Let's say a 1 MT nuke went off in the center of the US. One megaton is about 4 * 10^15 joules. Over a circle with a 2200 km radius (area 1.5 * 10^7 sq km), that's 2.7 * 10^8 joules per sq km, or 266 joules per meter. This assumes even distribution of energy, which won't happen -- much more than half the energy will be radiated into space.
I'm not sure how much actual damage that will do.
EMP lenses off of the ionosphere. The EMP itself shakes electrons loose and lenses them back down to the earth in massive quantities. There are three aspects to the attack, which include a ‘long wave’ that uses long conductors to transmit the energy to transformers and generators which can melt the windings.
This wouldn’t be all that bad, but we don’t make the transformers and generators here in the US that can replace them. They come from Asia now.
And the locomotives used to move them? Diesel electric now.
Also, there are nukes and their are nukes designed to maximize gamma yield.
You can design a fairly small nuke to maximize the gamma yield. The less high explosive you can get away with, the better. It’s the gamma yield that shakes the electrons loose.
The only damage that counts it the damage done to the electrical grid, particularly to transformers and generators. We already know it’s going to be hell on capacitors and IC’s.
We live in a ‘just-in-time’ supply chain now. Let the local Safeway run out of food and see what happens in a week.
ULF comms were justified particularly because you can harden them against EMP. The nuke triad ensured that if you did try to take us out with EMP, our subs would do the same, and worse. We could use ULF to communicate with the subs to ensure it happened. They’d be the eyes and ears figuring out who did what to whom.
One Second After is an excellent novel based on very good research on the topic. What’s funny about it is that it is likely optimistic. Good book.