An aftershock is a small earthquake that echoes or follows a larger one. After an earthquake, people often wait nervously to see if there will be an aftershock.
After the relief of surviving an earthquake, the tremors and shaking of aftershocks can be surprising and often dangerous. Sometimes houses and buildings that were damaged during the main earthquake can collapse during the aftershock that follows. Aftershocks are caused by the ground settling and adjusting to the changes in the faults that may have resulted from the original quake.