Monitor stations around the world can detect the tell-tell signs of a nuclear device done underground, the signals are different to mother nature, secondly. They can detect the radioactive, radiation in the air.
Yeah the seismic signature of a nuclear bomb is very different to that caused by plate tectonics and magma movement and very difficult to fake with conventional explosives.
Comparison of a typical ~5.8 earthquake type signature (over 145 seconds) to NK's recent test (over 60 seconds):
https://imgur.com/cLPl475
Enthusiasts all over the world run their own geophones, etc. so its not something you can really fake.
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