Real-time monitoring the hydrothermal outgassing of the active Campi Flegrei caldera through seismo-acoustic observations

Fumarolic and hydrothermal activities represent the shallow evidence of volcanic degassing within calderas, offering valuable insights for volcano monitoring. This phenomenon is particularly significant at the Pisciarelli fumarolic field in the Campi Flegrei caldera, Italy, where a continuous and vigorous release of hydrothermal and magmatic fluids occurs. This fumarolic outgassing is also associated with a persistent harmonic tremor and a persistent subtle infrasonic wavefield. Following the recent bradyseismic crisis at Campi Flegrei caldera, we established a permanent seismo-acoustic array to enhance real-time monitoring of temporal changes in fumarolic outgassing in Pisciarelli area. In particular, we exploited the seismo-acoustic wavefield produced by outgassing to assess its intensity and dynamics. Notably, we identified two distinct infrasonic sources, concurrently active and linked to 1) intense boiling in a water pool and 2) overpressurized steam release from fumarolic vents. The well-correlated temporal variations in infrasonic and seismic amplitudes offer insights into the hydrothermal system's outgassing mechanism. Specifically, they shed light on changes in the shallow hydrothermal reservoir pressure changes driving the outflow of hydrothermal gas. The integration of acoustic and seismic observations enhances our understanding of the dynamic nature of fumarolic outgassing at Campi Flegrei caldera. This improved understanding contributes to assessing volcanic risk for the caldera, as any modifications in fumarolic outgassing may indicate pressure changes in the hydrothermal reservoir.