Aftershocks

An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock. 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 formed as the crust around the displaced fault plane adjusts to the effects of the main shock. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

Most earthquakes form part of a sequence, related to each other in terms of location and time.[23] Most earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.
Almost any earthquake forecast requires proper accounting for earthquake clustering, mainly for aftershock occurrence, although foreshocks, if present, may be used to calculate a mainshock probability. Even if we are mainly interested in a long-term earthquake forecast, the influence of earthquake clustering on the results should be estimated. Moreover, faithful modeling of earthquake clustering is needed for any short-term earthquake forecasting.