Fig. 4

Mechanical data from velocity-stepping experiments conducted at elevated normal stresses. a Shear stress versus shear displacement for the samples deformed at 2 (orange), 5 (green), 10 (red) and 20 MPa (blue). The upper left inset displays a schematic of the double-direct shear configuration. b Steady-state shear stress (τ) as a function of the normal stress (σ) at a 1.5-mm shear displacement where a quasi-steady-state is achieved. The friction coefficient and cohesive strength determined by a least-square fit (coefficient of determination R2 = 0.9975) with the Mohr–Coulomb linear equation (dashed lines) were 0.43 (± 0.02) and 0.22 (± 0.17), respectively. c Quasi-steady-state friction coefficient corrected for the cohesive strength (0.22 MPa) as a function of normal stress