Yellowstone is oddly stable in its instability. What geologists and seismologists seem to be worried about these days (again) is the New Madrid Fault, which is the 2nd largest set of plates in North America, stretching across much of the midwest and mideast. The last time it fully erupted it changed the course of the Mississippi River near St Louis and created a new ridge in the Smokies. A full size eruption along the lines of the 1811 quakes could effectivly split the US. But even that could possibly be overcome.