Strain field analysis on Montserrat (WI) as tool for assessing permeable flow paths in the magmatic system of Soufriere Hills Volcano

Strain dilatometers have been operated on the volcanic island of Montserrat (West Indies) for more than a decade and have proven to be a powerful technique to approach short-term dynamics in the deformational field in response to pressure changes in the magmatic system of the andesitic dome-building Soufriere Hills Volcano (SHV). We here demonstrate that magmatic activity in each of the different segments of the SHV magmatic system (shallow dyke-conduit, upper and lower magma chambers) generates a characteristic strain pattern that allows the identification of operating sources in the plumbing system based on a simple scheme of amplitude ratios. We use this method to evaluate strain data from selected Vulcanian explosions and gas emission events that occurred at SHV between 2003 and 2012. Our results show that the events were initiated by a short phase of contraction of either one or both magma chambers and a simultaneous inflation of the shallow feeder system. The initial phase of the events usually lasted only tens to hundreds of seconds before the explosion/gas emission started and the system recovered. The short duration of this process points at rapid transport of fluids rather than magma ascent to generate the pressure changes. We suggest the propagation of tensile hydraulic fractures as viable mechanism to provide a pathway for fluid migration in the magmatic system at the observed time scale. Fluid mobilization was initiated by a sudden destabilization of large pockets of already segregated fluid in the magma chambers. Our study demonstrates that geodetic observables can provide unprecedented insights into complex dynamic processes within a magmatic system commonly assessed by theoretical modeling and petrologic observations.