ONGOING MAGMA INTRUSION BENEATH THE THREE SISTERS VOLCANIC CENTER, CENTRAL OREGON CASCADE RANGE, USA, INFERRED FROM SATELLITE INSAR

Images from satellite interferometric synthetic aperture radar (InSAR) reveal uplift of a broad ~10 km by 20 km area in the Three Sisters volcanic center of the central Oregon Cascade Range, ~130 km south of Mt. St. Helens. The uplift is centered ~5 km west of South Sister volcano, the youngest stratovolcano in the volcanic center. The last eruption in the Three Sisters area was ~1500 years ago. Multiple European Space Agency ERS-1 and ERS-2 satellite images from 1992 through 2001, used in this study, were selected based on orbital separation and time of year. Summer and early autumn scenes were necessary to avoid decorrelation from snow cover. Interferograms generated from these images indicate that most if not all of ~140 mm of observed uplift occurred between September 1998 and October 2001. We interpret the uplift as inflation caused by an ongoing episode of magma intrusion at a depth of 5-7 km.

Geochemical (water chemistry) anomalies, first noted ~1990, coincide with the area of uplift and suggest the existence of a magma reservoir prior to the uplift. High chloride and sulfate concentrations, a positive correlation between chloride concentration and spring temperature within the uplift area, and larger SO₄/Cl ratios in springs at higher elevations all occur within the area of uplift. These findings are indicative of a high-temperature hydrothermal system driven by magma intrusions.

The inflation episode observed with InSAR may lead to an eruption, however the more persistent geochemical evidence suggests that the episode is likely the latest in a series of magma intrusions. The most recent interferograms for the period October 2000 to October 2001, however, show that uplift continued at the same average rate (30-40 mm/yr) during that period as during 1998-2000. Continuous GPS data from a station near the center of deformation indicate uplift at the same rate through January 2002. The small volume of emplaced magma, inferred to date, is not a cause for alarm. However, the ongoing nature of the uplift serves to emphasize the need for intensified geodetic, seismic and geochemical monitoring, both to better understand the intrusion process and to continually update hazard assessments.