Can an aftershock have a larger magnitude than the main earthquake?

The term mainshock refers to the largest event in a sequence. However, as energy release within a seismogenic system proceeds, it may be found that an event that was thought to be the largest was not the mainshock and a larger event occurs. The category into which events within a sequence would fall can only be definitely set after the high level activity of the sequence is complete. Research is ongoing to try to distinguish between large magnitude precursory events and mainshocks as the events are occurring (i.e. in real time).