I work for a major telecom.
These systems *do* have backups and redundancy.
The moment that cable was cut, the system will have started an automated load-shed and re-route at the OC-48 level and above. You'll see messages from the OC-192 trunks shifting to new routes, jumping to spares, and generally trying to route around the damage.
The problem, is that these are OC-192 links. The smaller circuits riding them, such as 10-meg ethernet, OC3, DS3 and DS1 do not get shifted around to available trunks unless they happen to be on the 192 that gets shifted. They're essentially along for the ride.
*IF*...(and that's a very big "if") the smaller circuit is especially critical and vital, then they can TRY to arrange a re-route and stuff it onto one of the alt-routed links. But that takes authorization from people in business suits that fly out to their weekend home in the Hamptons.
So yes. There are backups. If there weren't, this outage would have been international news, and not just a blurb on Slashdot.