Weak rocks may raise quake risk in New Madrid zone

Although its been a little more than 200 years since the last big one hit, the New Madrid Seismic Zone is still a hot spot for earthquakes. New research conducted by U.S. Geological Survey scientists shows why the zone is unique and may continue to pose a higher earthquake risk than adjacent areas in the central United States.

Using new technology, scientists now have high-resolution imagery of the New Madrid Seismic Zone, allowing them to map the area in more detail, a USGS news release said. The maps allow for greater understanding of the weak rocks in the zone that are found at deeper depths in the Earth's mantle compared to surrounding areas. Scientists also determined that earthquakes and their effects are likely to be narrowly concentrated in the zone.

The New Madrid fault extends about 165 miles from Marked Tree, Ark., to Paducah, Ky. The southern end of the zone is about 35 miles northwest of Memphis, Tenn.

The USGS research specifically investigated the Reelfoot Rift area, a 500-million-year-old geologic feature that contains the New Madrid Seismic Zone in its northernmost part. Scientists took images of rocks deep beneath Earth's surface to see their characteristics and understand their mechanical behavior -- especially their ability to withstand huge stresses placed on them.

Findings were published in the Earth and Planetary Science Letters journal.

USGS scientists used data from USArray, a network of 400 seismometers that is part of the National Science Foundation's EarthScope Program. Fred Pollitz, lead author of the report, and co-author Walter Mooney took measurements of different earthquakes around the world.

A surprising finding, the release said, was that weak rocks underlie the fault lines in the crust of the Reelfoot Rift and extend more than 100 miles down into the mantle. In contrast, weak rocks in other ancient rift zones in the central and eastern United States bottom out at much shallower depths. These weak mantle rocks have low seismic velocity, meaning they are more susceptible to concentration of tectonic stress and more mobile.

In many other geophysical studies such as this, when someone observed a low velocity anomaly, they could attribute certain properties to those rocks. "One of those properties is they're weaker, so they can't sustain the stress" of higher-velocity rocks.

USGS scientists were unavailable for comment because of the federal government shutdown. However, Charles Langston, a professor and director of the Center for Earthquake Research and Information at the University of Memphis, read the USGS research and said further study would be needed to get a more definitive data.

"The result is really interesting. I really like it because it ... looks like something you can point to that makes the New Madrid Seismic Zone look different," Langston said.

A number of some of the largest historical earthquakes in the nation occurred in the New Madrid Seismic Zone, in particular three earthquakes greater than magnitude-7 during 1811 and 1812.

Witnesses said the earthquakes sounded like cannons going off, and people on riverboats watched the sides of the river appear to cave in. There also was the smell of brimstone in the air, Langston said.

He said at least 200 earthquakes a year occur in the New Madrid Seismic Zone, but they're generally 3.5-magnitude and smaller. "You have to be close to them to feel it," he said.

Along the New Madrid fault, Langston said, there is a 7 percent to 10 percent chance for a quake of magnitude-7.5 or larger in the next 50 years. There is a 25 percent to 40 percent chance of a magnitude-6 or larger quake in the next 50 years.

"So there's a good chance we could have a damaging quake in the next 50 years," Langston said.

But the average time between "really large" earthquakes along the fault is 300 to 500 years. There is evidence of earthquakes in the zone as far back as 2350 B.C. Langston said the sands and plays in the Mississippi River area create sand blows, the effects of which can still be seen in white spots in farm fields.

Before this research, the New Madrid Seismic Zone had been mapped by the USGS as an area of high seismic hazard, but those assessments were a consequence of a short -- about 4,500 years -- earthquake record for the area, the release said.

In the future, USGS scientists plan to map the seismic structure of the entire nation using USArray. This effort started in California in 2004, will focus on the East Coast next, then move to Alaska. All the USArray and other EarthScope efforts also will help draft the USGS National Seismic Hazard Maps.

rcampbell@semissourian.com

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