January 25, 2007

Researchers foresee death of quake zone

By Megan Fellman

New results about the tempera-tures of rock deep below the New Madrid Seismic Zone in the central United States shed light on the questions of why large earthquakes happened there in 1811 and 1812 and when they may happen again.

Scientists from Northwestern, 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.

“Hot rocks are weak,” says Seth A. Stein, William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences 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, 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 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.