Get some steel cable (maybe even that nice stuff thats wrapped in clear rubber)... trim and attach carabiners then string across the beams 6” to a foot above the shelf deck top. I’m assuming the beams have holes so you could set at whatever level works for the items on the shelf. That should at least help hold in large items in a quake.

I’d be more afraid of something falling on someone... games can be fixed, people not always.