Aftershocks are smaller earthquakes that occur following a larger seismic event, known as the mainshock. They are a common part of an earthquake sequence, representing the Earth’s crust adjusting to the initial rupture. These subsequent tremors happen in the same general area as the mainshock. Understanding aftershocks provides insight into the dynamic nature of Earth’s tectonic plates.
How Aftershocks Form
Aftershocks arise from the redistribution of stress within the Earth’s crust after a main earthquake. A mainshock involves a sudden release of accumulated stress along a fault, causing the ground to shake. This primary rupture does not relieve all stress uniformly across the affected region. Instead, the sudden movement can transfer or increase stress to adjacent areas or segments of the same fault system.
The crust then adjusts to this altered stress field. This adjustment leads to subsequent, smaller ruptures in these highly stressed areas, experienced as aftershocks. These tremors often occur on or near the mainshock fault plane, or on other smaller, subsidiary faults within the volume of rock affected by the main event. Aftershocks are the Earth’s way of settling into a new state of equilibrium following a major seismic disturbance.
Patterns of Aftershock Activity
Aftershocks exhibit predictable patterns following a mainshock. Their frequency and magnitude decrease over time, a phenomenon described by Omori’s Law. While many aftershocks may occur immediately after the main event, their number and intensity diminish rapidly in the hours, days, and weeks that follow.
Geographically, aftershocks are concentrated around the main earthquake’s fault rupture area. They commonly occur within one to two fault lengths from the mainshock fault, often delineating the main rupture’s extent. Larger and potentially damaging aftershocks can still occur, especially shortly after the main event, posing continued hazards. The duration of aftershock sequences varies widely, from weeks or months to several years, and in rare cases, even centuries, depending on the mainshock’s size and the area’s geological characteristics.
Identifying Aftershocks
Seismologists distinguish aftershocks from other seismic events, such as foreshocks or new mainshocks, primarily through their temporal and spatial relationship to a larger earthquake.
A foreshock is a smaller earthquake that precedes a larger mainshock in the same general location. The classification of an event as a foreshock is only possible retrospectively, once a larger earthquake has occurred. If an earthquake initially thought to be an aftershock turns out to be larger than the preceding “mainshock,” the classification is revised, and the larger event becomes the new mainshock, with the original event re-designated as a foreshock. This retrospective analysis of the sequence of events and their relative magnitudes is how seismologists categorize these seismic tremors.