Weak Crust Owing Past Magmatic Intrusions Beneath Campi Flegrei Identified: The Engine for Bradyseismic Movements?

Abstract Calderas often experience extended periods of unrest that are challenging to relate to a magmatic or hydrothermal origin, making it crucial to assemble a clear picture of these dynamics. Since 2005, Campi Flegrei caldera (Italy) has experienced accelerating ground uplift, seismicity rates, and degassing. Here we conduct petrological and 4D X‐ray microtomography investigations on cored rocks from a ∼3 km deep geothermal well located near the center of caldera, complemented by 3D high‐resolution seismic tomography. At a depth of ∼2.5–3.0 km we identify the transition to a weak tuff layer likely to trap magmatic fluids. Simulations of magma pathways indicate that stresses generated by caldera unloading may have arrested at the limestone/tuff transition past ascending dykes, which deformed, heated, and released magmatic fluids, deteriorating the surrounding rocks. This weak layer may play a crucial role in building up overpressure, causing deformation and seismicity, thus influencing the dynamics of recent unrests, and possible future magma ascent episodes.