After a significant earthquake, the surrounding crust adjusts to the stress changes produced by the rupture. The additional earthquakes that occur in the affected region are commonly called aftershocks. They can continue for days, months or even longer after very large events. Most are smaller than the mainshock, but they can still be damaging, especially where structures have already been weakened.
Why aftershock rates are high at first
Immediately after a mainshock, the earthquake rate around the ruptured area is often much higher than the prior background rate. It usually decreases with time. Statistical aftershock models use this tendency, the mainshock magnitude and the behavior of observed sequences to estimate the expected number and probability of later events over defined time windows.
Mainshock, aftershock and foreshock are retrospective labels
The largest event in a sequence is usually called the mainshock. A smaller event after it is an aftershock. If a later earthquake turns out to be larger, the earlier event may be relabeled a foreshock. That naming rule is important: scientists cannot know in real time that a particular earthquake will definitely remain the largest event in its sequence.
What forecasts can and cannot say
Aftershock forecasts are probabilistic. They can estimate how likely certain magnitude ranges are over a day, week, month or longer interval, but they do not provide the exact time and location of the next event. Quake Pulse sequence pages are descriptive unless an authoritative forecast product is explicitly cited.
Frequently asked questions
How long do aftershocks last?
The elevated rate may persist for days to years depending on the size and tectonic setting of the mainshock.
Can an aftershock be larger than the first earthquake?
Yes. If that happens, the larger event becomes the mainshock and the earlier one may be called a foreshock.
Do aftershocks get steadily smaller?
Their rate generally decreases, but individual magnitudes do not have to decrease in a smooth sequence.