Discovery News (July 29, 2010)
"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...."
Those early-1900s quakes were big ones. Back then, folks hadn't built all that many large, tall structures along the Mississippi watershed. These days, quite a few cities have popped up - without the sort of wake-up calls that encouraged places like San Francisco to figure earthquake effects into their building codes.
A 7.2+ temblor today would put the American Midwest on the front page: probably around the world.
Oddly enough, it looks like global warming is to blame for the 1812 quakes:
"...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 12 meters (39.4 feet) of sediment from a large swath of the New Madrid region...."
I think folks tend to forget that Earth was in an ice age until very recently. Whoever wrote the Discovery article apparently is in the camp that thinks that the current ice age is over: but there's some debate about that. The last I heard, we could be at the end of Earth's most recent period of continental glaciation - or in another interglacial period. (September 9, 2009)
The point is that, around 16,000 to 10,000 years ago, North America's glaciars were melting and the Mississippi and Ohio Rivers started eroding away a serious amount of material. Bedrock that had been pushed down by the weight of glaciers and soil - wasn't under as much pressure.
The idea is that, as the North American continent's interior eased back into a non-glaciated position, something gave: the New Madrid Seismic Zone.
The good news is that events like these seem to be one-off adjustments. It doesn't look like pressure is building up in the New Madrid Seismic Zone for another big quake.
The bad news is that another, as-yet-uncharted, place deep under the Midwest may be under stress - and hasn't cracked yet.
Well, that keeps life interesting.
- "Next Major Taupo Eruption Due - 800 Years Ago?!"
(July 18, 2010)
- "Earthquakes, Nuclear Winter, the End of the World, and All That"
(February 28, 2010)
- "Big Crack in Ethiopia: New Ocean in the Making"
(November 3, 2009)
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