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Thread: Source of Major Earthquakes Discovered Beneath U.S. Heartland

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    Source of Major Earthquakes Discovered Beneath U.S. Heartland

    Scientists have finally figured out what might have caused a series of devastating earthquakes that struck the Midwest nearly 200 years ago at a set of faults that has confused geologists for a long time.

    And the results suggest the region, still seismically active today, is going to keep shaking for a long time, and another big one will hit on the same 500-year cycle that has rocked the Heartland for as far back as records, legends and memory serve.

    The largest of three or four big seismic events that stretched from December 1811 to February 1812 is called the New Madrid Earthquake and had an estimated 8.0 magnitude, strong enough to cause the nearby Mississippi River to temporarily flow backward. Its epicenter was in the town of New Madrid in southeast Missouri, near the Kentucky and Tennessee state lines. Hundreds of aftershocks followed for several years.

    The damage from the New Madrid quake was bad enough in the early 19th century—half of the town was destroyed, but with many more people and buildings now in the area, a similar event in the region today would be devastating, seismologists and engineers agree.

    More to come

    The seismic zone today generates about 200 tiny quakes annually, but it also let loose a magnitude 4.1 quake in February 2005 and a magnitude 4.0 quake in June 2005. The U.S. Geological Survey says there is a 9-in-10 chance of a magnitude 6 or 7 temblor occurring in this area within the next 50 years.

    These mid-continent temblors have long fascinated seismologists because of the mysterious origin of earthquakes that occur not at the edges but in the center of tectonic plates such as the North American Plate that underlies the continent.

    One team of seismologists had thought that high density pillow lavas in the lower crust beneath the New Madrid region could have pulled the crust downward and thereby generated surface stresses that triggered the quakes.

    Now, Allessandro Forte of the Université du Québec à Montréal and his colleagues have arrived at a more dramatic mechanism—an ancient, giant slab of Earth called the Farallon slab that started its descent under the West Coast 70 million years ago and now is causing mayhem and deep mantle flow 360 miles beneath the Mississippi Valley where it effectively pulls the crust down an entire kilometer (.62 miles).

    "This remarkable localization of flow in the mantle below New Madrid, originating so deep below the surface, was completely unrecognized prior to our work," Forte told LiveScience.

    Slabs like this that sink oceanic crust are called subduction zones, and those adjacent to Japan produce intense and damaging seismic activity.

    "We have discovered an analogous subduction zone, deep inside the Earth below the central Mississippi River Valley," Forte said.

    Forte and his colleagues at the University of Toronto and the University of Texas based their findings on high-resolution seismic tomography images that were used to predict the topography and viscous flow of the mantle under and around North America. They used the model to focus on the New Madrid seismic zone and propose that the descending slab and associated mantle flow directly below the New Madrid seismic zone strains the overlying crust, causing seismic ruptures.

    The results were published in a recent issue of Geophysical Research Letters.

    Still diving

    The Farallon plate will continue to descend into the deep mantle and thus to cause mantle downwelling in the New Madrid region for a long time.

    "[This] suggests that the seismic risk in the New Madrid region will not fade with time," Forte said.

    The fault structure under the New Madrid region is a "failed rift" created by the opening of the ocean that later became the Atlantic Ocean 650 to 600 million years ago, Forte said.

    That activity also caused rifts in the St. Lawrence, Saguenay and Ottawa river valleys in Canada, where there is similar mid-continental quaking, he said. Another set of faults far from the boundaries of the North American Plate are associated with the Keweenawan Rift, a 1240-mile-long rift in the area surrounding Lake Superior.



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    Earthquake in US Heartland

    I waas shaken out of bed about 10 minutes ago by an earthquake. In Indiana. No damage but strong enough to shake everyone out of bed. I can't get any info yet, I hope this isn't the big one predicted for the Saint Louis -Memphis area.

    Edit: They are now reporting it was a 5.4 on the richter scale, the epicenter somewhere in southern Illinois. It shook people out of bed as far east as Louisville, Ky.

