Fig. 4

a Changes in the shear stresses at the three points denoted in Fig. 2d including the period during the triggering perturbation (shown by the expanded time axes in Fig. 3), the following period, and the triggered instability. b Changes in velocities. c Changes in displacements. The parameters are the same as those in Fig. 2. Figure 3 shows expansions of these plots around t = 0