Frictional properties of the shallow Nankai Trough accretionary sediments dependent on the content of clay minerals

<jats:title>Abstract</jats:title> <jats:p>We conducted triaxial friction experiments on the shallow Nankai Trough accretionary sediments at confining pressures, pore water pressures, temperatures close to their <jats:italic>in situ</jats:italic> conditions, and axial displacement rates (<jats:italic>V</jats:italic> <jats:sub>axial</jats:sub>) changed stepwise among 0.1, 1, and 10µm/s. The results revealed that their frictional properties change systematically according to the content of clay minerals, smectite in particular. The steady-state friction coefficient (<jats:italic>μ</jats:italic> <jats:sub>ss</jats:sub>) at <jats:italic>V</jats:italic> <jats:sub>axial</jats:sub> = 1µm/s decreases with increasing clay mineral content, shown in weight percent, from 0.82 for a sandstone sample (6 wt%), through 0.71 for a tuff sample (≈17 wt%), and 0.53 to 0.56 for siltstone samples (29 to 34 wt%), to 0.25 for a claystone sample (42 wt%). Slip-dependent frictional behavior changes accordingly from slip hardening for the sandstone sample, through quasi steady-state slip for the tuff and siltstone samples, to distinct slip weakening for the claystone sample. Although all samples exhibit velocity-strengthening behavior upon stepwise changes in sliding velocity, the ratio of the (<jats:italic>a</jats:italic> − <jats:italic>b</jats:italic>) value to the velocity dependence of steady-state friction (∆<jats:italic>μ</jats:italic> <jats:sub>ss</jats:sub>/∆ln<jats:italic>V</jats:italic> <jats:sub>sliding</jats:sub>) decreases with increasing clay mineral content, which implies that the friction component decreases while the flow component increases accordingly. Thus, faulting in the shallow Nankai Trough accretionary prism is likely controlled by the clay mineral content, in particular the smectite content, in the sediments as well as in the fault zones.</jats:p>