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. It can be weeks or decades.
Bigger earthquakes have more and larger aftershocks.
The bigger the mainshock, the bigger the largest aftershock, on average . The difference in magnitude between the mainshock and largest aftershock can be 0.1 to 3 or more, but averages 1.2. There are many more small aftershocks than large ones. We sometimes see large aftershocks months or even years after the mainshock. We can use these patterns to calculate probabilities of aftershocks that can be quite high.