Tracking Transient Changes in the Plumbing System at Campi Flegrei Caldera

The Campi Flegrei caldera is experiencing a dramatic episode of unrest with a sharp constant increase in seismicity, ground deformation and gas emission. The ongoing escalation of seismicity in such a densely populated area, with the most energetic earthquake of the last 40 years occurring a few months ago, is generating increasing concern in the community. Until now, we never detected the injection of fresh magma in the shallow system, leaving uncertainties on the proximity of an eruption. We developed a fully non-linear and trans-dimensional approach to 4D tomography (in space and time), with a self-adaptive parametrization, which enables to overcome intrinsic drawbacks of the tomography based on linearized solvers and fixed model parametrization. This novel method enhances the resolution capability of seismic imaging, highlighting the structure of the plumbing system and revealing for the first time the main shallow magma reservoir beneath the caldera. Transient changes in Vp and Vp/Vs depict a progressive accumulation of magmatic fluids beneath the resurging area and the ascent of magma batches at shallow depth. The existence of a similar signature also during the previous 1982-84 unrest 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 forecast and model how restless calderas evolve and may become prone to erupt.