Does Deep Magma Power the Campi Flegrei Caldera? Evidence from Seismic Tomography and Anisotropy

The Campi Flegrei caldera is currently experiencing significant ground deformation and bradyseismic activity, yet the subsurface processes driving this unrest remain debated. Competing interpretations invoke magmatic intrusion, hydrothermal circulation, or regional tectonic stress, with no consensus on the role of magma at depth.Here, we present results from a new probabilistic high-resolution seismic tomography of the Campi Flegrei caldera, imaging both isotropic velocity structure and seismic anisotropy using P-wave arrival times from a recently published, machine-learning–derived seismic catalog (Tan et al., 2025). The model reveals a pronounced low-velocity volume extending from shallow levels down to approximately 7 km depth. Within this region, anisotropy patterns are characterized by vertically oriented fast axes and predominantly horizontal slow axes, consistent with aligned vertical cracks or dike-like structures. The magnitude and spatial coherence of the inferred anisotropy, together with the observed low velocities, are difficult to reconcile with purely hydrothermal or tectonic processes. Instead, they suggest a stress regime compatible with upward pressurization from depth, potentially associated with magmatic intrusion. These interpretations are further supported by numerical modeling that tests alternative source configurations and reproduces the observed anisotropy orientations only when deep magmatic pressurization is included. While alternative mechanisms cannot be fully excluded, our results indicate that magma may play an active role in driving the ongoing deformation of the Campi Flegrei caldera.These findings provide new constraints on the physical processes underlying caldera unrest and have important implications for hazard assessment in one of the most densely populated volcanic regions in the world. Tan, X., Tramelli, A., Gammaldi, S., Beroza, G. C., Ellsworth, W. L., & Marzocchi, W. (2025). A clearer view of the current phase of unrest at Campi Flegrei caldera. Science, 390(6768), 70-75.