Two ways for open decks:
1. Add 2X flat diagonal bracing under the deck joists, nailed to the underside of the joists, or
2. Add 2X diagonal flush blocking with metal strapping over, nailed to blocking and joists.
Either way has worked well for me in the past. No worries.
Another way is for the deck is to be covered with plywood, then the plywood serves as a cantilever floor diaphragm. But this is obviously not feasible unless the deck is waterproof.
Mike McCann
McCann Engineering