A Staff Report from the Straight Dope Science Advisory Board

What's the difference between an earthquake and an aftershock?

September 22, 1999

Dear Straight Dope:

What is the difference between an "earthquake" and an "aftershock"? More to the point, when does an aftershock qualify as an earthquake in its own right? I mean, I can understand if a place gets nailed with a 4.3 earthquake and there are several smaller temblors over the next few days. But I read in the newspaper the other day that a 4.3 earthquake that hit a certain area was classified as an aftershock to a prior earthquake in the same place--that happened four years earlier! Wouldn't an aftershock coming along four years after a quake be considered a separate earthquake? Or does this fall into the category of an "is Greenland an island or a continent?" sort of question?

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. Make sense? 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. Sounds shaky to me.

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