Aftershock

In the field of seismology, an aftershock is a smaller earthquake that follows a larger event. This larger event is known as the main shock. Aftershocks occur in the same area as the main shock. They happen because the displaced crust must adjust to the effects of the primary movement. When a massive earthquake occurs, it shifts the Earth's crust significantly. The crust then settles into a new position through a series of smaller movements.

Large earthquakes can produce hundreds or even thousands of detectable aftershocks. These events follow a consistent pattern where both their magnitude and frequency steadily decrease over time. Most aftershocks are located over the full area of the fault rupture. They occur either along the fault plane or on other faults within the volume affected by strain. Typically, aftershocks appear within a distance equal to the rupture length from the fault plane. This distribution is useful for scientists. The pattern helps confirm the total size of the area that slipped during the main shock. For example, the 2004 Indian Ocean earthquake and the 2008 Sichuan earthquake showed asymmetric rupture propagation. In both cases, the epicenter lay at one end of the final slip area.

Scientists use specific empirical laws to describe how aftershocks behave. One of the most important is Omori's law. This was first described by Fusakichi Omori in 1894. It states that the frequency of aftershocks decreases roughly with the reciprocal of time after the main shock. A modified version, the Utsu-Omori law, was proposed by Utsu in 1961. This version includes a constant, $p$, which modifies the decay rate. This rate typically falls between 0.7 and 1.5. If $p$ equals 1, the probability of an aftershock on the second day is half that of the first day. On the tenth day, the probability is approximately one-tenth of the first day.

While these laws describe statistical behavior, the actual timing and location of each shock are stochastic, meaning they are random. The Utsu-Omori law can be derived theoretically through differential equations. One interpretation involves the deactivation of faults near the main shock. Another rule is Båth's law. This law states that the difference in magnitude between a main shock and its largest aftershock is nearly constant. This difference is typically 1.1 to 1.2 on the Moment magnitude scale. This constant difference holds regardless of how large the main shock is.

Another key concept is the Gutenberg–Richter law of size scaling. This law describes the relationship between magnitude and the total number of earthquakes in a region over a specific time. It shows that there are many more small aftershocks than large ones.

Scientists use this to understand the distribution of seismic events. To study these cascading sequences of aftershocks and foreshocks, seismologists use tools like the Epidemic-Type Aftershock Sequence model, or ETAS. This helps model how one event can trigger another in a chain reaction.

Aftershocks present real dangers to people and infrastructure. They are often unpredictable and can reach large magnitudes. A dangerous effect is that aftershocks can collapse buildings already weakened by the main shock. The duration of an aftershock sequence varies greatly. In some cases, sequences can last for years. For instance, in the New Madrid seismic zone, events following the 1811–1812 main shocks were considered aftershocks nearly 200 years later. In contrast, the San Andreas Fault averages about one year of land movement, though aftershocks there may top out at 10 years. An aftershock sequence is only considered finished when the rate of seismicity drops back to a background level.

Sometimes, the effects of earthquakes extend to human psychology. Many people report feeling "phantom earthquakes" even when no seismic activity is occurring. This condition is called earthquake sickness. It is thought to be related to motion sickness. These sensations usually fade away as the actual seismic activity tails off. Understanding these patterns helps scientists distinguish between true aftershocks and other seismic events like doublet earthquakes. In a doublet earthquake, the main rupture happens in two or more steps. These are distinguished from aftershocks because they have similar magnitudes and nearly identical seismic waveforms.