<p>I was able to figure out the Criteria , by a little armchair research &amp; inputs from user &quot;nate&quot; &amp; from user &quot;Universal_learner&quot; &amp; from <a href="https://www.britannica.com/science/aftershock-geology" rel="nofollow noreferrer">Britannica</a> &amp; from <a href="https://en.wikipedia.org/wiki/Aftershock" rel="nofollow noreferrer">Wiki</a> :</p>
<p>Particularly , the Wiki Section <a href="https://en.wikipedia.org/wiki/Aftershock#Effect_of_aftershocks" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Aftershock#Effect_of_aftershocks</a> gave the Major Hint :</p>
<p>&quot;Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area; see, for example, the New Madrid Seismic Zone, where events still follow Omori's law from the main shocks of 1811–1812. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level; i.e., no further decay in the number of events with time can be detected.&quot;</p>
<p>Thus The Main Criterion (in addition to Position &amp; type &amp; Etc) is whether the seismic activity has returned to normal background level.</p>
<p>When background &quot;hum&quot; or &quot;rumble&quot; or seismic activity is higher than normal when multiple earth quake occur , then all are treated in one large &quot;sequence&quot; , with the &quot;largest&quot; called main &amp; the others called foreshocks &amp; aftershocks.</p>
<p>When background seismic activity goes back to normal , then that &quot;sequence&quot; is deemed over.<br />
That terminal event might take Days or Weeks or Months or even 200 Years.</p>
<p>[[ I will try to added Charts &amp; Images later ]]</p>
<p>Of course , the &quot;sequence&quot; must occur within the same Position &amp; must have same triggers , though the background seismic activity decides whether it is new &quot;sequence&quot; or not.</p>

CC BY-SA 4.0
2023-11-28T17:19:15.303Z
2023-11-28T17:19:15.303Z
Prem
<p>That's an interesting question. <em>Without</em> checking literature, this is my take for an answer...</p>
<p>A given earthquake has a defined, estimated focus and epicenter. That being said, I assume that an &quot;aftershock&quot; event (from an initial earthquake) is one where displacement occurs at or near that corresponding focus/epicenter from the initial earthquake - also, perhaps even with the same or similar <a href="https://www.iris.edu/hq/inclass/animation/focal_mechanisms_explained" rel="nofollow noreferrer">focal mechanism</a>.</p>

CC BY-SA 4.0
2023-11-28T15:29:32.770Z
Thank you for the Inputs. Position is a main Criteria. When we go by that thinking alone , then almost everything will be aftershock because earthquakes have occurred almost everywhere. Then we will have to introduce new Criteria for time , like within few days or few months. The Question will remain why something is still aftershock after 200 years.
Prem
CC BY-SA 4.0
2023-11-26T19:36:00.547Z
2023-11-26T19:36:00.547Z
nate
<p>I saw a recent paper where it is claimed that aftershocks can occur years, decades or centuries after the main earthquake.</p>
<p>When seismic activity is observed, what criteria determines the classification of whether that earthquake is new or an aftershock ?</p>
<p>Some stories about the topic:</p>
<ul>
<li><a href="https://science.slashdot.org/story/23/11/22/165204/aftershocks-can-occur-centuries-after-original-earthquake-says-study" rel="noreferrer">Slashdot</a></li>
<li><a href="https://v" rel="noreferrer">Guardian</a></li>
<li><a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023JB026482" rel="noreferrer">AGU paper</a></li>
</ul>

CC BY-SA 4.0
2023-11-23T15:46:35.070Z
I did check https://en.wikipedia.org/wiki/Earthquake & https://en.wikipedia.org/wiki/Aftershock where after shocks ~~ 10 Years & ~~ 200 Years are indeed listed , though I could not see the Criteria.
Prem
<geology><geophysics><earthquakes><measurements><earth-observation>
What criteria determines whether earthquakes are classified as new or aftershocks?
Prem
# What criteria determines whether earthquakes are classified as new or aftershocks?

(Asked by: username_0 on 2023-11-23)

I saw a recent paper where it is claimed that aftershocks can occur years, decades or centuries after the main earthquake.

When seismic activity is observed, what criteria determines the classification of whether that earthquake is new or an aftershock ?

Some stories about the topic:

* [Slashdot](https://science.slashdot.org/story/23/11/22/165204/aftershocks-can-occur-centuries-after-original-earthquake-says-study)
* [Guardian](https://v)
* [AGU paper](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023JB026482)

**Tags:** geology, geophysics, earthquakes, measurements, earth-observation

> **Comments:**
> **username_0**: I did check https://en.wikipedia.org/wiki/Earthquake & https://en.wikipedia.org/wiki/Aftershock where after shocks ~~ 10 Years & ~~ 200 Years are indeed listed , though I could not see the Criteria.  

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## 2 answers

### Answer by username_0 on 2023-11-28. Score: 1

I was able to figure out the Criteria , by a little armchair research & inputs from user "username_1" & from user "Universal\_learner" & from [Britannica](https://www.britannica.com/science/aftershock-geology) & from [Wiki](https://en.wikipedia.org/wiki/Aftershock) :

Particularly , the Wiki Section <https://en.wikipedia.org/wiki/Aftershock#Effect_of_aftershocks> gave the Major Hint :

"Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area; see, for example, the New Madrid Seismic Zone, where events still follow Omori's law from the main shocks of 1811–1812. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level; i.e., no further decay in the number of events with time can be detected."

Thus The Main Criterion (in addition to Position & type & Etc) is whether the seismic activity has returned to normal background level.

When background "hum" or "rumble" or seismic activity is higher than normal when multiple earth quake occur , then all are treated in one large "sequence" , with the "largest" called main & the others called foreshocks & aftershocks.

When background seismic activity goes back to normal , then that "sequence" is deemed over.  
That terminal event might take Days or Weeks or Months or even 200 Years.

[[ I will try to added Charts & Images later ]]

Of course , the "sequence" must occur within the same Position & must have same triggers , though the background seismic activity decides whether it is new "sequence" or not.


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### Answer by username_1 on 2023-11-26. Score: 1

That's an interesting question. *Without* checking literature, this is my take for an answer...

A given earthquake has a defined, estimated focus and epicenter. That being said, I assume that an "aftershock" event (from an initial earthquake) is one where displacement occurs at or near that corresponding focus/epicenter from the initial earthquake - also, perhaps even with the same or similar [focal mechanism](https://www.iris.edu/hq/inclass/animation/focal_mechanisms_explained).

> **Comments:**
> **username_0**: Thank you for the Inputs. Position is a main Criteria. When we go by that thinking alone , then almost everything will be aftershock because earthquakes have occurred almost everywhere. Then we will have to introduce new Criteria for time , like within few days or few months. The Question will remain why something is still aftershock after 200 years.  

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