Amend Posse Comitatus for natural disaster situations, if it hasn't been already!
I don't see anything in Posse Comitatus that prohibits "rescue" activity.
These soldiers would be sent in simply to secure the area and get those people out of there.
They're not going in there to "arrest" anyone or conduct other "police activities".
If some armed thug wants to interfere, it would be simple self-defense for a Marine to take him out.
After all, even the reporters from the MSM have armed escorts on the scene.