I am told, from the days of the first Test Ban Treaty, that methods were developed to go for much more lower yields to test nukes, which are not particularly detectable. Apparently they have enough data from the early full-scale tests, Los Alamos and such, thus dispensing with full-scale tests. For info, just reading 'The Samson Option' by Sy Hersh.

Otoh, Iran is liable to earthquakes. Quite a number of seismic fault lines there ... from memory.

Take your pick ..