Assessing Gravity Changes at Mt. Baker, WA, 1975-2006

Significant micro-gravity changes have been detected at Mt. Baker, WA, from 1975-2006. Dynamic micro- gravity measurements were initiated by Steve Malone, Univ. of Washington, in March, 1975 in response to increased fumarolic activity and continued through 1981. Malone's unpublished data show that during this time, gravity decreased by 820 μgal at the Crag View station on the southeastern flank. On the south rim of the crater, gravity decreased by 440 μgal from 1975-1977 and increased by 220 μgal from 1977- 1980. For this project, the Crag View and South Rim stations were reoccupied in summer 2005, and six new stations were established to be a baseline for future gravity surveys. This reoccupation of the Crag View and South Rim stations revealed maximum gravity increases of 610 and 1810 μgal, respectively, since the previous surveys by Malone. Subsequent measurements made in 2006 confirm these findings, and errors ranged from 10-450 μgal. Large closure errors result from long-distance transport of the gravimeter on foot. Reoccupation of the six stations established in 2005 one year later reveals almost no change at 5 of the 6 stations. At Railroad Grade station on the southern flank, a decrease of 1190 μgal was observed that may be caused in part by the presence of a new, small body of water (~15 m3) near the gravity station and/or differences in the amount of snow present near the station. Mt. Baker's remote location and large extent of glacial ice limit the number of possible stations and increase instrumental error, limiting our ability to model gravity change. Simple spherical and cylindrical models were used to model the gravity change at the crater from 1977-2006. To account for this entire gravity change, an elevation decrease of 4.9 m, a mass increase of 1010 -1013 kg, or a density increase of up to 2.0 g/cm3 is necessary. Results of sensitivity analyses also indicate that effects from changes in snow cover and ice fluctuations could increase the observed gravity signal by as much as 125%. The observed change between 2005 and 2006 at Railroad Grade station suggests that effects from groundwater fluctuations could be significant.