(A reprise from Rosetta Stones, especially for Robert B., as this answers part of the question posed: “But what’s up with South Carolina and the Mississippi/Ohio River confluence?”)
Malachite asked an excellent question I’m actually well-placed to address without further research. Yay!
New curiosity: what the heck is that danger zone where Missouri meets Tennessee?
Heh. Pretty startling, innit?
USGS National Seismogenic Hazard Map. Image courtesy USGS.
That great big target painted on Middle America, my friends, is the New Madrid Seismic Zone. In 1811, it broke in a big way, so big it caused the Mississippi River to run backwards for a bit. Lots of interesting things happened that weren’t quite so interesting to the people who lived through it. More terrifying. And since then, people have watched that fault with a wary stare. It still kicks from time to time, letting us know the earth isn’t as stable as we’d like. But some studies suggest that those may just be aftershocks, long after the main event, and nothing much to worry about. I wrote that up here, a long time ago when I was a young, fresh science blogger.
The thing about New Madrid is this: it was so dramatic, so unexpected, that we’ve approached it with an overabundance of caution ever since. And until further studies confirm it’s no longer a threat, I personally think we’d be wise to continue to treat it as a potential, even if not probable, problem. And this is an excellent place to study intercontinental earthquakes, which are odd and intriguing, so let the science continue!
Here are some additional links should you wish to investigate further.
Nature: Seth Stein: The quake killer.
Nature: Long aftershock sequences within continents and implications for earthquake hazard assessment (pdf).
Highly Allochthonous: Earthquakes within plates: we don’t know when, and we may not know where.
+/- Science: An Abbreviated Numerical History of the Great New Madrid Earthquakes.
Geologic diagram of the Reelfoot Rift. Image courtesy USGS.