Evolution of the plumbing system at Campi Flegrei caldera (Italy) during the 2007-2023 unrest

Campi Flegrei Caldera (Italy) is affected by a still ongoing unrest characterized by increasing seismicity rates, gas emissions and ground deformations. Multi-technique geodetic data (satellite, inland and seafloor GNSS measurements) are collected to understand the evolution of the unrest from its beginning in 2007 to the end of 2023, to get insights on the plumbing system. The data show increasing rates of deformation in the last 16 years, focusing in the caldera center. 3D finite element models are employed in a Bayesian inversion framework, including the elastic heterogeneous structure of the underlying medium based on the newest seismic tomography of the area. The analysis divides the whole period into seven distinct time intervals, based on the changes in ground displacement time series. The modeling approach implements a double-source plumbing system: a shallower source with its depth and geometry determined by the inversion framework, and a second deeper tabular source at 8 km depth, consistent with the petrological and geochemical evidence of melt and massive degassing from the deeper portion of the magmatic system. The results reveal that the shallower source beneath Pozzuoli exhibits a gradual decrease in depth, from 5.9 km during 2007-2010 to 3.9 km during 2015-2023. The shallower seismicity, above 3 km depth, is influenced by the long-term straining of the local crust due to the continuous inflation of the shallower source. Concurrently, the deeper tabular source at 8 km depth experiences a limited but constant deflation over time. The depicted plumbing system evidences a sequential growth of horizontal opening and volumetric expansion of the shallower source, likely fed by the deeper source for 16 years at least, for a total volume variation of about 60 million cubic meters. To determine whether the shallower source is fed by fluids and/or magma, we performed calculations of volatiles and/or magma transfer from 8 km upwards, considering different settings. The most plausible scenario accounts for magma ascent associated with degassing and outgassing. Indeed, the volumetric variation calculated from geodetic data for the shallower source reflects the injection of an equivalent volume of magma and a deep source that is continuously refilled by a flux of magma comparable to the ascending one.