Seismically induced soft-sediment deformation phenomena during the volcano-tectonic activity of Campi Flegrei caldera (southern Italy) in the last 15 kyr

We report, for the first time, evidence of seismically induced soft-sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine-transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1 km long tunnel in the Pozzuoli area. The successions host several soft-sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55 - 4.28 kyr BP interval. The volcano-sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors, from the base progressing upwards we recognize; (1) a massive tuff; (2) marine-transitional sands of the La Starza unit and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano-sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft-sediment deformation within the volcano-sedimentary record suggests that moderate earthquakes could occur during future volcano-seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises.