BOUNDARY EFFECTS IN A RANDOM NEIGHBOR MODEL OF EARTHQUAKES

We introduce spatial inhomogeneities (boundaries) in a random neighbor version of the Olami, Feder, and Christensen model [Phys. Rev. Lett. 68, 1244 (1992)] and study the distributions of avalanches starting bo th from the bulk and from the boundaries of the system. Because of the ir clear geophysical interpretation, two different boundary conditions have been considered (named free and open, respectively). In both cas es the bulk distribution is described by the exponent tau similar or e qual to 3/2. Boundary distributions are instead characterized by two d ifferent exponents tau'similar or equal to 3/2 and tau'similar or equa l to 7/4, for free and open boundary conditions, respectively. These e xponents indicate that the mean-field behavior of this model is correc tly described by a recently proposed inhomogeneous form of the critica l branching process.