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[1] At Mount Baker, elevated gas and heat fluxes from fumaroles in Sherman Crater indicate the presence of a degassing magma reservoir. Campaign Global Positioning System (GPS) surveys in 2006 and 2007 provide slope distance measurements of 19 trilateration lines and provide baseline positions for future GPS study on Mount Baker. Comparison of slope distance measurements acquired in 1981 and 1983 with electronic distance meters (EDM) indicates that significant surface deformation has occurred on Mount Baker during the past quarter century. Slope distances have predominantly shortened around the edifice at rates <2 mm/yr. The greatest slope length change detected is −17 ± 4 ppm on the northern flank of the volcano. A uniform surface strain rate model fit to the weighted slope change data shows contractional strain, with an areal dilation rate of −420 ± 140 nanostrain/yr. The observed strain rate is an order of magnitude greater than that expected from tectonic sources. Elastic dislocation models are used to invert for the location and strength of a point source at depth. The optimal model predicts a volume change of −11 × 10 6 m 3 , located 1500 m east-northeast of the summit, at a depth of ∼5.8 km. The model can account for most of the deformation detected, suggesting that the magmatic and hydrothermal system at Mount Baker has depressurized since 1981, from the combined result of densification and devolatilization.

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