Three‐dimensional Seismic velocity tomography of Montserrat from the SEA-CALIPSO offshore/onshore experiment

Abstract

The Sea-Calipso offshore/onshore experiment was conducted in December 2007 to image the seismic velocity structure of the island of Montserrat. Seismic signals were generated by a towed airgun array at 60 second intervals along a series of circles and radial lines around the island. A total of 4,414 shots were recorded by a network consisting of 204 one-component vertical seismometers in linear deployment, 28 three-component seismometers in a grid deployment, and 10 Ocean Bottom Seismometers (OBS). A first-arrival tomographic inversion covering 45x50 km target area was conducted using the recorded data. In order to avoid a spatial bias, a subset of the data consisting of 58 stations was used. The first-arrival signals were easily identified in most cases. However, in seismic stations deployed close to the shore and/or the volcano, first-arrival signals were less easily identified due to poor signal-to-noise ratio. First-arrival data from 115,158 raypaths were used in a damped smoothed tomographic inversion to produce a 3-dimensional image of the P-seismic velocity to a depth of about 5 km. Inversion results showed areas of high velocity below each of the three volcanic centers of the island at a depth of 2-3 kilometers. Also visible were two areas of low velocity in the northeast and southwest flanks of the island. Modeled land velocities under the island were higher than expected, resulting in raypaths bottoming out at shallower depths without sampling a potential magma chamber at greater depths.