Major quake likely in central U.S.

The possibility of an earthquake with a magnitude of 7 or 8 is increasing according to a study released this week.

BY HEATHER MOYER | BALTIMORE | June 24, 2005



"The probability of getting an 1812-type earthquake is between 7 to 10 per cent in the next 50 years."

—Michael Ellis


The possibility of a magnitude 7 or 8 earthquake striking the Central U.S. is very high, researchers said this week.

The rate of strain building up in the New Madrid Seismic Zone (NMSZ) is similar to other seismic zones in the country. That announcement came Wednesday in a study by scientists from the University of Memphis Center for Earthquake Research and Information (CERI). The study was detailed in the journal Nature.

According to study member Dr. Michael Ellis, this new research overturns previous studies that claimed otherwise.

"The most important point is that for the first time, these results confirm what the geological evidence has been showing for decades now "- that strain is accumulating," said Ellis, a geology professor at the university and program director of land use dynamics for the National Science Foundation.

"Earlier results did not confirm this, so earlier people suggested that the seismic hazard there should be reduced -- we can throw that out the window now and move on."

The NMSZ is named for the small town of New Madrid, MO, where three devastating magnitude 8 earthquakes struck in the winter of 1811-1812. The quakes were felt in 27 states and as far away as Boston and Charleston, S.C. According to witness accounts in journals and newspapers from the time, the ground rolled in waves and sections of the earth sank or rose. Thousands of aftershocks also plagued the region during the winter as well.

Seismologists estimate the 1811-1812 earthquakes were felt strongly over 50,000 square miles and moderately across nearly one million square miles. By comparison, the historic San Francisco earthquake of 1906 was felt moderately over 60,000 square miles.

What makes the NMSZ unusual is that it is not a seismic zone based on plate tectonics -- meaning that its seismic potential is not based on plates in the Earth's crust moving against each other. In fact, the nearest plate boundary to the NMSZ is more than 1,200 miles away.

Ellis said the surface of the NMSZ is accumulating strain by moving in a manner that resembles someone squeezing a block very slowly. He noted in the Nature article that how earthquakes happen within a plate interior is not understood.

"Seismic zones like the New Madrid are not very common, which is why the whole thing is enigmatic and has stirred up so much controversy," explained Ellis, whose research has already been greeted with some disagreement by researchers who previously studied the area.

In Ellis' study, data was collected over four years using 11 GPS data transmitting stations along the NMSZ. Satellites recorded any movement of those transmitters. He says previous studies used transmitters that were not placed deep enough in the ground. Critics say Ellis' results only come from a few of the transmitters and not all.

The previous studies had used data to argue that the seismic hazard in the NMSZ is not high.

Yet scientists are saying that another significant earthquake is possible for that region. Small earthquakes occur along the NMSZ every day, including magnitude one and two tremors felt in Arkansas, Tennessee, Missouri, and Kentucky. On Monday, June 20, an earthquake with a magnitude of 3.9 was reported just east of Cairo, IL and a 3.6 quake was reported in Western Kentucky.

If an earthquake comparable to those of 1811 and 1812 struck the NMSZ today, the results would be far different than then due to the increased population of the region.

"Unfortunately, it would do a considerable amount of damage," said Jim Wilkinson, executive director of the Central U.S. Earthquake Consortium (CUSEC). "The region has just grown tremendously since that point. Seismic building codes didn't start showing up in the region until the late 1980s and 1990s.

"We have a lot of buildings out there that just weren't built to withstand earthquakes.

Wilkinson added that just what kind of earthquake strikes -- whether it's from deep within the ground or right along the surface -- will also have an impact on how severe the damage might be.

"There are a lot of variables," he noted. "That's the difficulty in all this: figuring out how all these variables play out so we can put together good risk reduction efforts to make our communities safer."

Wilkinson said Wednesday's study also just validates what the consensus has been for years: that history and science have shown that the region has experienced significant earthquakes before.

The timing of this study will help garner interest, he added. The recent increased seismic activity around the globe -- from Southeast Asia to Southern California -- has made the public want to learn more about earthquakes.

The NMSZ has experienced more seismic activity recently, too, with more magnitude 4 quakes than average shaking the area in the past year.

Study Member Michael Ellis said no one should be making predictions, though.

"We're not predicting anything. We're just saying that the established level of probability for earthquakes of the size of those in 1812 -- those levels of probability remain the same, which is in contrast to earlier studies," he explained. "The probability of getting an 1812-type earthquake is between 7 to 10 per cent in the next 50 years."

Because this study supports that, the authors are pushing for builders and community planners to not overlook or lessen building codes that could make life safer in the event of an earthquake. "They should not reduce that seismic hazard or their motivation to adopt better building codes," Ellis said.

He also stressed that the public should not panic about these findings, but rather pay attention and be aware of seismic hazards.


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Related Links:

Recent earthquakes reported in Central US region

Central U.S. Earthquake History takes a detailed look at the history of quakes in the New Madrid Seismic Zone.

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