Reservoir Structure and Hydraulic Properties of the Campi Flegrei Geothermal System Inferred by Audiomagnetotelluric, Geochemical, and Seismicity Study

The Campi Flegrei caldera is a large volcanic complex lying in the Campanian Plain, Southern
Italy. During its history the caldera experienced episodes of bradyseism and intense swarm seismicity. The
mechanism leading to unrest episodes is still debated, and great efforts are ongoing to improve the
knowledge of this structure and its evolution due to the high volcanic risk in such a densely populated area.
Here we present a resistivity model from a two‐dimensional inversion of audiomagnetotelluric data acquired
along an approximately 5.6‐km long profile crosscutting the Solfatara‐Pisciarelli district and the Agnano
plain. The resistivity model shows (1) very low resistivity values confined in the first 500 m of depth both in
correspondence of the Solfatara‐Pisciarelli districts and the Agnano depression; (2) a resistive plume that
extends underneath the Solfatara crater down to 2,000‐ to 3,000‐m depth, and (3) an adjoining relative
conductive unit eastward. We discuss the resistivity structures in a multidisciplinary framework integrating
inedited geochemical and seismological observations with existing surface geology and subsurface
information. The Solfatara‐Pisciarelli district and the Agnano plain, both being expression of intense
hydrothermal activity, show different characteristics. Below the Solfatara‐Pisciarelli area, the shallow
conductive zone is interpreted as a faulted clay cap that overlies a highly active vapor‐dominated reservoir
characterized by a convective mechanism. Conversely, below the Agnano plain, a liquid phase seems to
prevail in the reservoir. The spatiotemporal variations of seismicity imply a combined action of preexisting
tectonic lineaments and fluid interaction between the gas/steam reservoir and the outflow zone.