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New Madrid seismic zone in the center of the country.

 
Anonymous Coward
06/23/2005 07:16 AM
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New Madrid seismic zone in the center of the country.
Seismic activity in country´s center sparks debate

After California quakes, attention turns to New Madrid zone

By KC Wildmoon
CNN

Wednesday, June 22, 2005; Posted: 8:34 p.m. EDT (00:34 GMT)

[link to www.cnn.com]

ATLANTA, Georgia (CNN) -- Earthquakes on the West Coast have prompted fresh speculation about "the big one" -- a quake that could dump a huge section of California into the sea.

A few have been large enough to shake the faith of skeptics -- a magnitude 7.2 quake on June 15, followed two days later by a magnitude 6.7, both off the coast near the California-Oregon border.

Doomsayers have warned about the Pacific Coast for years. But only a few have raised concerns about an area with the potential to be more dangerous than California -- the New Madrid seismic zone in the center of the country.

It´s a 120-mile-long system of three to five faults stretching from 40 miles northwest of Memphis to southern Illinois, near Cairo.

"The system is capable of producing a quake near 4.0 magnitude every three years," said Gary Patterson, a geologist and information services director for the Center for Earthquake and Research Information in Memphis, Tennessee. "And they´ll cause minimal damage."

But New Madrid already has spawned four earthquakes this year of similar size, along with nearly 100 smaller quakes. Patterson said such activity may or may not be the precursor to a much larger quake.

The recent activity is an anomaly, he said.

"It´s unusual, and we don´t have any reason to believe there is increased risk," Patterson said. "But any time you have this kind of activity in an area that has a 25 [percent] to 40 percent chance of a 6.0 or greater in the next 50 years, it will draw attention."

And the region is ill-prepared for a strong quake, he added.
Under pressure

Scientists know little about how the New Madrid seismic zone works, but in the early 19th century, it was the source of the most violent series of earthquakes known in North American history.

The zone, named for the town of New Madrid, Missouri, is hundreds of miles from a tectonic plate boundary, which Patterson said defies the logic of coastal earthquake science.

"Plate tectonic theory can account for large quakes on the edges of plate boundaries, but plate boundary theory assumes a rigid continental plate," he said. "Madrid is in the middle of a continental plate, not on the boundaries."

Three large quakes happened in the winter of 1811-1812, and strong rumbles hit several times until near the end of the 19th century.

These quakes were felt keenly over more than 2 million square miles -- people in Boston, Massachusetts, felt one or more of the three main quakes, the first of which struck in three shocks on the morning of December 16, 1811.

Two more large shocks struck the area -- on January 23, 1812, and the largest and most devastating of all hit February 7, 1812, destroying the town of New Madrid.

By contrast, the 1906 earthquake in San Francisco, California, was felt over 60,000 square miles.

Patterson said the incredible distance the quakes reached was largely due to the cold, solid rocks "that make this continent float," a different environment from the plate boundaries on the coast.

"On the boundaries, the rock is hot, molten and broken up," he said. The solid rock carries the movement farther from the epicenter.

Earthquake researcher Otto Nuttli estimated 200 moderate to large earthquakes on the New Madrid fault between December 16, 1811, and March 15, 1812, and about 1,800 earthquakes of slightly lesser strength.

The stronger quakes lifted parts of the land high or dropped them down, and drew the Mississippi´s waters in and threw them back far over the river banks. In some areas, the upheaval beneath the surface was so violent that it caused the mighty river to flow backward.

Whole islands in the river -- and entire towns -- disappeared.

The strongest quake in the area since 1895 was a magnitude 5.5 in 1968. New Madrid is "a sleeping giant we don´t understand very well," Patterson said.

"But we realize the need to understand is very important," he added. "It´s a challenge. If we understand this question, then we´ve really put in a piece of how the Earth works as a system."
Trouble for Memphis, other cities?

Patterson said he saw no reason for a "high level of concern" at the moment but added that so little is known about New Madrid that it´s even more unpredictable than its coastal cousins.

The area isn´t prepared for an earthquake of magnitude 6 or higher, specialists said. Damage from such quakes would be significant over a multistate area, Patterson said, with the likelihood of significant infrastructure disruption and damage to population centers and municipalities that would have huge economic impact.

Memphis, and to a slightly lesser degree, St. Louis, Missouri, could be seriously hurt by a strong quake, "especially when you have old infrastructure and a lot of buildings that predate 1940, when unreinforced masonry was a typical style," Patterson said.

"Our building inventory is very vulnerable and has not been shaken significantly," he said. "It´s potentially a large disaster even from a magnitude 6."

Ted Ilsley, manager of the plan review section of Shelby County, Tennessee´s building code enforcement division, said the building code is adequate for an earthquake the size of the 1811-1812 ones. This code has been in effect since 1989 in the county where Memphis is located.

The county is preparing to adopt an amended version of the International Building Code, a requirement to receive funding from the U.S. Department of Homeland Security.

The city of Memphis also likely will adopt something similar to Shelby County. Calls to the city´s code enforcement division were not returned.

Although strong earthquakes strike the West Coast more frequently, New Madrid is "an active seismic zone," Patterson said, and population centers in the area should be concerned -- not with the frequency -- but with the consequences if one does strike.

"It does not take the big one to do a lot of damage," he said. "The most damaging quake in the United States, in 1994 [the Northridge quake in Southern California], causing $30 billion, some say $40 billion in damage, was a 6.7 in a place that´s prepared generally for earthquakes."

There´s no doubt that the New Madrid seismic zone has the potential to spawn catastrophic earthquakes. The question, as with most fault areas, is when it will occur.

Whenever it occurs, the quake likely will be felt far from its epicenter. The one in 1968, centered in southeastern Illinois near the confluence of the Tennessee and Ohio rivers, caused moderate damage, but it was felt across 23 states -- as far as the Carolinas -- and into Canada.

CNN researcher Anne Pifko contributed to this report.
Anonymous Coward
12/08/2005 10:14 AM
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