Geophysical responses to an environmentally-boosted volcanic unrest

Campi Flegrei caldera, one of the most dangerous volcanoes in the world, is experiencing the strongest seismic and deformation unrest of the last 40 years. Geophysical, environmental, and geochemical responses during volcanic unrest at any volcano are difficult to couple: this is especially true at a caldera where deformation signals and earthquakes are attributed to magma and fluid/rock interactions at depths as well as tidal and meteoric forcing at the surface. Here, we applied Empirical Orthogonal Functions analysis, a technique developed within climatic and environmental Sciences, to GPS data. The technique allows the spatiotemporal separation of the dominant deep-sourced inflation from environmentally controlled signals associated with extension at Campi Flegrei caldera. This separation bridges the gap between deformation, seismic and geochemical responses, clarifying the processes that started the ongoing volcanic unrest. Persistent meteoric forcing during the 2017-18 hydrological year, located in the middle of a five-year drought, changed the decadal trend of seismic energy and secondary deformation components, pairing their spatial patterns. The result was a block in the carbon dioxide released in 2018 at Solfatara, the primary stress-release valve at the caldera. The subsequent overpressure weakened the fractured eastern caldera, opening pathways for hot materials produced by the dominant deep deformation source to reach the surface. Our results show how environmental forcing can favour volcanic unrest in pressurised calderas.