An aftershock is a smaller earthquake that follows a larger mainshock in the same region, occurring as the crust adjusts to the stress changes caused by the main event. Aftershocks can persist for months or years and pose significant risk to already-damaged structures.
An aftershock is an earthquake that occurs after a larger mainshock in the same geographic region, as the crust adjusts to the stress perturbations caused by the main event. The mainshock-aftershock sequence is one of the most robust statistical patterns in seismology: the frequency of aftershocks decays approximately according to Omori's Law, where the rate of aftershocks decreases inversely with time after the mainshock. Modified versions (Omori-Utsu) and extensions (ETAS β Epidemic-Type Aftershock Sequence) describe this decay and the triggering of secondary aftershocks more accurately.
Aftershocks occur because the mainshock redistributes stress in the surrounding crust. Some regions experience increased Coulomb stress (encouraging failure), while others experience decreased stress (discouraging it). Aftershocks cluster in the regions of increased stress β typically at the ends of the rupture, in the hanging wall of thrust faults, and around the edges of the rupture zone. The largest aftershock is often about one magnitude unit smaller than the mainshock (BΓ₯th's Law), though this is a statistical tendency rather than a strict rule. Aftershock sequences can persist for months to years, with occasional damaging events β the 2011 Christchurch earthquake in New Zealand, for example, was itself an aftershock of the 2010 Darfield earthquake, and caused far more damage than its predecessor.
Aftershocks pose a severe risk to already-damaged structures and to emergency response operations. Buildings weakened by the mainshock may collapse under smaller aftershocks, and rescue workers face ongoing danger. For critical facilities β hospitals, bridges, pipelines, and lifelines β aftershock hazard must be explicitly considered in emergency planning. Modern seismic networks publish aftershock forecasts within hours of a major earthquake, using statistical models calibrated by the mainshock magnitude and regional seismicity. These forecasts β probabilistic estimates of the number and location of aftershocks above a given magnitude β inform decisions about evacuation, building access, and restoration of services. In engineering, aftershocks are increasingly considered in performance-based design for structures whose post-earthquake functionality matters, such as hospitals and emergency operations centers.