Fumarolic degassing dynamics revealed by coupled seismo-acoustic observation (Pisciarelli, Campi Flegrei Caldera, Italy)

Hydrothermal activity is a natural manifestation of volcanic degassing at calderas, testified by the presence of fumarolic fields, boiling pools, steaming ground and soil diffuse degassing, which are of interest for volcano monitoring and surveillance as they can be related to the magma dynamics within the caldera reservoirs. Campi Flegrei (Italy) is a half submerged resurgent caldera with a nested structure located at the western edge of the bay of Naples. Since its last eruption in 1538, several episodes of ground uplift accompanied by seismic swarms and intense degassing have been reported. The last uplift phase started in 2005 and is still ongoing. The Pisciarelli fumarolic field is a key area of the Campi Flegrei caldera where a continuous and vigorous degassing of hydrothermal fluids, of magmatic origin, takes place. Such fumarolic degassing is associated with a persistent harmonic tremor showing within the last decade an increasing amplitude trend that correlates well with the geochemical and geodetic unrest indicators of the caldera. In the framework of the DPC-INGV 2012-2021 Agreement and the LOVE-CF Project, we investigated the seismo-acoustic wavefield produced by fumarolic degassing with the aim of characterizing the source process that produces the harmonic tremor, and to propose a potential seismo-acoustic based tool to estimate the fumarolic gas fluxes in real time.  At this aim, we performed a series of temporary geophysical experiments with the deployment of 4-element small aperture seismo-acoustic arrays equipped, at each array element, by a short-period three-component seismometer and a broadband infrasonic pressure sensor. We show that the harmonic tremor source is located within the fumarolic field at shallow depth (<100m) and is strongly controlled by the dynamics of the water level within the fumarolic conduits. We detected for the first time the nearly continuous acoustic wavefield produced by Pisciarelli’s degassing activity. We recognize two distinct acoustic sources that are active at the same time and associated with 1) the intense bubbling from a water pool and with 2) the over-pressurized vapour degassing from the fumarolic vents. Integration between acoustic and seismic observation allowed us to propose a potential mechanism for tremor generation through a bubble collapse as soon as the volcanic gas approaches the earth surface while ascending through the conduit. Coupled acoustic and seismic observation has brought to a better understanding on the dynamics of fumarolic degassing at Campi Flegrei, paving the way to the design of an innovative tool for the real time monitoring of the fumarolic degassing. This will improve our capability to assess the volcanic risk for the Campi Flegrei Caldera, as any changes in fumarolic degassing may be related to a change in the on-going unrest dynamics.