In seismology, doublet earthquakes—and more generally, multiple earthquakes or twin earthquakes—were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as distinct earthquake sequences having two main shocks with similar/slightly different magnitudes that occurred twice in a single moment, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude—often within 0.4 magnitude—distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock and decrease in magnitude and frequency according to known laws.