I imagine for a large tech company, it would go something like this:
1. Immediately call extra hands in other locations (datacenters, customer service, etc); begin over-time scheduling to alleviate need for workers in disaster zone. Doesn't require any coordination of disaster zone, just remaining executives located outside.
2. Offer two special payments to workers in the impacted zone: 1. Disaster recovery assistance, to help them cushion emergency needs and get their family/belongings/etc recovered faster; 2. Relocation bonus, to create an incentive for quick relocation to temporary (or new) office.
3. Spend as much as you to have to get extra emergency services in to the area, and generally assist in government/regional recovery area. This will free up the last of your staff for temporary relocation.
I doubt that a large company (we're talking Google, Microsoft, Facebook, etc) would take more than a couple days to have other sites working overtime to cover the load, less than a week to have already transported some of their staff to a new site, and less than a month to have brought almost all of the office back online.
This timeline, of course, assumes that the problem wasn't that their office fell on all their employees, killing them or something of that nature.
Likely, we'd see a hiccup in the development cycle, but in terms of keeping infrastructure online, emergency bug fixes, etc, we'd probably see relatively little damage.