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Reading jeffronicus' answer about how seismic stations record bomb explosions, particularly the reminder that the intensity of earthquakes drops by distance, made me start wondering what scale of earthquakes can be missed if they occur in just the right place, due to gaps in the observing network and the energy dropping too low by the time it reaches the nearest seismic stations.

Being a novice, I have no idea about typical resolution of instruments, the density/gaps in the observing network, nor whether earthquakes have other wave features that are more coherent and consistent than the impacts of weaponry explosions. But I figure there must be an intensity and location that would be currently undetectable? So how strong of an earthquake can be missed these days?

  • For simplicity's sake, lets focus on fairly shallow earthquakes (in/near the crust?), as I'm figuring depth is an additional factor... and some quite strong movements thousands of km down go undetected!?
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    $\begingroup$ I started writing an answer, but my computer crashed & I lost references. Apparently a mag 4 quake produces a displacement of 10 nanometers - very small. Detection & non-detection depends on local density of seismographs. The number of seismographs globally is vast & there are a large number of seismograph networks. It would be difficult to miss a quake, but it might happen in large deep oceans. $\endgroup$
    – Fred
    Nov 19, 2023 at 1:35
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    $\begingroup$ @Fred Fortunately there are ocean-bottom seismometers to help with some of those. $\endgroup$ Nov 20, 2023 at 0:25
  • $\begingroup$ That will very much depend on where the earthquake happens. If it's in Southern California, with its dense network of seismometers, you will see a lot of things that you will surely miss if the earthquake happened in the northern Pacific or southern Indian Ocean where there are few islands with seismometers. Ocean bottom seismometers may help for some locales, but these are so expensive to build and operate that their numbers are vastly smaller than land-based ones. $\endgroup$ Nov 28, 2023 at 16:17

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What you are looking for is called the Magnitude of Completeness or Completeness Magnitude. There are several ways to estimate this quantity, one of the common ways is to look at the power law distribution the events such as earthquakes are supposed to follow, and check below which magnitude the actual detected distribution begins to deviate from the theoretical case.

See e.g. https://pubs.geoscienceworld.org/ssa/bssa/article/95/2/684/146883/Assessing-the-Quality-of-Earthquake-Catalogues

As mentioned in other comments, this value varies greatly depending on the catalogue or network you are looking at. There is also some variability depending on how the value is calculated.

Just as rough numbers: for an arbitrary earthquake happening anywhere on the planet (i.e. global catalogue), the completeness magnitude is around 5.5. For local networks this value can improve to between 2 to 4. For local network specifically designed to monitor active areas, such as in California, Japan, this value can be from 1 to 2.

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