what about a zero-yield or "test stand level yield" explosion. or is there a ton TNT limit or something

@EvanHarper The market specifies "a live-fire nuclear explosion," which implies that there actually has to be criticality. While it might appear as an earthquake on sensors, it would be distinguishable from the release of short-lived radioactive isotopes. (Also, earthquakes aren't usually localized to near the surface under test sites.)

The above market includes tests.

https://www.telegraph.co.uk/world-news/2022/10/03/nuclear-weapons-convoy-sparks-fears-putin-could-preparing-test/