Earthquakes at Campi Flegrei have been low magnitude and sparse since 1985, denying onshore monitoring observations of their usual source for structural constraint: seismic tomography. Here we used continuous seismic records from 2011–2013 to reconstruct period-dependent Rayleigh wave group velocity maps of the volcano. The Neapolitan Yellow Tuff rim faults bound high-velocity intracrater products of historical eruptions, which act as a barrier for deep fluid migration. The anomaly with lowest group velocity is aseismic and corresponds to the portion of a fluid storage zone that was fractured during the 1984 volcanic unrest under Pozzuoli town. Earthquake locations show that fluids migrate from this area toward the Solfatara and Pisciarelli fumaroles along shallower low-velocity fractures. The aseismic anomaly is likely fed by a deep-seated offshore magmatic source. Its spatial relation with regional dynamics and observations from historical unrests mark the area as the most likely feeder pathway for fluid and magmatic inputs from depth.
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