In seismology, not all earthquakes are the same, and not every sequence of tremors follows the same pattern. As a rule, after a major earthquake there are aftershocks of lower magnitude that gradually decrease over time. But there is a rarer and more unsettling phenomenon: the seismic doublet.
As seismologist Lucía Lozano of Spain's National Seismic Network explained, a seismic doublet occurs when "two earthquakes of very similar magnitude take place, very close together in time and very close together in space".
