3D image of an active magma chamber beneath Montserrat, Lesser Antilles, from first-arrival travel-time tomography (Invited)

Andesitic magma erupted at island arc strato-volcanoes is stored in the shallow crust prior to eruption. Constraining the size, location and characteristics of magma reservoirs is key to forecasting the likelihood and characteristics of future eruptions. The island of Montserrat, in the Lesser Antilles, has been the subject of an active-source seismic tomography experiment (the SEA-CALIPSO experiment) with the main aim of studying the magmatic system of the active Soufriere Hills Volcano (SHV). We present the results of the three-dimensional travel-time inversion of the dataset. We are able to image the main features of the volcanic system of the island including SHV and the two older and extinct volcanoes. The shallow structure is dominated by the presence of high-velocity cores beneath the three volcanic centres. Beneath SHV at depth between 5 and 8 km we observe a negative velocity anomaly (NVA) roughly 4 km across. Checkerboard tests show that a pattern with wavelength similar to the size of the anomaly is well resolved at up to 7 km depth therefore the lower limit of the low velocity region is not constrained by our data. We show that in this case the resolution limit is in part determined by the bandwidth of the illuminating seismic source, as well as the experiment geometry. Temperature estimates at the centre of the NVA calculated with respect to the average velocity structure of the island indicate that a significant region is at temperature above the solidus of andesite and thus require the presence of melt. To further constrain the nature of the NVA we compare the observed seismic velocities with those predicted by thermal and geochemical models of melt intrusions in the upper crust. The thermal models show that the observed NVA is compatible with the presence of a magma storage region of 20-40 cubic kilometers, formed by repeated emplacement of horizontal sills over thousands of years. This shallow magma storage region is likely to correspond to the source of andesitic magma in the shallow crust feeding the current eruption.