mainshock

A mainshock is the largest earthquake in a cluster or sequence that occurs in a given region over a relatively short time. It is a term used in seismology to describe the principal event around which aftershocks occur and, in retrospective studies, around which foreshocks may be identified. The designation of a mainshock depends on the chosen spatial area and time window; a sequence can be reinterpreted if a larger event occurs later in the same region.

In a typical sequence, smaller earthquakes that precede the mainshock are called foreshocks, while those that follow are called aftershocks. Aftershocks occur as the crust adjusts to the new stress state after the mainshock and usually decline in frequency and magnitude over time, a pattern described by empirical decay laws. The mainshock itself is not necessarily the largest possible event in a region, but it is the largest event within the specific sequence under consideration.

Mainshock–aftershock activity is studied to understand fault systems, assess seismic hazard, and model expected shaking. The magnitude distribution of earthquakes in a sequence often follows the Gutenberg–Richter relation, and aftershock sequences tend to exhibit predictable decay, though exact patterns vary by region and tectonics. In practice, the term mainshock is a descriptive label that helps organize and compare seismic sequences, rather than a property of the earthquake itself.