An
earthquake following and occuring in the same area as the 'main' earthquake.
The largest cause of aftershocks is the redistribution of stress to faults which have been opened or weakened by the main earthquake. The number of aftershocks depends on the magnitude of the main earthquake and decrease exponentially with time.
Aftershocks are generally of smaller magnitude than the parent event, with further aftershocks becoming less and less powerful. Aftershocks can still cause large amounts of damage due to the weakening of structures and fault lines after the parent event.
An aftershock can occur minutes to weeks after the main earthquake.