Boundary-induced anisotropy of the avalanches in the sandpile automaton

We study numerically the avalanches in a two--dimensional critical height sandpile model with sand grains added at the center of the system. Smaller avalanches near the center of the system are isotropic. Larger avalanches are, however, affected by the boundary of the system, to a degree that increases with the avalanche size. Up to linear system size $L=1001$, we did not find an obvious indication for lattice--induced anisotropy.