HONORING THE MEMORY OF DAVID VARNES: THE BOXING DAY 1997 SECTOR COLLAPSE, DEBRIS AVALANCHE, AND DEPRESSURIZATION BLAST ON MONTSERRAT, BWI

Dave's contributions to the engineering geology of landslides were legion, and among his interests were landslide classifications, hazards and risk zonation, failure prediction, and even the giant collapses of volcanic edifices. He participated in hazards evaluations at Mount St Helens volcano in early 1980, and I'm sure he would have enjoyed examining a similar, and rare, situation that developed on the Caribbean island of Montserrat in 1996-1997. The southern sector of Soufrière Hills Volcano on Montserrat failed on 26 December 1997 (Boxing Day), after a year of disturbance culminating in a devastating eruptive episode. Sector collapse produced a ~50 million m³ volcanic debris avalanche and depressurized the interior of the lava dome, which exploded to generate a powerful pyroclastic blast. The growth of a lava lobe and build-up of lava-block talus, since early November 1997, brought the hydrothermally weakened southern sector to a condition of marginal stability. Stability and stress-deformation analyses, constrained by geomechanical testing of edifice and debris avalanche materials, suggest that the sector collapse was triggered by the 24-26 December pulse of co-seismic lava emplacement, with slip-surface localization influenced by strain-weakening. The resulting debris avalanche was emplaced in 3 minutes at an average speed of 40 m/s; the velocity of the pyroclastic blast exceeded 80 m/s and generated damage to concrete buildings analogous to, remarkably, that at Hiroshima.

The possibility of an edifice collapse was recognized in 1996, and a precautionary evacuation in late 1996 prevented loss of life in this violent event. A periodic 6-7 week cycle of strongly oscillating pressurized extrusion had also been recognized from tilt and seismic monitoring since Summer 1997, the beginnings of which were correlated with major dome collapse and/or explosion events; as a result, occurrence of a major event in mid to late December was anticipated by the scientific staff.