17:15 〜 18:30
[SCG39-P05] Frictional coefficient and its velocity dependence of cuttings retrieved from the Nankai accretionary prism at IODP Site C0002
キーワード：摩擦、南海トラフ、粘土鉱物
To understand the process of Nankai megathrust earthquakes, the Nankai Trough Seismogenic Zone Excavation Project (NanTroSEIZE) has been conducted as part of the IODP since 2007 to 2019. Previous studies of friction experiments were performed using core samples collected by ODP/IODP drilling project (e.g., Takahashi et al., 2014; Ikari et al., 2020), however, these samples were retrieved form the shallow parts of the Nankai accretional prism. In order to make detailed depth profile of frictional properties, we carried frictional experiments on cuttings samples, which were collected from 875 to 3262 meters below seafloor (mbsf) during riser operations in the IODP Expeditions 348 and 358.
Frictional tests (currently about 40 cuttings) were conducted by biaxial frictional machine at Hiroshima University under wet conditions (brine, 0.5 mol/L) at in-situ effective normal stress assuming the hydrostatic condition (σe = 9–34 MPa). The cuttings samples were ground and the grain size was adjusted to less than 106 µm. The gouge sample was sandwiched by gabbro blocks in double–direct configuration. After slip behavior reached a steady state at 3 µm/s, velocity was changed stepwise between 0.3, 3, and 33 µm/s. In these tests, frictional coefficient at a steady state and the velocity dependence parameters (a, b, and Dc) defined by the rate- and sate-dependent friction constitutive law were examined. To obtain the velocity dependence parameters, we used software “RSFit3000” (Skarbek and Savage,2019). The relationship between frictional properties and clay contents were also investigated using clay content data reported by Underwood 2017.
Frictional coefficient of the cuttings samples were ranging from 0.472 to 0.657, in which frictional coefficients tend to slightly as the depth increased. Frictional coefficients show a negative correlation with increasing smectite content, which is consistent with the pervious study (Takahashi et al., 2014). Velocity dependence of friction(a-b) were positive ranging from 0.001 to 0.006, indicating a velocity strengthening behavior. Dc tends to decrease with depth, which is likely related to decreasing smectite contents. From these experimental results, the frictional properties at Site C0002 are governed by the clay mineralogy and its content. The relatively large Dc and positive velocity dependence at shallow depth may favor the occurrence of slow slip that have been observed around the drilling site.
Frictional tests (currently about 40 cuttings) were conducted by biaxial frictional machine at Hiroshima University under wet conditions (brine, 0.5 mol/L) at in-situ effective normal stress assuming the hydrostatic condition (σe = 9–34 MPa). The cuttings samples were ground and the grain size was adjusted to less than 106 µm. The gouge sample was sandwiched by gabbro blocks in double–direct configuration. After slip behavior reached a steady state at 3 µm/s, velocity was changed stepwise between 0.3, 3, and 33 µm/s. In these tests, frictional coefficient at a steady state and the velocity dependence parameters (a, b, and Dc) defined by the rate- and sate-dependent friction constitutive law were examined. To obtain the velocity dependence parameters, we used software “RSFit3000” (Skarbek and Savage,2019). The relationship between frictional properties and clay contents were also investigated using clay content data reported by Underwood 2017.
Frictional coefficient of the cuttings samples were ranging from 0.472 to 0.657, in which frictional coefficients tend to slightly as the depth increased. Frictional coefficients show a negative correlation with increasing smectite content, which is consistent with the pervious study (Takahashi et al., 2014). Velocity dependence of friction(a-b) were positive ranging from 0.001 to 0.006, indicating a velocity strengthening behavior. Dc tends to decrease with depth, which is likely related to decreasing smectite contents. From these experimental results, the frictional properties at Site C0002 are governed by the clay mineralogy and its content. The relatively large Dc and positive velocity dependence at shallow depth may favor the occurrence of slow slip that have been observed around the drilling site.