Since you seem to be knowledgeable, do you think we have the ability to do a “focused” EMP blast that would disable all electronics nearest the NK/SK border but not on the SK side?
Seems like if we did, we could disable a lot of the communications at all those artillery sites pointed at Seoul.
That Question could never be answered.
The strength of an electric field falls off at the square of the distance from a point source, as one over the distance from a line source... and from a planar source it doesn't fall off at all. This would be the effect seen directly under a high-altitude nuclear or thermo-nuclear burst, and for some distance away in any direction.
The Starfish Prime explosion of July 1962 damaged electrical systems in Hawaii, almost 900 miles away. This long before the intricate and sensitive microprocessor circuitry on which so much of our civilization relies (but also long before fiber optic cables, that are immune to EMP effects and serve to break up large conductive loops in communications networks, thereby improving their EMP strength). That event resulted from the detonation of a 1.4 MT hydrogen bomb at an altitude of 250 miles.
The only way you could "focus" these effects better would be to set off many large bombs simultaneously at high altitude, which we can almost certainly do. I don't think the North Koreans can do anything like that at present, but I'm also pretty sure that they are working on something like that, or worse.