The Gutenberg-Richter power law distribution for energy released at earthquakes can be understood as a consequence of the earth crust being in a self-organized critical state. A simple cellular automaton stick-slip type model yields D(E)≈E-&tgr; with &tgr;≈1.0 and &tgr;≈1.35 in two and three dimensions, respectively. The size of earthquakes in unpredictable since the evolution of an earthquake depends crucially on minor details of the crust. ¿ American Geophysical Union 1989