Earthquakes come in clusters.
The stress on the mainshock's fault changes during the mainshock and that fault produces most of the aftershocks. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well. Aftershocks are other earthquakes triggered either on the mainshock fault or at a distance from the mainshock fault no greater than the length of that fault. An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour. We call an earthquake an aftershock as long as the rate of earthquakes in the area is greater than it was before the mainshock.