Mass addition at Mount St. Helens, Washington, inferred from repeated gravity surveys.
Relative gravity measurements were made at 12 sites on Mount St. Helens and 4 sites far afield during the summers of 2010, 2012, 2014, and 2016. Positive residual gravity changes of 0.05–0.06 ± 0.01 mGal from 2010 to 2016—a sign of mass addition—remain at proximal sites after accounting for the effects of changes in Crater Glacier shape and mass. Modeling of the 2010–2016 monitoring data indicates mass addition in the volcano magma reservoir, the volcano conduit, and/or the shallow hydrothermal aquifer.