This topic is a bit far afield from backpacking, but I'll bite.

"Don't have to carry it" is the key phrase here. 90 days worth of food don't have to be freeze dried! It is much cheaper to buy "normal" shelf stable food like pasta, rice, dry beans, oil, canned goods, etc. Take a lesson from the Mormons and store food that you normally eat anyway, so it won't expire and go to waste if the earthquake doesn't happen.

It is hard to imagine that any place in the US would be isolated for more than a week or two, even after a major disaster (hurricanes and floods are good examples). Total preparation could include a well, solar water heating (or a long black garden hose), and maybe solar panels to run the well pump etc.