SIGNIFICANT GRAVITY CHANGE DETECTED AT MOUNT BAKER, WASHINGTON, 1975-2005

In May 1975 geothermal activity increased suddenly at Mount Baker volcano in northwest Washington, releasing large vapor plumes from summit fumaroles in Sherman Crater and instigating many investigations, including repeated micro-gravity surveys. These surveys detected a decrease in gravity of ~300 µgal over 4 months during the summer of 1975; however, they lacked elevation control. Since 1975, Mt. Baker has continued to release large amounts of CO₂ and H₂S (187 t/d and 5.5 t/d respectively), corresponding to an approximate mass loss of 2.1 x10⁹ kg over the last 30 years. This investigation seeks to determine the gravity change at Sherman Crater since the 1975 surveys and to develop model(s) to account for the observed gravity change.

Eight micro-gravity stations were established on and around Mt. Baker during the summer of 2005, including two from the 1975 surveys, and GPS data were collected at each station for elevation control. After correcting for elevation, latitude, and earth tides, increases of 240 and 1380 µgal were found on the southeastern flank and south rim of Sherman Crater, respectively, since August 1975. Both spherical and cylindrical geometries are used to model potential volcanic sources of the gravity change. An elevation decrease of 4.5 m, significant mass intrusion (10¹⁵-10¹⁷ kg), or major densification of the magma source (+ 0.09-0.9 g/cm³) is required to account for the observed gravity change. Ranges in values represent variations in size or depth of the source. Snow, ice, and groundwater variations may also contribute to the observed gravity change. We will present results of sensitivity analyses using snow depth and precipitation data from the National Resource Conservation Service's Snowpack Telemetry (SNOTEL) sites at Mt. Baker and glacial variation data from the North Cascades Glacial Climate Project to determine the potential gravity signal observed due to these sources. Further monitoring of Mt. Baker will be necessary to conclusively determine what processes have been active at the volcano over the last 30 years, for which this project can be used as a baseline.