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New Mexico State University professor heads study of the Rio Grande Rift

New Mexico State University geophysicist James Ni is directing a project aimed at providing the most detailed description yet of the Earth's structure beneath the Rio Grande Rift, a 700-mile-long depression in the Earth's crust that runs through New Mexico.



New Mexico State geophysicist James Ni poses with a seismometer on the university campus. Ni heads a project aimed at getting the most comprehensive picture yet of the Earth's structure beneath the Rio Grande Rift, a geological formation in which most of

is a failure of the Earth's surface that occurs along faults. The Rio Grande Rift is one of the Earth's major continental rifts. Running from central Colorado to south of Juarez, Mexico, it includes the area on both sides of the Rio Grande, including Santa Fe, Albuquerque, Alamogordo, Las Cruces, El Paso and Juarez.

To geoscientists, the Rio Grande Rift is one sign that the North American plate -- that part of the Earth on which this continent sits -- is breaking apart, Ni said.


The Colorado Plateau, on the plate's western side, is moving away from the Great Plains. The movement is very slow, although just how slow is uncertain, he said.

"Although, the Rio Grande Rift is caused by an extension of the crust, the underlying process is by no means clear," Ni said. "At present, two competing hypotheses have been proposed, active rifting and passive rifting. Active rifting is the result of a thermal upwelling of a deeper part of the mantle we call the asthenosphere. Passive rifting is a response of the area near the rift to forces acting on the boundary of the plate. In both processes, the determining factor is the structures of the Earth beneath the rift. One purpose of this study is to get accurate information about those structures."

To get that information, along with the answers to other questions, Ni and colleagues from four other institutions set up a line of 54 solar-powered seismometers during the fall and winter of 1999. The seismometers ran across the rift from slightly northwest of Pecos, Texas, to Lake Powell, Utah, a distance of about 600 miles, Ni said.

Ni's collaborators are Scott Baldridge of Los Alamos National Laboratory, Rick Aster and John Schlue of New Mexico Tech, Steve Semken of Dine College and Steve Grand of the University of Texas at Austin.

A seismographic array like the one the researchers set up records data about the Earth in a manner similar to the way a CAT scan records data about the human body, Ni explained.

"Seismometers detect ground motion and translate that motion into a measurement of velocity. We aligned our seismometers with an earthquake zone called the "Ring of Fire" that runs around the Pacific Rim from the Aleutian Islands to South America. The sound and shear waves from that zone traveled through the Earth and were picked up by the seismometers. But they had different travel times to different seismometers and from that difference we can infer structural differences in the Earth beneath," he said.

"It's really not that different from showing a picture of the brain in human medicine," he said.

The line of seismometers was dismantled in May and researchers will spend the next five years analyzing the data it recorded, Ni said.

"This is the first study designed to provide a vertical profile of the subsurface structure down to 400 miles," he said.

"An understanding of the underlying structures is essential to understanding the rift's origin. With new information, it is then possible to resolve the debate over active or passive rifting in the Rio Grande Rift. We'd also like to know more about the Earth's crust beneath the Great Plains, which in eastern New Mexico and western Texas is a source of oil and gas. Also, while we know the Colorado Plateau is at a high elevation, it is not well known what mechanisms support that elevation," he said.

"Part of it is basic human curiosity -- how did this land become the way it is and what's going on beneath us?" he said.

While it operated, the seismic array ran through rugged country on federal, private, state of New Mexico and Navajo land. Ni said getting to where the seismometers were planted in the ground sometimes required four-wheeled vehicles, but wasn't nearly the ordeal New Mexico State researchers faced four years ago, when they set up a similar array across the world's highest plateau in central Tibet.

"You didn't have to travel five days just to get there or work in altitudes that made you barely able to walk," he joked.