Can subterranean termites live on wet wood only without access to the ground?
You should ask a professional, well established pest control company.  I can tell you only about my experience in Florida that Subterranean termites can establish colonies by building mud tubes up the sides of concrete block, to reach wood inside your home. In Fl. A lot of the big companies use Termidor, which is injected into the ground surrounding your home.  They treat your house every ten years and will offer a warranty if you pay about $125. Per year and they inspect your house yearly for signs of damage.  What we call dry-wood termites fly into your house through openings and colonize after they drop their wings. Often they can be treated with a type of heat gun and other means if the infestation is localized.  If they are all through your house they usually treat by tenting the entire house and gassing them.
I'm sure there are other opinions and treatments, esp. in different climates.