VOLCANO MADNESS: Re: [Geology2] Re: Mississippi River gets blame for giant 1811 quakes

Thursday, July 29, 2010

Re: [Geology2] Re: Mississippi River gets blame for giant 1811 quakes

I also have a problem with this article. Not one word about the Mississippi Embayment is mentioned, nor the effects of the movement of plates post Pangea. In fact, the concept in this article has been around for ages; I was told years ago that the removal of the weight of the sand, laid down by glacial thawing during the previous ice ages was the direct cause of the NM 1811-12 quakes. However, most of the detritus that flowed down south from said glacial melts was in the form of loess aka glacial flour. Here in Jackson, TN, particularly on the north side of town, one can find deep deposits of loess, via roadcuts. Not all loess is airborne, or rather, it doesn't move far before gravity takes hold.

Erosion becomes an even bigger problem in figuring these late quakes, as any local geology student can tell you that the Mississippi River has altered its course tens of times since it first began to flow. Oxbows break away, the river makes a shortcut, and thanks to the Coriolis Effect, the process is repeated a multitude of times. The Mississippi Embayment caused the Mississippi River to enter the gulf along the TN/MO statelines, and that is where we received our own (& E. Arkansas & the bootheel of Missouri) deposits of sand and loess. I can't imagine anything, including the Mississippi River, having such power as to move that much land so far. Most deposits would have formed sandbars and berms within the curves of the river, itself. The weight of the water, alone, would be a better, but not sound, explanation for earth movement on the NMFZ.

The NMFZ is, of course, not just one fault, but a series of over 200 known faults. At some point below, some of them may converge; who really knows? As the deepest of those faults lies down near the base of the continent, the only conclusion I can draw is that a little sand is not going to cause the isostatic rebound of such a large land mass. But then, what do I know? I'm just a rank amateur. :)

Lin

On Thu, Jul 29, 2010 at 11:58 AM, Diorite <diorite@yahoo.com> wrote:

I have serious doubts about this one. So does one of my colleagues who I shared this with. They're going to have to do a lot more work to impress this geologist.

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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