Cascade Range Current Update

U.S. Geological Survey Cascades Volcano Observatory, Vancouver, Washington, and Volcano Hazards Team, Menlo Park, California U.S. Geological Survey Earthquake Hazards Team, Menlo Park, California University of Washington, Geophysics Program, Seattle, Washington

May 8, 2001 THREE SISTERS, OREGON, INFORMATION STATEMENT

USGS scientists have detected a slight swelling, or uplift, of the ground surface over a broad area centered 5 kilometers (3 miles) west of South Sister volcano in the Three Sisters region of the central Oregon Cascade Range. The Three Sisters region is located 35 kilometers (22 miles) west of Bend, Oregon, and 100 kilometers (60 miles) east of Eugene, Oregon. The uplift, which occurred between 1996 and 2000, covers an area about 15 to 20 kilometers (9 to 12 miles) in diameter and the maximum amount of uplift at its center is about 10 centimeters (4 inches). It is too broad and low to be noticed from the ground, and several close aerial inspections of the area have revealed no unusual surface features.

The relatively new technique employed by the scientists, Satellite Radar Interferometry (or InSAR), uses satellite data to make radar images of a portion of the Earth's surface. Images acquired at different times, but from the same location in space, can be used to detect even minor (a few centimeters or one inch) changes in the elevation of the ground. The images that reveal the 10-centimeter uplift near South Sister were obtained in 1996 and 2000. The exact timing of the uplift, or whether it is continuing at present, is unknown, but is being studied further.

The specific cause of the uplift is uncertain. Because the Three Sisters region is a volcanic area, the uplift may reflect intrusion of a small volume of magma (molten rock) deep under the surface-probably at a depth of about 7 kilometers (4 miles). Such a process, which keeps volcanic areas "alive" and prepares them for future eruptions, is a common occurrence under volcanoes, but until development of techniques such as InSAR, it has been difficult to detect. If intrusion of magma were to continue, it could eventually lead to a volcanic eruption; however, an eruption is unlikely without significant precursory activity. In addition to continued or accelerating uplift, precursors to an eruption would include earthquakes, typically swarms of small events generated by fracturing of rock as magma moves upward, and large emissions of volcanic gases, such as carbon dioxide, which are released from the magma. At present, earthquake activity appears to be at or near background level and gas emissions are low. In order to be prepared to more accurately detect possible precursors and to better understand this uplift phenomenon, USGS plans to enhance the existing monitoring network. Installation of one or more additional seismometers and a Global Positioning System (GPS) receiver, resurvey of existing benchmarks and installation of new benchmarks, and periodic airborne and ground-based sampling of gases are currently being considered in consultation with managers of the Willamette and Deschutes National Forests.

A number of public officials and agencies in the State of Oregon and Lane and Deschutes Counties have been briefed on these findings and they and scientists will work together to address any questions or concerns the public may have.

Additional information, including maps and a volcanic-hazards assessment, may be found on the WORLD WIDE WEB at URL: http://vulcan.wr.usgs.gov/Volcanoes/Sisters/framework.html http://volcanoes.usgs.gov/About/What/Monitor/Deformation/InSAR.html and http://www.geophys.washington.edu/SEIS/PNSN/

Information contacts: U.S. Geological Survey-Cascades Volcano Observatory, Vancouver, Washington (360) 993-8900

U.S. Geological Survey-Volcano Hazards Team, Menlo Park, California (650) 329-5227

Pacific Northwest Seismograph Network, University of Washington, Seattle, Washington (206) 685-2255

Oregon Department of Geology and Mineral Industries, Portland, Oregon (503) 731-4100



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