Mass variations in response to magmatic stress changes at Soufrière Hills Volcano, Montserrat (W.I.): Insights from 4-D gravity data

The eruption of Soufrière Hills Volcano (Montserrat, West Indies) has been ongoing for more than a decade, yet routine monitoring of the activity did not include gravity surveillance for most of the time. In June/July 2006, we installed a new elevation-controlled microgravity network, which we re-occupied in January/February 2007 and August/September 2008. Residual gravity changes of up to 74 μGal between the surveys allow us to infer net mass and/or density changes beneath the central part of the island. Data inversion for causative source parameters indicates mass changes along NW-SE elongated structures beneath the Centre Hills at a minimum depth of 700 m. We suggest the observed gravity variations to be related to poroelastic dynamics involving groundwater migration and/or fracture opening/closing along a hitherto unrecognized fault zone. The perturbations appear to be triggered by changes in the stress field of the shallow plumbing system of Soufrière Hills Volcano.