Shallow crustal mechanics from volumetric strain data: Insights from Soufrière Hills Volcano, Montserrat

Volumetric strain data from the 29 July 2008 Vulcanian explosion of the Soufrière Hills Volcano provide an excellent opportunity to probe the mechanical properties of the volcanic edifice and the shallow crust beneath Montserrat. We use Finite Element Analysis to constrain the geometry of the shallow plumbing system as well as edifice and crustal mechanical properties for mechanically plausible pressure drops associated with dilatational volumetric strains recorded during the explosive activity. Our results from both forward and inverse models indicate a conduit radius of ∼40 m and a length of ∼1500 m, and hence much larger conduit dimensions than previously suggested. In order to fit the syneruptive volumetric strain data for a conduit pressure drop of <10 MPa, the conduit needs to be surrounded by a halo of mechanically compliant rocks for which the best fit models indicate a width of ∼600 m and a Young's modulus of ∼1 GPa. Young's moduli for the main edifice and the shallow crust up to ∼1000 m below sea level are found to be ∼10 GPa. Our best fit inverse model predicts a syneruptive radial conduit contraction by 0.24 m with a corresponding volume loss of ∼0.1 Mm³, implying only partial emptying of the conduit upon the explosion. This study demonstrates the critical role of edifice and shallow crustal mechanics for strain partitioning on Montserrat. Our findings may have implications for the assessment of conduit processes at other dome-building volcanoes.