User blog comment:IamJakuhoRaikoben/Chapter 388: Erza vs. Minerva, Review./@comment-24053170-20140617062927/@comment-7418318-20140617072004

Don't take those science fiction stories too serious. EMP doesn't just deactivate machines and everything will go back to normal a few minutes later. An EMP will fry unprotected and active electronic machines and also mess up anything that works with magnets (like hard-drives for example).

But I doubt EMP make a good comparison in this case. I doubt it will create something as "simple" as an interference.

More likely the Faces either change the Eternano to something Humans can't use (perhaps into that which the Demons use to fuel their curses) or it might damage, destroy or seal the magic containers within the Mages. Or maybe it's just creating a massive magicl shockwave that pushes the Eternano away from the epicentre, but due to its magical nature doesn't affect anything that isn't magical. Or it might work like the reverse anima did and send the magic power off to a parallel dimension, maybe even Edolas.

As for the range thing, I think that's exactly why there are thousands of Faces. One might not have the range and/or power to effect the entire continent, but all of them together surely do.