Daniel J. Johnson1
In the spring of 2001, USGS volcano researchers discovered a zone of recent crustal uplift immediately west of South Sister stratovolcano, central Oregon Cascade Range, using the technique of satellite interferometric synthetic aperture radar (InSAR). In response, the USGS has installed a Global Positioning System (GPS) instrument near the center of uplift, reoccupied electronic distance measurement (EDM) and tilt networks last measured in 1986, and scrutinized additional InSAR images. These efforts have confirmed that uplift is presently continuing at a pace of approximately 40 mm/year at the center of uplift and that it began in ~1998. Modeling of the uplift data indicates that the inflation source lies 5 to 7 km below the surface. Currently it is unknown if the inflation is due to injection of new magma, or due to expansion - for example vesiculation - of older magma at depth, or simply due to changes within a hydrothermal system. To help answer these questions, a series of repeated precise gravity and GPS surveys will be conducted at monitoring points established in the uplift region. The primary observation is the ratio of gravity change to vertical uplift, which may be interpreted in terms of mass and density change at depth. If it is new magma injection that is producing the uplift, then an important second goal will be to characterize the magma chamber. Does the chamber contain only material added within the past few years, or a significant volume of magma that accumulated earlier? What kind of magma is present and what is its gas content?
1 Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926