aftershocks

Aftershocks are smaller earthquakes that follow a larger earthquake, known as the mainshock, and occur in the same general area. They result from ongoing adjustments in the crust after the initial rupture. Aftershocks can occur within minutes to hours after the mainshock and may continue for days, weeks, or months, with the frequency usually decreasing over time. While most aftershocks are smaller than the mainshock, a few can be relatively large and cause additional damage in already weakened areas.

Seismologists describe the decay in aftershock rate with time using empirical relations such as Omori's law, which states that the frequency of aftershocks decreases roughly inversely with time since the mainshock. Variants consider background seismicity and regional stress. Spatially, aftershocks tend to cluster around the fault segment that ruptured, gradually widening as stress redistributes.

The triggering of aftershocks is linked to stress changes in the crust caused by the mainshock slip. Static stress transfer can bring nearby faults closer to failure; dynamic triggering from seismic waves can also provoke them. Aftershocks tend to be smaller as the crust relaxes, but complex fault networks can generate sequences with variable magnitudes and occasional large events still.

Aftershocks pose ongoing hazards to investigators, responders, and damaged structures; they can cause further collapse, hinder rescue efforts, and trigger secondary hazards such as falls, landslides, or tsunamis in subduction zones. Officials use aftershock forecasts to guide safety measures and inspections, though precise prediction remains unreliable.

Foreshocks are smaller earthquakes that precede a mainshock and are not a reliable predictor of a larger event, whereas aftershocks occur after the mainshock as the crust adjusts.