Analysis of shear-wave splitting from volcano-tectonic events at Soufrière Hills volcano, Montserrat.

Abstract:

Here we investigate seismic anisotropy of the upper crust in the vicinity of Soufrière Hills volcano using shear wave splitting (SWS) analysis from volcano-tectonic (VT) events. Soufrière Hills, which is located on the Island of Montserrat in the Lesser Antilles, became active in 1995 and has been erupting ever since with 5 major phases of extrusive activity. We use data recored on a network of seismometers between 1996 and 2007 partially spanning three eruptive phases. Shear-wave splitting in the crust is often assumed to be controlled by either by structural features, or by stress aligned cracks. In such a case the polarisation of the fast shear wave (phi) would align parallel to the strike of the structure, or to the maximum compressive stress direction. Previous studies analysing SWS in the region using regional earthquakes observed temporal variations in phi which were interpreted as being caused by stress perturbations associated with pressurisation of a dyke. Our analysis, which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust, shows no clear temporal variation. However, temporal effects cannot be ruled out, as large fluctuations in the rate of VT events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess. Average delay times of approximately 0.2 seconds, similar in magnitude to that reported for much deeper slab events, suggest that the bulk of the anisotropy is in the shallow crust. We observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left-lateral transtensional array of faults which crosses the volcanic complex.