A related question is Hydrogen bomb radiation output, but my question is based on the seismograph data (or any other data) possibly producing a distinctive pattern.
Is it possible to tell the difference, if the test occurs inside a sealed chamber? (The obvious point here is, can a unverifiable claim be made, so it is in the interest of the "tester" not to allow radioactive byproducts to escape.)
Another obvious reply is that the answer is yes, based on previous tests and/or computer simulated tests. But I don't know if this is true or not, although I suspect a different vibration pattern is involved.
To clarify this, I thought initially (and naïvely) that the time delay as described below might show up in some fashion in seismological data, but I doubt if even LIGO (with suitable adaption), could detect the trigger explosion:
The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", with the detonation of each stage providing the energy to ignite the next stage. At a bare minimum, this implies a primary section that consists of an implosion-type fission bomb (a "trigger"), and a secondary section that consists of fusion fuel. The energy released by the primary compresses the secondary through a process called "radiation implosion", at which point it is heated and undergoes nuclear fusion.
Picture and text source: Teller-Ulam Configuration