NEW RESULTS FROM A PROPOSED PBO CASCADE VOLCANO CLUSTER II: INSAR, GPS, AND TILT-LEVELING DATA FROM THE THREE SISTERS AREA, CENTRAL OREGON

Although the Three Sisters volcanic center was not among the sites in the Cascade Range initially proposed for Plate Boundary Observatory (PBO) instrument clusters (Mount St. Helens and Medicine Lake-Mount Shasta) in 2000, the discovery by InSAR in April 2001 of a broad area of uplift centered about 5 km west of South Sister volcano makes it an attractive target that should be considered in the final PBO implementation plan. In cooperation with the U.S. Forest Service (USFS) and the Pacific Northwest Seismograph Network, the USGS Cascades Volcano Observatory (CVO) installed a seismometer and continuous GPS station about 3 km north of the center of uplift in June 2001. In August 2001, CVO measured EDM and tilt-leveling networks (the former with GPS) at South Sister, which were established in 1985, and also established a broader GPS network that spans the entire deforming area. The amount of deformation measured at South Sister since 1985 is comparable to that revealed by InSAR, which indicates that the uplift started sometime after the first InSAR observation in 1995. In September 2002, the expanded GPS network was re-measured and a new tilt-leveling network was established, consisting of 3 traverses totaling ~7 km that straddle the center of uplift. The location and depth (5.7 km) of the pressure sources estimated independently from InSAR (1995-2001) and GPS (2001-02) data are nearly identical. The average inflation rate has been 0.005 ± 0.001 km³/yr and the total volume increase through September 2002 was 0.027 ± 0.005 km³. The continuous GPS data indicate that inflation, which has been nearly aseismic, is continuing at approximately the same rate. We will present results of GPS and tilt-leveling surveys to be conducted in September 2003, and also the most recent interferograms available. The deforming area is entirely within the Three Sisters Wilderness Area, so any proposal to establish a PBO instrument cluster at Three Sisters must be carefully coordinated with the USFS.