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The setting is current day Los Angeles... except for the goblin city that exists beneath it. Its chambers range from about 500-2000ft underground and has a population of several thousand. A lot of what the goblins have comes from things they steal from humans at night: radios, power tools, babies, and whatever other odds and ends they find lying around. Because of this, the kinds of sounds you hear in the goblin city will be a lot like what you hear in the human city, just deeper. While they certainly make a fair amount of noise, when I wrote this setting, I assumed that whatever noise they made would be mostly muffled by hundreds of feet of Earth and blend with whatever noise the humans were making up above.

But then I read this https://xkcd.com/2344/ this morning and realized that Los Angeles probably has some pretty dedicated seismologists keeping an eye on things. This got me wondering:

Would seismologists notice the goblin city?

Assume the humans do not generally know that there are goblins in their city at all, so they would not be actively looking for them.

Well, I'm convinced at this point that taking no special precautions would lead to the city being discovered, but if you have any ideas about how to the Goblins could take special precautions to hide, see related question: How to hide this underground city from seismologists, oil prospectors, and ground surveying projects?

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  • $\begingroup$ Is this city a system of small underground passages, like in "Minecarft", or it's one huge cavern? $\endgroup$
    – Alexander
    Commented Aug 11, 2020 at 16:51
  • $\begingroup$ Sort of a mixture of both, there are several fairly large caverns interconnected by tunnels. The largest caverns if I recall were about about 30-50ft high and 200-300 hundred feet across in either direction, but most caverns were much smaller and more tunnel like. $\endgroup$
    – Nosajimiki
    Commented Aug 11, 2020 at 17:03
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    $\begingroup$ @user535733 While that is a valid concern, I found a perfectly scientific solution for this. The water table is only 20-100ft thick in most of California; so, the city lies entirely beneath it. This means there is only a small but manageable waterfall of fresh water near the main entrance were the goblins dug through the bedrock to reach the surface. This then feeds down a bunch of streams that function kind of like an aqueduct system for the city, and ends with portal to a pocket dimension ruled by a Sídhe Lord who uses the water to build his ever expanding kingdom of ice in the faerie world. $\endgroup$
    – Nosajimiki
    Commented Aug 11, 2020 at 19:45
  • $\begingroup$ I would not want to use water downstream from a goblin city for /anything/ unless I had some top-tier water treatment systems/magic in place. $\endgroup$
    – Salda007
    Commented Aug 12, 2020 at 8:00
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    $\begingroup$ The caverns themselves would be noticed. City noises would be noticed. Seismologists study this area because of how active the faults are. They want to give notice before the next huge quake. Incidentally, the caverns should have collapsed in the last big quake. $\endgroup$
    – NomadMaker
    Commented Aug 12, 2020 at 16:27

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The problem is not the sounds the goblins make. Since the goblins need to move around (and probably breath as well), they will be moving through a gaseous medium. That is much less dense than rock, and will easily be detected when analyzing seismic waves (even the ones with epicenters far from the Goblin city).

Seismic waves, like any other wave, refract when they move from one medium to another. When analyzing such waves, seismologists will notice there is a pocket of air where the city is. Even if you make the goblins aquatic and fill the city with water, it would still be quite noticeable.

Notice that some waves go all the way through the planet. Also from this link:

Primary and secondary waves are body waves that travel within the Earth. The motion and behavior of both P-type and S-type in the Earth are monitored to probe the interior structure of the Earth. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition. Differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earth's inner structure.

So there is basically nowhere underground where the goblins could hide. Seismographers everywhere would be studying the weird set of chambers because it would stand out very clearly.

Here is a paper where scientists use wave refraction to detect and detail a water collection shaft. I think that with current technology, the goblin city could not be mapped in detail, but it would be possible to see it in a similar way that you can see a baby in the womb with ultrasound. You would definitely know there is something there.

Also notice that muography is increasingly becoming a thing. With patience and money we could already map the whole interior of the Earth in 3D. From the wiki:

Film muography enabled them to obtain the first interior imaging of an active volcano in 2007, revealing the structure of the magma pathway of Asama volcano.

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Methinks they'd be detected

Seismometers are by definition extremely sensitive, recording all that is going on around them whether seismic or not. (Source)

Considering seismometers can accurately detect small things miles away, the answer is most certainly "no." It's theoretically possible that the goblins could set up a random pattern (like walking on sand in Dune) that would hide the noise, but that gets less and less likely with population, which eventually develops "habits" (like devoted walkways/transport).

You could assume that they're incredibly quiet critters, but the first time they cracked rock for expansion, it would be detected.

Ultimately, a seismograph is only as sensitive as the accuracy of the "proof weight" and the balancing technology. But humans have been very accurate with those for some time now. Think "pendulum clocks.")

Alternatively...

On the other hand, let's assume the goblins were smart enough to move into the subway, sewers, water overflow tunnels, etc., or build close to them. Their activities might be hidden in the operational noise of those existing structures. How big are your goblins? I can't even imagine the number of rats, dogs, alligators, already living in those places. So, if you don't push them so far away — making them parasites of humanity — they might be hidden for some time.

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There are people who know about the goblins.

But it is not publicized. Besides goblins there are other very good reasons for humans not to start poking around the underworld.

The party line given to geologists and seismographers is that there is an acoustically reflective layer beneath the city, which turns out to be true and experimentally demonstrable with test noises. Unusual sounds detected from the goblins are thus attributed to faint echoes of human noise coming down from the city then being reflected back. These noises are altered in the transmission and reflections but bear the characteristics of human type noises and so are routinely filtered out and ignored.

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Other answers have given good reasons why caverns under Los Angeles would be detected. A second part of the question would be whether there were any cities where the caverns might go undetected. It might be easier for something like this to go undetected in a city like Paris or Johannesburg, which is built on top of a large network of former mines. Although depending on how long the goblins have been there, it may be hard to explain how the miners missed them when digging the mines.

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