What is the term for a smaller earthquake that follows a larger one?

A smaller earthquake that follows a larger one is called an aftershock. Aftershocks occur in the same general region as the main earthquake, as the surrounding crust adjusts to stresses changed by the main rupture.

Aftershock sequences can include hundreds or thousands of detectable earthquakes. Their frequency usually decreases with time, although a strong aftershock can still occur days, weeks or even months after the mainshock. Some aftershocks are powerful enough to collapse buildings already weakened by the first earthquake, making them a major hazard for rescuers and survivors.

The terms foreshock and aftershock depend on the final sequence. An earthquake is called a foreshock only if a larger, related earthquake later occurs. The largest event is generally identified as the mainshock. A swarm is a cluster of earthquakes without one clearly dominant mainshock, so it is not simply another word for aftershock.