# How long after an earthquake can an aftershock occur?

**URL:** <https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294>\
**Category:** Factual Questions\
**Created:** [October 19, 2007, 1:58pm UTC](https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294 "2007-10-19T13:58:20Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Skammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/skammer/32/143_2.png) [@Skammer](https://boards.straightdope.com/u/Skammer)\
**Post date:** [October 19, 2007, 1:58pm UTC](https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294/1 "2007-10-19T13:58:20Z")

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Yesterday, a mild (2.5) earthquake was recorded in Massachusetts. According to a local [news story](http://www.thebostonchannel.com/news/14375072/detail.html), there was also a 1.8 quake in the area earlier this month. According to the story, “Officials said that [earlier] quake was a late aftershock of an earthquake from 1727 in Newburyport.”

Wow! An earthquake can produce an aftershock two hundred and eighty years later?

How do seismologists distinguish between an aftershock and a new, unrelated earthquake in the same area?

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**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\
**Post date:** [October 19, 2007, 2:16pm UTC](https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294/2 "2007-10-19T14:16:20Z")

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Someone goofed. Either someone misspoke, was misquoted or there was a misprint.

I do not know precisely how long after an earthquake am aftershock may occur but there is actually an equation called [Omori’s Law](http://en.wikipedia.org/wiki/Aftershock) that follows this and it has a time component.

There is no way that was an aftershock of a 280 year old earthquake.

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**Author:** ![Skammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/skammer/32/143_2.png) [@Skammer](https://boards.straightdope.com/u/Skammer)\
**Post date:** [October 19, 2007, 3:20pm UTC](https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294/3 "2007-10-19T15:20:55Z")

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Okay, based on [this](http://www.boston.com/news/local/articles/2007/10/10/a_link_to_1727_quake_rattles_merrimack_valley/) story, I’ve identified the unnamed “official”. The reporter says,

> [@The Boston Globe](#):
>
> The quake registered 1.8 on the Richter scale, said John Ebel, director of Boston College’s Weston Observatory. It struck a few miles from the epicenter of the quake in Newburyport on Oct. 29, 1727, which was a 5.5 magnitude and shook the earth from Maine to Philadelphia.
> 
> Combing through Colonial records, Ebel said he counted more than 100 aftershocks that immediately followed the 1727 quake.
> 
> The frequency decreased, but aftershocks have continued, with four small quakes alone in January 1999. The persistence of seismic activity indicates that the quakes are connected, Ebel said.
> 
> “In a sense, it is a part of history,” he said. “It could also be a warning about something that’s going to happen.”

Ebel identifies the quakes of 1999 as aftershocks and implies that these are as well, at least according to the reporter. I can easily imagine, though, that he said the recent quakes are connected (having roughly the same epicenter) and the reporter jumped to the conclusion “aftershocks.”

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**Author:** ![Rain\_Soaked](https://avatars.discourse-cdn.com/v4/letter/r/e19adc/32.png) [@Rain\_Soaked](https://boards.straightdope.com/u/Rain_Soaked)\
**Post date:** [October 19, 2007, 4:29pm UTC](https://boards.straightdope.com/t/how-long-after-an-earthquake-can-an-aftershock-occur/423294/4 "2007-10-19T16:29:07Z")

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See:

[http://www.straightdope.com/mailbag/maftershock.html](http://www.straightdope.com/mailbag/maftershock.html)

for more info.