On the Edge: Southwest Shakes

By Tracy Cozzens

Using a large network of GPS stations, a team of researchers has found that the Rio Valley Rift in the Southwest United States — previously suspected to be dead — is slowly expanding, at a rate of about 0.1 millimeter per year.

The Rio Grande Rift extends from Colorado’s central Rocky Mountains to Mexico.

The study was conducted by scientists at the Cooperative Institute for Research in the Environmental Sciences (CIRES) at the University of Colorado at Boulder, 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 study author Anne Sheehan, CIRES Fellow and associate director of CIRES Solid Earth Sciences Division. “We use continuous measurements of GPS sites from across the Rio Grande Rift, Great Plains, and Colorado Plateau to estimate present-day surface velocities and strain rates,” Sheehan said.

Using GPS instruments at 25 sites in Colorado and New Mexico, the team tracked the rift’s miniscule movements from 2006 to 2011. The team 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 millimeter per year extension over a 1000-kilometer length.“If you picked two points in New Mexico, and one of them lies 100 kilometers to the west of the other, then they would be moving apart at a rate of 0.1 millimeter per year,” explained researcher Henry Berglund.

Researchers used data from 25 continuous GPS stations installed as part of the EarthScope Rio Grande Rift GPS experiment, supplemented by data from other GPS monuments in the southwestern U.S., resulting in a data set of daily position estimates of 284 GPS monuments for the years 2006 through 2010.

“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 slow rates of motion made previous attempts to determine tectonic activity difficult. 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,” Sheehan said. “This is the most comprehensive and accurate set of geodetic measurements in this area to date.”

The extensional deformation is not concentrated in a narrow zone centered on the Rio Grande Rift. Instead, it is distributed broadly from the western edge of the Colorado Plateau into the western Great Plains — a span of more than 370 miles. “This unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain-rate deformation within continental interiors is accommodated,” Sheehan said. “Questions we wanted to answer are: how is the Rio Grande Rift deforming? Is it alive or dead? Is it opening or not?”

Along the rift, spreading motion in the crust has caused magma to rise to the surface, creating long basins susceptible to earthquakes. “The rift is still active,” Sheehan said.

The team plans to continue monitoring the Rio Grande Rift, and may attempt to determine vertical as well as horizontal activity to determine whether the Rocky Mountains are still uplifting.

University of Colorado (Boulder) student Henry Berglund services GPS site RG20 west of Silverton, Colorado.

The study’s findings shed light on how continents deform away from plate boundaries, Sheehan said. At plate boundaries scientists can clearly see what is going on. “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.”

“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.”

The National Science Foundation funded the study. EarthScope and UNAVCO provided instruments, equipment, and engineering services. Results of the study were published in the January 2012 issue of Geology magazine.

GPS monuments in the vicinity of the Rio Grande Rift and southern Rocky Mountains. The study included construction of 25 GPS monuments (blue circles) in Colorado and New Mexico in 2006 and 2007. Regional EarthScope Plate Boundary Observatory and Continuously Operating Reference Station monuments are shown by gray triangles.