipelagic and pelagic mud samples collected from the cover sediments on the Pacific plate off Sanriku, a chert sample cored from the footwall of the plate boundary thrust near the Japan Trench, and a basalt sample cored from the oceanic basement of the Philippine Sea plate off Kii Peninsula. We then fitted the friction data for each step change in V by the rate- and state-dependent friction constitutive law, and obtained the optimized (a − b) value, i.e., an indicator of frictional stability, at each V.
The results show that steady-state friction coefficient μss decreases with increasing content of clay minerals (wt%) from 0.58−0.61 of the chert sample (0 wt%), through 0.52−0.53 of the basalt sample (≈21 wt%) and 0.33−0.36 of the hemipelagic mud sample (≈55 wt%), to 0.21−0.32 of the pelagic mud sample (≈89 wt%). μss of a given sample tends to increase with increasing T, the amount of which is greater for the mud samples than that for the chert and basalt samples. Because the mud samples contain significant amounts of smectite (μss ≈0.2), which is transformed into illite (μss ≈0.5) at temperatures of 50−150°C, more amount of μss increase is expected with increasing T for these samples than that for the chert and basalt samples. For the chert and basalt samples with a small or no amount of smectite, the μss increase with increasing T is possibly due to increasing gouge lithification, which is promoted by thermally activated dissolution–precipitation creep.
Our results also show that (a − b) value tends to increase with increasing content of clay minerals, while (a − b) value of a given sample tends to decrease with increasing T. The former is attributable to the stabilizing effect of clay minerals, while the latter is likely due to the effects of dissolution–precipitation creep, because its activity increases gouge lithification with increasing T or decreasing V, resulting in higher μss. The transition T from a − b > 0 to a − b < 0 also increases with increasing content of clay minerals; 50°C < T < 100°C in the chert sample, T ≈ 100°C in the basalt sample, 100°C < T < 150°C in the hemipelagic mud sample, 150°C < T in the hemipelagic mud sample. This implies that the transition from stable aseismic faulting to unstable, possible seismic faulting occurs with increasing T at the Japan Trench subduction zone, but the transition T is different among incoming sediments/rocks.
The results show that steady-state friction coefficient μss decreases with increasing content of clay minerals (wt%) from 0.58−0.61 of the chert sample (0 wt%), through 0.52−0.53 of the basalt sample (≈21 wt%) and 0.33−0.36 of the hemipelagic mud sample (≈55 wt%), to 0.21−0.32 of the pelagic mud sample (≈89 wt%). μss of a given sample tends to increase with increasing T, the amount of which is greater for the mud samples than that for the chert and basalt samples. Because the mud samples contain significant amounts of smectite (μss ≈0.2), which is transformed into illite (μss ≈0.5) at temperatures of 50−150°C, more amount of μss increase is expected with increasing T for these samples than that for the chert and basalt samples. For the chert and basalt samples with a small or no amount of smectite, the μss increase with increasing T is possibly due to increasing gouge lithification, which is promoted by thermally activated dissolution–precipitation creep.
Our results also show that (a − b) value tends to increase with increasing content of clay minerals, while (a − b) value of a given sample tends to decrease with increasing T. The former is attributable to the stabilizing effect of clay minerals, while the latter is likely due to the effects of dissolution–precipitation creep, because its activity increases gouge lithification with increasing T or decreasing V, resulting in higher μss. The transition T from a − b > 0 to a − b < 0 also increases with increasing content of clay minerals; 50°C < T < 100°C in the chert sample, T ≈ 100°C in the basalt sample, 100°C < T < 150°C in the hemipelagic mud sample, 150°C < T in the hemipelagic mud sample. This implies that the transition from stable aseismic faulting to unstable, possible seismic faulting occurs with increasing T at the Japan Trench subduction zone, but the transition T is different among incoming sediments/rocks.