Diorite
--- In geology2@yahoogroups.com, Lin Kerns <linkerns@...> wrote:
>
> Mississippi River gets blame for giant 1811 quakes New finding could
> help scientists' understanding of temblors' fault systems
> by Brett Israel
> OurAmazingPlanet
> updated 7/28/2010 5:10:29 PM ET
>
> Massive earthquakes that struck the town of New Madrid, Mo., in 1811 can be
> traced to the actions of the mighty Mississippi River thousands of years
> earlier, a new study in the journal Nature suggests.
>
> The work could affect scientists' understanding of the fault systems that
> caused the quakes.
>
> Such mid-continent
> temblors<http://www.livescience.com/php/multimedia/imagegallery/igviewer.php?imgid=207&gid=16&index=0>have
> fascinated seismologists because they occur not at the points where
> tectonic plates interact — as the 2010 Haiti quake did — but in the center
> of plates. New Madrid lies atop the center of the North American Plate.
>
> The new thinking works like this: At the end of the last ice age around
> 16,000 years ago, the Mississippi River washed away tons of soil, taking a
> giant weight off the central portions of the continent. Over 6,000 years,
> the river dredged 39 feet of sediment from the river basin — a "quite
> dramatic" erosion event that set in motion the events that would lead to the
> giant quakes, said geologist and study team member Roy Van Arsdale of the
> University of Memphis.
>
> Where the sediment eroded, the earth buckled because of the released weight,
> just like a stick that is bent with two hands, said another team member,
> geophysicist Andrew Freed of Purdue University.
>
> In the middle of the stick, Freed said, the upward curving top part is
> stretched and the bottom part is compressed. The land at New Madrid bent the
> same way as the sediment eroded. The area of land that was stretched
> contained faults, or cracks in the rocky plates of the Earth's crust. These
> faults were already close to rupturing, and when they eventually failed in
> 1811 they unleashed violent earthquakes.
>
> *The odds of repeating*
> Unlike in the places where two pieces of the Earth's crust butt heads, an
> earthquake is unlikely to hit again anytime soon on the same faults in New
> Madrid, the study suggests. But other evidence may dispute this conclusion.
> "The theory is certainly interesting enough to merit further consideration,"
> Susan Hough, a geologist with the U.S. Geological Survey who was not
> involved with the work, told OurAmazingPlanet, but she added that geological
> evidence "suggests strongly" that a set of faults in the area has produced
> multiple earthquakes over the last 2,000 years (with quakes coming in
> roughly 500-year intervals), implying some regularity in these faults as
> well. More work is necessary to explain that, she said.
>
> The study also suggests other faults in this zone may be close to failure
> and could erupt because of the sediment erosion, which means the danger zone
> could be more widespread than previously thought.
>
> "We predict that whenever you have local erosion and uplift, you can have
> earthquakes," Freed told OurAmazingPlanet.
>
> But scientists can't calculate the probability or severity of earthquakes at
> faults in the middle of tectonic plates, as they can for faults at plate
> boundaries.
>
> *Danger zone*
> The New Madrid earthquakes that struck between December 1811 and February
> 1812 were some of the strongest seismic
> events<http://www.livescience.com/history/080506-natural-disasters.html>ever
> recorded in the continental United States.
>
> The largest of these quakes was estimated between magnitude 7.0 and 8.0 and
> made the Mississippi River flow
> backwards<http://www.livescience.com/environment/070502_newmadrid_quake.html>temporarily.
> Earthquakes in this part of the country rumble over a larger
> area than they do on the West Coast, due to the makeup of the underlying
> ground. When the quakes hit New Madrid, according to legend, church bells
> rang in Boston.
>
> The temblors ripped through the heart of the United States in the New Madrid
> Seismic Zone — the country's most earthquake-prone region outside
> California. The zone runs 100 miles along the Mississippi and borders eight
> Midwestern and Southeastern states: Illinois, Indiana, Kentucky, Tennessee,
> Alabama, Mississippi, Arkansas and Missouri.
>
> New Madrid still rumbles with roughly 200 tiny quakes annually — a
> 5.2-magnitude quake hit in 2008 — but a recent study suggested that what the
> town is feeling are actually 200-year-old aftershocks, since the fault may
> be shutting down<http://www.livescience.com/environment/090325-new-madrid.html>.
>
>
> The New Madrid faults move more than 100 times more slowly than the San
> Andreas Fault; the slower a fault moves, the longer the aftershocks last.
> That's because the tectonic plates can't "reload" the fault, wiping out the
> effects of a previous earthquake and suppressing aftershocks.
> Source<http://www.msnbc.msn.com/id/38454786/ns/technology_and_science-science/>
>
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>
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