The injuries are bad enough, I’m sure. But I hope no one was downstairs.
Any engineer will tell you that any type of span, roof, floor, bridge etc. if it is hit with rhythmic waves of vibration will develop a sympathetic sway or bending movement that will take out pretty well anything holding it together. Enough weight of bodies with feet moving in a synchronized rhythm will just start the floor joists a bouncing.
If the were, they aren’t anymore. That’d be a crusher.