Seismic Velocity Structure of the Island of Montserrat From Reflection/Refraction Tomography

The Soufrière Hills Volcano (SHV) on the island of Montserrat, Lesser Antilles, resumed its activity in 1995 and has since been studied in great detail, but knowledge of its deep structure is scarce. In December 2007 the SEA-CALIPSO land-sea seismic experiment was conducted to investigate the seismic velocity structure of the island and better understand the magma system feeding the volcano. An array consisting of 28 3- component Refteks, 209 1-component Texans and 10 LC-2000 ocean bottom seismometers (OBSs) was installed to record the signals generated by an array of 8 airguns firing every 60 s at a pressure of 2000 psi and with total volume of 2600 cu in.. A total of 4414 shots were recorded over 77 hours. The survey covered an area of about 50 × 40 km. A subset of the data, consisting of four OBSs and four land stations on a south-east to north-west line, has been processed and travel times have been inverted to obtain a two-dimensional seismic velocity section through the island. Identified phases include crustal and sediment p-waves and their multiples, basement reflections and a reflection from a shallow sediment layer. A regularized inversion approach has been used, where the data misfit and the model roughness are minimized simultaneously to give a minimum-structure model. The inversion process starts with a highly smoothed model on a regular grid and uses a layer stripping approach. The resulting velocity model reveals the presence of a high velocity body with velocity contrast of up to 1.5 km/s underneath the island, about 10 km wide and extending from the surface to a depth of 8.0 km. This could be explained by the presence of intrusions. The basement reflector is observed at a depth of about 1200 m, presenting a depression under the island due to edifice loading. In the superficial layer observed velocities vary from 1.6 to 3.0 km/s at sea and from 3.0 to 4.5 km/s on land. A shallower reflector is observed at about 500 m depth possibly marking the unconformity between Soufrière Hills and the older volcanic edifices. The results so far provide a constraint on the upper crustal structure up to a depth of 9 km that will help understand volcanism at Montserrat and other island arc volcanoes.