[1] The SEA-CALIPSO experiment in December 2007 incorporated a sea-based airgun source, and seismic recorders both on Montserrat and on the adjacent sea floor. A high quality subset of the data was used for a first arrival P-wave velocity tomographic study. A total of more than 115,000 traveltime data from 4413 airgun shots, and 58 recording stations, were used in this highresolution tomographic inversion. The experiment geometry limited the depth of well resolved structures to about 5 km. The most striking features of the tomography are three relatively high velocity zones below each of the main volcanic centers on Montserrat, and three low velocity zones flanking Centre Hills. We suggest that the high velocity zones represent the solid andesitic cores of the volcano complexes, characterized by wave speeds faster than adjacent volcaniclastic material. The low velocity zones may reflect porous volcaniclastic material and/or alteration by formerly active hydrothermal systems.Citation: Shalev, E., et al. (2010), Three-dimensional seismic velocity tomography of Montserrat from the SEA-CALIPSO offshore/onshore experiment, Geophys. Res. Lett., 37, L00E17,

S U M M A R YTo better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seismic experiment (Seismic Experiment with Airgun-source -Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufriére Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5-5.5 km s −1 ) beneath the island, with highest velocities beneath the Soufriére and Centre Hills, corresponding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the offshore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s −1 , consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ∼1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0-6.0 km s −1 and is inferred to correspond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ∼5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes.

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[1] During the December 2007, SEA-CALIPSO experiment we collected seismic reflection profiles offshore of Montserrat. Off the east coast, we imaged deep fans of volcaniclastic debris from three volcanoes progressively active from ∼2 Ma to present. Near-shelf sedimentation rates of 8-9 cm/ka are approximated following cessation of local volcanic activity. The fans were deposited on sediments with apparent dips towards the ESE-trending Montserrat-Havers fault system (MHFS) in southern Montserrat. The MHFS encloses the Soufrière Hills Volcano, has elevated crustal blocks at Roche's Bluff, St. Georges Hill, and Garibaldi Hill, and extends off the west coast. Off the west coast, the N-dip of two faults supports a N-dip interpretation for a major component of MHFS, the Belham Valley fault. We propose that local deformation is affected by stress redistributions consistent with a right-stepping, sinistral en-echelon fault system, but the interplay of transtension and magmatism has resulted in complex and evolving stress regimes.