Characterizing an Active Magma Chamber at South Sister Volcano, Oregon: Constraints From Gravity and GPS Measurements

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Characterizing an Active Magma Chamber at South Sister Volcano, Oregon: Constraints From Gravity and GPS MeasurementsEAR-0208490PI: Johnson Modeling of InSAR data by the USGS has determined that an uplift of ~0.022 km 3 has developed since approximately 1998 due to an inflation source at 5-7 km depth. This uplift has not been accompanied by seismic activity. A goal of this project is to determine if this uplift is attributable to new magma injection, expansion of an existing magma body, or hydrothermal processes. If magma injection is producing the uplift, then an important second goal will be to characterize the magma chamber. Important questions to be addressed are: 1) does the magma chamber at 5-7 km depth contain only material added within the past few years? 2) what is the volume of the magma? 3) is the magma basic or silicic? 4) what is the gas content of the magma? These questions will be addressed by modeling the proposed gravity measurements. Three years of gravity observations on three radial profiles of the uplift will be completed. Vertical control with leveling and GPS surveys is planned by the USGS and will be available to this project. The siting of stations and survey schedule will be optimized for the characterization and removal of the annual groundwater and soil moisture signal from this data set. Central to modeling of the gravity changes is observing the ratio of gravity change to vertical uplift at monitored sites, which may be interpreted in terms of mass and density change at depth.