What Is the Difference Between an Earthquake and an Aftershock?
Any earthquake after the main quake in an area is an aftershock. The term, aftershock, is used to define the sequence of the quakes. Quakes detected before the major one are called foreshocks. The main earthquake is the one with the largest magnitude.
The shock waves caused by earthquakes are measured with an instrument called a seismograph. In a great earthquake, the shocks caused by the sudden movement of rock beneath the earth may be felt a thousand miles away. Seismographs on the other side of the world pick up the waves.
Earthquakes usually come in clusters divided into foreshocks, mainshocks and aftershocks. If an aftershock is stronger than the mainshock, it becomes the mainshock and the mainshock becomes a foreshock. Basically they’re all earthquakes, but they’re related. Aftershocks must occur geographically near the mainshock, though they can occur on another nearby fault, triggered by the stress on the mainshock’s fault.
According to the seismology/geography glossary, aftershocks must occur “after a larger earthquake (a mainshock), within one rupture-length of the original fault rupture” (or within what is called an “aftershock zone” in some places).
Another big difference between a mainshock and the aftershocks is that we expect aftershocks. The vast majority of aftershocks happen within hours of the mainshock, and the chance of them occurring decreases rapidly as time passes. Generally speaking, there are half as many aftershocks the second day after a mainshock and 1/10 the number of aftershocks 10 days afterwards.
But it’s also true on average that the larger the mainshock, the stronger its biggest aftershock and the more of them there will be. Though aftershocks aren’t as strong, they can still cause quite a bit of damage, especially if the main quake was a big one.
Aftershocks presumably related to a mainshock may be felt weeks, months, or even years afterwards. There’s a rule of thumb that an earthquake is an aftershock–part of the same cluster–if it occurs during a time period when the seismicity rate in an area is higher than before the mainshock, even if that’s a decade later.
In other words, when the earthquake activity in a region is back down to the background rate before the mainshock, any new earthquake is a new mainshock or a foreshock to a new mainshock.