U.S. Geological Survey
David A. Johnston Cascades Volcano Observatory
New Evidence Indicates Continuing Uplift at Three Sisters, Oregon
Update -- March 18, 2002
Scientist's from the U.S. Geological Survey's (USGS) Cascades Volcano Observatory and Central Washington University, in cooperation with staff from the U.S. Forest Service's (USFS) Willamette and Deschutes National Forests, have confirmed that slow uplift of a broad area centered about 5 kilometers (3 miles) west of South Sister volcano in the central Oregon Cascade Range is continuing at approximately the same rate as previously reported (i.e., a maximum rate of approximately 1 inch/year). The uplift was discovered by a satellite radar imaging technique during the spring of 2001, which showed that the ground surface had risen about 10 cm (4 inches) from 1996 to 2000. In May 2001, USGS and Pacific Northwest Seismograph Network (PNSN) scientists installed a seismometer and a GPS instrument near the center of uplift to monitor earthquake activity and ground movement, respectively (see Figure 1). These deployments were followed by surveys of ground deformation, spring-water chemistry, and volcanic gas emissions in Summer 2001 to ascertain whether the uplift was continuing and to determine its likely cause. In addition, satellite radar images for 2000-2001 were analyzed and compared to the 1996-2000 results.
The September 2001 ground-tilt and GPS surveys, when compared to similar surveys made in 1985 and 1986, are consistent with the radar results (i.e., the same model that fits the 1996-2001 radar observations also fits the 1986-2001 ground-tilt and GPS observations). This implies that the uplift did not start until sometime after 1996. The most recent radar data confirm that uplift continued through the summer of 2001, and daily data from the continuous GPS station installed in May 2001 show that uplift continues to the present. Meanwhile, seismicity in the area has remained at a low, background level. Analyses of spring-water samples collected in late summer 2001 are similar to those from earlier surveys but isotopic studies of carbon and helium in the most recent samples, which were not done previously, suggest a magmatic source.
Taken together, the ground-deformation, seismic, spring-water chemistry, and gas emission results suggest that uplift is caused by slow accumulation of magma at a depth of 6-7 km (4 miles) beneath the surface. If intrusion of magma were to continue, it could eventually lead to a volcanic eruption; however, an eruption is unlikely without months to years of 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 is released from the magma. At present, earthquake and gas emissions remain at low, background levels while steady uplift continues. USGS and PNSN scientists are working with USFS staff to improve monitoring of the area and, with State and local agencies, to develop an emergency-response plan.
More information about the Three Sisters region and its recent activity is available from the following sources:
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