# What criteria determines whether earthquakes are classified as new or aftershocks?

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 ?

I was able to figure out the Criteria , by a little armchair research & inputs from user "nate" & from user "Universal_learner" & from Britannica & from Wiki :

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.

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.

• 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
Nov 28, 2023 at 15:29

It's always beneficial to refer to what the USGS states:

"Foreshock" and "aftershock" are relative terms.

Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.

Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes.

Source: USGS

From this, I've learned that there are:

• Foreshocks, which precede larger earthquakes.
• Major earthquakes.
• Aftershocks, which are adjustments of the fault plane.

My background was in geochemistry, not geophysics, at university, so my ability to elaborate further is limited, and this might seem like a link-only answer.

I believe that the readjustment of the fault plane is the criterion for classifying an earthquake as an aftershock.

• Thank you for the Inputs. It is indeed useful to check with USGS. It is a Criteria to check readjustment of the fault plane. When we go by that thinking alone , then classifying as aftershock will require lots of measurements to conclude that there is readjustment of the fault plane. It will take time & effort. Yet , geologists routinely claim something is aftershock almost immediately after it occurs.
– Prem
Nov 28, 2023 at 15:39
• @Prem You are welcome. I am happy to help you continue learning about earthquakes. Nov 29, 2023 at 4:58