The Mississippi River may be mightier than anyone ever imagined. It may have been behind the baffling 1811-1812 earthquake in the New Madrid Seismic Zone, a region of Earthly unrest where by rights no earthquakes should be found.
Yet in December of 1811, a pair of massive magnitude 7.2+ temblors ripped through the Mississippi River Valley near the corners of Missouri, Kentucky, Arkansas, Tennessee, and Illinois.
January and February of 1812 brought two more strong quakes that caused severe damage to buildings in St. Louis and sparked reports of shaking felt as far away as Maine. Middle America was, inexplicably, in tumult.
A new paper in the journal Nature suggests the river's erosive action may be to blame. Between 16,000 and 10,000 years ago, as North America emerged from the last ice age, the Mississippi and Ohio Rivers conspired to remove 39.4 feet of sediment from a large swath of the New Madrid region.
Eric Calais of Purdue University in Indiana and a team of researchers think Earth's crust relaxed slowly under the lightened load. As the millennia passed, ancient, buried faults that had been compressed and locked tight began inching closer to catastrophic failure. Then, suddenly, they broke.
Though intensely studied, the quakes have baffled scientists for decades — with a few exceptions, strong earthquakes occur exclusively where tectonic plates are grinding past one another. But New Madrid is in the middle of the North America plate, which has been stable for tens of millions of years. Precise GPS measurement tools used to measure movement in faults zones confirm this — New Madrid hasn't budged.
This is good news: it means tectonic stresses aren't building up in the region. These "relaxation quakes" aren't likely to occur again any time soon, because there is little to no new strain accumulating along the faults. Other faults in the New Madrid zone that haven't ruptured in thousands of years still could pop, but they should all be one-offs.