VULCANIAN EXPLOSION DYNAMICS IN 2008-09 AT SOUFRIERE HILLS VOLCANO, MONTSERRAT, FROM STRAINMETER DATA

The Soufrière Hills Volcano (SHV), Montserrat, has been erupting for over 14 years, with three semi-continuous periods of extrusion (~1-2 yr) separated by three long pauses. The third pause ended in late July 2008 but subsequent activity was not continuous, and instead comprised two short periods of extrusion with explosive activity. Vulcanian explosions at the Soufriere Hills Volcano on Montserrat in July and December 2008 and on 3 January 2009 were recorded on the CALIPSO array with Sacks-Evertson dilatometers. (The CALIPSO array contains specialized instruments in four strategically located ~200m-deep-boreholes installed to investigate the dynamics of the SHV magmatic system). The explosions resulted in clear, coherent strain-steps (amplitude range ca. ~5-140 nanostrain). We report here on observations of strainmeter and microbarometer data, which allow us to describe the evolution of the explosions. The strain data are corrected for air pressure effects, and compared to seismic observations, plume heights to >12 km, and mass erupted, in order to constrain the conduit source, overpressure, and the dynamics of the events. The various events share many similarities and strain data for these explosions are (mainly) characterized by contractive dilatation offset at all sites, from 5.4 to 9.3 km from the crater. Barometer data show the influence of atmospheric pressure changes on the dilatometer strains, and give further information on eruption dynamics. The strain steps are correlated with explosion plume height and mass erupted and may be useful for rapid assessment of aviation hazards from airborne ash.