Crustal structure of Montserrat and Surrounding Area From Integrated Onshore, Ocean Bottom and Multichannel Reflection Seismic Data

Seismic tomography of active volcanic islands can provide information about the size and distribution of intrusions, the response of the lithosphere to edifice loading, the extent to which material has been added on top of versus within the pre-existing crust, and the presence of magma chambers. In December 2007, we carried out such an experiment at the island of Montserrat. Seismic signals were generated by a tuned 8- airgun 2600 cu in. array fired at a 60 s (c. 140 m) interval along a series of radial and circular paths around the island. Shots were recorded on 237 land stations and 10 ocean bottom seismometers (OBSs). OBSs were equipped with a hydrophone and a 4.5 Hz 3-component geophone or a 2 Hz vertical geophone and were deployed at 750-1100 m water depths from a 13-m boat, predominantly to the leeward, western side of the island. Shots were also recorded on a 600 m, 48-channel streamer. First arrivals are identifiable in OBS data at offsets up to 50 km, corresponding to rays turning beneath the island at a depth of about 9 km. Other observed phases include p-waves turning in the upper sediment layer, basement reflections, a reflection from a shallow interface in the sediments and a reflection from 15-20 km depth, possibly corresponding to the Moho. We will show examples of OBS data, their integration with reflection data to determine the shape of the basement surface around the island, and some initial models of the deeper structure.