According to the Southern California Earthquake Center (SCEC), the difference between an earthquake, also known as the mainshock, and an aftershock is that an aftershock follows closely in the wake of a larger earthquake and in approximately the same area as that earthquake. Earthquakes are usually more powerful and longer lasting than aftershocks. An aftershock will not occur unless there is a mainshock that occurs first.
Aftershocks can occur as regularly as earthquakes that happen within a certain area. Aftershocks happen over a period of weeks, months and even years, notes the SCEC. Generally, there are bigger, longer and more frequent aftershocks when the main earthquake is large.
The SCEC also explains that an aftershock is a normal earthquake in every physical detail. The only difference between an "arbitrary earthquake" and an "aftershock" is that an aftershock follows closely after a larger earthquake. Seismologists label an earthquake as an aftershock using two guidelines. First, the earthquake must occur within an "aftershock zone," which is usually defined as the region within one fault-rupture length of the mainshock rupture surface or the area defined by seismologists based on previous aftershock activity. Second, it must happen within this zone before the seismicity rate in the area returns to its pre-mainshock level. An earthquake that meets these two specific criteria is considered an aftershock.