Aftershock zones can be defined in two different ways...
An aftershock is actually just a normal earthquake in every physical detail. Out of context, there is no way to tell the difference between any arbitrary earthquake and an "aftershock". The only real difference between the two is that an aftershock follows closely in the wake of a larger earthquake, and in roughly the same location as its predecessor. That larger, initial earthquake is usually referred to as the "mainshock".
More specifically, there are two guidelines for labelling an earthquake as an aftershock. First, it must occur within an "aftershock zone." This is sometimes defined as within one fault-rupture length of the mainshock rupture surface, or alternatively, within an area defined by seismologists based upon early aftershock activity. Second, it must occur within that designated area -- the "aftershock zone" -- before the seismicity rate in that area returns to its "background", meaning pre-mainshock, level. If an earthquake meets these two criteria, seismologists consider it an "aftershock."
