New results about the temperatures of rock deep below the New Madrid Seismic Zone in the central United States shed light on the puzzling questions of why large earthquakes happened there in 1811 and 1812 and when they may happen again.
Scientists from Northwestern University, the U.S. Army Engineer Research and Development Center and the University of Illinois at Chicago have found that New Madrid appears to be cold and dying. They presented their findings Dec. 13 at the annual meeting of the American Geophysical Union (AGU) in San Francisco.
"Hot rocks are weak," says Seth A. Stein, William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences at Northwestern and a coauthor of the study. "So people suggested that the reason large earthquakes occur in the New Madrid area rather than in the many similar geologic settings in other parts of the eastern United States is that the New Madrid rocks are hotter."
But the researchers discovered this is not the case. They looked at data used in the new edition of the Geothermal Map of North America (American Association of Petroleum Geologists, 2004), which shows all the measurements of the heat coming to the Earth's surface (heat flow) taken from boreholes. They found that thermally New Madrid is surprisingly similar to other areas of the eastern United States.
"The New Madrid data are essentially no different from other sites in the eastern United States," explains coauthor Jason R. McKenna from the U.S. Army Engineer Research and Development Center. "Although we'd like to have more measurements to be sure, at this point, there's no reason to believe New Madrid rocks are hotter and therefore weaker than rock in other parts of the eastern United States."
One of the most difficult aspects of assessing the earthquake hazard is deciding whether New Madrid is a special place or simply where central U.S. earthquakes have occurred in the past few thousand years. "When we look at things like geology, gravity or the magnetic field, there's no obvious difference between New Madrid and similar places in the eastern United States that haven't had large earthquakes recently," McKenna notes. "Now we see the same for heat flow."
The new heat flow results fit into a growing idea that earthquakes can migrate among similar faults, some of which -- such as the Meers fault in Oklahoma -- appear to have been active about 10,000 years ago but show no activity today. Geological studies find that New Madrid earthquakes comparable to those of 1811-1812 occurred about 1450 and 900 AD. However, because this fault system has not generated significant topography, it is likely to have "turned on" relatively recently, perhaps within the past few thousand years.
With this view, say the researchers, prior earthquakes were concentrated on other faults, and future earthquakes will occur somewhere else when the New Madrid system "shuts down." Once this happens, it may be a very long time -- thousands of years or longer -- before New Madrid becomes active again.
"Although we don't know when the New Madrid fault system will shut down, it may be dying today," says Stein. "The recent cluster of earthquakes may be coming to an end."
Migrating earthquakes also occur in the interior of other continents, such as Australia. This is very different from the way earthquakes occur on boundaries between plates, like the San Andreas fault along the boundary between the Pacific and North American plates. Because the plates keep moving, earthquakes continue to occur on the boundaries in the same places.
Precise measurements taken by Stein, coworkers and other investigators using the Global Positioning System (GPS) show that motion across the New Madrid Seismic Zone currently is either very slow or at zero. Because this motion has to accumulate for many years to cause a large earthquake, it will be at least hundreds of years, and perhaps much longer, before another large earthquake happens.
"Until recently about all we could say was that future earthquakes might occur in places where past ones had," says Stein. "Now we can actually test that idea by looking at the motion accumulating for possible future earthquakes. Although we can't be sure yet, the longer the GPS data continue to show essentially no motion, the more likely it seems that the fault is shutting down and won't cause large earthquakes for a very long time. It's time to start thinking about this possibility and to use what we're learning to improve estimates of the hazard from future earthquakes."
The possibility of the fault shutting down is important for assessing the earthquake hazard in the central United States. Large earthquakes (magnitude 7) occurred in 1811 and 1812, causing shaking across much of the area. Houses collapsed in the tiny Mississippi river town of New Madrid, Mo., and minor damage occurred in St. Louis, Louisville and Nashville. The smaller earthquakes that continue in the area today are typically more of a nuisance than a catastrophe, say the researchers. The largest in the past century, the 1968 southern Illinois earthquake (magnitude 5.5), was widely felt and caused some damage but no fatalities. However, if large earthquakes like those of 1811-12 occurred again, they would be very destructive.
The third coauthor of the study, "No thermal weakening under the New Madrid Seismic Zone," is Carol A. Stein, professor of Earth and environmental sciences at the University of Illinois at Chicago.
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