Interevent Time Analyses for Avalanches on a Conical Bead Pile

A conical bead pile subject to slow driving is used as a model critical system to experimentally investigate the distributions of avalanche sizes and time between events. The pile is composed of roughly 20 000 steel beads, 3 mm in diameter; the pile is driven by dropping one bead at a time from a small height onto the apex of the pile. Changes in pile mass are recorded over the course of tens of thousands of bead drops to characterize the distribution of avalanche sizes. An external magnetic field may be applied, inducing cohesion among the beads. Expanding previous work by Kelly Kim, the Bi test and Omori’s law were applied to additional avalanche data to investigate trends in bead drop height and cohesion. The Bi test analyzes the behavior of large avalanches throughout time. We find that at lower drop heights or with increased cohesion, large avalanches are more uniformly distributed in time. Omori’s law analyzes the rate of aftershocks following a main avalanche. Higher drop heights yielded larger Omori exponents. We also analyze the progression of the average size of avalanches over small intervals between two main shocks. As the intervals grew closer to the next main shock, the average sizes of the avalanches increased.