Probing the interior of an active volcano: Three-dimensional seismic tomography at Montserrat

Constraining the magmatic systems of active volcanoes is important for comprehending the mechanisms that drive magma supply. Particularly important are the geometry and characteristics of magma storage regions in the upper and middle crust, which can influence the style and time-scales of eruptions. Seismic tomography can provide detailed resolution of the physical characteristics of complex three-dimensional structures in the subsurface, and has been successfully employed at several active volcanoes to constrain the structure of their magmatic systems. The island of Montserrat, Lesser Antilles, has been the subject of an land-sea active-source seismic experiment to constrain its upper crustal structure, with particular focus on the magma reservoir feeding the current eruption. A two-dimensional inversion of seismic travel-times from a subset of the data collected, including wide-angle refractions and reflections, constrains the upper crust to a depth of 5-6 km, along a south-east to north-west section. The two-dimensional model delineates the high-velocity cores of the volcanic edifices that make up the island, the slower surrounding volcaniclastic deposits and pelagic sediments, and the underlying upper crust. Analysis of field recordings and synthetic waveforms calculated with a viscoelastic finite-difference method indicates that the high-velocity cores correspond to a geologically heterogeneous region which causes anomalous scattering of the seismic wave field. A three-dimensional seismic velocity model from first-arrival travel-time tomography delineates the high-velocity cores in greater detail and reveals a low-velocity volume at 4 to 8 km depth beneath the active Soufriere Hills volcano, inferred to correspond to the active upper-crustal magma chamber. The magma chamber is further constrained with the help of numerical models of incremental magma chamber growth, which suggests that a magma chamber of about 18 km3 formed at 5.5 to at least 7.5 km depth by incremental intrusion of sills over a few thousand years and is therefore larger and deeper than previously inferred. The new tomographic results are integrated with previous geological, petrological and geodetic constraints to provide one of the most complete models of the magmatic system of an active volcano from the magma source in the mantle wedge to the volcanic edifice at the surface. The volume estimate and chamber location are critical parameters required for models of eruption dynamics, which in turn are key to forecasting the likelihood and characteristics of future eruptions and the mapping of hazard