January 11, 2012
Earthquake potential in Colorado and New Mexico
The Rio Grande Rift—the north-trending continental rift zone that extends from Colorado’s central Rocky Mountains to Mexico—is not dead but geologically alive and active, according to a new study by scientists at the Cooperative Institute for Research in the Environmental Sciences (CIRES) in collaboration with the University of New Mexico, New Mexico Tech, Utah State University, and UNAVCO.
“We don’t expect to see a lot of earthquakes, or big ones, but we will have some earthquakes,” said author Anne Sheehan, CIRES Fellow and Associate Director of CIRES Solid Earth Sciences Division.
Along the rift, spreading motion in the crust has led to the rise of magma—the molten rock material under Earth’s crust—to the surface, and to the creation of long, fault-bounded basins that are susceptible to earthquakes. Sheehan and her team, along with colleagues at the University of New Mexico, studied the region to assess the potential earthquake hazards.
Using semi-permanent Global Positioning System instruments at 25 sites in Colorado and New Mexico, the team tracked the rift's miniscule movements from 2006 to 2011. “Questions we wanted to answer are: how is the Rio Grande Rift deforming? Is it alive or dead? Is it opening or not?” Sheehan said.
The high-precision instrumentation has provided unprecedented data about the volcanic activity in the region as the slow rates of motion had made previous attempts to determine tectonic activity particularly challenging, she said. Previously, geologists had estimated the rift had spread apart by up to 5 millimeters each year but the errors introduced by the measuring instrumentations were significant. “The GPS has reduced the uncertainty dramatically,” she said. “This is the first real set of real space geodetic measurements in this area.”
Using the high-precision instrumentation, the scientists found an average strain rate of 1.2 nanostrain each year across the experimental area. A nanostrain is a change in length of one part per billion, thus 1.2 nanostrain per year is equivalent to 1.2 mm/yr extension over a 1000 km length. “It is lower than we thought but it does exist,” Sheehan said. “Some people thought it was zero but we are seeing things are extending slowly.”
The researchers also found the extensional deformation—stretching— is not concentrated in a narrow zone centered on the Rio Grande Rift but is distributed broadly from the western edge of the Colorado Plateau well into the western Great Plains. “The surprising thing to come out of the study was that the strain was spread out,” Sheehan said. The results of the study are published in the January edition of Geology.
This finding sheds some light on the mystery of how continents deform away from plate boundaries, Sheehan said. At plate boundaries scientists can observe what is going on pretty clearly, she said. “Things move past each other and crash into each other—at active plate boundaries the rates of motion detected by GPS can be centimeters per year—compare that with the fraction of a millimeter per year that we have measured for the Rio Grande Rift.”
The team plans to continue monitoring the Rio Grande Rift, probing whether the activity remains constant over time, said lead author Henry Berglund of UNAVCO, who was a graduate student at CIRES when he completed this stage of the research. Also, the scientists may attempt to determine vertical as well as horizontal activity, to determine whether the Rocky Mountains are still uplifting or not, he said.
“Present day measurements of deformation within continental interiors have been difficult to capture due to the typically slow rates of deformation within them,” Berglund said. “Now, with the recent advances in space geodesy, we are finding some very surprising results in these previously unresolved areas.”
As far as the potential for future earthquakes in the region, the study’s results are unequivocal, however. “The rift is still active,” Sheehan said.
The new paper, " Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau,” is also authored by CIRES Fellow Steven Nerem, UNAVCO’s Frederick Blume, Anthony Lowry of the Department of Geology at Utah State University, Mousumi Roy of the Department of Earth & Planetary Sciences at the University of New Mexico, and Mark Murray of the Department of Earth & Environmental Science at New Mexico Tech.
The National Science Foundation provided the funding for this study and EarthScope and UNAVCO provided instruments, equipment and engineering services.