Aftershock

Sometimes a big earthquake happens.

Then, smaller shakes follow. These are called aftershocks. They happen as the ground settles into place.

There can be many aftershocks. Some big shakes have thousands of them. Most of these shakes are small. Fewer of them are large.

Aftershocks happen near the first shake. They can happen for a long time. Some can last for years.

These shakes can be dangerous. They can knock down damaged buildings.

People may even feel fake shakes. This is called earthquake sickness.

It is good to know about them. They help us learn about our world.

When a large earthquake hits, the ground moves a lot. This movement can leave the Earth's crust out of place. As the ground adjusts to its new position, smaller shakes happen. We call these aftershocks.

Aftershocks usually happen in the same area as the main shock. They often occur along the fault plane. A fault plane is the surface where the ground slipped.

Scientists use math to study how aftershocks behave. One rule is Omori's law. This law says that aftershocks happen less often as time goes by. They also get smaller over time. Another rule is Båth's law. It says the biggest aftershock is usually a set amount smaller than the main shock.

Most aftershocks are small. Only a few are large. This follows the Gutenberg–Richter law.

Aftershocks can be dangerous. They might knock down buildings that were hurt by the first shake. Some shakes can last for years. In some places, they can even last for hundreds of years! Aftershocks end when the number of shakes drops to a normal level.

An aftershock is a smaller earthquake that follows a larger one. These shakes happen in the same area as the main shock. They occur because the Earth's crust is trying to adjust. The main earthquake moves the ground out of its usual place. The crust must settle into a new position after this move. This settling process creates many smaller shakes.

Aftershocks follow a very specific pattern. Most of them happen along the fault plane. A fault plane is the surface where the ground slipped. They can also happen on other faults nearby. These shakes usually stay within a distance equal to the rupture length. The pattern helps scientists see how much the ground slipped. In the 2004 Indian Ocean earthquake, the pattern showed the rupture was asymmetric.

Scientists use special rules to understand these shakes. Fusakichi Omori described a rule in 1894 called Omori's law. This law says the number of aftershocks drops as time passes. For example, the second day has half the chance of the first. A newer version was made by Utsu in 1961. Another rule is Båth's law. It says the biggest aftershock is usually 1.1 to 1.2 units smaller than the main shock.

There are also rules about the size of the shakes. The Gutenberg–Richter law describes how many earthquakes happen in a region. This law shows there are many small aftershocks. There are far fewer large aftershocks.

This helps scientists estimate the chance of future shakes. Even though they follow patterns, the exact timing is still random. Scientists use models like ETAS to study these cascading events.

Aftershocks can be quite dangerous for people. They can collapse buildings that were already damaged. Some sequences can last for many years. In the New Madrid seismic zone, events lasted for a long time. Some were thought to be aftershocks 200 years after 1812. On the San Andreas Fault, they might last 10 years. People may even feel "phantom earthquakes" after the real ones stop. This is called earthquake sickness.

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.