In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two (or more) main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within four-tenths of a unit of magnitude – distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock (Båth's law), and decrease in magnitude and frequency according to known laws.