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 five major phases of extrusive activity. We use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases. Shear-wave splitting in the crust is often assumed to be controlled either by structural features, or by stress aligned cracks. In such a case the polarization of the fast shear wave (ϕ) would align parallel to the strike of the structure, or to the maximum compressive stress direction. Previous studies analyzing SWS in the region using regional earthquakes observed temporal variations in ϕ which were interpreted as being caused by stress perturbations associated with pressurization 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 s, similar in magnitude to those 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.
Highlights
Active volcanoes experience dynamic processes such as dyke pressurization, and the migration of magmatic, hydrothermal and meteoric fluids, which may produce complex heterogeneous stress fields
The island of Montserrat lies between the islands of Guadeloupe and Nevis in the Lesser Antilles arc, which was formed as a result of subduction of the North American plate beneath the Caribbean plate
Wadge (1986) mapped dyke orientations throughout the Lesser Antilles and found evidence of a change in S H from approximately arc-normal in the northern portion of the arc to arc-parallel in the south, with Montserrat lying near the transition between these two zones
Summary
Active volcanoes experience dynamic processes such as dyke pressurization, and the migration of magmatic, hydrothermal and meteoric fluids, which may produce complex heterogeneous stress fields. This may be further complicated through their interaction with local tectonic structures such as active faults. Boness and Zoback, 2006; Johnson et al, 2011) This approach may be used to investigate spatial and temporal changes in rock properties, providing a powerful tool for monitoring volcanoes. We investigate seismic anisotropy of the upper crust in the vicinity of Soufrière Hills Volcano (SHV) on the island of Montserrat, using SWS analysis from volcano-tectonic (VT) events. Mapping of fault systems in the upper plate in the region around Montserrat (Feuillet et al, 2002, 2010; Kenedi et al, 2010) indicates localized tectonic domains
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