USGS RESPONSE TO ONGOING CRUSTAL UPLIFT AT THREE SISTERS VOLCANIC CENTER, OREGON

The April 2001 discovery by radar interferometry of a broad area of uplift centered 5 km west of South Sister volcano created an opportunity to investigate processes that might be early precursors to an eruption. Initial analysis suggested that uplift began after 1996, but was it still underway? Modeling suggested a volume increase of ~ 107 m3 at ~ 6 km depth, but was magma intrusion its cause? A sparse seismic network had not triggered on any earthquakes in the area, but was there any microseismicity? Spring-chemistry measurements in 1990 and 1999 suggested the presence of a shallow hydrothermal system, but were there gas emissions or geochemical changes that might confirm intrusion of magma? If intrusion were going on, would it lead to eruption? If so, where and when? Were there public safety concerns? The uplift was almost entirely within the Three Sisters Wilderness Area, so field investigations were restricted.

USGS scientists alerted officials from the Willamette and Deschutes National Forests and consulted them about appropriate responses. A public-information release in early May created a stir when the uplift was christened “the bulge” in the media, raising public concern that something akin to Mount St. Helens’ infamous bulge was forming. Owing to potential safety concerns, difficult access, lack of real-time monitoring, and sparse recreational use in May, USFS allowed scientists from USGS and the Pacific Northwest Seismograph Network (PNSN) to install a 3-component seismometer and a continuous GPS station near the center of uplift. The seismometer soon confirmed the near lack of microseismicity. By late summer, spring-water studies yielded results similar to those of earlier surveys, but C and He isotopic studies, not done previously, suggested a magmatic source. Airborne surveys for magmatic gases detected only trace to background CO2 levels but no other gases. Ground-based geodetic surveys, continuous GPS, and 2000-01 interferograms confirmed that uplift was continuing at ~ 30 mm/yr. Public meetings were held in nearby communities to discuss potential hazards, current conditions, and possible outcomes. Local newspapers, radio and TV stations are keeping citizens informed. USGS and PNSN are working with USFS to improve monitoring and, with State and local agencies, to develop an emergency-response plan.