Tracking transient changes in the plumbing system at Campi Flegrei Caldera

Increase in seismicity, gas emission and ground uplift are symptoms of unrest in Quaternary calderas. Since the raising of the alert level in 2012, their continuous increase in the Campi Flegrei caldera is generating growing concerns for such a densely populated area. Until now, we never detected the injection of fresh magma in the shallow system, leaving uncertainties on the proximity of an eruption. Here, we show evidence for episodic refills of magma and magmatic fluids in shallow and deep reservoirs beneath the resurging area, revealed by a new fully non-linear 4D (in space and time) seismic tomography. Transient signatures in tomograms depict the ascent of magma batches at shallow depth, which could have started in 2019, perturbing the shallow circulation of hydrothermal fluids. The existence of a similar signature also during the 1982–84 episode, suggests that accumulation of magma batches may be a common way of caldera to grow. Anyway, the ability to fast track changes associated with magma ascent is crucial to forecasting and modeling how restless calderas evolve and may become prone to erupt.