1. Scientists would start working on a solution to how we all ended up in some alternate reality where North Korea was able to not only build a nuke that big but actually get it all the way to the Atlantic. However, no one would be surprised that they completely missed every continent along the flight path.
2. There would be a lot of dead fish, radioactive water, and some radioactive clouds heading along to jetstream to Europe. Keep in mind that 60 years ago, the nuclear powers detonated nukes in the middle of the Pacific on a routine basis.
3. If those scientists concluded that we weren't all transported to the mirror universe, then a lot of people would be pissed at North Korea. If anyone else were president, I'd say that Kim would get a stern rebuke, but I wouldn't put it beyond Trump to turn North Korea into a parking lot.