Man made EMPs are more problematic. Suitcased sized EMPs will fry all electronics within a few blocks. The nation wide catastrophe is if they deployed these suit-cased sized EMPs next to the 150-200 power junction points connecting the regional power grids - local power production goes on, but the balancing of supply and demand is destroyed. So unless your town gets most of its power from a steady hydroelectric source or natural gas plant that can moderate power generation to power demand or a local power grid with multiple power sources, we risk an India scale blackout until new transformers are in place. Areas that use more power than can be locally supplied risk brown outs and blackouts. Areas with greater supply than demand can scale back production but face challenges mediating power supply.
The other use for EMPs like this is someone surrounding Google server farms with these things in the back of a bunch of trucks and setting them off, shutting down a major portion of the internet and crippling, but not killing, the economy. Conversely, the same small scale EMPs could be used around the NSA / federal data center being built in Utah to cripple that.
Very large EMPs are the size of big bombs. Someone could probably ship those in via Mexican trucks or Chinese shipping containers. You'd end up with most high end electronics in a major city permanently toasted, a local blackout that takes weeks to repair as you spend time replacing everything burnt out, and deaths from pacemakers, insulin pumps and elevators failing. If there was an attack similar to a large nuclear scale strike on major cities, the same places that are safe from nuclear blasts due to distance and protective mountain ranges are safe from EMP blasts. As with all radiation, twice as far away reduces its strength to a quarter. A number of BIG EMP blasts in the major cities, or even just big ports like Boston, New York, New Orleans and San Francisco could cripple the nation. Not many people would die from these blasts. But if it were set off in a port, you're also in range of the connecting railroad stations and within a short distance of airports, highways and the transportation control centers that keep our Just in Time Economy going. Imagine a traffic jam like people trying to flee Hurricane Katrina, but it is all the people who want to leave AND all the trains are stopped AND all the planes are stuck AND most ships are dead in the water.
People inland and in rural areas are physically fine. Their local water, sewer and probably power are unaffected at first. Then the impact of a JIT delivery system hits. Stores empty from lack of deliveries from the coasts, businesses suffer from challenges shipping anything out, and anyone dependent upon imports suffers. Imagine the business disruptions because of the Japanese tsumani leaving their suppliers short, and then do it to several major manufacturing cities and the hubs that connect the rest of the country. Even if your factory is fine, if you have trouble getting key components, you're idle. And if someone sets of a large EMP weapon somewhere like Houston and Lake Charles, LA at the same time, they've just destroyed a huge proportion of our refining capacity.
And the so called "smart grid" where all appliances are smarter and everything talks to everything else to monitor and manage power usage just makes us MORE vulnerable to EMP strikes.
Very large EMPs are the size of big bombs. Someone could probably ship those in via Mexican trucks or Chinese shipping containers.That's going to be wholly ineffective (a ground burst) in generating EMP.
October is coming...