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    5.4 earthquake rocks Illinois; felt 350 miles away

    WEST SALEM, Ill. - A 5.4 magnitude earthquake that appeared to rival the strongest recorded in the region rocked people up to 450 miles away early Friday, surprising residents unaccustomed to such a powerful Midwest temblor.

    The quake just before 4:37 a.m. was centered six miles from West Salem, Ill., and 66 miles from Evansville, Ind. It was felt in such distant cities as Chicago, Cincinnati, Milwaukee, and Des Moines, Iowa, 450 miles northwest of the epicenter, but there were no early reports of injuries or significant damage.

    "It shook our house where it woke me up," said David Behm of Philo, 10 miles south of Champaign. "Windows were rattling, and you could hear it. The house was shaking inches. For people in central Illinois, this is a big deal. It's not like California."

    Bonnie Lucas, a morning co-host at WHO-AM in Des Moines, said she was sitting in her office when she felt her chair move. She grabbed her desk, and then heard the ceiling panels start to creak. The shaking lasted about 5 seconds, she said.

    The quake shook skyscrapers in Chicago's Loop, 240 miles north of the epicenter, and in downtown Indianapolis, about 160 miles northeast of it.

    Irvetta McMurtry of Cincinnati said she felt the rattling for up to 20 seconds.

    "All of a sudden, I was awakened by this rumbling shaking," said McMurtry, 43. "My bed is an older wood frame bed, so the bed started to creak and shake, and it was almost like somebody was taking my mattress and moving it back and forth."

    Lucas Griswold, a dispatcher in West Salem, said the Edwards County sheriff's department received reports of minor damage and no injuries.

    "Oh, yeah, I felt it. It was interesting," Griswold said. "A lot of shaking."

    Indiana State Police spokesman Sgt. Todd Ringle in Evansville said there were no immediate reports of damage.

    The quake occurred in the Illinois basin-Ozark dome region that covers parts of Indiana, Kentucky, Illinois, Missouri, and Arkansas and stretches from Indianapolis and St. Louis to Memphis, according to the U.S. Geological Survey.

    The organization's Web site said earthquakes occur irregularly in the area, and that the largest historical earthquake in the region — also a magnitude 5.4 — caused damage in southern Illinois in 1968.

    http://news.yahoo.com/s/ap/20080418/...est_earthquake


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    We just had an aftershock. It lasted for just a few seconds & was much milder then the one this morning but this is really weird for this part of the country.

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    Quote Originally Posted by Æmeric View Post
    We just had an aftershock. It lasted for just a few seconds & was much milder then the one this morning but this is really weird for this part of the country.

    Yeah, they're not very common out your way, are they?

    We get little ones here from time to time here but they never disrupt anything. There's actually an old fault line that runs basically between Vermont and New Hampshire...but it isn't very active.

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    Wtf? just watched the news didnt get anything about the earthquake.
    Damn cant imagine how it would feel, we dont have earthquakes here we actually dont have any of those things here except water.

    Werent you scared or what? i would be there go's my cool imageeyes:

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    Each part of America has it's fair share of creative natural disasters. I noticed Europe seems to have a significantly lower amount of disasters. Over here we have volcanic eruptions (more like organic nuclear explosions), wind storms and Earthquakes.

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    I was surprised & disoriented. I was shaken awake & the front porch was rattling pretty loudly & there was a rumbling sound all around. My 4-year old panic because he thought it was a tornado. My first thought it was the big one predicted on the New Madrid fault near the Mississippi. But it was actually just across the Wabash in Illinois, not far from Vincennes. It was strong enough to wake people up in Louisville & they had some damage there. But so far no reports of serious injuries & most of the damage was done to masonry block buildings & chimneys. I had no damage at my house aside from some items that fell off shelves. There is a little bit of nervousness that this could be the precursor to the big one. The aftershock an hour ago was 4.5 & was about 6-miles from the 5.2 that accured at 4:34 this morning.

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