Seismic expression of magma-induced crustal strains and localized fluid pressures during initial eruptive stages, Soufriere Hills Volcano, Montserrat

We explore volcanotectonic earthquake activity during the early stages of volcanic activity (1995-96) at Soufriere Hills volcano. We focus on several zones of temporallyconfined seismic activity that comprise assemblages of small scale structural elements, >2-4 km distant from the volcano, at depths 2-4 km below sea level, and consider seismicity in relation to regional tectonics and heterogeneity of stiffness and strength as revealed by the SEA-CALIPSO tomography experiment. The clustered seismicity and relatively aseismic zones are interpreted to reflect a broad weak-ened tectonic zone of ESE trend that crosses Montserrat, and the ascent of a magmatic dike of NNE trend, which altered the stress distribution to promote localized fault movements and caused localized dilatation with changes in pore-fluid pressures, to either weaken or strengthen the rock mass depending on the local polarity of strain.