  After a large earthquake, the likelihood of successive strong aftershocks
needs to be estimated. Exploiting similarities with critical phenomena, we
introduce a scaling law for the decay in time following a main shock of the
expected number of aftershocks greater than a certain magnitude. Empirical
results that support our scaling hypothesis are obtained from analyzing the
record of earthquakes in California. The proposed form unifies the well-known
Omori and Gutenberg-Richter laws of seismicity, together with other
phenomenological observations. Our results substantially modify presently
employed estimates and may lead to an improved assessment of seismic hazard
after a large earthquake.}
