  We report on experiments investigating the dynamics of a slider that is
pulled by a spring across a granular medium consisting of a vertical layer of
photo-elastic disks. The motion proceeds through a sequence of discrete events,
analogous to seismic shocks, in which elastic energy stored in the spring is
rapidly released. We measure the statistics of several properties of the
individual events: the energy loss in the spring, the duration of the movement,
and the temporal profile of the slider motion. We also study certain
conditional probabilities and the statistics of mainshock-aftershock sequences.
At low driving rates, we observe crackling with Omori-Utsu, Bath, and waiting
time laws similar to those observed in seismic dynamics. At higher driving
rates, where the sequence of events shows strong periodicity, we observe
scaling laws and asymmetrical event shapes that are clearly distinguishable
from those in the crackling regime.
