When something like this happens, does the traffic get distributed on other operational fibers that have sufficient unused bandwidth or do the operators fire up some dark fibers to carry the load? Or maybe it’s both?
It depends. Generally the larger carriers and consumers of fibers have a network that is able to route around outages (fiber cuts like this are basically routine) just the same as on land. If they don't have enough, then they'll need to either reduce demand or rent some capacity from someone else.