May 02, 2007

Let's Get Ready To Ruuuuumble!!!

Kewel. A team of scientists has come up with a new theory to explain the great New Madrid earthquake of 1811 and similar rocking n' rolling in the Mississippi River Valley:

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.

I love stories like this that take a peek into geological time. Indeed, I remember just before Tina Brown took over the New Yorker reading a multi-part series that started out considering some minor fault line near San Francisco and gradually spread in scope until one was envisioning the crumpling of the western third of the entire continent from the forces of plate tectonics. Gives one chills to think about it all.

Posted by Robert at May 2, 2007 12:46 PM | TrackBack

Back in my FEMA days, a repeat of the New Madrid quake was among the doomsday scenarios. Why? Well, almost none of the structures mid-continent are earthquake resistant. Places like St. Louis would be complete rubble after a quake LA or SF could basically brush off.

Posted by: Hucbald at May 2, 2007 07:00 PM

YOU can love this story. I live about 150 miles from New MAdrid - Little more personal here. :-}

Posted by: Joe Weber at May 2, 2007 08:26 PM