Weak crustal layer beneath Campi Flegrei (Italy) identified: what's its impact on unrest?

Calderas often experience prolonged periods of unrest that are challenging to attribute to magmatic or hydrothermal origins, making it critical to develop a clear picture of these dynamics. The Campi Flegrei caldera (CFc), in southern Italy, is a striking example. Since 2005, the caldera has been undergoing its most recent phase of unrest, marked by increased ground uplift, seismicity and hydrothermal activity, the nature of which remains under debate. Understanding the past and ongoing behaviour of this volcanic system is far from trivial, yet it is of crucial importance. To address this, we focus on investigating the chemico-physical and mechanical properties as well as the dynamics of the CFc shallow crustal structure (first 4 km of depth), where most earthquakes and deformation occur, and their role during periods of unrest. Our study employs a multiscale analysis, integrating results from 4D X-ray microtomography (i.e., 3D imaging during in-situ experiments) of core samples from a ~3 km-deep geothermal well located near the caldera’s center with 3D high-resolution seismic tomography, complemented by magmatic dyke path simulations. At a depth of approximately 2.5–3.0 km, we identify a transition to a weak tuff layer likely acting as a trap for magmatic fluids. Simulations of magma pathways suggest that past intrusions may have stalled at these depth, influenced by caldera unloading stress and magma's neutral buoyancy level. These stalled dykes deformed, heated, and released magmatic fluids, weakening the surrounding rocks. This weak layer may play a pivotal role in generating overpressure, driving deformation and seismicity, and shaping the dynamics of recent CFc unrest episodes, as well as potential future magma ascent events.