Spatial propagation (from top to bottom) of the fracture front during a perturbation of the loading. Pixels cover a region of 20×20 μ m2. The color code is the time that the front is waiting at a given pixel (waiting time in seconds) which is inversely proportional to the local speed of the front. Circles mark ‘optical’ events made of accelerated patches with respect to the average speed of the front. This image corresponds to the propagation of the front from time equals 754 s as referred to in Figure 4b.