Aftershock

An aftershock is a smaller earthquake that happens after a bigger one. It occurs in the same place as the larger earthquake, called the main shock, because the Earth's crust is still adjusting after being moved by the main earthquake. Big earthquakes can cause many aftershocks, sometimes hundreds or even thousands that we can detect with instruments. These aftershocks usually get smaller and happen less often over time, following a pattern that scientists have studied.

Sometimes, the main earthquake might actually be two big quakes that happen close together. These are called doublet earthquakes, and they are different from aftershocks because they have about the same size and very similar earthquake waves. Understanding aftershocks helps scientists learn more about how the Earth's surface moves and changes after big earthquakes.

Distribution of aftershocks

Most aftershocks happen near the area where the big earthquake started. They occur along the same fault or nearby faults affected by the main earthquake. Scientists study these aftershocks to understand how big the main earthquake's impact was.

In major earthquakes like the 2004 Indian Ocean earthquake and the 2008 Sichuan earthquake, the pattern of aftershocks showed that the starting point of the earthquake was at one end of the area that moved during the quake. This helps experts learn more about how earthquakes spread.

Aftershock size and frequency with time

Aftershocks, the smaller earthquakes that follow a larger one, happen less often as time goes by. Scientists have found patterns in how often they occur. One pattern, called Omori's law, shows that the number of aftershocks drops quickly after the main earthquake. For example, the chance of an aftershock on the second day is about half of what it was on the first day.

Another pattern, Båth's law, tells us that the biggest aftershock is usually about 1.1 to 1.2 units smaller than the main earthquake on the Moment magnitude scale. Finally, the Gutenberg–Richter law explains that there are many more small aftershocks than big ones.

Main article: Gutenberg–Richter law

Effect of aftershocks

Aftershocks are smaller earthquakes that happen after a big earthquake. They can be dangerous because they are hard to predict and can sometimes be quite strong. These aftershocks can cause more damage to buildings that were already weakened by the main earthquake. Big earthquakes often have many aftershocks, and these can continue for years, especially in areas that don’t usually have many earthquakes.

For example, in the New Madrid seismic zone, aftershocks were still happening nearly 200 years after a big earthquake in 1812. In places like the San Andreas Fault in California, the ground moves more quickly, and aftershocks tend to stop after about 10 years. In New Madrid, the ground moves much slower, only about 0.2 mm each year.

Foreshocks

Main article: Foreshock

Some scientists have studied foreshocks, which are small earthquakes that happen before a larger one, to try and predict upcoming earthquakes. One of their successes was with the 1975 Haicheng earthquake in China. On the East Pacific Rise, certain faults show predictable foreshock behavior before the main earthquake. These areas tend to have fewer aftershocks but more foreshocks compared to other types of faults on continents.

Modeling

Seismologists use special tools like the Epidemic-Type Aftershock Sequence model, or ETAS, to understand how smaller earthquakes happen after a big one. These tools help them see patterns in how the ground shakes and moves after the main earthquake.

Psychology

After a big earthquake and its aftershocks, some people feel like they are experiencing another earthquake when there isn't one. This feeling is called "earthquake sickness" and is similar to motion sickness. It usually goes away as the earthquakes stop happening.