Well, if they used a nuclear bomb to destroy the building, nobody's going near it. That might be the real reason for the big silence - the first use of nuclear weapons in war since WWII.
If they used a small tacnuke set to ground burst, the EMP wouldn’t even be that large, nor would there have been that many large scale effects. Seismographs would “see” an event comparable to a large conventional explosion.
FYI, there have been at least two nuclear weapons used in combat since WW2. During the Sino-Soviet border wars (WW2-1993) had two events recorded on seismographs in the West that could only have been large tactical nukes or small strategic ones.
Nobody’s talking about that.