What Is the Difference Between Earthquake and Aftershock?

According to the Southern California Earthquake Center (SCEC), the difference between an earthquake, also known as the mainshock, and an aftershock is that an aftershock follows closely in the wake of a larger earthquake and in approximately the same area as that earthquake. Earthquakes are usually more powerful and longer lasting than aftershocks. An aftershock will not occur unless there is a mainshock that occurs first.