The correct answer is - after shocks.
The after shocks are essentially minor earthquakes that appear after the main earthquake. They appear because of the further adjustments into the crust. The after shocks vary is strength. These minor vibrations can last for only few seconds, or few minutes, but their time span can be anywhere from a single day, few days, few weeks, or even a couple of months after the main earthquake appeared.
Occurrences like these can be noticed on places that are close to plate boundaries, where there's often earthquakes, and lot of adjustments inside the crust.
