The term for smaller earthquakes that occur after the main shock is aftershock. Aftershocks happen as the Earth's crust adjusts to the sudden stress changes caused by the main earthquake, usually near the original rupture zone.
Aftershock sequences can begin within minutes and continue for days, months, or even years after a very large earthquake. They generally become less frequent over time, although a later earthquake can occasionally be larger than the first event, changing which quake is called the main shock.
Aftershocks are not the same as foreshocks. A foreshock is an earthquake later followed by a larger main shock, while a tremor is a broader term for ground shaking. An earthquake swarm is a cluster of earthquakes without one clearly dominant main shock. Seismologists commonly describe aftershock frequency using Omori's law.