I was doing some reading on an EMP and assuming there was a big enough burst at a high enough elevation that most of NA was effected the authors estimated that 90% of the population would be dead in a year. I thought that was ridiculous on it's face when I read it...and still think it's way too high...but in any case life will not be pretty in the face of such a catastophe. There will be millions of people who think they are prepared for such an event but there will also be hundreds of millions who aren't prepared to last a week. And many of those folks will descend on everyone else like packs of ravenous wolves when they get hungry enough, cold enough and scared enough. Imagine what it would be like to be stuck in Phlly or NYC when THSHTF. Once they've destroyed those cities they'll move out to the burbs and finally the countryside. Ugly scenario.
A few months ago someone here on FR recommended the novel One Second After, about an EMP that takes out all of North America. Downright bonechilling read. IIRC, the author has intelligence experience and wrote it to warn about the dangers and how ill prepared we are from the top down to deal with an EMP.
Haiti