aftershock

An aftershock is an earthquake that follows a larger event, the mainshock, occurring in the same general area. Aftershocks are typically smaller than the mainshock, but they can be sizable and may continue for days, weeks, months, or even years as the crust adjusts after rupture.

The mechanism involves redistribution of stress on nearby faults after the mainshock, triggering additional ruptures as rocks settle into new stress conditions. This stress transfer can activate faults that were stressed but not yet ruptured, producing a sequence of earthquakes in the surrounding region.

Aftershock rate declines with time according to Omori's law or the Omori-Utsu variant, with the number of aftershocks decreasing roughly as 1/(t+c)^p. The mainshock magnitude influences the overall productivity of aftershocks—the larger the mainshock, the more aftershocks are typically expected. Aftershocks can themselves trigger further ruptures, prolonging the sequence.

Most aftershocks are smaller than the mainshock, but relatively large aftershocks can occur and cause renewed damage. Forecasts estimate the probabilities of additional events in the near term and rely on historical sequences, current seismicity, and physical models to update risk assessments as a sequence evolves.

Aftershocks can pose continued hazards by damaging already weakened structures, complicating rescue, relief, and assessment efforts. Seismological agencies monitor sequences and issue alerts or updates to reflect evolving risk in the aftermath of the mainshock.

Foreshocks are events that precede a mainshock in some sequences; not every mainshock has foreshocks, and distinguishing true foreshocks from unrelated seismic activity in real time can be challenging.