Surface Deformation at the Soufriere Hills Volcano, Montserrat, B.W.I.: Short Term Analysis of Continuous GPS Data During Periods of High Activity

Continuous Global Positioning System (CGPS) data have been collected at the Soufriere Hills volcano, Montserrat, and used to measure surface deformation during the current eruption (1995-2000). The data were reprocessed to determine whether absolute point positioning averaged over 24, 12, and 4 hour periods is sufficiently precise to record significant magmatic events as manifested by surface activity and seismicity. A rapid change in the position of the continuous sites was observed during the explosion of September 17-18, 1996. The CGPS site at Reid's Hill Estate (REID), located about 3 km southwest of the active growth area, moved north about 30 mm (\pm2 mm) and subsided 45-60 mm (\pm8 mm). This change was followed by a period of strain relaxation, when the site moved back to its original trend. The site at Hermitage Estate (HERM), located less than 1.5 km northeast of the dome, subsided about 40 mm (\pm15 mm), but no short term trends were found in the horizontal component. Reprocessing the data in shorter intervals (12 and 4 hours) confirmed the results. The detection limit of the GPS technique is likely between 12 and 4 hours for similar magnitude magmatic events at stratovolcanoes in the tropics. A major change in the GPS data was observed before the second phase of dome growth started (November 1999), serving as a significant precursor to this important event. Other measurements (i.e. dome growth parameters, extrusion and ascent rates) are not taken with enough temporal precision to correlate with the observed changes recorded by the CGPS.