Aftershocks Explained: How Long Do They Last?

By Quake Tracker Live Editorial · Published November 9, 2026

After a large earthquake, the ground does not simply go quiet. Over the next hours, days and months, smaller earthquakes keep occurring near the original rupture. These are aftershocks, and they are one of the most predictable features of seismology. Scientists cannot say when a specific aftershock will strike, but they can describe how many to expect and how the rate fades with time.

What is an aftershock?

An aftershock is a smaller earthquake that follows a larger one (the mainshock) in the same general area, and which is considered part of the same sequence. A mainshock changes the stress in the surrounding rock: it relieves stress on the part of the fault that slipped, but it transfers stress to the edges of the rupture and to nearby faults. Aftershocks happen as the crust adjusts to the new stress pattern.

The terms are relative. An earthquake is only labeled a mainshock, foreshock or aftershock once the sequence is complete. A foreshock is a smaller earthquake that precedes a larger one. In July 2019, for example, a magnitude 6.4 earthquake near Ridgecrest, California, on July 4 was followed about 34 hours later by a magnitude 7.1 mainshock on July 6, and the first one was then relabeled a foreshock. There is no reliable way to know in advance whether an earthquake is a foreshock.

Three empirical laws of aftershocks

Aftershock sequences follow patterns so consistent that they are called laws. They are statistical regularities, not physical laws like gravity, but they work remarkably well around the world.

Omori's law: aftershocks fade with time

In 1894, the Japanese seismologist Fusakichi Omori studied the aftershocks of the 1891 Nōbi earthquake and found that their rate decreased roughly in proportion to the time since the mainshock. The modern version, called the modified Omori law or the Omori-Utsu law, says that the rate of aftershocks is proportional to 1 divided by the time elapsed (with small adjustments for the exact shape).

In practice, this means that if a sequence produces about 100 aftershocks on the first day, you can expect around 50 on the second day, around 10 on the tenth day and roughly 1 per day after 100 days. The decay is fastest at the beginning and slower later, which is why the first hours are the most active, and why sequences have long tails.

Båth's law: the biggest aftershock is about 1.2 smaller

In 1965, Markus Båth observed that the largest aftershock in a sequence is, on average, about 1.2 magnitude units smaller than the mainshock. After a magnitude 7.0 earthquake, the largest aftershock is typically around magnitude 5.8. It is an average, and individual sequences vary a lot. Some have larger aftershocks, and in rare cases an aftershock is larger than the first earthquake, in which case the sequence is relabeled.

The Gutenberg-Richter relation: many small, few large

For every aftershock of a given magnitude, there are about ten times as many one magnitude unit smaller. This is the same size distribution seen in earthquakes in general. The consequence is that although large aftershocks are less likely than small ones, they are still possible, and a long sequence with thousands of small events is more likely to include a rare large one.

How long do aftershocks last?

There is no fixed answer, because it depends on the size of the mainshock and on how fast the local tectonic plates are moving. The rule of thumb is that bigger earthquakes have longer sequences.

• Magnitude 5 to 6 earthquakes: the felt aftershocks usually fade within days to weeks, though small ones continue to be recorded for months.
• Magnitude 7 earthquakes: aftershocks can be felt for months, and the sequence is above background for years. The 2018 Anchorage earthquake (M7.1) produced thousands of aftershocks.
• Magnitude 9 earthquakes: the sequence can last for decades. Japan's 2011 Tohoku earthquake (M9.1) still had an elevated rate of earthquakes more than a decade later.
• Slow-moving regions: where plates move slowly, such as the central United States, a large earthquake can have an aftershock sequence that lasts for centuries. Some researchers argue that part of the small-earthquake activity in the New Madrid zone today may still be aftershocks of 1811-1812. This is debated.

Can aftershocks be bigger than the mainshock?

By definition, the biggest earthquake in a sequence is the mainshock, so an aftershock "bigger than the mainshock" means the original one is relabeled as a foreshock. This happens in a small fraction of sequences. The USGS estimates that, in any given sequence, there is a chance (usually a few percent) that an even larger earthquake will follow, which is why the agency tells people to stay prepared after a strong earthquake instead of assuming the worst is over.

Aftershock forecasts

Since 2018, the USGS has issued operational aftershock forecasts for significant U.S. earthquakes. They give the probability of earthquakes of different sizes over the following day, week, month and year. The forecasts combine the three laws above with data from the ongoing sequence, so they are updated as the sequence unfolds. The numbers describe chances, not predictions. A forecast of a 5 percent chance of a larger earthquake means that it is unlikely, but not that it is impossible.

Why aftershocks matter for safety

An aftershock does not need to be large to be dangerous. Buildings that were damaged by the mainshock may be weakened, and a moderate aftershock can bring down a damaged wall or chimney. Rescue and recovery teams are also at risk. In coastal areas, a strong aftershock can generate its own local tsunami.

• Treat every aftershock like an earthquake. Drop, Cover and Hold On each time. See our guide on what to do during an earthquake.
• Stay out of damaged buildings until a professional has inspected them.
• Keep your emergency supplies handy for the following days. Our preparedness checklist lists them.
• Expect sleeplessness and anxiety. Frequent small shocks can be exhausting. This is a common reaction.

See aftershocks on the map

On Quake Tracker Live, the zone and city pages list every recent earthquake in the area, so after a major event you can see the sequence evolve. A typical pattern is a burst of activity in the first hours that tapers off. Try the Alaska page or California page to see how active a region is on an ordinary day, or look at the biggest recent earthquakes for the latest mainshocks. For the idea of what a given magnitude means, see our magnitude guide.

Frequently asked questions

Sources and further reading

• USGS: Operational aftershock forecasting
• FEMA / Ready.gov: Earthquakes

This guide is general information, not professional or emergency advice. Quake Tracker Live is not an official warning system; follow instructions from local authorities.