# The longest interval between earthquake main shock & after shock

On August 21, 2022, the Western Australian town of Cadoux experienced a magnitude 4.4 earthquake. According to the report of the incident, that earthquake might be an aftershock of a magnitude 6.1 earthquake the town experienced 43 years earlier, in June 1979.

According to Geoscience Australia,

it was quite common for aftershocks to be experienced decades later, and this most recent tremor could have been an aftershock from the 6.1-magnitude event more than 40 years ago.

If such delays in the occurrence of aftershocks are quite common what is regarded as being the longest interval between the main quake and aftershock? Secondly, what is the reason for such delays?

• your problem is defining an aftershock, aftershocks are just earthquakes triggered by the change in forces caused by other earthquakes, so every earthquake could technically be an aftershock.
– John
Aug 28, 2022 at 14:44

This is a difficult question to answer because the causal relationship between earthquakes is not something that is measured, but rather something that is modeled.

Nonetheless, we often will say that aftershocks can persist for a very long time, after a larger earthquake in particular, wherein the rate of seismicity is higher than some nominal (or regional) background rate and decays with time. In Japan, aftershocks have been reported for more than 100 years following the Nobi earthquake.

A typical model used to describe earthquake occurrence states that all earthquakes have some probability to be a 'mainshock', an aftershock, or an aftershock of an aftershock, (and so on). In this way, under the assumptions of this model, we can provide a probabilistic assessment of whether an earthquake is an aftershock, but not an absolute or deterministic assessment.

As for the reason for the delay, there are many potential mechanisms. It is likely a combination of mechanisms. These may include the fact that frictional failure can start with slow nucleation (e.g. rate and state friction), fluids very slowly migrating through the crust as a response to the mainshock, a viscous response of the crust or mantle, and more.

• Thank you! Much appreciated. I was hoping someone would provide an answer.
– Fred
Nov 2, 2022 at 4:58