Gravimetric Constraints on the Hydrothermal System of the Campi Flegrei caldera

The Campi Flegrei caldera (Italy) has been undergoing unrest over the past five decades including episodes of rapid ground deformation, seismicity and variations in gas emissions. Hydrothermal fluids and gases are released most vigorously in the central sector of the caldera at the fumarolic fields of Solfatara volcano and Pisciarelli. We conducted a high‐precision gravity survey coupled with inverse modelling to image the shallow (<2 km depth) structure of the hydrothermal feeder system. Results indicate the presence of three anomalously low density bodies beneath Pozzuoli, Astroni volcano and the Solfatara/Pisciarelli fumarolic fields. The first two are inferred to be sealed hydrothermal systems trapped beneath impermeable cap rock while the latter depicts a plume‐like geothermal feeder system reaching the surface via a combination of Solfatara's maar‐diatreme structure and the intersection of NW‐SE and NE‐SW trending regional faults. The density contrasts of the reservoirs from background values are best explained by a multi‐phase mixture of caldera‐fill containing a secondary and interconnected void volume fraction of between 0.2 and 0.3 that hosts a vapour volume fraction 휓v of between 0.38 and 1 and a liquid volume fraction 휓l fraction of between 0 and 0.62.} This work highlights the control of volcano‐tectonic structures on fluid movement in the shallow crust of hydrothermally active volcanic systems undergoing sustained or periodic unrest.