Crustal structure beneath the Montserrat region of the Lesser Antilles island arc

We use receiver function analysis to estimate the first-order subsurface seismic structure in the vicinity of the island of Montserrat, located in the northern Lesser Antilles arc and home of the active Soufrière Hills Volcano. Near-surface complexity in the island structure inhibits our ability to resolve lateral variations in seismic structure, so we average the signals from eight stations in two directions to produce a first-order regional seismic velocity model. Although we are unable to resolve a precise sharpness of the crust-mantle transition, we can limit its thickness to less than ∼4 km. Lateral variations in shallow structure complicate a receiver function based estimate of Poisson's ratio, but the data indicate that in both directions sampled by the receiver functions, the lower crustal Poisson's ratio is greater than 0.27 and more likely in the range 0.29-0.30. We estimate a mean crustal thickness of ∼30 km, and the observations suggest that the crust may be slightly thinner northwest of the island (∼26-30 km) than it is to the south (∼30-34 km). The high P wave speed and Poisson's ratio indicate a generally mafic lower crust, with rocks of intermediate composition not precluded in the upper part of the lower crust. We did not find evidence for a thick high-speed lower crust (>7.4 km/s) as has been inferred in some other arcs.