Seismic precursors of a basaltic paroxysmal explosion track deep gas accumulation and slug upraise

Basaltic volcanoes, though fed with low-viscosity magma, occasionally produce sudden paroxysmal explosions that are unforecasted and whose triggering mechanism remains poorly elucidated. Here we report on the first detection of seismic signals precursory to such an explosion on 5 April 2003, at Stromboli volcano (Italy). This strongest event in the past 73 years was preceded by similar to 25 h of seismic tremor variation, broadly coincident with strong geochemical anomalies in crater plume emissions, followed by similar to 15 h of tilt-related long-period inflation pulses of increasing amplitude. These precursory signals are best explained by accelerating growth and leakage of a bubble melt "foam" layer formed at similar to 10 km depth, whose collapse triggered the fast ascent of CO2-rich gas slugs and then the explosion. Our results open new perspectives for the forecasting of such paroxysmal explosions and associated hazards.