  We monitor the stick-slip displacements of a very slowly driven moveable
perforated top plate which interacts via shearing with a packing of identical
glass beads confined in a tray. When driven at a constant stress rate, the
distributions of large event displacements and energies triggered by the
stick-slip instabilities exhibit power law responses reminiscent of the
Gutenberg-Richter law for earthquakes. Small events are quasi-size independent,
signaling crossover from single-bead transport to collective behavior.
