Aftershock (seismic event)

Characteristics

Aftershocks have a number of empirical patterns that seismologists use to describe them. They tend to be clustered close to the mainshock rupture, decrease in frequency with time, and show a distribution of magnitudes skewed toward smaller values relative to the mainshock. Two widely used empirical descriptions are Omori's law, which quantifies the temporal decay in the rate of aftershocks, and the magnitude-frequency relationship that predicts many small events and few larger ones.

Mechanism and causes

Aftershocks result from changes in stress and strain on nearby faults caused by the mainshock. When one segment of the crust slips, it alters the stress field on adjacent faults and in surrounding rock; parts of the system that were brought closer to failure may then produce aftershocks. Other factors such as fluid pressure changes within faults can also promote slip after a large earthquake.

Hazards and impacts

• Structural: Buildings and infrastructure weakened by the mainshock can suffer further damage or collapse during aftershocks.
• Operational: Aftershocks complicate rescue, recovery and repair work and can cause repeated disruption to utilities and transport.
• Secondary effects: They can trigger landslides, rockfalls, liquefaction or, in offshore cases, tsunamis when conditions permit.

Monitoring, forecasting and mitigation

Seismic networks rapidly catalog aftershocks to map the evolving rupture zone and to update hazard assessments. While exact prediction of individual aftershocks is not possible, probabilistic forecasts estimate the likelihood of strong aftershocks based on established statistical models and past behavior. Public safety measures focus on rapid damage assessment, avoiding damaged structures, and designing buildings and lifelines to withstand not only a mainshock but the expected sequence of subsequent events.

Distinctions and notable facts

By definition the mainshock is the largest event in a sequence; smaller preceding events are foreshocks and smaller following events are aftershocks. Not all earthquakes in time and space are aftershocks: distant earthquakes triggered by stress changes may be considered triggered events rather than true aftershocks if they lie far from the main rupture. Understanding aftershocks is important for emergency response, engineering design and scientific study of fault behaviour.

For additional general information about seismic events and how they are classified, consult sources on earthquakes.