U.S. Geological Survey,
David A. Johnston Cascades Volcano Observatory,
Vancouver, Washington, and
University of Washington, Pacific Northwest Seismograph Network,
Three Sisters, Oregon
This report provides an update of U.S. Geological Survey (USGS) and partnersí (chiefly the Pacific Northwest Seismic Network at the University of Washington and University of Oregon) activities to monitor and assess ongoing volcanic unrest in the Three Sisters region, which began in late 1997 or early 1998. It was discovered in 2001, when analysis of satellite radar data (using a technique called InSAR) revealed that an area about 10 miles in diameter had risen about 4 inches at its center, which lay 3 miles west of the summit of South Sister volcano. Uplift continued at average rates of about 1 inch per year through 2004.
Two continuously recording GPS receivers and annual field surveys during summer 2005 and 2006 confirm that over the past two years this rate has slowed somewhat. InSAR and leveling measurements indicate that the rate of vertical motion has slowed by about one-half, while GPS measurements also show that the rate of vertical motion has slowed, but not by as much. The uplift and extension are likely driven by intrusion of a modest volume of magma, or molten rock, at a depth of about 4 miles. Slowing of ground deformation implies a decrease in the rate of magma intrusion.
Work by USGS scientists in 1990 showed that magmatic heat and gas influenced spring-water temperature and chemistry in this same area. Therefore, the area may be affected episodically by intrusions. The current episode has not yet had a measurable effect on spring waters, which are sampled and analyzed yearly. Seismicity continues at the low rate typical since the March 2004 earthquake swarm, which was located near Middle and South Sisters and tallied more than 300 small earthquakes over several days. In the past year, 7 small earthquakes were located in and near the area of uplift; maximum magnitude was 1.8. Since 2001, scientists have installed 7 additional seismic stations in the area. Two more will come on line this summer and improve our ability to detect and accurately locate future earthquakes. The duration and outcome of the current episode of intrusion is impossible to forecast, and only continued monitoring can track future developments. Intrusions in volcanic areas require close scrutiny because some eventually lead to eruption, but only after weeks to months of intensifying unrest involving earthquakes, ground uplift, and release of volcanic gases. Scientists will work with the U.S. Forest Service to enhance monitoring networks as needed, and continue to work with local agencies to develop a plan for responding to future volcanic events.
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