How long do aftershocks last

After a significant earthquake, the ground does not simply go quiet. The rupture leaves the surrounding rock stressed in new ways, and it adjusts through a series of smaller earthquakes called aftershocks. The short answer to how long they last is that the rate falls off quickly at first but the sequence can continue for months, years, or, after the very largest earthquakes, decades. The reason follows a pattern that has been understood for more than a century.

Omori's law: fast at first, then a long tail

In 1894 the Japanese seismologist Fusakichi Omori noticed that aftershock rates decay in a predictable way: roughly in proportion to one divided by the time since the mainshock. In practice that means the day after a large earthquake is by far the busiest, the first week sees most of the notable aftershocks, and the rate then tapers into a long, slowly thinning tail. You might feel several aftershocks an hour at first, then several a day, then a few a week, trailing off over months. The decay is steep early and gentle late, so the last aftershocks can arrive long after the sequence seems finished.

How long is that in practice

The duration scales with the size of the mainshock. A moderate magnitude 5 earthquake might produce a handful of felt aftershocks over a few days to weeks. A magnitude 7 can generate aftershocks for a year or more. The greatest earthquakes leave the crust adjusting for a very long time; scientists still attribute some small earthquakes in the central United States today to the great earthquakes of 1811 and 1812. A useful rule of thumb is that the bigger the mainshock, the longer and more energetic the aftershock sequence, and the larger the area it covers.

Bath's law: how big the biggest aftershock is

There is also a pattern to aftershock size. Bath's law observes that the largest aftershock is typically about 1.2 magnitude units smaller than the mainshock, regardless of the mainshock's size. So a magnitude 7 earthquake tends to have a largest aftershock near magnitude 5.8, itself a damaging event. This is only an average, and individual sequences vary, but it is a helpful expectation: the biggest aftershock is usually clearly smaller than the mainshock, yet still large enough to matter, especially to structures already weakened.

Can an aftershock be bigger than the mainshock

Occasionally an earthquake in a sequence is larger than the one before it. When that happens, seismologists relabel the events. The larger earthquake becomes the mainshock, and the earlier ones are reclassified as foreshocks. In other words, an aftershock cannot, by definition, be bigger than its mainshock, because the moment a bigger earthquake occurs, the naming shifts to make the largest event the mainshock. This is more than wordplay: it is why any earthquake can, in principle, be a foreshock to something larger, and why officials avoid promising that the strongest shaking is over.

Pattern	What it describes
Omori's law	Aftershock rate decays roughly as one over the time since the mainshock: intense at first, then a long, thinning tail.
Bath's law	The largest aftershock is on average about 1.2 magnitude units below the mainshock.
Foreshock reclassification	If a later earthquake is larger, it becomes the mainshock and earlier events are relabelled foreshocks.
Duration	Scales with mainshock size, from days for a moderate event to years or decades for the greatest.

What this means for safety

The practical lesson is that the risk does not end when the main shaking stops. Aftershocks, and the small chance that the first earthquake was a foreshock, are why guidance is to treat the hours and days after a large earthquake with continued care: expect more shaking, be wary of buildings and slopes already weakened, and follow official updates. Monitoring agencies now publish aftershock forecasts after significant earthquakes, giving the probability of further strong shaking over the next day, week, and year, and those forecasts are built directly on the Omori and Bath patterns described above. They are best read as odds, not promises: a quiet day does not mean the sequence is over, and a busy one does not mean the worst is still coming. You can watch a sequence unfold in real time, aftershocks clustering around the mainshock, on the live globe, or check what has been recorded near you. To understand how the sizes in a sequence translate into felt shaking, see the guides to what a magnitude feels like and the Modified Mercalli intensity scale.