To put this in perspective: It's not the "New Madrid fault line"; it's the "New Madrid Seismic Zone" (NMSZ). The region around New Madrid (located in the "bootheel" of Missouri, where the borders of Missouri, Arkansas, and Tennessee meet) is where, millions of years ago, the North American Plate started to split on a northeast/southwest line. The two sides resolved their differences and stayed together, but the region had been pulled like taffy and so is weakened. You can think of the ground a couple of miles underground as being "foamy" and every so often the weight of the ground above it "crushes" the foam just a little. This means there is no one fault line. There are a series of smaller faults that run northwest/southeast and another set of faults at a different level almost perpendicular to those, which is why it's a "seismic zone". Earthquakes there are so much more destructive because the ground doesn't just move up and down; it moves left and right and forward and backward all at the same time. St. Louis is about 300 miles from the center of the Seismic Zone; by the time the seismic waves reach it they should be reduced in amplitude by around 30%, maybe 40%. So if the original earthquake is 8.5, the earthquake that hits St. Louis will be around 6, maybe 5.5 on the modified Richter scale (currently, the Moment Magnitude Scale). Make no mistake about it: there will be massive death and destruction up and down the Mississippi River. Any building that is not reinforced (which accounts for maybe 100% of the buildings that have been grandfathered in or are part of "historic zones") will collapse. People that make it out to the streets will be sliced by falling panes of glass. Houses that are not firmly attached to foundations (i.e., those built on stem walls or stone foundations with dirt-floor basements) will be shaken off their foundations, breaking both water and gas mains. We can expect almost as many deaths from being burned in fires as from collapsed buildings. What rarely gets discussed is how much of the destruction will travel upstream in both the Missouri River and the Ohio River Valleys, not to mention the Tenn-Tom Waterway. All bridges, dams, and levies up and down all of these waterways will need to be closed until inspected and cleared, not only after the initial shock but all the aftershocks, which will last for months afterwards. Damage on the Ohio River Valley may extend as far as the Pennsylvania border. All the states in the southeastern portion of the US may be effectively cut off by land. The underground fire in St. Louis is unrelated to the New Madrid Seismic Zone. What happened is that the nuclear waste was illegally buried in an existing landfill. The landfill then caught fire and had been going sometime before someone realized it was moving towards the illegal nuclear waste site. If the NMSZ were to go now, it might increase the rate at which the nuclear waste is released into the environment (since some of the containers MAY be breached), but otherwise won't affect it, except it may delay plans to contain and re-route the fire.