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

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.