  Applying mechanical perturbations in a granular assembly may rearrange the
configuration of the particles. However, the spatial propagation of an event is
not related to the size of the external perturbation alone. Thus, the
characteristic length scale of an event is not well defined. In this study, we
trigger rearrangements by driving two intruders through a vertical
two-dimensional packing of disks. The amplitude of the rearrangements of the
granular assembly appearing around the two evolving intruders is related to
their separating distances. We show that there exists a characteristic distance
between intruders under which the dynamics of the grains above one intruder is
influenced by the other. The size of the intruders has little effect on this
characteristic length. Finally, we show that the correlation between the
movements of the grains decreases with the distance away from the intruders
over a larger length scale.
