GRAVITY INCREASE OBSERVED AT MOUNT BAKER VOLCANO, 1975-2006

Eight micro-gravity stations were established on Mt. Baker volcano during the summer of 2005, two of which were sites occupied by 1975-1980 dynamic surveys (Malone 1979). Increases of 1810 and 610 microgal were found at the Sherman Crater and Crag View stations (on the southeastern flank), respectively, since the 1975-1980 surveys. These values were confirmed in summer 2006. Modeled effects of snow and ice variations suggest these values could underestimate the gravity change by as much as 125%. Reoccupation of the other six micro-gravity stations in 2006 revealed decreases of 892 and 1186 microgal at two flank stations, and no change outside of error at the other stations. We attribute these changes to differences in late-summer snow pack and/or local groundwater variations.

Cylindrical and spherical source models were used to determine possible end-member volcanic sources of the gravity change at the Sherman Crater station from 1975-2006. These include an elevation decrease of 4.7-7.0 m, a mass increase of 1010-1012 kg, or a density increase of no more than 1.5 g/cm^3. Errors in these surveys range from 10-450 microgal and are primarily the result of Mt. Baker's remote location, requiring long-distance transport of the gravity meter on foot. Although these errors are large, they are well below the magnitude of the observed change at the crater, which suggests that one or more of these volcanic processes has contributed to this change.