Metastability and instability in model grain piles: Scaling below and at the threshold

We present a careful simulational study of the onset of instabilities in a toy model grain pile as it is tilted through a threshold angle. Our simulations on a system of 1196 grains reveal qualitative differences in the number of grains undergoing translation over extended length scales below and at the threshold angle. The results are consistent with available experimental data and with the mean field treatments of self-organized critical behavior.