It would probably be Yellowstone National Park or, to be more exact, the Yellowstone Caldera that hides beneath the scenic park. The caldera is a certified supervolcano whose tectonic activities causes between 1,000 to 3,000 earthquakes a year. An eruption of the Yellowstone Supervolcano would certainly be a uncomfortable day in the US of A.
RE: What is the most dangerous geological fault threatening the United States?