It is not an EM emission from the nuclear device itself which causes EMP.
Rather, gamma rays emitted during the detonation cause Compton scattering in the upper atmosphere (stripping electrons off of atoms and molecules in the air).
The earth's magnetic field then accelerates these electrons to near-relativistic speeds; the moving electrons create large magnetic fields, which induce short but intense currents in electrical materials, leading to all kinds of fun.
“The earth’s magnetic field then accelerates these electrons to near-relativistic speeds”
This alone tells me you don’t know what you are talking about. I don’t mean this as criticism of you. You read this someplace and believed it. It just isn’t true...
A magnetic field will bend (deflect) the path of an electron. It will not accelerate it. It takes an electric field to accelerate an electron.