Assume that each earthquake can produce a series of aftershock independently of its size according to its local Omori's law with exponent 1+&thgr;. Each aftershock can itself trigger other aftershocks and so on. The global observable Omori's law is found to have two distinct power law regimes, the first one with exponent p-=1-&thgr; for time t1. ¿ 1999 American Geophysical Union