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Ground still swells near South Sister

PORTLAND, Ore. (AP) -- Federal researchers will consider whether to place new seismic equipment near the South Sister after evidence that molten rock has continued to seep into the area over the past year.

Scientists say the influx of volcanic rock doesn't necessarily mean the South Sister will erupt, but the subterranean magma three to four miles below the surface could trigger earthquakes as it moves.

New measuring devices would allow further study of the volcanic area, as well as protect the public, said Daniel Dzurisin of the U.S. Geological Survey's Cascades Volcano Observatory.

"If it continues, it could eventually culminate in an eruption, and we need to be prepared for that," said Dzurisin, who works in Vancouver, Washington.

Volcanic vents near the South Sister in Central Oregon last spilled magma more than 1,200 years ago and lie in a region that has spewed more volcanic debris than any other in the Pacific Rim.

The U.S. Geological Survey would need permission from the U.S. Forest Service to place the seismic equipment.

Satellite images earlier this year revealed that a region nine to 12 miles across had risen about four inches from 1996 to 2000. Geologists said it was the most striking geological change in the Cascade Range since the 1980 eruption of Mount St. Helens.

At the time, there were no current satellite images to show whether the swelling was continuing. In September, however, a European satellite circling about 500 miles above Earth snapped a new picture that includes precise measurements of the surface topography.

Comparing it with earlier images, researchers found that the area centered three miles west of South Sister had continued bulging upward at more than an inch a year. That brings its total swelling to about five inches in five years -- a breakneck pace in geologic terms.

Other volcanoes are known to swell and shrink without erupting, but in some cases small surges of molten rock have triggered eruptions.

A relatively small infusion of 30 million cubic yards of magma could have caused the swelling, but it could also come from a larger reservoir of molten rock that has been under the Cascades for hundreds of years.

"We know from other places that there can be magma bodies present for some time, where a small increase causes the original body to erupt," Dzurisin said. "If the process were to continue, it would be like stretching a rubber band. Eventually it's going to break. It may take five years or it may take 10 or 20."

William E. Scott, scientist-in-charge of the Cascades Volcano Observatory, emphasized that more research would help unravel details of the uplift. He said scientists need to find out if the rate of the swelling is increasing, decreasing or holding steady over time.

A new seismic-monitoring station installed in April on The Husband, a peak west of South Sister, only registered a 1.9 quake on August 21.

In addition to monitoring ground movement, researchers have sampled springs in the area for volcanic gases that would be a signature of an underground magma reservoir. Those gases act as the driving force of eruptions.

Stephen E. Ingebritsen, a hydrologist with the Geological Survey, said carbon and helium isotopes in samples collected this summer "showed pretty conclusively" that magma is present.

A small amount of carbon dioxide also has been detected in the atmosphere above the region, which is consistent with the idea that magma is responsible for the uplift.

South Sister, the youngest of the Three Sisters, erupted twice about 2,000 years ago.

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