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Aquifers are poroelastic bodies that respond to strain by changes in pore pressure. Crustal deformation due to volcanic processes induces pore pressure variations that are mirrored in well water levels. Here, we investigate water level changes in the Belham valley on Montserrat over the course of two years (2004-2006). Using finite element analysis, we simulate crustal deformation due to different volcanic strain sources and the dynamic poroelastic aquifer response. While some additional hydrological drivers cannot be excluded, we suggest that a poroelastic strain response of the aquifer system in the Belham valley is a possible explanation for the observed water level changes. According to our simulations, the shallow Belham aquifer responds to a steadily increasing sediment load due to repeated lahar sedimentation in the valley with rising aquifer pressures. A wholesale dome collapse in May 2006 on the other hand induced dilatational strain and thereby a short-term water level drop in a deeper-seated aquifer, which caused groundwater leakage from the Belham aquifer and thereby induced a delayed water level fall in the wells. The system thus responded to both gradual and rapid transient strain associated with the eruption of Soufrière Hills Volcano (Montserrat). This case study gives field evidence for theoretical predictions on volcanic drivers behind hydrological transients, demonstrating the potential of hydrological data for volcano monitoring. Interrogation of such data can provide valuable constraints on stress evolution in volcanic systems and therefore complement other monitoring systems. The presented models and inferred results are conceptually applicable to volcanic areas worldwide.

Although the persistently active Soufrière Hills Volcano (Montserrat, West Indies) is one of the most extensively studied active stratovolcanoes, a local Bouguer gravity map of the volcano and the island of Montserrat has yet to be constructed. We collected 157 new gravity data, which we analyzed and inverted in order to constrain the island's subsurface density distribution. Our model results reveal high-density material beneath the centers of the extinct volcanic complexes - presumably related to exposed dome cores - while the volcanic flanks and the active Soufrière Hills Volcano are underlain by low-density material. Volcaniclastic deposits and subsurface melt aggregations, respectively, may explain these negative gravity anomalies. Our results are in good agreement with previous structural observations from seismic tomography; however, a higher spatial density of the gravity survey network has allowed us to additionally capture smaller, shallow-seated anomalies in the gravity field that relate to tectonic structures and fluvial filling deposits. Key Points First local Bouguer anomaly map of Montserrat from 157 new gravity data Data inversion provides high resolution of density contrasts at shallow depths Anomalies image old dome cores, volcaniclastic aprons and tectonic structures ©2013. American Geophysical Union. All Rights Reserved.

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.