mainshockaftershock

Mainshock-aftershock is a term used in seismology to describe the typical sequence of earthquakes that follows a large event on a fault. In such a sequence, the largest event is the mainshock, and smaller earthquakes that occur in the same region during the hours to years after the mainshock are the aftershocks. Some sequences also include foreshocks, smaller earthquakes that precede the mainshock, but foreshocks are not required in all cases.

Aftershocks tend to be located near the rupture zone of the mainshock and their frequency decreases with time. This temporal decay is often described by Omori's law (also called the Omori-Utsu law), in which the rate falls roughly as t^-p, with p around 1. The magnitudes of aftershocks are typically smaller than that of the mainshock, and the largest aftershock is commonly 1 to 2 magnitudes smaller, though exceptions occur. The size distribution of events in an aftershock sequence generally follows the Gutenberg-Richter relation.

Statistical models such as the ETAS model treat aftershock sequences as a branching process in which earthquakes can trigger further events, creating clusters of activity. These models are used in probabilistic forecasting to estimate the likelihood of subsequent earthquakes in the days to years after a mainshock.

Physical mechanisms: The rupture associated with the mainshock redistributes stress in the surrounding crust, changing Coulomb failure stress on nearby faults and fault segments, which can trigger aftershocks as the crust settles toward a new equilibrium.

Impact and relevance: Understanding mainshock-aftershock sequences helps in hazard assessment and emergency response, and provides insight into earthquake triggering processes. While the mainshock usually dominates the sequence, aftershocks can themselves be large enough to cause additional damage and may require separate response planning.