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. This happens because the Earth's crust is still adjusting after the main earthquake.

Big earthquakes can cause many aftershocks. Sometimes there can be hundreds or even thousands of these smaller quakes. These aftershocks usually get smaller and happen less often over time. Scientists study these patterns to learn more about how the Earth's surface moves and changes after big earthquakes.

Sometimes, the main earthquake might actually be two big quakes that happen close together. These are called doublet earthquakes. They are different from aftershocks because they are about the same size and have very similar earthquake waves.

Distribution of aftershocks

Most aftershocks happen close to where the big earthquake began. They happen along the same fault or nearby faults that were affected by the main earthquake. Scientists study these aftershocks to learn more about the main earthquake's impact.

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

Aftershock size and frequency with time

Aftershocks are smaller earthquakes that happen after a bigger one. They happen less often as time goes by. Scientists have found patterns in how often they occur.

One pattern is called Omori's law. It 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 is Båth's law. It tells us that the biggest aftershock is usually 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 hard to predict and sometimes are strong. Aftershocks can damage buildings that were already weakened by the main earthquake. Big earthquakes often have many aftershocks. These aftershocks can continue for years, especially in areas that don’t often have 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 usually 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 study foreshocks. These are small earthquakes that happen before a bigger one. They hope this can help predict bigger earthquakes. One success was with the 1975 Haicheng earthquake in China. On the East Pacific Rise, some faults show predictable foreshock behavior before the main earthquake. These areas usually have fewer aftershocks but more foreshocks than other types of faults on continents.

Modeling

Seismologists use special tools to study smaller earthquakes that happen after a big one. These tools help them learn about the patterns of how the ground shakes and moves.

Psychology

After a big earthquake and its aftershocks, some people might feel like they are experiencing another earthquake even when they are not. This feeling is called "earthquake sickness" and is similar to motion sickness. It usually goes away once the earthquakes stop happening.