“But an emp blast is not.”
THis is true....but to generate an EMP electromagnetic field you need to start with gamma rays that come from a nuclear detonation.
You can’t hide the nuclear detonation.
Probably in the form of a massive aurora borealis. Interesting the interaction of the earth's magnetic field too with all of the charged particles from the blast which would induce massive currents on the ground. The light show would be impressive, though.