Fig. 1

Setup to perform the numerical simulations. The upper layer (100–1000 m) comprises particle A, whereas the bottom layer (0–100 m) comprises particle B. The left side wall pushes the particles to the right side at a small rate of displacement (0.009 m for each calculation time step). The moving wall has a slit (0–60 m) to produce a detachment (décollement) within the bottom layer. The shear drag factor of particle B increases from the left-hand side to the right-hand side in response to the displacement of the moving wall (modified after Miyakawa et al. 2010)