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An aftershock is an earthquake that occurs after a previous earthquake (the main shock). An aftershock is in the same region of the main shock, but is per definition of smaller magnitude strength. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are smaller earthquakes formed as the crust around the displaced fault plane adjusts to the effects of the main shock.