THE ONGOING ERUPTION OF THE SOUFRIERE HILLS VOLCANO, MONTSERRAT: A MECHANICAL PERSPECTIVE

Systematic changes in local stress field orientation preceding volcanic eruptions have been documented independently through analysis of fault-plane solutions (FPS) for high-frequency volcanic-tectonic (VT) earthquakes and through analysis of split shear waves from regional tectonic earthquakes. Observed ~90° reorientations of maximum compression preceding eruption or accompanying shallow intrusion have been modeled as the result of inflation of a dike oriented parallel to regional maximum compression. A critical test of this model is the application of both shear-wave splitting analysis and VT FPS analysis to a single volcano-seismic data set, to confirm the occurrence and timing of ~90° stress field reorientation by two independent techniques. Here, we present results from an analysis of shear-wave splitting in regional and local earthquakes recorded during the 1995-present eruption of the Soufriere Hills Volcano, Montserrat. A completed study of VT FPS for this eruption documented ~90° local stress field reorientations occurring prior to all three phases of the Soufriere Hills eruption as well as prior to a major shift from effusive to explosive activity in 1997. Three-component waveforms of all regional earthquakes occurring within the shear-wave window during the period October 1996-April 1999 were analyzed using the splitting code SHEBA, resulting in 44 high-quality measurements of fast wavelet polarization direction (FWPD), a proxy for the orientation of maximum compression. During the period leading up to the onset of a sequence of Vulcanian eruptions in June 1997 the FWPD was NW-SE, similar to an ephemeral trend in FPS p-axis orientations (another proxy for the orientation of maximum compression) observed prior to each eruption phase. During a period of residual extrusive activity from April 1998-April 1999, the FWPD was NE-SW, again similar to the orientation of FPS p-axes for this period. Agreement between the two independent proxies for local maximum compressive stress orientation confirms the ~90° change in local stress field orientation between periods of volcanic activity and periods of quiescence at Soufriere Hills, and provides strong support for a causative model involving inflation of an ideally-oriented dike beneath the Soufriere Hills vent.