Secondary seismic waves from timed explosions helped University of Oregon scientists capture a three-dimensional view of Newberry Volcano near Bend, allowing them to see where magma dwells below the caldera.
There is magma but in low volumes, possibly from refilling some 50 years ago but since quiet, says Emilie Hooft, assistant professor of geological sciences.
"Newberry last erupted 1,300 years ago. It is a quiet system right now," Hooft says. "Another episodic refilling might stress the system, leading to little earthquakes, which would be a warning that the system is refilling. But South Sister has been inflating for years, though more slowly since 2004, and there are no earthquakes associated with that." So, she adds, Newberry's refilling could occur without the earthquakes.
Newberry — one of the nation's biggest and youngest volcanoes — is a shield volcano with gradual sloping flanks. It rises 7,986 feet about 20 miles southeast of Bend. South Sister, one of the three Sisters peaks, is the state's third-tallest mountain, rising to 10,358 feet 28.5 miles west of Bend.
The new subsurface view of Newberry indicates that magma regeneration has occurred at shallower depths than under South Sister.
The magma chamber beneath Newberry is at about four kilometers (2.5 miles); under Sisters, refilling is at the four-mile depth (about seven kilometers). Hooft says a bigger question is what is happening at deeper depths, say 15-30 kilometers (nine to 19 miles).
Viscosity refers to the consistency of the magma, in the case of volcanoes, and the liquid's ability to resist flow. Rock types and temperatures also play a role.
Initially, she and her team tapped seismic wave data compiled from 17 locations where the U.S. Geological Survey had set off explosions in the 1980s.
These primary waves pass through the ground and capture anomalies such as loose rock, magma or rubble, but the resulting distortion quickly "heals" in data readouts as the waves move away. The resulting tomographic images lack precision.
Hooft wondered what returning, secondary seismic waves might add to the picture.
Along with revisiting earlier data, Hooft added a new location for an explosion by piggybacking with another group doing seismic studies near the volcano. Her part was done with a USGS Venture Capital Grant and funding from the National Science Foundation.
After adding the new data with the old, Hooft had a much clearer picture, creating the opportunity to produce the 3D view.
The new information was detailed in October 2012 in the Journal of Geophysical Research.
"The paper drew a lot of interest," Hooft says. "Petrologists have been excited by this new view under the volcano. They are saying that the new structural information might give them clues on the chemistry of the lavas and is useful for what they do.
"The image that is forming is that these magmatic systems are multiple layers with multi levels of chambers. What we can see is basically what is under the caldera," she added.
More perspective on what the information really means, Hooft adds, could emerge with similar data drawn from other active volcanoes.
- by Jim Barlow, University Communications