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  • Seismic data and metadata. Grant abstract: More than 500 million people live close to active volcanoes. Evidence suggests that, throughout history, societies have been affected and destroyed by catastrophic eruptions. In the 1900s alone almost 100000 people were killed by volcanic explosions and their associated hazards. Explosive eruptions inject enormous columns of ash and debris into the atmosphere and discharge fast avalanches of hot gas and rocks on the slopes of volcanic edifices. Lava dome eruptions represent a style of volcanism of distinctive interest because of their potentially catastrophic effects. The hazards from this type of eruptions are well-known, due to the unpredictable transitions from slow effusion of viscous lava to violent explosive activity, and to the propensity of volcanic domes to suddenly collapse spawning devastating pyroclastic flows. Over the past few decades shifts in eruptive style were reported at several lava dome volcanoes worldwide. The underlying processes driving these transitions, however, remain poorly understood, and geophysical measurements documenting them are also very rare. The Santiaguito lava dome complex in Guatemala has been continuously erupting since 1922 and it has switched several times between effusive and explosive eruption regimes, even displaying the two types of activity simultaneously. At the time of this writing Santiaguito is undergoing a major transition from effusive to explosive behaviour marked by some the largest eruptive events ever recorded at this lava dome complex. The new activity started with a large explosion on 11 April, 2016, which produced an ash column that rose to a height in excess of 4.5 km above the vent and was clearly visible in satellite images. Preliminary estimates by local scientists suggest that this explosion was two orders of magnitude more energetic than anything recorded at Santiaguito over the past 5-6 years. The new activity offers a rare opportunity to document and investigate the geophysical fingerprint of a sudden switch in eruptive style at a lava dome volcano, and to decipher its underlying mechanisms. A geophysical deployment, including seismic, deformation and acoustic measurements is the ideal framework to seize an opportunity that is not frequently available. The proposed experiment will help addressing key scientific questions on activity at lava dome volcanoes, with impact on hazard assessment and risk mitigation in this and other eruption prone areas. The pool of target beneficiaries is broad and includes scientists within academia, civil defense authorities, policy makers and communities living nearby active volcanoes.